Technical Field
The present invention relates to diagnostic imaging and more particularly,
to a radiography X-ray detector having an X-ray shield which protects the detector
electronics and reduces or eliminates the impact of backscattered X-rays during
the exposure of the subject to the X-ray source.
Background Art
X-ray imaging is a non-invasive technique to capture medical images of
patients or animals as well as to inspect the contents of sealed containers, such
as luggage, packages, and other parcels. To capture these images, an X-ray
beam irradiates an object. The X-rays are then attenuated as they pass through
the object. The degree of attenuation varies across the object as a result of
variances in the internal composition and/or thickness of the object. The
attenuated X-ray beam impinges upon an X-ray detector designed to convert the
attenuated beam to a usable shadow image of the internal structure of the object.
Increasingly, radiography flat panel detectors (RFPDs) are being used to
capture images of objects during inspection procedures or of body parts of
patients to be analyzed. These detectors can convert the X-rays directly into
electric charges (direct conversion direct radiography - DCDR), or in an indirect
way (indirect conversion direct radiography - ICDR).
In direct conversion direct radiography, the RFPDs convert X-rays directly
into electric charges. The X-rays are directly interacting with a photoconductive
layer such as amorphous selenium (a-Se).
In indirect conversion direct radiography, the RFPDs have a scintillating
phosphor such as CsI:Tl or Gd2O2S which converts X-rays into light which then
interacts with an amorphous silicon (a-Si) semiconductor layer, where electric
charges are created.
The created electric charges are collected via a switching array, comprising
thin film transistors (TFTs). The transistors are switched-on row by row and
column by column to read out the signal of the detector. The charges are
transformed into voltage, which is converted in a digital number that is stored in a
computer file which can be used to generate a softcopy or hardcopy image.
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Recently Complementary Metal Oxides Semiconductors (CMOS) sensors are
becoming important in X-ray imaging. The detectors based on CMOS are already
used in mammography, dental, fluoroscopy, cardiology and angiography images.
The advantage of using those detectors is a high readout speed and a low
electronic noise.
Generally, the imaging array including TFTs as switching array and
photodiodes (in case of ICDR) is deposited on a thin substrate of glass. The
assembly of scintillator or photoconductor and the imaging array on the glass
substrate does not absorb all primary radiation, coming from the X-ray source and
transmitted by the object of the diagnosis. Hence the electronics positioned under
this assembly are exposed to a certain fraction of the primary X-ray radiation.
Since the electronics are not sufficiently radiation hard, this transmitted radiation
may cause damage.
Moreover, X-rays which are not absorbed by the assembly of scintillator or
photoconductor and the imaging array on the glass substrate, can be absorbed in
the structures underneath the glass substrate. The primary radiation absorbed in
these structures generates secondary radiation that is emitted isotropically and
that thus exposes the imaging part of the detector. The secondary radiation is
called "backscatter" and can expose the image part image of the detector, thereby
introducing artefacts into the reconstructed image. Since the space under the
assembly is not homogeneously filled, the amount of scattered radiation is
position dependent. Part of the scattered radiation is emitted in the direction of the
assembly of scintillator or photoconductor and imaging array and may contribute
to the recorded signal. Since this contribution is not spatially homogeneous this
contribution will lead to haze in the image, and, therefore, reduce the dynamic
range. It will also create image artefacts.
To avoid damage to the electronics and image artefacts due to scattered
radiation, an X-ray shield may be applied underneath the assembly of scintillator
or photoconductor and imaging array. Because of their high density and high
intrinsic stopping power for X-rays, metals with a high atomic number are used as
materials in such an X-ray shield. Examples of these are sheets or plates from
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tantalum, lead or tungsten as disclosed in EP1471384B1, US2013/0032724A1
and US2012/0097857A1.
However, metals with a high atomic number also have a high density.
Hence, X-ray shields based on these materials have a high weight. Weight is an
important characteristic of the RFPD especially for the portability of the RFPDs.
Any weight reduction is, therefore, beneficial for the users of the RFPDs such as
medical staff.
US7317190B2 discloses a radiation absorbing X-ray detector panel support
comprising a radiation absorbing material to reduce the reflection of X-rays of the
back cover of the X-ray detector. The absorbing material containing heavy atoms
such as lead, barium sulphate and tungsten can be disposed as a film via a
chemical vapour deposition technique onto a rigid panel support or can be mixed
via injection moulding with the base materials used to fabricate the rigid panel
support.
In US5650626, an X-ray imaging detector is disclosed which contains a
substrate, supporting the conversion and detection unit. The substrate includes
one or more elements having atomic numbers greater than 22. Since the
detection array is directly deposited on the substrate, the variety of suitable
materials of the substrate is rather limited.
In US5777335, an imaging device is disclosed comprising a substrate,
preferably glass containing a metal selected from a group formed by Pb, Ba, Ta or
W. According to the inventors, the use of this glass would not require an additional
X-ray shield based on lead. However, glass containing sufficient amounts of
metals from a group formed by Pb, Ba, Ta or W is more expensive than glass
which is normally used as a substrate for imaging arrays.
US7569832 discloses a radiographic imaging device, namely a RFPD,
comprising two scintillating phosphor layers as scintillators each one having
different thicknesses and a transparent substrate to the X-rays between said two
layers. The use of an additional phosphor layer at the opposite side of the
substrate improves the X-ray absorption while maintaining the spatial resolution.
The presence of the additional phosphor layer as disclosed is not sufficient to
absorb all primary X-ray radiation to prevent damage of the underlying electronics
5
and to prevent backscatter. An extra X-ray shield will still be required in the design
of this RFPD.
In US2008/011960A1 a dual-screen digital radiography apparatus is
claimed. This apparatus consists of two flat panel detectors (front panel and back
panel) each comprising a scintillating phosphor layer to capture and process Xrays.
The scintillating phosphor layer in the back panel contributes to the image
formation and has no function as X-ray shield to protect the underlying electronics.
This dual-screen digital flat panel, still requires an X-ray shield to protect the
underlying electronics and to avoid image artefacts due to scattered radiation.
WO2005057235A1 describes a shielding for an X-ray detector wherein
lead or another suitable material is disposed in front of the processing circuits in a
CT-device.
WO20051055938 discloses a light weight film, with an X-ray absorption at
least equivalent to 0.254 mm of lead and which has to be applied on garments or
fabrics for personal radiation protection or attenuation, such as aprons, thyroid
shields, gonad shields, gloves, etc. Said film is obtained from a polymer latex
mixture comprising high atomic weight metals or their related compounds and/or
alloys. The suitable metals are the ones that have an atomic number greater than
45. No use of this light weight film in a RFPD is mentioned. Although a light weight
film is claimed, the metal particles used in the composition of the film still
contribute to a high extend to the weight of the shield.
US6548570 discloses a radiation shielding composition to be applied on
garments or fabrics for personal radiation protection. The composition comprised
a polymer, preferably an elastomer, and a homogeneously dispersed powder of a
metal with high atomic number in an amount of at least 80% in weight of the
composition as filler. A loading material is mixed with the filler material and
kneaded with the elastomer at a temperature below 180ºC resulting in a radiation
shielding composition that can be applied homogeneously to garments and fabrics
on an industrial scale. The use of metals is however increasing the weight of the
shield of this invention considerably.
WO2009/0078891 discloses a radiation shielding sheet which is free from
lead and other harmful components having a highly radiation shielding
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performance and an excellent economical efficiency. Said sheet is formed by
filling a shielding material into an organic polymer material, the shielding material
being an oxide powder containing at least one element selected from the group
consisting of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),
samarium (Sm), europium (Eu) and gadolinium (Gd) and the polymer being a
material such as rubber, thermoplastic elastomer, polymer resin or similar. The
volumetric amount of the shielding material filled in the radiation shielding sheet is
40 to 80 vol. % with respect to the total volume of the sheet. No use of this light
weight film in a RFPD is mentioned.
From the foregoing discussion, it should be apparent that there is a need
for a RFPD with an X-ray shield to protect the underlying electronics and to
absorb the scattered radiation produced by the underlying structures to avoid
image artefacts in the imaging area, but which has a low weight, a low cost and
which can be produced in an economically efficient way.
Summary of invention
It is therefore an object of the present invention to provide a solution for the
high weight contribution of the X-ray shield in a radiography flat panel detector
having a single imaging array and to provide at the same time a solution for
producing the X-ray shield on an economically efficient way. The object has been
achieved by a radiography flat panel detector as defined in claim 1. The X-ray
shield is the combination of the 2nd substrate and the X-ray absorbing layer as
defined in claim 1.
An additional advantage of the RFPD as defined in claim 1, is that the
thickness of said X-ray shield can be adjusted in a continuous way to the required
degree of the X-ray shielding effect instead of in large steps as it is in the case of
shielding metal sheets commercially available with standard thicknesses. Even
though plates with custom made thickness can be purchased, the price of those
metal plates is still very high because of the customization.
In accordance with another aspect of the present invention, the
composition of the X-ray shield leads to an X-ray absorbing layer which is
mechanically strong enough to avoid sealing the layer with a second substrate or
7
which do not require expensive moulding techniques. Moreover, the X-ray shield
comprises a second substrate, the X-ray shield contributes thus to the mechanical
strength of the whole RFPD and more specifically of the thin fragile glass
substrate of the single imaging array.
According to another aspect, the present invention includes a method of
manufacturing a radiography flat panel detector. The method includes providing a
substrate and coating on said substrate a binder with at least one chemical
compound having a metal element with an atomic number of 20 or more and one
or more non-metal elements.
Other features, elements, steps, characteristics and advantages of the
present invention will become more apparent from the following detailed
description of preferred embodiments of the present invention. Specific
embodiments of the invention are also defined in the dependent claims.
Brief description of drawings
Fig. 1 represents a cross-section of a RFPD according to one embodiment
of the present invention and the underlying electronics, wherein:
1 is the scintillator or photoconductive layer
2 is the single imaging array
3 is the first substrate
4 is the second substrate
5 is the X-ray absorbing layer
6 is the underlying electronics
Fig. 2 represents a cross-section of a RFPD, according to one embodiment
of the present invention, wherein:
1 is the scintillator or photoconductive layer
2 is the single imaging array
3 is the first substrate
4 is the second substrate
5 is the X-ray absorbing layer
6 is the underlying electronics
8
Description of embodiments
The present invention relates to radiography flat panel detector (RFPD)
comprising a scintillator or photoconductive layer, a single imaging array on a first
substrate and an X-ray shield having an X-ray absorbing layer comprising a binder
and a chemical compound having a metal element with an atomic number of 20 or
more and one or more non-metal elements coated on a substrate (2nd substrate).
X-ray absorbing layer
It has been found that X-ray shields can be made with the same X-ray
stopping power but with considerably less weight than X-ray shields consisting of
metals only by use of a layer comprising a binder and one or more chemical
compounds having a metal element with an atomic number of 20 or more and one
or more non-metal elements. Preferably these compounds are oxides or salts
such as halides, oxysulphides, sulphites, carbonates of metals with an atomic
number of 20 or higher. Examples of suitable metal elements with an atomic
number higher than 20 that can be used in the scope of the present invention are
metals such as Barium (Ba), Calcium (Ca), Cerium (Ce), Caesium (Cs),
Gadolinium (Gd), Lanthanum (La), Lutetium (Lu), Palladium (Pd), Tin (Sn),
Strontium (Sr), Tellurium (Te), Yttrium (Y), and Zinc (Zn). A further advantage of
the invention is that these compounds are relatively inexpensive and are
characterised by a low toxicity.
Examples of preferred compounds having a metal element with an atomic
number of 20 or more and one or more non-metal elements, are Caesium iodide
(CsI), Gadolinium oxysulphide (Gd2O2S), Barium fluorobromide (BaFBr), Calcium
tungstate (CaWO4), Barium titanate (BaTiO3), Gadolinium oxide (Gd2O3), Barium
chloride (BaCl2), Barium fluoride (BaF2), Barium oxide (BaO), Cerium oxides,
Caesium nitrate (CsNO3), Gadolinium fluoride (GdF2), Palladium iodide (PdI2),
Tellurium dioxide (TeO2), Tin iodides, Tin oxides, Barium sulphides, Barium
carbonate (BaCO3), Barium iodide, Caesium chloride (CsCl), Caesium bromide
(CsBr), Caesium fluoride (CsF), Caesium sulphate (Cs2SO4), Osmium halides,
Osmium oxides, Osmium sulphides, Rhenium halides, Rhenium oxides, Rhenium
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sulphides, BaFX (wherein X represents Cl or I), RFXn (wherein RF represents
lanthanides selected from: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,
Yb, Lu and X represents halides selected from: F, Cl, Br, I), RFyOz, RFy(SO4)z,
RFySz and/or RFy(WO4)z, wherein n, y, z are independently an integer number
higher than 1. These compounds can produce lower weight X-ray shields and are
easy to handle due to their low hygroscopicity than their pure metal analogues.
The most preferred metallic compounds are: Gd2O2S, Gd2O3, Ce2O3, CsI, BaFBr,
CaWO4, YTaO4 and BaO.
It is another advantage of the present invention that the range of metal
elements which can be used for the X-ray absorbing layer, is much larger than the
corresponding range of the pure metals and/or alloys, since many of them are not
stable in their elemental form. Examples are the alkali metals, the alkaline earth
metals and the rare-earth metals. Additionally the X-ray shield of the present
invention allows the flexible adjustment to X-ray apparatus in such a way that the
thickness of the X-ray absorbing layer is determined in reference to the allowable
dose limit required to attenuate radiation.
The chemical compounds having a metal element with an atomic number
of 20 or more and one or more non-metal elements may be used in the X-ray
absorbing layer of the present invention as powder dispersed in a binder. The
amount of the binder in the X-ray absorbing layer in weight percent can vary in the
range from 1% to 50%, preferably from 1% to 25%, more preferably from 1% to
10%, most preferably from 1% to 3%.
Suitable binders are e.g. organic polymers or inorganic binding
components. Examples of suitable organic polymers are polyethylene glycol
acrylate, acrylic acid, butenoic acid, propenoic acid, urethane acrylate, hexanediol
diacrylate, copolyester tetracrylate, methylated melamine, ethyl acetate, methyl
methacrylate. Inorganic binding components may be used as well. Examples of
suitable inorganic binding components are alumina, silica or alumina
nanoparticles, aluminium phosphate, sodium borate, barium phosphate,
phosphoric acid, barium nitrate.
Preferred binders are organic polymers such as cellulose acetate butyrate,
polyalkyl (meth)acrylates, polyvinyl-n-butyral, poly(vinylacetate-co-vinylchloride),
10
poly(acrylonitrile-co-butadiene-co-styrene), poly(vinyl chloride-co-vinyl acetate-covinylalcohol),
poly(butyl acrylate), poly(ethyl acrylate), poly(methacrylic acid),
poly(vinyl butyral), trimellitic acid, butenedioic anhydride, phthalic anhydride,
polyisoprene and/or a mixture thereof. Preferably, the binder comprises one or
more styrene- hydrogenated diene block copolymers, having a saturated rubber
block from polybutadiene or polyisoprene, as rubbery and/or elastomeric
polymers. Particularly suitable thermoplastic rubbers, which can be used as blockcopolymeric
binders, in accordance with this invention, are the KRATON™ G
rubbers, KRATON™ being a trade name from SHELL.
In case the coating of the X-ray absorbing layer is to be cured, the binder
includes preferably a polymerizable compound which can be a monofunctional or
polyfunctional monomer, oligomer or polymer or a combination thereof. The
polymerizable compounds may comprise one or more polymerizable groups,
preferably radically polymerizable groups. Any polymerizable mono- or
oligofunctional monomer or oligomer commonly known in the art may be
employed. Preferred monofunctional monomers are described in EP1637322 A
paragraph [0054] to [0057]. Preferred oligofunctional monomers or oligomers are
described in EP1637322A paragraphs [0059] to [0064]. Particularly preferred
polymerisable compound are urethane (meth)acrylates and 1,6-
hexanedioldiacrylate. The urethane (meth)acrylates are oligomer which may have
one, two, three or more polymerisable groups.
Suitable solvents, to dissolve the binder being an organic polymer during
the preparation of the coating solution of the X-ray absorbing layer can be
acetone, hexane, methyl acetate, ethyl acetate, isopropanol, methoxy propanol,
isobutyl acetate, ethanol, methanol, methylene chloride and water. The most
preferable ones are toluene, methyl-ethyl-ketone (MEK) and methyl cyclohexane.
To dissolve suitable inorganic binding components, water is preferable as the
main solvent. In case of a curable coating liquid, one or more mono and/or
difunctional monomers and/or oligomers can be used as diluents. Preferred
monomers and/or oligomers acting as diluents are miscible with the above
described urethane (meth)acrylate oligomers. The monomer(s) or oligomer(s)
used as diluents are preferably low viscosity acrylate monomer(s).
11
The X-ray absorbing layer of the present invention may also comprise
additional compounds such as dispersants, plasticizers, photoinitiators,
photocurable monomers, antistatic agents, surfactants, stabilizers oxidizing
agents, adhesive agents, blocking agents and/or elastomers.
Dispersants which can be used in the present invention include nonsurface
active polymers or surface-active substances such as surfactants, added
to the binder to improve the separation of the particles of the chemical compound
having a metal element with an atomic number of 20 or more and one or more
non-metal elements and to further prevent settling or clumping in the coating
solution. Suitable examples of dispersants are Stann JF95B from Sakyo and
Disperse Ayd™ 1900 from Daniel Produkts Germany. The addition of dispersants
to the coating solution of the X-ray absorbing layer improves further the
homogeneity of the layer.
Suitable examples of plasticizers are Plastilit™ 3060 from BASF, Santicizer
™ 278 from Solutia Europe and Palatinol™ C from BASF. The presence of
plasticizers to the X-ray absorbing layer improves the compatibility with flexible
substrates.
Suitable photo-initiators are disclosed in e.g. J.V. Crivello et al. in “
Photoinitiators for Free Radical, Cationic & Anionic Photopolymerisation 2nd
edition”, Volume III of the Wiley/SITA Series In Surface Coatings Technology,
edited by G. Bradley and published in 1998 by John Wiley and Sons Ltd London,
pages 276 to 294.Examples of suitable photoinitiators can be Darocure™ 1173
and Nuvopol™ PI-3000 from Rahn. Examples of suitable antistatic agents can be
Cyastat™ SN50 from Acris and Lanco™ STAT K 100N from Langer.
Examples of suitable surfactants can be Dow Corning™ 190 and Gafac
RM710, Rhodafac™ RS-710 from Rodia. Examples of suitable stabilizer
compounds can be Brij™ 72 from ICI Surfactants and Barostab™ MS from
Baerlocher Italia. An example of a suitable oxidizing agent can be lead (IV) oxide
from Riedel De Haen. Examples of suitable adhesive agents can be Craynor™
435 from Cray Valley and Lanco™ wax TF1780 from Noveon. An example of a
suitable blocking agent can be Trixene™ BI7951 from Baxenden. An example of a
suitable elastomer compound can be Metaline™ from Schramm).
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The thickness of the X-ray absorbing layer can vary as well and depends
on the necessary shielding power and/or the space available to incorporate the Xray
shield in the design of the RFPD. In the present invention, the thickness of the
X-ray absorbing layer can be at least 0.1 mm, more preferably in the range from
0.1 mm to 1.0 mm.
Depending on the application, the coating weight of the chemical
compound having a metal element with an atomic number of 20 or more and one
or more non metal elements the X-ray shields can be adjusted and in case of
using a RFPD for medical purposes, this coating weight is preferably at least 100
mg/cm2, more preferably at least 200 mg/cm².
Substrate for the X-ray absorbing layer
The substrate for the X-ray absorbing layer of the X-ray shield according to
the invention, hereafter denoted as the second substrate, can be either rigid or
flexible, such as an aluminium plate, an aluminium foil, a film of polyethylene
terephthalate (PET), polyethylene naphthalate (PEN), polyimide (PI),
polyethersulphone (PES), a metal foil, a carbon fibre reinforced plastic (CFRP)
sheet, glass, flexible glass, triacetate and a combination thereof or laminates
thereof. Preferred materials for the second substrate of the invention are PET,
glass and aluminium due to their low weight, their low cost and their availability.
Suitable substrates for the invention also include substrates which are
substantially not transparent to light by incorporating a light absorbing or light
reflecting material into the substrate.
More preferable substrates are flexible sheets obtained from for example
PET, aluminium or flexible glass. The application of an X-ray absorbing layer onto
the substrate (2nd substrate) as described above is preferably done by means of a
coating method. Coating is an economically efficient technique of application of
one or more layers onto a substrate. By means of coating techniques, the X-ray
absorbing layer can be applied together with light absorbing or light reflecting
layers, adhesion layers etc. Flexible substrates are particularly suitable for a
continuous coating process. Moreover, flexible substrates can be available as rolls
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and they can be wound and un-wound in the production process of coating and
drying or curing.
White coloured layers may be used to reflect light emitted by the
scintillating phosphor in the X-ray absorbing layer. Layers comprising TiO2 are
preferably used to reflect 90% or more light at the wavelength(s) of the light
emitted by the scintillating phosphor. The solid content of TiO2 in the light
reflecting layer is preferably in the range of 25 to 50 (wt.)%. and the thickness is
preferably in the range of 5 to 40 μm. More preferably, the solid content of the
TiO2 is 33 to 38(wt.)% of the total solid content of the layer and the layer thickness
is between 13 and 30 μm.
In another preferred embodiment of the invention, black coloured layers
can be used to absorb light emitted by a scintillating phosphor in the X-ray
absorbing layer because of their high efficiency to absorb light. Black particles,
such as fine carbon black powder (ivory black, titanium black, iron black), are
suitable to obtain sufficient absorption of emitted light by the scintillating phosphor.
Preferably the solid content of carbon black is in the range of 3 to 30 (wt.)% and a
layer thickness of 2 to 30 μm will absorb 90% or more of the emitted light by the
scintillating phosphor. More preferably the range of the solid content of the carbon
black is in the range of 6 to 15 (wt.)% and the layer thickness between 5 and15
μm. In another embodiment of the invention, coloured pigments or dyes absorbing
specifically at the maximal wavelength of the emitted light by the scintillating
phosphor in the X-ray absorbing layer can be used.
Scintillator
In the RFPD for indirect conversion direct radiography according to the
present invention, the scintillator comprises optionally a support and provided
thereon, a scintillating phosphor such as Gd2O2S:Tb, Gd2O2S:Eu, Gd2O3:Eu,
La2O2S:Tb, La2O2S, Y2O2S:Tb, CsI:Tl, CsI:Eu, CsI:Na, CsBr:Tl, NaI:Tl, CaWO4,
CaWO4:Tb, BaFBr:Eu, BaFCI:Eu, BaSO4:Eu, BaSrSO4, BaPbSO4, BaAI12O19:Mn,
BaMgAl10O17:Eu, Zn2Si04:Mn, (Zn, Cd)S:Ag, LaOBr, LaOBr:Tm, Lu2O2S:Eu,
Lu2O2S:Tb, LuTa04, HfO2:Ti, HfGe04:Ti, YTa04, YTa04:Gd, YTa04:Nb, Y2O3:Eu,
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YBO3:Eu, YBO3:Tb, or (Y,Gd)BO3:Eu, or combinations thereof. Besides crystalline
scintillating phosphors, scintillating glass or organic scintillators can also be used.
When evaporated under appropriate conditions, a layer of doped CsI will
condense in the form of needle like, closely packed crystallites with high packing
density onto a support. Such a columnar or needle-like scintillating phosphor is
known in the art. See, for example, ALN Stevels et al. , "Vapor Deposited CsI:Na
Layers: Screens for Application in X-Ray Imaging Devices, " Philips Research
Reports 29:353-362 (1974); and T. Jing et al, "Enhanced Columnar Structure in
CsI Layer by Substrate Patterning", IEEE Trans. Nucl. Sci. 39: 1195-1198 (1992).
More preferably, the scintillating phosphor layer includes doped CsI.
A blend of different scintillating phosphors can also be used. The median
particle size is generally between about 0. 5 μm and about 40 μm. A median
particle size of between 1 μm and about 20 μm is preferred for ease of
formulation, as well as optimizing properties, such as speed, sharpness and
noise. The scintillator for the embodiments of the present invention can be
prepared using conventional coating techniques whereby the scintillating
phosphor powder, for example Gd2O2S is mixed with a solution of a binder
material and coated by means of a blade coater onto a substrate. The binder can
be chosen from a variety of known organic polymers that are transparent to Xrays,
stimulating, and emitting light. Binders commonly employed in the art include
sodium o-sulfobenzaldehyde acetal of poly(vinyl alcohol); chloro-sulfonated
poly(ethylene); a mixture of macromolecular bisphenol poly(carbonates) and
copolymers comprising bisphenol carbonates and poly(alkylene oxides);aqueous
ethanol soluble nylons; poly(alkyl acrylates and methacrylates) and copolymers of
poly(alkyl acrylates and methacrylates with acrylic and methacrylic acid);
poly(vinyl butyral); and poly(urethane) elastomers. Other preferable binders which
can be used are described above in the section of the X-ray absorbing layer. Any
conventional ratio phosphor to binder can be employed. Generally, the thinner
scintillating phosphor layers are, the sharper images are realized when a high
weight ratio of phosphor to binder is employed. Phosphor-to-binder ratios in the
range of about 70:30 to 99:1 by weight are preferable.
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The photoconductive layer
In the RFPD for direct conversion direct radiography according to the
present invention, the photoconductive layer is usually amorphous selenium,
although other photoconductors such as HgI2, PbO, PbI2, TlBr, CdTe and
gadolinium compounds can be used. The photoconductive layer is preferentially
deposited on the imaging array via vapour deposition but can also been coated
using any suitable coating method.
The imaging array and first substrate
The single imaging array used in the invention for indirect conversion direct
radiography is based on an indirect conversion process which uses several
physical components to convert X-rays into light that is subsequently converted
into electrical charges. First component is a scintillating phosphor which converts
X-rays into light (photons). Light is further guided towards an amorphous silicon
photodiode layer which converts light into electrons and electrical charges are
created. The charges are collected and stored by the storage capacitors. A thinfilm
transistor (TFT) array adjacent to amorphous silicon read out the electrical
charges and an image is created. Examples of suitable image arrays are
disclosed in US5262649 and by Samei E. et al., “General guidelines for
purchasing and acceptance testing of PACS equipment”, Radiographics, 24, 313-
334 . Preferably, the imaging arrays as described in US2013/0048866, paragraph
[90-125] and US2013/221230, paragraphs [53-71] and [81-104] can be used.
The single imaging array used in the invention for direct conversion direct
radiography is based on a direct conversion process of X-ray photons into electric
charges. In this array, an electric field is created between a top electrode, situated
on top of the photoconductor layer and the TFT elements. As X-rays strike the
photoconductor, the electric charges are created and the electrical field causes to
move them towards the TFT elements where they are collected and stored by
storage capacitors. Examples of suitable image arrays are disclosed by Samei E.
16
et al., “General guidelines for purchasing and acceptance testing of PACS
equipment”, Radiographics, 24, 313-334.
For both the direct and indirect conversion process, the charges must be
read out by readout electronics. Examples of readout electronics in which the
electrical charges produced and stored are read out row by row, are disclosed by
Samei E. et al., Advances in Digital Radiography. RSNA Categorical Course in
Diagnostic Radiology Physics (p. 49-61) Oak Brook, Ill.
The substrate of the imaging array of the present invention, hereafter
denoted as the ‘first substrate’ is usually glass. However, imaging arrays
fabricated on substrates made of plastics, metal foils can also be used. The
imaging array can be protected from humidity and environmental factors by a
layer of silicon nitride or polymer based coatings such as fluoropolymers,
polyimides, polyamides, polyurethanes and epoxy resins. Also polymers based on
B-staged bisbenzocyclobutene-based (BCB) monomers can be used.
Alternatively, porous inorganic dielectrics with low dielectric constants can also be
used.
The underlying electronics
The underlying electronics, situated under the X-ray absorbing layer
comprise a circuit board which is equipped with electronic components for
processing the electrical signal from the imaging array, and/or controlling the
driver of the imaging array and is electrically connected to the imaging array.
Method of making the radiographic flat panel detector
Method of making the X-ray shield
The X-ray shield of the present invention can be obtained by applying a
coating solution comprising at least one chemical compound having a metal
element with an atomic number of 20 or more and one or more non-metal
elements and a binder onto a substrate (2nd substrate) by any known methods,
17
such as knife coating, doctor blade coating, spin-coating, dip-coating, spraycoating,
screen printing and lamination. The most preferable method is the doctor
blade coating.
In a preferred embodiment the coating solution is prepared by first
dissolving the binder in a suitable solvent. To this solution the chemical compound
having a metal element with an atomic number of 20 or more and one or more
non-metal elements is added. To obtain a homogenous coating solution or
lacquer, a homogenization step or milling step of the mixture can be included in
the preparation process. A dispersant can be added to the binder solution prior to
the mixing with the chemical compound having a metal element with an atomic
number of 20 or more and one or more non-metal elements. The dispersant
improves the separation of the particles in the coating solution and prevents
settling or clumping of the ingredients in the coating solution. The addition of
dispersants to the coating solution of the X-ray absorbing layer decreases the
surface tension of the coating solution and improves the coating quality of the Xray
absorbing layer.
In another embodiment of the invention, the binder being a polymerisable
compound can be dissolved in diluents comprising one or more mono and/or
difunctional monomers and/or oligomers.
After stirring or homogenization the coating solution is applied onto the
substrate preferably using a coating knife or a doctor blade. The substrate can be
the first substrate or a second substrate. If the coating solution is coated on the
first substrate, the coating is preferably performed on the side opposite to the
imaging array. After the coating of the X-ray absorbing layer onto a substrate, the
X-ray absorbing layer can be dried via an IR-source, an UV-source, a heated
metal roller or heated air. When photocurable monomers are used in the coating
solution, the coated layer can be cured via heating or via an UV-source. After
drying or curing, the X-ray shield which is coated on a second substrate can be
cut into sheets of appropriate size.
The obtained X-ray shield, comprising a substrate and an X-ray absorbing
layer comprising a binder and a chemical compound having a metal element with
an atomic number of 20 or more and one or more non-metal elements, can be
18
used as to shield electronics in medical apparatus and non-destructive testing
apparatus from X-rays. The combination of the X-ray absorbing layer with a
substrate, gives the whole X-ray shield sufficient mechanical strength to be used
as a self-contained component in medical devices or non-destructive testing
devices.
Method of making the RFPD for indirect conversion direct radiography
The RFPD for indirect conversion direct radiography according to the
invention is made by assembling the different components which are described
above. A preferred method is now described.
In a first step, the scintillator, which comprises a scintillating phosphor and
a support, is coupled via gluing onto the single imaging array situated on the first
substrate, preferably glass. Gluing is done with pressure sensitive adhesives or
hot melts. Preferably a hot melt is used. Suitable examples of hot melts are
polyethylene-vinyl acetate, polyolefins, polyamides, polyesters, polyurethanes,
styrene block copolymers, polycarbonates, fluoropolymers, silicone rubbers,
polypyrrole. The most preferred ones are polyolefins and polyurethanes due to
the higher temperature resistance and stability. The hot melt is preferably thinner
than 25 μm. The hot melt with a lining is placed onto the surface of the imaging
array. The imaging array on the first substrate, together with the hot melt is then
heated in an oven at a prescribed temperature. After cooling, the lining is removed
and releases a melted hot melt with a free adhesive side. The scintillator is
coupled to the imaging array by bringing the scintillating phosphor layer in contact
with the adhesive side of the hot melt and by applying a high pressure at a high
temperature. To achieve a good sticking over the complete area of the imaging
array, a pressure in a range from 0.6 to 20 bars has to be applied and a
temperature value in a range from 80 – 220°C, during between 10 and 1000 s is
required. A stack of scintillator-imaging array-first substrate is thereby formed. In
one preferred embodiment of the invention, this stack can be positioned above the
underlying electronics which perform the processing of the electrical signal from
the imaging array, or the controlling of the driver of the imaging array.
19
In the last step, the X-ray shield is coupled to the first substrate at the
opposite side of the single imaging array. Either the second substrate or the X-ray
absorbing layer of the X-ray shield can be contacted with the first substrate. A
preferable method is after making of the contact, to immobilise the components of
the obtained stack by cold roll lamination or heated roll lamination using foils
having a protective ability. The best suitable foils are the polyethylene, polyester,
polyvinylchloride or acrylic based foils with a thickness of maximum 100μm.
Another preferable method is using a pressure sensitive glue or hot melt. A hot
melt with a lining is placed onto either the substrate (2nd substrate) or the X-ray
absorbing layer of the X-ray shield. The X-ray shield is then heated, preferably in
an oven at the prescribed temperature. After cooling, the lining is removed and
releases a melted hot melt with a free adhesive side. The X-ray shield is coupled
to the imaging array by bringing the first substrate of the stack into contact with
the adhesive side of the hot melt and by applying a high pressure at a high
temperature. To achieve a good sticking over the complete area of the
components to be glued, a pressure in a range from 0.6 to 20 bars has to be
applied and a temperature value in a range from 80 – 220°C, during between 10
and 1000 s is required.
In a preferred embodiment of the invention, the scintillating phosphor is
directly applied on the single imaging array via a coating or deposition process.
This method has the advantage that no gluing is required and hence omits at least
one step in the production process of the RFPD.
Method of making the RFPD for direct conversion direct radiography
The FPD for direct conversion direct radiography according to the invention
is made by assembling the different components which are described above.
A preferred method is as follows: the photoconductor, preferably
amorphous selenium is deposited onto the single imaging array situated on the
first substrate which is preferably glass. Examples of deposition methods are
disclosed in Fischbach et al.,’Comparison of indirect CsI/a:Si and direct a:Se
digital radiography’, Acta Radiologica 44 (2003) 616-621. After providing a top
20
electrode on top of the photoconductive layer, the single imaging array with the
photoconductor is coupled with the X-ray shield. This can be done according to
the same methods as described for making the RFPD for indirect conversion
direct radiography.
EXAMPLES
1. Measurement methods
1.1 X-ray shielding capacity of the X-ray shields
The X-ray shielding capacity of X-ray shields according to the present
invention (INV) and of a commercially available metal based X-ray shield (COMP)
was measured based on measurements of the optical density of a radiographic
film placed between scintillator and X-ray shield, after X-ray exposure and
development. The radiographic film is commercially available from Agfa
Healthcare (AGFAHDRC1824) and is a green sensitive film with one radiation
sensitive side. The X-ray exposure was performed with a Philips Optimus 80 X-ray
source. The X-ray shield was usually positioned in the following configuration:
scintillator – radiographic film – X-ray shield – scattering elements comprising a
printed circuit board (PCB), a lead strip and a PMMA block. This configuration is
called the standard configuration of the RFPD. The default scintillating phosphor
used was a commercially available GOS scintillator (CAWO Superfine 115 SW,
from CAWO). The scintillating phosphor layer was put in contact with the radiation
sensitive side of the radiographic film. The underlying electronics of a RFPD were
simulated by the PCB with discrete components, the lead strip and the block of
poly(methylmethacrylate). Poly(methylmethacrylate) is used due to its very high
scattering properties.
To achieve good contact between the components, each X-ray shield was
sealed in a black polyethelene bag (PE, Type B, 260x369 mm, 0,19mm thickness,
from Cornelis Plastic) together with the scintillator and the radiographic film by
means of vacuum. The substrate of the X-ray shield was always in contact with
21
the non-radiation sensitive side of the radiographic film, unless otherwise
specified. The package prepared like this way, is called a basic RFPD.
The X-ray source, the basic RFPD and the scattering elements were
mounted on a horizontal bench. The basic RFPDs were placed at 1.5 m from said
X-ray source. Behind the basic RFPD, the block of PMMA, the strip of lead of 3
mm thickness and the PCB, were placed next to each other to simulate the
underlying electronics of the RFPD. The distance between the scattering elements
and the basic RFPD was less than 0.2 cm. The reference measurement was done
with the basic RFPD configuration without scattering elements behind the RFPD.
Following standard radiation X-ray beam qualities were used: RQA3 (10
mm Al, 52kV), RQA5 (21 mm Al, 73kV), RQA7 (30 mm Al, 88kV), and RQA9 (40
mm Al, 117kV), RQA X-ray beam qualities as defined in IEC standard 61267, 1st
Ed. (1994).
After exposure each film was developed in G138i (Agfa Healthcare) at 33°
C for 90s. and placed in a MacBeth densitometer, type TR-924 to measure the
optical density. The higher the measured optical density, the more backscatter of
X-rays was taking place.
1.2. Weighing of the X-ray shields:
The X-ray shields prepared according to the present invention (INV) and
the comparative X-ray shield (COMP) were weighed on the laboratory scales
(Mettler Toledo PG5002-S) with a resolution of 0.01g.
1.3. X-ray absorption of the X-ray shields:
The X-ray absorption of the X-ray shields was measured with a Philips
Optimus 80 apparatus together with a Triad dosimeter having a 30cc volume cell.
The measuring cell was placed at 1.5 m distance from the X-ray source directly
behind the X-ray shield. The X-ray shield in both cases was placed with its
substrate directed to the X-ray source. Data for each screen were collected
22
multiple times and the average value was calculated together with the standard
deviation.
All tests were done for standard radiation X-ray beam qualities (RQA X-ray
beam qualities as defined in IEC standard 61267, 1st Ed. (1994)): RQA3 (10 mm
Al, 52kV), RQA5 (21 mm Al, 73kV), RQA7 (30 mm Al, 88kV), and RQA9 (40 mm
Al, 117kV) unless otherwise specified.
2. Materials
The materials used in the following examples were readily available from
standard sources such as ALDRICH CHEMICAL Co. (Belgium), ACROS
(Belgium) and BASF (Belgium) unless otherwise specified. All materials were
used without further purification unless otherwise specified.
• Gadolinium oxysulphide (Gd2O2S) or GOS: (CAS 12339-07-0) powder
was obtained from Nichia, mean particle size: 3.3 μm.
• CaWO4, powder was obtained from Nichia, mean particle size: 7.0
μm.
• YTaO4, powder was obtained from Nichia, mean particle size: 4.4 μm.
• White PET substrate: polyethylene terephthalate (PET) film with a
thickness of 0.19 mm, obtained from Mitsubishi, trade name
Hostaphan WO.
• Black PET substrate: polyethylene terephthalate (PET) film a
thickness of 0.188 mm, obtained from Toray, trade name Lumirror
X30.
• Disperse Ayd™ 9100 (Disperse Ayd™ W-22), anionic surfactant/Fatty
Ester dispersant (from Daniel Produkts Company).
• Kraton™ FG1901X (new name = Kraton™ FG1901 GT), a clear,
linear triblock copolymer based on styrene and ethylene/butylene with
a polystyrene content of 30%, from Shell Chemicals.
• Default GOS scintillator, CAWO Superfine 115 SW, from CAWO.
• Caesium Iodide (CsI): (CAS 7789-17-5) powder from Rockwood
Lithium, 99.999%.
23
• Aluminium 318G: plate from Alanod having a thickness of 0.3 mm.
• Imaging array: TFT (according US2013/0048866, paragraph [90-125]
and US2013/221230, paragraphs [53-71] and [81-104]) on Corning
Lotus™ Glass having a thickness of 0.7 mm.
• Radiographic film: AGFAHDRC1824, from Agfa Healthcare
• PMMA: poly(methylmethacrylate), 7 cm thick, 30x30cm, compliant
with ISO 9236-1 standard
• Lead strip: 13cm x 2.5 cm, thickness is 0.3 cm.
• PCB: 13cm x 4.5 cm
3. Preparation of X-ray shields
3.1 Preparation of the solution for coating the X-ray absorbing layer:
4.5 g of binder (Kraton™ FG1901X) was dissolved in 18 g of a solvent
mixture of toluene and MEK (ratio 75:25 wt. / wt.) and stirred for 15 min at a rate
of 1900 r.p.m. The chemical compound having a metal element with an atomic
number of 20 or more and one or more non-metal elements, was added thereafter
as a powder, in an amount of 200g and the mixture was stirred for another 30
minutes at a rate of 1900 r.p.m.
3.2 Preparation of X-ray shields SD-01 to SD-20 (INV):
The coating solution as obtained in § 1.1 was coated with a doctor blade at
a coating speed of 4 m/min onto several PET substrates (white and black) to
obtain different dry layer thicknesses variable from 100 to 450 μm to obtain X-ray
shields SD-01 to SD-20 (see Table 1). Subsequently, the X-ray shields were dried
at room temperature during 30 minutes. In order to remove volatile solvents as
much as possible the coated X-ray shields were dried at 60°C for 30 minutes and
again at 90°C for 20 to 30 minutes in a drying oven. The total thickness of the Xray
absorbing layer was controlled by adjusting the wet layer thickness and/or the
number of layers coated on top of each other after drying each layer. The wet
24
layer thickness has a value between 220 μm and 1500 μm. The size of the
obtained shields was 18 cm X 24 cm.
After coating, each X-ray shield was weighed and the coating weight of the
chemical compound having a metal element with an atomic number of 20 or more
and one or more non metal elements was obtained by applying formula 1. The
results are reported in Table 1
Formula 1
Where:
WF is the weight of an X-ray shield (2nd substrate + X-ray absorbing layer),
WS is the weight of the substrate (2nd substrate) of the X-ray shield,
AS is the surface area of the substrate (2nd substrate),
P% is the amount in weight % of the chemical compound having a metal
element with an atomic number of 20 or more and one or more non-metal
elements in the X-ray absorbing layer.
3.3 Molybdenum X-ray shield SD-21 (COMP)
An X-ray shield consisting of a plate of Molybdenum was obtained from one
of the commercially available RFPDs on the market. The thickness of the
Molybdenum plate was 0.3 mm and the size was 18 cm X 24 cm. The
Molybdenum plate did not contain a substrate. The composition of the plate was
99.85% (wt.) of Mo, and below 0.05% (wt.) of Na, K, Ca, Ni, Cu, and Bi.
The coating weight for this Mo plate was calculated based on formula 1
taking into account that P% is 100 and WS is 0. The results of the calculated
coating weight of the Mo plate, hereafter denoted as SD-21 were reported in
25
Table 1.
Table 1: Coating weights of the inventive X-ray shields (SD-01 to SD-20)
and of Mo in the comparative X-ray shield (SD-21).
Table 1
X-ray shield Chemical compound having a metal
element with an atomic number of
20 or more and one or more nonmetal
elements in the X-ray
absorbing layer
Substrate Coating weight
(mg/cm2)
SD-01 GOS Black PET 110
SD-02 GOS Black PET 152
SD-03 GOS White PET 129
SD-04 GOS White PET 121
SD-05 GOS White PET 116
SD-06 GOS White PET 108
SD-07 GOS White PET 100
SD-08 GOS White PET 96
SD-09 GOS White PET 81
SD-10 GOS White PET 115
SD-11 GOS White PET 40
SD-12 GOS White PET 80
SD-13 GOS White PET 171
SD-14 GOS White PET 195
SD-15 GOS White PET 145
SD-16 GOS White PET 230
SD-17 GOS Black PET 115
SD-18 GOS Black PET 155
SD-19 CaWO4 White PET 75
SD-20 YTaO4 White PET 82
SD-21 - - 302
26
3.4 Preparation of X-ray shields with and without a dispersant
To illustrate the difference between X-ray shields based on GOS and
prepared with or without a dispersant in the coating solution of the X-ray
absorbing layer, two X-ray shields were prepared according to the method
described in §3.1. In both cases a white PET substrate was used. The coating
weight of GOS was 172 mg/cm² for both shields. Shield SD-00.1 was prepared
without dispersant in the coating solution and SD-00.2 was prepared with
dispersant (Disperse Ayd™ 9100) added to the coating solution. Firstly, 0.5 g of
dispersant was dissolved in 11.21 g of a toluene and methyl-ethyl-ketone (MEK)
solvent mixture, having a ratio of 75:25 (w/w) and mixed with the binder solution
as prepared in §3.1. The further preparation steps are the same as in §3.1 and §
3.2. The X-ray absorption of both shields was determined according to the
measuring method 3 with a RQA5 X-ray beam quality and a load of 6.3mAs. The
results are shown in Table 2.
Table 2: X-ray absorption of GOS X-ray shields prepared with or without
dispersant.
Table 2
X-ray
shield
Dispersant Homogeneity of
coated X-ray
absorbing layer
Thickness of
the X-ray
absorbing layer
(μm)
Weight
(g)
X-ray
absorption
(%)
SD-00.1
(INV)
No good 325 152.15 65.51±2.5
SD-00.2
(INV)
Yes perfect 325 152.50 63.80±3.0
As shown in Table 2, the X-ray shield prepared with the dispersant present
in the coating solution had a more homogeneous X-ray absorbing layer for a
comparable weight and X-ray absorption as the X-ray shield prepared without
dispersant. The presence of the dispersant is advantageous for the preparation
27
process of the shields since it further reduces the surface tension and prevents
the floating of μm size particles.
4. X-ray shielding capacity of inventive X-ray shields in comparison with the
comparative shield.
The X-ray shielding capacity of the inventive X-ray shields SD-17 and SD-
18 was therefore measured according measuring method 1.1 in a standard
configuration of the RFPD in comparison with the comparative Mo plate X-ray
shield (SD-21). The X-ray shielding capacity of the inventive X-ray shields SD-19
and SD-20 was measured according measuring method 1.1 in a configuration
wherein the scattering elements consist of a lead strip and a PMMA block, in
comparison with the comparative Mo plate X-ray shield (SD-21). The X-ray
shields, which had the same surface, were weighed according to measuring
method 1.2. The results are shown in Table 3.
Table 3: Difference in optical density of the radiographic film with the inventive Xray
shields compared to the comparative shield SD-21.
Table 3
No. X-ray beam quality X-ray shield Weight (g) Difference in
optical density
7 RQA3 and RQA5 SD-17(INV) 74.69 Decrease
8 RQA7 SD-17(INV) 74.69 Slight increase
9 RQA9 SD-17(INV) 74.69 Increase
10 RQA3 and RQA5 SD-18(INV) 102.47 Decrease
11 RQA7 SD-18(INV) 102.47 Equal
12 RQA9 SD-18(INV) 102.47 Slight increase
13 RQA3 SD-19(INV) 109.00 Decrease
14 RQA5 SD-19(INV) 109.00 Decrease
15 RQA3 SD-20(INV) 119.00 Decrease
16 RQA5 SD-20(INV) 119.00 Equal
17 - SD-21(COMP) 163.46 -
28
These results show that the shielding capacity of the inventive shields are
equal or higher than the comparative X-ray shield based on a Molybdenum plate
and in some cases only a bit lower, but that their weight is significantly lower than
the comparative X-ray shield.
Example 1: Preparation of RFPDs comprising different X-ray shields
RFPDs for indirect conversion direct radiography were prepared by
bringing a scintillator in contact with the above mentioned imaging array on a
glass substrate (Corning Lotus™ Glass). Subsequently this package was brought
into contact with different X-ray shields SD-01 to SD-18 and the Molybdenum
metal plate SD-21.
To assure good optical contact between each layer of the RFPDs, a hot
melt layer based on polyurethane and not thicker than 25 μm, was used. Two
types of scintillators were used: i) a powder-based scintillating phosphor GOS
(CAWO Superfine 115 SW from CAWO) and ii) a needle-based scintillating
phosphor CsI deposited on the aluminium 318G substrate with a coating weight
of CsI of 120 mg/cm2. The CsI based scintillator was prepared as follows: 400g of
CsI was placed in a container in a vacuum deposition chamber. The pressure in
the chamber was decreased to 5.10-5 mbar. The container was subsequently
heated to a temperature of 680°C and the CsI was deposited on the aluminium
support Al318G having a size of 24cm x 18cm. The distance between the
container and the substrate was 20 cm. During evaporation, the substrate was
rotated at 12 r.p.m. and kept at a temperature of 140°C. During the evaporation
process argon gas was introduced into the chamber. The duration of the process
is 160 min. After the evaporation process the X-ray shield was placed in the oven
and kept for 1h at 170°C.
The scintillator was first coupled to the imaging array on the glass. The
coupling was achieved by placing hot melt with a lining on the surface of the
imaging array on the glass. The glass with the imaging array was then put into an
oven and kept at a temperature of 85°C for 10 minutes. After cooling, the lining
was removed to release the adhesive side of the melted hot melt. Subsequently,
29
the scintillating phosphor layer of the scintillator was brought into contact with the
adhesive surface of the hot melt at high pressure and high temperature. To
achieve a good sticking over the complete area a pressure in a range of 0.8 bar
was applied, at a temperature of 115°C, for 15 min.
In the following step, the X-ray shields were coupled to the glass substrate
- imaging array - scintillator package. A polyurethane based hot melt of maximum
25μm thickness with a lining is placed onto the substrate (2nd substrate) of the Xray
shields SD-01 to SD-18 at the side opposite to the X-ray absorbing layer. With
the use of the comparative shield, SD-21, the hot melt is applied directly on one
side of the metal plate. The X-ray shields were put into the oven and kept at a
temperature of 80°C for 10 minutes. After cooling, the lining was removed to
release the adhesive side of the melted hot melt. Subsequently, the glass
substrate carrying the imaging array and scintillator was brought into contact with
the adhesive surface of the hot melt at a high pressure and a high temperature.
To achieve a good sticking over the complete area a pressure of 0.8 bar was
applied, with a temperature of 115°C, for 15 min.
Following RFPDs have been prepared according to the above described method:
a) DRGOS-01 to DRGOS-18: GOS scintillator + GOS X-ray shields SD-01 to SD-
18 ,
b) DRCSI-01 to DRCSI-18: CsI scintillator + GOS X-ray shields SD-01 to SD-18,
c) DRGOS-19: GOS scintillator + Mo X-ray shield SD-21
d) DRCSI-19 : CsI scintillator + Mo X-ray shield SD-21
Example 2: X-ray shielding capacity of different X-ray shields
This example illustrates the X-ray shielding capacity of X-ray shields with
different coating weights and different substrates (2nd substrate) in a standard
configuration of the RFPD with different scattering elements. Therefore the ability
of the inventive X-ray shields to reduce the backscatter of several X-ray shields
prepared according to §3.1 – 3.3 and assembled in the standard RFPD
configuration as described in measurement method 1.1, is demonstrated. The
optical densities of the radiographic film exposed in the standard RFPD
30
configurations are compared to the optical densities of the radiographic film
exposed in a RFPD configuration without scattering elements. The tests were
done with RQA X-ray beam qualities as described in the measurements method 1
and with loads for RQA3 – 12.5 mAs, RQA5 – 6.3 mAs, RQA7 – 5.6 mAs, and
RQA9 – 3mAs. Table 4 shows the measured X-ray shielding capacities.
Table 4: Difference in optical density of the radiographic film with the X-ray shields
in a specific configuration of the RFPD with respect to the X-ray shield in a RFPD
configuration without scattering elements.
Table 4
No. RFPD configuration X-ray shield Difference in optical
density
1 standard SD-01; SD-02 Slight decrease
2 standard SD-08; SD-09;SD-
11;SD-12
Slight decrease
3 standard SD-03 to SD-07;
SD-10,
SD-13 to SD-16
Strong decrease
4 Standard with only PMMA
block
SD-03 to SD-16 Decrease
5 Standard with only PMMA
block
SD-01; SD-02 Strong decrease
The results show that all inventive X-ray shields in a RFPD are able to
reduce the backscatter of X-rays originating from scattering elements which
simulate the underlying electronics of the RFPD.
31

Description
Radiography flat panel detector having a low weight X-ray shield and the method
of production thereof.
Technical Field
[0001] The present invention relates to diagnostic imaging and more particularly,
to a radiography X-ray detector having an X-ray shield which protects the
detector electronics and reduces or eliminates the impact of backscattered
X-rays during the exposure of the subject to the X-ray source.
Background Art
[0002] X-ray imaging is a non-invasive technique to capture medical images of
patients or animals as well as to inspect the contents of sealed containers,
such as luggage, packages, and other parcels. To capture these images,
an X-ray beam irradiates an object. The X-rays are then attenuated as
they pass through the object. The degree of attenuation varies across the
object as a result of variances in the internal composition and/or thickness
of the object. The attenuated X-ray beam impinges upon an X-ray detector
designed to convert the attenuated beam to a usable shadow image of the
internal structure of the object.
[0003] Increasingly, radiography flat panel detectors (RFPDs) are being used to
capture images of objects during inspection procedures or of body parts of
patients to be analyzed. These detectors can convert the X-rays directly
into electric charges (direct conversion direct radiography - DCDR), or in
an indirect way (indirect conversion direct radiography - ICDR).
[0004] In direct conversion direct radiography, the RFPDs convert X-rays directly
into electric charges. The X-rays are directly interacting with a
photoconductive layer such as amorphous selenium (a-Se).
[0005] In indirect conversion direct radiography, the RFPDs have a scintillating
phosphor such as CshTI or Gd202S which converts X-rays into light which
then interacts with an amorphous silicon (a-Si) semiconductor layer, where
electric charges are created.
[0006] The created electric charges are collected via a switching array,
comprising thin film transistors (TFTs). The transistors are switched-on
row by row and column by column to read out the signal of the detector.
The charges are transformed into voltage, which is converted in a digital
number that is stored in a computer file which can be used to generate a
softcopy or hardcopy image. Recently Complementary Metal Oxides
Semiconductors (CMOS) sensors are becoming important in X-ray
imaging. The detectors based on CMOS are already used in
mammography, dental, fluoroscopy, cardiology and angiography images.
The advantage of using those detectors is a high readout speed and a low
electronic noise.
[0007] Generally, the imaging array including TFTs as switching array and
photodiodes (in case of ICDR) is deposited on a thin substrate of glass.
The assembly of scintillator or photoconductor and the imaging array on
the glass substrate does not absorb all primary radiation, coming from the
X-ray source and transmitted by the object of the diagnosis. Hence the
electronics positioned under this assembly are exposed to a certain
fraction of the primary X-ray radiation. Since the electronics are not
sufficiently radiation hard, this transmitted radiation may cause damage.
[0008] Moreover, X-rays which are not absorbed by the assembly of scintillator or
photoconductor and the imaging array on the glass substrate, can be
absorbed in the structures underneath the glass substrate. The primary
radiation absorbed in these structures generates secondary radiation that
is emitted isotropically and that thus exposes the imaging part of the
detector. The secondary radiation is called "backscatter" and can expose
the image part image of the detector, thereby introducing artefacts into the
reconstructed image. Since the space under the assembly is not
homogeneously filled, the amount of scattered radiation is position
dependent. Part of the scattered radiation is emitted in the direction of the
assembly of scintillator or photoconductor and imaging array and may
contribute to the recorded signal. Since this contribution is not spatially
homogeneous this contribution will lead to haze in the image, and,
therefore, reduce the dynamic range. It will also create image artefacts.
[0009] To avoid damage to the electronics and image artefacts due to scattered
radiation, an X-ray shield may be applied underneath the assembly of
scintillator or photoconductor and imaging array. Because of their high
density and high intrinsic stopping power for X-rays, metals with a high
atomic number are used as materials in such an X-ray shield. Examples of
these are sheets or plates from tantalum, lead or tungsten as disclosed in
EP1471 384B1 , US2013/0032724A1 and US2012/0097857A1 .
[0010] However, metals with a high atomic number also have a high density.
Hence, X-ray shields based on these materials have a high weight. Weight
is an important characteristic of the RFPD especially for the portability of
the RFPDs. Any weight reduction is, therefore, beneficial for the users of
the RFPDs such as medical staff.
[001 1] US7317190B2 discloses a radiation absorbing X-ray detector panel
support comprising a radiation absorbing material to reduce the reflection
of X-rays of the back cover of the X-ray detector. The absorbing material
containing heavy atoms such as lead, barium sulphate and tungsten can
be disposed as a film via a chemical vapour deposition technique onto a
rigid panel support or can be mixed via injection moulding with the base
materials used to fabricate the rigid panel support.
[0012] In US5650626, an X-ray imaging detector is disclosed which contains a
substrate, supporting the conversion and detection unit. The substrate
includes one or more elements having atomic numbers greater than 22.
Since the detection array is directly deposited on the substrate, the variety
of suitable materials of the substrate is rather limited.
[0013] In US5777335, an imaging device is disclosed comprising a substrate,
preferably glass containing a metal selected from a group formed by Pb,
Ba, Ta or W. According to the inventors, the use of this glass would not
require an additional X-ray shield based on lead. However, glass
containing sufficient amounts of metals from a group formed by Pb, Ba, Ta
or W is more expensive than glass which is normally used as a substrate
for imaging arrays.
[0014] US7569832 discloses a radiographic imaging device, namely a RFPD,
comprising two scintillating phosphor layers as scintillators each one
having different thicknesses and a transparent substrate to the X-rays
between said two layers. The use of an additional phosphor layer at the
opposite side of the substrate improves the X-ray absorption while
maintaining the spatial resolution. The presence of the additional phosphor
layer as disclosed is not sufficient to absorb all primary X-ray radiation to
prevent damage of the underlying electronics and to prevent backscatter.
An extra X-ray shield will still be required in the design of this RFPD.
[0015] In US2008/01 1960A1 a dual-screen digital radiography apparatus is
claimed. This apparatus consists of two flat panel detectors (front panel
and back panel) each comprising a scintillating phosphor layer to capture
and process X-rays. The scintillating phosphor layer in the back panel
contributes to the image formation and has no function as X-ray shield to
protect the underlying electronics. This dual-screen digital flat panel, still
requires an X-ray shield to protect the underlying electronics and to avoid
image artefacts due to scattered radiation.
[0016] WO2005057235A1 describes a shielding for an X-ray detector wherein
lead or another suitable material is disposed in front of the processing
circuits in a CT-device.
[001 7] WO20051 055938 discloses a light weight film, with an X-ray absorption at
least equivalent to 0.254 mm of lead and which has to be applied on
garments or fabrics for personal radiation protection or attenuation, such
as aprons, thyroid shields, gonad shields, gloves, etc. Said film is obtained
from a polymer latex mixture comprising high atomic weight metals or their
related compounds and/or alloys. The suitable metals are the ones that
have an atomic number greater than 45. No use of this light weight film in
a RFPD is mentioned. Although a light weight film is claimed, the metal
particles used in the composition of the film still contribute to a high extend
to the weight of the shield.
[0018] US6548570 discloses a radiation shielding composition to be applied on
garments or fabrics for personal radiation protection. The composition
comprised a polymer, preferably an elastomer, and a homogeneously
dispersed powder of a metal with high atomic number in an amount of at
least 80% in weight of the composition as filler. A loading material is mixed
with the filler material and kneaded with the elastomer at a temperature
below 180°C resulting in a radiation shielding composition that can be
applied homogeneously to garments and fabrics on an industrial scale.
The use of metals is however increasing the weight of the shield of this
invention considerably.
[0019] WO2009/0078891 discloses a radiation shielding sheet which is free from
lead and other harmful components having a highly radiation shielding
performance and an excellent economical efficiency. Said sheet is formed
by filling a shielding material into an organic polymer material, the
shielding material being an oxide powder containing at least one element
selected from the group consisting of lanthanum (La), cerium (Ce),
praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu) and
gadolinium (Gd) and the polymer being a material such as rubber,
thermoplastic elastomer, polymer resin or similar. The volumetric amount
of the shielding material filled in the radiation shielding sheet is 40 to 80
vol. % with respect to the total volume of the sheet. No use of this light
weight film in a RFPD is mentioned.
[0020] From the foregoing discussion, it should be apparent that there is a need
for a RFPD with an X-ray shield to protect the underlying electronics and
to absorb the scattered radiation produced by the underlying structures to
avoid image artefacts in the imaging area, but which has a low weight, a
low cost and which can be produced in an economically efficient way.
Summary of invention
[0021] It is therefore an object of the present invention to provide a solution for
the high weight contribution of the X-ray shield in a radiography flat panel
detector having a single imaging array and to provide at the same time a
solution for producing the X-ray shield on an economically efficient way.
The object has been achieved by a radiography flat panel detector as
defined in claim 1. The X-ray shield is the combination of the 2nd substrate
and the X-ray absorbing layer as defined in claim .
[0022] An additional advantage of the RFPD as defined in claim 1, is that the
thickness of said X-ray shield can be adjusted in a continuous way to the
required degree of the X-ray shielding effect instead of in large steps as it
is in the case of shielding metal sheets commercially available with
standard thicknesses. Even though plates with custom made thickness
can be purchased, the price of those metal plates is still very high because
of the customization.
[0023] In accordance with another aspect of the present invention, the
composition of the X-ray shield leads to an X-ray absorbing layer which is
mechanically strong enough to avoid sealing the layer with a second
substrate or which do not require expensive moulding techniques.
Moreover, the X-ray shield comprises a second substrate, the X-ray shield
contributes thus to the mechanical strength of the whole RFPD and more
specifically of the thin fragile glass substrate of the single imaging array.
[0024] According to another aspect, the present invention includes a method of
manufacturing a radiography flat panel detector. The method includes
providing a substrate and coating on said substrate a binder with at least
one chemical compound having a metal element with an atomic number of
20 or more and one or more non-metal elements.
[0025] Other features, elements, steps, characteristics and advantages of the
present invention will become more apparent from the following detailed
description of preferred embodiments of the present invention. Specific
embodiments of the invention are also defined in the dependent claims.
Brief description of drawings
[0026] Fig. 1 represents a cross-section of a RFPD according to one embodiment
of the present invention and the underlying electronics, wherein:
1 is the scintillator or photoconductive layer
2 is the single imaging array
3 is the first substrate
4 is the second substrate
5 is the X-ray absorbing layer
6 is the underlying electronics
[0027] Fig. 2 represents a cross-section of a RFPD, according to one
embodiment of the present invention, wherein:
1 is the scintillator or photoconductive layer
2 is the single imaging array
3 is the first substrate
4 is the second substrate
5 is the X-ray absorbing layer
6 is the underlying electronics
Description of embodiments
[0028] The present invention relates to radiography flat panel detector (RFPD)
comprising a scintillator or photoconductive layer, a single imaging array
on a first substrate and an X-ray shield having an X-ray absorbing layer
comprising a binder and a chemical compound having a metal element
with an atomic number of 20 or more and one or more non-metal elements
coated on a substrate (2nd substrate).
X-ray absorbing layer
[0029] It has been found that X-ray shields can be made with the same X-ray
stopping power but with considerably less weight than X-ray shields
consisting of metals only by use of a layer comprising a binder and one or
more chemical compounds having a metal element with an atomic number
of 20 or more and one or more non-metal elements. Preferably these
compounds are oxides or salts such as halides, oxysulphides, sulphites,
carbonates of metals with an atomic number of 20 or higher. Examples of
suitable metal elements with an atomic number higher than 20 that can be
used in the scope of the present invention are metals such as Barium (Ba),
Calcium (Ca), Cerium (Ce), Caesium (Cs), Gadolinium (Gd), Lanthanum
(La), Lutetium (Lu), Palladium (Pd), Tin (Sn), Strontium (Sr), Tellurium
(Te), Yttrium (Y), and Zinc (Zn). A further advantage of the invention is that
these compounds are relatively inexpensive and are characterised by a
low toxicity.
[0030] Examples of preferred compounds having a metal element with an atomic
number of 20 or more and one or more non-metal elements, are Caesium
iodide (Csl), Gadolinium oxysulphide (Gd202S), Barium fluorobromide
(BaFBr), Calcium tungstate (CaW0 4) , Barium titanate (BaTiOs),
Gadolinium oxide (Gd203), Barium chloride (BaCl2), Barium fluoride
(BaF2), Barium oxide (BaO), Cerium oxides, Caesium nitrate (CSNO3),
Gadolinium fluoride (GdF2), Palladium iodide (Pd ), Tellurium dioxide
(Te02), Tin iodides, Tin oxides, Barium sulphides, Barium carbonate
(BaCOs), Barium iodide, Caesium chloride (CsCI), Caesium bromide
(CsBr), Caesium fluoride (CsF), Caesium sulphate (CS2SO4), Osmium
halides, Osmium oxides, Osmium sulphides, Rhenium halides, Rhenium
oxides, Rhenium sulphides, BaFX (wherein X represents CI or I), RFXn
(wherein RF represents lanthanides selected from: La, Ce, Pr, Nd, Pm,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and X represents halides selected
from: F, CI, Br, I), RFyOz, RFy(S0 )z, RFyS and/or RFy(W0 )z, wherein n,
y, z are independently an integer number higher than 1. These
compounds can produce lower weight X-ray shields and are easy to
handle due to their low hygroscopicity than their pure metal analogues.
The most preferred metallic compounds are: Gd202S, Gd203, Ce203, Csl,
BaFBr, CaW0 , YTa0 4 and BaO.
[0031] It is another advantage of the present invention that the range of metal
elements which can be used for the X-ray absorbing layer, is much larger
than the corresponding range of the pure metals and/or alloys, since many
of them are not stable in their elemental form. Examples are the alkali
metals, the alkaline earth metals and the rare-earth metals. Additionally
the X-ray shield of the present invention allows the flexible adjustment to
X-ray apparatus in such a way that the thickness of the X-ray absorbing
layer is determined in reference to the allowable dose limit required to
attenuate radiation.
[0032] The chemical compounds having a metal element with an atomic number
of 20 or more and one or more non-metal elements may be used in the Xray
absorbing layer of the present invention as powder dispersed in a
binder. The amount of the binder in the X-ray absorbing layer in weight
percent can vary in the range from 1%to 50%, preferably from 1%to 25%,
more preferably from 1% to 10%, most preferably from 1% to 3%.
[0033] Suitable binders are e.g. organic polymers or inorganic binding
components. Examples of suitable organic polymers are polyethylene
glycol acrylate, acrylic acid, butenoic acid, propenoic acid, urethane
acrylate, hexanediol diacrylate, copolyester tetracrylate, methylated
melamine, ethyl acetate, methyl methacrylate. Inorganic binding
components may be used as well. Examples of suitable inorganic binding
components are alumina, silica or alumina nanoparticles, aluminium
phosphate, sodium borate, barium phosphate, phosphoric acid, barium
nitrate.
[0034] Preferred binders are organic polymers such as cellulose acetate butyrate,
polyalkyl (meth)acrylates, polyvinyl-n-butyral, poly(vinylacetate-covinylchloride),
poly(acrylonitrile-co-butadiene-co-styrene), polyvinyl
chloride-co-vinyl acetate-co-vinylalcohol), poly(butyl acrylate), poly(ethyl
acrylate), poly(methacrylic acid), polyvinyl butyral), trimellitic acid,
butenedioic anhydride, phthalic anhydride, polyisoprene and/or a mixture
thereof. Preferably, the binder comprises one or more styrenehydrogenated
diene block copolymers, having a saturated rubber block
from polybutadiene or polyisoprene, as rubbery and/or elastomeric
polymers. Particularly suitable thermoplastic rubbers, which can be used
as block-copolymeric binders, in accordance with this invention, are the
KRATON™ G rubbers, KRATON™ being a trade name from SHELL
[0035] In case the coating of the X-ray absorbing layer is to be cured, the binder
includes preferably a polymerizable compound which can be a
monofunctional or polyfunctional monomer, oligomer or polymer or a
combination thereof. The polymerizable compounds may comprise one or
more polymerizable groups, preferably radically polymerizable groups. Any
polymerizable mono- or oligofunctional monomer or oligomer commonly
known in the art may be employed. Preferred monofunctional monomers
are described in EP1637322 A paragraph [0054] to [0057]. Preferred
oligofunctional monomers or oligomers are described in EP1637322A
paragraphs [0059] to [0064]. Particularly preferred polymerisable
compound are urethane (meth)acrylates and 1,6-hexanedioldiacrylate.
The urethane (meth)acrylates are oligomer which may have one, two,
three or more polymerisable groups.
[0036] Suitable solvents, to dissolve the binder being an organic polymer during
the preparation of the coating solution of the X-ray absorbing layer can be
acetone, hexane, methyl acetate, ethyl acetate, isopropanol, methoxy
propanol, isobutyl acetate, ethanol, methanol, methylene chloride and
water. The most preferable ones are toluene, methyl-ethyl-ketone (MEK)
and methyl cyclohexane. To dissolve suitable inorganic binding
components, water is preferable as the main solvent. In case of a curable
coating liquid, one or more mono and/or difunctional monomers and/or
oligomers can be used as diluents. Preferred monomers and/or oligomers
acting as diluents are miscible with the above described urethane
(meth)acrylate oligomers. The monomer(s) or oligomer(s) used as diluents
are preferably low viscosity acrylate monomer(s).
[0037] The X-ray absorbing layer of the present invention may also comprise
additional compounds such as dispersants, plasticizers, photoinitiators,
photocurable monomers, antistatic agents, surfactants, stabilizers
oxidizing agents, adhesive agents, blocking agents and/or elastomers.
[0038] Dispersants which can be used in the present invention include nonsurface
active polymers or surface-active substances such as surfactants,
added to the binder to improve the separation of the particles of the
chemical compound having a metal element with an atomic number of 20
or more and one or more non-metal elements and to further prevent
settling or clumping in the coating solution. Suitable examples of
dispersants are Stann JF95B from Sakyo and Disperse Ayd™ 1900 from
Daniel Produkts Germany. The addition of dispersants to the coating
solution of the X-ray absorbing layer improves further the homogeneity of
the layer.
[0039] Suitable examples of plasticizers are Plastilit™ 3060 from BASF,
Santicizer™ 278 from Solutia Europe and Palatinol™ C from BASF. The
presence of plasticizers to the X-ray absorbing layer improves the
compatibility with flexible substrates.
[0040] Suitable photo-initiators are disclosed in e.g. J.V. Crivello et al. in "
Photoinitiators for Free Radical, Cationic & Anionic Photopolymerisation
2nd edition", Volume III of the Wiley/SITA Series In Surface Coatings
Technology, edited by G. Bradley and published in 1998 by John Wiley
and Sons Ltd London, pages 276 to 294. Examples of suitable
photoinitiators can be Darocure™ 173 and Nuvopol™ PI-3000 from
Rahn. Examples of suitable antistatic agents can be Cyastat™ SN50 from
Acris and Lanco™ STAT K 100N from Langer.
[0041] Examples of suitable surfactants can be Dow Corning™ 190 and Gafac
RM710, Rhodafac™ RS-7 0 from Rodia. Examples of suitable stabilizer
compounds can be Brij™ 72 from ICI Surfactants and Barostab™ MS from
Baerlocher Italia. An example of a suitable oxidizing agent can be lead (IV)
oxide from Riedel De Haen. Examples of suitable adhesive agents can be
Craynor™ 435 from Cray Valley and Lanco™ wax TF 780 from Noveon.
An example of a suitable blocking agent can be Trixene™ BI7951 from
Baxenden. An example of a suitable elastomer compound can be Metaline
™ from Schramm).
[0042] The thickness of the X-ray absorbing layer can vary as well and depends
on the necessary shielding power and/or the space available to
incorporate the X-ray shield in the design of the RFPD. In the present
invention, the thickness of the X-ray absorbing layer can be at least 0.1
mm, more preferably in the range from 0.1 mm to .0 mm.
[0043] Depending on the application, the coating weight of the chemical
compound having a metal element with an atomic number of 20 or more
and one or more non metal elements the X-ray shields can be adjusted
and in case of using a RFPD for medical purposes, this coating weight is
preferably at least 00 mg/cm2, more preferably at least 200 mg/cm2.
Substrate for the X-ray absorbing layer
[0044] The substrate for the X-ray absorbing layer of the X-ray shield according to
the invention, hereafter denoted as the second substrate, can be either
rigid or flexible, such as an aluminium plate, an aluminium foil, a film of
polyethylene terephthalate (PET), polyethylene naphthalate (PEN),
polyimide (PI), polyethersulphone (PES), a metal foil, a carbon fibre
reinforced plastic (CFRP) sheet, glass, flexible glass, triacetate and a
combination thereof or laminates thereof. Preferred materials for the
second substrate of the invention are PET, glass and aluminium due to
their low weight, their low cost and their availability.
[0045] Suitable substrates for the invention also include substrates which are
substantially not transparent to light by incorporating a light absorbing or
light reflecting material into the substrate.
[0046] More preferable substrates are flexible sheets obtained from for example
PET, aluminium or flexible glass. The application of an X-ray absorbing
layer onto the substrate (2nd substrate) as described above is preferably
done by means of a coating method. Coating is an economically efficient
technique of application of one or more layers onto a substrate. By means
of coating techniques, the X-ray absorbing layer can be applied together
with light absorbing or light reflecting layers, adhesion layers etc. Flexible
substrates are particularly suitable for a continuous coating process.
Moreover, flexible substrates can be available as rolls and they can be
wound and un-wound in the production process of coating and drying or
curing.
[0047] White coloured layers may be used to reflect light emitted by the
scintillating phosphor in the X-ray absorbing layer. Layers comprising T1O2
are preferably used to reflect 90% or more light at the wavelength(s) of the
light emitted by the scintillating phosphor. The solid content of T1O2 in the
light reflecting layer is preferably in the range of 25 to 50 (wt.)%. and the
thickness is preferably in the range of 5 to 40 mh . More preferably, the
solid content of the T1O2 is 33 to 38(wt.)% of the total solid content of the
layer and the layer thickness is between 13 and 30 mih .
[0048] In another preferred embodiment of the invention, black coloured layers
can be used to absorb light emitted by a scintillating phosphor in the X-ray
absorbing layer because of their high efficiency to absorb light. Black
particles, such as fine carbon black powder (ivory black, titanium black,
iron black), are suitable to obtain sufficient absorption of emitted light by
the scintillating phosphor. Preferably the solid content of carbon black is in
the range of 3 to 30 (wt.)% and a layer thickness of 2 to 30 mhi will absorb
90% or more of the emitted light by the scintillating phosphor. More
preferably the range of the solid content of the carbon black is in the range
of 6 to 15 (wt.)% and the layer thickness between 5 and 15 m. In another
embodiment of the invention, coloured pigments or dyes absorbing
specifically at the maximal wavelength of the emitted light by the
scintillating phosphor in the X-ray absorbing layer can be used.
Scintillator
[0049] In the RFPD for indirect conversion direct radiography according to the
present invention, the scintillator comprises optionally a support and
provided thereon, a scintillating phosphor such as Gd202S:Tb, Gd20 2S:Eu,
Gd203:Eu, La20 2S:Tb, La20 2S, Y20 2S:Tb, CshTI, CshEu, Csl:Na, CsBnTI,
NahTI, CaW0 4, CaW0 4:Tb, BaFBr:Eu, BaFChEu, BaS0 :Eu, BaSrSCU,
BaPbS0 , BaAli 20i9:Mn, BaMgAli 0O 7:Eu, Zn2Si04:Mn, (Zn, Cd)S:Ag,
LaOBr, LaOBr:Tm, Lu202S:Eu, Lu20 2S:Tb, LuTa0 , Hf0 2:Ti, HfGe04:Ti,
YTa04, YTa0 :Gd, YTa04:Nb, Y203:Eu, YB0 3:Eu, YB0 3:Tb, or
(Y,Gd)B03:Eu, or combinations thereof. Besides crystalline scintillating
phosphors, scintillating glass or organic scintillators can also be used.
[0050] When evaporated under appropriate conditions, a layer of doped Csl will
condense in the form of needle like, closely packed crystallites with high
packing density onto a support. Such a columnar or needle-like scintillating
phosphor is known in the art. See, for example, ALN Stevels et al. , "Vapor
Deposited CshNa Layers: Screens for Application in X-Ray Imaging
Devices, " Philips Research Reports 29:353-362 (1974); and T. Jing et al,
"Enhanced Columnar Structure in Csl Layer by Substrate Patterning",
IEEE Trans. Nucl. Sci. 39: 1195-1 198 (1992). More preferably, the
scintillating phosphor layer includes doped Csl.
[0051] A blend of different scintillating phosphors can also be used. The median
particle size is generally between about 0. 5 mih and about 40 mhh . A
median particle size of between 1 miti and about 20 mhi is preferred for
ease of formulation, as well as optimizing properties, such as speed,
sharpness and noise. The scintillator for the embodiments of the present
invention can be prepared using conventional coating techniques whereby
the scintillating phosphor powder, for example Gd202S is mixed with a
solution of a binder material and coated by means of a blade coater onto a
substrate. The binder can be chosen from a variety of known organic
polymers that are transparent to X-rays, stimulating, and emitting light.
Binders commonly employed in the art include sodium osulfobenzaldehyde
acetal of polyvinyl alcohol); chloro-sulfonated
poly(ethylene); a mixture of macromolecular bisphenol poly(carbonates)
and copolymers comprising bisphenol carbonates and poly(alkylene
oxides);aqueous ethanol soluble nylons; poly(alkyl acrylates and
methacrylates) and copolymers of poly(alkyl acrylates and methacrylates
with acrylic and methacrylic acid); polyvinyl butyral); and poly(urethane)
elastomers. Other preferable binders which can be used are described
above in the section of the X-ray absorbing layer. Any conventional ratio
phosphor to binder can be employed. Generally, the thinner scintillating
phosphor layers are, the sharper images are realized when a high weight
ratio of phosphor to binder is employed. Phosphor-to-binder ratios in the
range of about 70:30 to 99:1 by weight are preferable.
The photoconductive layer
In the RFPD for direct conversion direct radiography according to the
present invention, the photoconductive layer is usually amorphous
selenium, although other photoconductors such as Hg , PbO, Pb , TIBr,
CdTe and gadolinium compounds can be used. The photoconductive layer
is preferentially deposited on the imaging array via vapour deposition but
can also been coated using any suitable coating method.
The imaging array and first substrate
[0053] The single imaging array used in the invention for indirect conversion
direct radiography is based on an indirect conversion process which uses
several physical components to convert X-rays into light that is
subsequently converted into electrical charges. First component is a
scintillating phosphor which converts X-rays into light (photons). Light is
further guided towards an amorphous silicon photodiode layer which
converts light into electrons and electrical charges are created. The
charges are collected and stored by the storage capacitors. A thin-film
transistor (TFT) array adjacent to amorphous silicon read out the electrical
charges and an image is created. Examples of suitable image arrays are
disclosed in US5262649 and by Samei E. et al., "General guidelines for
purchasing and acceptance testing of PACS equipment", Radiographics,
24, 313-334 . Preferably, the imaging arrays as described in
US201 3/0048866, paragraph [90-125] and US201 3/221 230, paragraphs
[53-71] and [81-104] can be used.
[0054] The single imaging array used in the invention for direct conversion direct
radiography is based on a direct conversion process of X-ray photons into
electric charges. In this array, an electric field is created between a top
electrode, situated on top of the photoconductor layer and the TFT
elements. As X-rays strike the photoconductor, the electric charges are
created and the electrical field causes to move them towards the TFT
elements where they are collected and stored by storage capacitors.
Examples of suitable image arrays are disclosed by Samei E. et al., "
General guidelines for purchasing and acceptance testing of PACS
equipment", Radiographics, 24, 313-334.
[0055] For both the direct and indirect conversion process, the charges must be
read out by readout electronics. Examples of readout electronics in which
the electrical charges produced and stored are read out row by row, are
disclosed by Samei E. et al., Advances in Digital Radiography. RSNA
Categorical Course in Diagnostic Radiology Physics (p. 49-61 ) Oak Brook,
III.
[0056] The substrate of the imaging array of the present invention, hereafter
denoted as the 'first substrate' is usually glass. However, imaging arrays
fabricated on substrates made of plastics, metal foils can also be used.
The imaging array can be protected from humidity and environmental
factors by a layer of silicon nitride or polymer based coatings such as
fluoropolymers, polyimides, polyamides, polyurethanes and epoxy resins.
Also polymers based on B-staged bisbenzocyclobutene-based (BCB)
monomers can be used. Alternatively, porous inorganic dielectrics with low
dielectric constants can also be used.
The underlying electronics
[0057] The underlying electronics, situated under the X-ray absorbing layer
comprise a circuit board which is equipped with electronic components for
processing the electrical signal from the imaging array, and/or controlling
the driver of the imaging array and is electrically connected to the imaging
array.
Method of making the radiographic flat panel detector
Method of making the X-ray shield
[0058] The X-ray shield of the present invention can be obtained by applying a
coating solution comprising at least one chemical compound having a
metal element with an atomic number of 20 or more and one or more nonmetal
elements and a binder onto a substrate (2nd substrate) by any
known methods, such as knife coating, doctor blade coating, spin-coating,
dip-coating, spray-coating, screen printing and lamination. The most
preferable method is the doctor blade coating.
[0059] In a preferred embodiment the coating solution is prepared by first
dissolving the binder in a suitable solvent. To this solution the chemical
compound having a metal element with an atomic number of 20 or more
and one or more non-metal elements is added. To obtain a homogenous
coating solution or lacquer, a homogenization step or milling step of the
mixture can be included in the preparation process. A dispersant can be
added to the binder solution prior to the mixing with the chemical
compound having a metal element with an atomic number of 20 or more
and one or more non-metal elements. The dispersant improves the
separation of the particles in the coating solution and prevents settling or
clumping of the ingredients in the coating solution. The addition of
dispersants to the coating solution of the X-ray absorbing layer decreases
the surface tension of the coating solution and improves the coating
quality of the X-ray absorbing layer.
[0060] In another embodiment of the invention, the binder being a polymerisable
compound can be dissolved in diluents comprising one or more mono
and/or difunctional monomers and/or oligomers.
[0061] After stirring or homogenization the coating solution is applied onto the
substrate preferably using a coating knife or a doctor blade. The substrate
can be the first substrate or a second substrate. If the coating solution is
coated on the first substrate, the coating is preferably performed on the
side opposite to the imaging array. After the coating of the X-ray absorbing
layer onto a substrate, the X-ray absorbing layer can be dried via an IRsource,
an UV-source, a heated metal roller or heated air. When
photocurable monomers are used in the coating solution, the coated layer
can be cured via heating or via an UV-source. After drying or curing, the Xray
shield which is coated on a second substrate can be cut into sheets of
appropriate size.
[0062] The obtained X-ray shield, comprising a substrate and an X-ray absorbing
layer comprising a binder and a chemical compound having a metal
element with an atomic number of 20 or more and one or more non-metal
elements, can be used as to shield electronics in medical apparatus and
non-destructive testing apparatus from X-rays. The combination of the Xray
absorbing layer with a substrate, gives the whole X-ray shield sufficient
mechanical strength to be used as a self-contained component in medical
devices or non-destructive testing devices.
Method of making the RFPD for indirect conversion direct radiography
[0063] The RFPD for indirect conversion direct radiography according to the
invention is made by assembling the different components which are
described above. A preferred method is now described.
[0064] In a first step, the scintillator, which comprises a scintillating phosphor and
a support, is coupled via gluing onto the single imaging array situated on
the first substrate, preferably glass. Gluing is done with pressure sensitive
adhesives or hot melts. Preferably a hot melt is used. Suitable examples of
hot melts are polyethylene-vinyl acetate, polyolefins, polyamides,
polyesters, polyurethanes, styrene block copolymers, polycarbonates,
fluoropolymers, silicone rubbers, polypyrrole. The most preferred ones are
polyolefins and polyurethanes due to the higher temperature resistance
and stability. The hot melt is preferably thinner than 25 m t i. The hot melt
with a lining is placed onto the surface of the imaging array. The imaging
array on the first substrate, together with the hot melt is then heated in an
oven at a prescribed temperature. After cooling, the lining is removed and
releases a melted hot melt with a free adhesive side. The scintillator is
coupled to the imaging array by bringing the scintillating phosphor layer in
contact with the adhesive side of the hot melt and by applying a high
pressure at a high temperature. To achieve a good sticking over the
complete area of the imaging array, a pressure in a range from 0.6 to 20
bars has to be applied and a temperature value in a range from 80 - 220°
C, during between 0 and 1000 s is required. A stack of scintillatorimaging
array-first substrate is thereby formed. In one preferred
embodiment of the invention, this stack can be positioned above the
underlying electronics which perform the processing of the electrical signal
from the imaging array, or the controlling of the driver of the imaging array.
[0065] In the last step, the X-ray shield is coupled to the first substrate at the
opposite side of the single imaging array. Either the second substrate or
the X-ray absorbing layer of the X-ray shield can be contacted with the first
substrate. A preferable method is after making of the contact, to
immobilise the components of the obtained stack by cold roll lamination or
heated roll lamination using foils having a protective ability. The best
suitable foils are the polyethylene, polyester, polyvinylchloride or acrylic
based foils with a thickness of maximum 100 m. Another preferable
method is using a pressure sensitive glue or hot melt. A hot melt with a
lining is placed onto either the substrate (2nd substrate) or the X-ray
absorbing layer of the X-ray shield. The X-ray shield is then heated,
preferably in an oven at the prescribed temperature. After cooling, the
lining is removed and releases a melted hot melt with a free adhesive side.
The X-ray shield is coupled to the imaging array by bringing the first
substrate of the stack into contact with the adhesive side of the hot melt
and by applying a high pressure at a high temperature. To achieve a good
sticking over the complete area of the components to be glued, a pressure
in a range from 0.6 to 20 bars has to be applied and a temperature value
in a range from 80 - 220°C, during between 10 and 1000 s is required.
[0066] In a preferred embodiment of the invention, the scintillating phosphor is
directly applied on the single imaging array via a coating or deposition
process. This method has the advantage that no gluing is required and
hence omits at least one step in the production process of the RFPD.
Method of making the RFPD for direct conversion direct radiography
[0067] The FPD for direct conversion direct radiography according to the
invention is made by assembling the different components which are
described above.
[0068] A preferred method is as follows: the photoconductor, preferably
amorphous selenium is deposited onto the single imaging array situated
on the first substrate which is preferably glass. Examples of deposition
methods are disclosed in Fischbach et al.,'Comparison of indirect Csl/a:Si
and direct a:Se digital radiography', Acta Radiologica 44 (2003) 616-621 .
After providing a top electrode on top of the photoconductive layer, the
single imaging array with the photoconductor is coupled with the X-ray
shield. This can be done according to the same methods as described for
making the RFPD for indirect conversion direct radiography.
EXAMPLES
1. Measurement methods
1. 1 X-ray shielding capacity of the X-ray shields
[0069] The X-ray shielding capacity of X-ray shields according to the present
invention (INV) and of a commercially available metal based X-ray shield
(COMP) was measured based on measurements of the optical density of a
radiographic film placed between scintillator and X-ray shield, after X-ray
exposure and development. The radiographic film is commercially
available from Agfa Healthcare (AGFAHDRC1824) and is a green
sensitive film with one radiation sensitive side. The X-ray exposure was
performed with a Philips Optimus 80 X-ray source. The X-ray shield was
usually positioned in the following configuration: scintillator - radiographic
film - X-ray shield - scattering elements comprising a printed circuit board
(PCB), a lead strip and a PMMA block. This configuration is called the
standard configuration of the RFPD. The default scintillating phosphor
used was a commercially available GOS scintillator (CAWO Superfine 15
SW, from CAWO). The scintillating phosphor layer was put in contact with
the radiation sensitive side of the radiographic film. The underlying
electronics of a RFPD were simulated by the PCB with discrete
components, the lead strip and the block of poly(methylmethacrylate).
Poly(methylmethacrylate) is used due to its very high scattering properties.
[0070] To achieve good contact between the components, each X-ray shield was
sealed in a black polyethelene bag (PE, Type B, 260x369 mm, 0,19mm
thickness, from Cornells Plastic) together with the scintillator and the
radiographic film by means of vacuum. The substrate of the X-ray shield
was always in contact with the non-radiation sensitive side of the
radiographic film, unless otherwise specified. The package prepared like
this way, is called a basic RFPD.
[0071] The X-ray source, the basic RFPD and the scattering elements were
mounted on a horizontal bench. The basic RFPDs were placed at 1.5 m
from said X-ray source. Behind the basic RFPD, the block of PMMA, the
strip of lead of 3 mm thickness and the PCB, were placed next to each
other to simulate the underlying electronics of the RFPD. The distance
between the scattering elements and the basic RFPD was less than 0.2
cm. The reference measurement was done with the basic RFPD
configuration without scattering elements behind the RFPD.
[0072] Following standard radiation X-ray beam qualities were used: RQA3 (10
mm Al, 52kV), RQA5 (21 mm Al, 73kV), RQA7 (30 mm Al, 88kV), and
RQA9 (40 mm Al, 117kV), RQA X-ray beam qualities as defined in IEC
standard 61267, 1 Ed. (1994).
[0073] After exposure each film was developed in G138i (Agfa Healthcare) at 33°
C for 90s. and placed in a MacBeth densitometer, type TR-924 to measure
the optical density. The higher the measured optical density, the more
backscatter of X-rays was taking place.
1.2 . Weighing of the X-ray shields:
[0074] The X-ray shields prepared according to the present invention (INV) and
the comparative X-ray shield (COMP) were weighed on the laboratory
scales (Mettler Toledo PG5002-S) with a resolution of 0.01 g.
1.3. X-ray absorption of the X-ray shields:
[0075] The X-ray absorption of the X-ray shields was measured with a Philips
Optimus 80 apparatus together with a Triad dosimeter having a 30cc
volume cell. The measuring cell was placed at 1.5 m distance from the Xray
source directly behind the X-ray shield. The X-ray shield in both cases
was placed with its substrate directed to the X-ray source. Data for each
screen were collected multiple times and the average value was calculated
together with the standard deviation.
[0076] All tests were done for standard radiation X-ray beam qualities (RQA X-ray
beam qualities as defined in IEC standard 61267, 1st Ed. (1994)): RQA3
(10 mm Al, 52kV), RQA5 (21 mm Al, 73kV), RQA7 (30 mm Al, 88kV), and
RQA9 (40 mm Al, 117kV) unless otherwise specified.
2. Materials
[0077] The materials used in the following examples were readily available from
standard sources such as ALDRICH CHEMICAL Co. (Belgium), ACROS
(Belgium) and BASF (Belgium) unless otherwise specified. All materials
were used without further purification unless otherwise specified.
• Gadolinium oxysulphide (Gd20 2S) or GOS: (CAS 12339-07-0) powder
was obtained from Nichia, mean particle size: 3.3 miti .
CaW0 4, powder was obtained from Nichia, mean particle size: 7.0 m h .
YTa0 4, powder was obtained from Nichia, mean particle size: 4.4 miti .
White PET substrate: polyethylene terephthalate (PET) film with a
thickness of 0.19 mm, obtained from Mitsubishi, trade name
Hostaphan WO.
Black PET substrate: polyethylene terephthalate (PET) film a thickness
of 0.188 mm, obtained from Toray, trade name Lumirror X30.
• Disperse Ayd™ 9100 (Disperse Ayd™ W-22), anionic surfactant/Fatty
Ester dispersant (from Daniel Produkts Company).
• Kraton™ FG1901X (new name = Kraton™ FG1901 GT), a clear, linear
triblock copolymer based on styrene and ethylene/butylene with a
polystyrene content of 30%, from Shell Chemicals.
• Default GOS scintillator, CAWO Superfine 115 SW, from CAWO.
• Caesium Iodide (Csl): (CAS 7789-17-5) powder from Rockwood
Lithium, 99.999%.
Aluminium 3 18G: plate from Alanod having a thickness of 0.3 mm.
• Imaging array: TFT (according US201 3/0048866, paragraph [90-125]
and US201 3/221 230, paragraphs [53-71] and [81-104]) on Corning
Lotus™ Glass having a thickness of 0.7 mm.
• Radiographic film: AGFAHDRC1824, from Agfa Healthcare
PMMA: poly(methylmethacrylate), 7 cm thick, 30x30cm, compliant with
ISO 9236-1 standard
Lead strip: 13cm x 2.5 cm, thickness is 0.3 cm.
• PCB: 13cm x 4.5 cm
3. Preparation of X-ray shields
3.1 Preparation of the solution for coating the X-ray absorbing layer:
[0078] 4.5 g of binder (Kraton™ FG1901X) was dissolved in 18 g of a solvent
mixture of toluene and MEK (ratio 75:25 wt. / wt.) and stirred for 15 min at
a rate of 1900 r.p.m. The chemical compound having a metal element with
an atomic number of 20 or more and one or more non-metal elements,
was added thereafter as a powder, in an amount of 200g and the mixture
was stirred for another 30 minutes at a rate of 1900 r.p.m.
3.2 Preparation of X-ray shields SD-01 to SD-20 (INV):
[0079] The coating solution as obtained in § 1. 1 was coated with a doctor blade
at a coating speed of 4 m/min onto several PET substrates (white and
black) to obtain different dry layer thicknesses variable from 100 to 450 miti
to obtain X-ray shields SD-01 to SD-20 (see Table 1) . Subsequently, the
X-ray shields were dried at room temperature during 30 minutes. In order
to remove volatile solvents as much as possible the coated X-ray shields
were dried at 60°C for 30 minutes and again at 90°C for 20 to 30 minutes
in a drying oven. The total thickness of the X-ray absorbing layer was
controlled by adjusting the wet layer thickness and/or the number of layers
coated on top of each other after drying each layer. The wet layer
thickness has a value between 220 m t i and 1500 mih . The size of the
obtained shields was 18 cm X 24 cm.
[0080] After coating, each X-ray shield was weighed and the coating weight of the
chemical compound having a metal element with an atomic number of 20
or more and one or more non metal elements was obtained by applying
formula 1. The results are reported in Table 1
(W - W , )—
* %
Formula 1
Where:
F is the weight of an X-ray shield (2nd substrate + X-ray absorbing layer),
Ws is the weight of the substrate (2nd substrate) of the X-ray shield,
As is the surface area of the substrate (2nd substrate),
P% is the amount in weight % of the chemical compound having a metal
element with an atomic number of 20 or more and one or more non-metal
elements in the X-ray absorbing layer.
3.3 Molybdenum X-ray shield SD-21 (COMP)
[0081] An X-ray shield consisting of a plate of Molybdenum was obtained from
one of the commercially available RFPDs on the market. The thickness of
the Molybdenum plate was 0.3 mm and the size was 18 cm X 24 cm. The
Molybdenum plate did not contain a substrate. The composition of the
plate was 99.85% (wt.) of Mo, and below 0.05% (wt.) of Na, K, Ca, Ni, Cu,
and Bi.
[0082] The coating weight for this Mo plate was calculated based on formula 1
taking into account that P% is 00 and Ws is 0. The results of the
calculated coating weight of the Mo plate, hereafter denoted as SD-21
were reported in Table 1.
Table 1: Coating weights of the inventive X-ray shields (SD-01 to SD-20)
and of Mo in the comparative X-ray shield (SD-21 ) .
Table 1
X-ray shield Chemical compound having a metal Substrate Coating weight
element with an atomic number of (mg/cm2)
20 or more and one or more nonmetal
elements in the X-ray
absorbing layer
SD-01 GOS Black PET 110
SD-02 GOS Black PET 152
SD-03 GOS White PET 129
SD-04 GOS White PET 121
SD-05 GOS White PET 116
SD-06 GOS White PET 108
SD-07 GOS White PET 100
SD-08 GOS White PET 96
SD-09 GOS White PET 8 1
SD-10 GOS White PET 115
SD-1 GOS White PET 40
SD-12 GOS White PET 80
SD- 3 GOS White PET 171
SD-14 GOS White PET 195
SD-15 GOS White PET 145
SD-16 GOS White PET 230
SD-17 GOS Black PET 115
SD-18 GOS Black PET 155
SD-19 CaWO4 White PET 75
SD-20 YTaO White PET 82
SD-21 - - 302
3.4 Preparation of X-ray shields with and without a dispersant
83] To illustrate the difference between X-ray shields based on GOS and
prepared with or without a dispersant in the coating solution of the X-ray
absorbing layer, two X-ray shields were prepared according to the method
described in §3.1 . In both cases a white PET substrate was used. The
coating weight of GOS was 172 mg/cm2 for both shields. Shield SD-00.1
was prepared without dispersant in the coating solution and SD-00.2 was
prepared with dispersant (Disperse Ayd™ 9100) added to the coating
solution. Firstly, 0.5 g of dispersant was dissolved in 11.21 g of a toluene
and methyl-ethyl-ketone (MEK) solvent mixture, having a ratio of 75:25
(w/w) and mixed with the binder solution as prepared in §3.1 . The further
preparation steps are the same as in §3.1 and §3.2. The X-ray absorption
of both shields was determined according to the measuring method 3 with
a RQA5 X-ray beam quality and a load of 6.3mAs. The results are shown
in Table 2 .
Table 2: X-ray absorption of GOS X-ray shields prepared with or without
dispersant.
Table 2
[0084]
[0085] As shown in Table 2, the X-ray shield prepared with the dispersant present
in the coating solution had a more homogeneous X-ray absorbing layer for
a comparable weight and X-ray absorption as the X-ray shield prepared
without dispersant. The presence of the dispersant is advantageous for
the preparation process of the shields since it further reduces the surface
tension and prevents the floating of m size particles.
4 . X-ray shielding capacity of inventive X-ray shields in comparison with
the comparative shield.
The X-ray shielding capacity of the inventive X-ray shields SD-17 and SD-
18 was therefore measured according measuring method 1. 1 in a standard
configuration of the RFPD in comparison with the comparative Mo plate Xray
shield (SD-21 ) . The X-ray shielding capacity of the inventive X-ray
shields SD-19 and SD-20 was measured according measuring method 1. 1
in a configuration wherein the scattering elements consist of a lead strip
and a PMMA block, in comparison with the comparative Mo plate X-ray
shield (SD-21). The X-ray shields, which had the same surface, were
weighed according to measuring method 1.2. The results are shown in
Table 3.
Table 3: Difference in optical density of the radiographic film with the
inventive X-ray shields compared to the comparative shield SD-21 .
Table 3
These results show that the shielding capacity of the inventive shields are
equal or higher than the comparative X-ray shield based on a
Molybdenum plate and in some cases only a bit lower, but that their weight
is significantly lower than the comparative X-ray shield.
Example : Preparation of RFPDs comprising different X-ray shields
[0087] RFPDs for indirect conversion direct radiography were prepared by
bringing a scintillator in contact with the above mentioned imaging array on
a glass substrate (Corning Lotus™ Glass). Subsequently this package
was brought into contact with different X-ray shields SD-01 to SD-18 and
the Molybdenum metal plate SD-21 .
[0088] To assure good optical contact between each layer of the RFPDs, a hot
melt layer based on polyurethane and not thicker than 25 m, was used.
Two types of scintillators were used: i) a powder-based scintillating
phosphor GOS (CAWO Superfine 5 SW from CAWO) and ii) a needlebased
scintillating phosphor Csl deposited on the aluminium 3 8G
substrate with a coating weight of Csl of 20 mg/cm2. The Csl based
scintillator was prepared as follows: 400g of Csl was placed in a container
in a vacuum deposition chamber. The pressure in the chamber was
decreased to 5.1 0 5 mbar. The container was subsequently heated to a
temperature of 680°C and the Csl was deposited on the aluminium
support AI318G having a size of 24cm x 18cm. The distance between the
container and the substrate was 20 cm. During evaporation, the substrate
was rotated at 12 r.p.m. and kept at a temperature of 140°C. During the
evaporation process argon gas was introduced into the chamber. The
duration of the process is 160 min. After the evaporation process the X-ray
shield was placed in the oven and kept for 1h at 170°C.
[0089] The scintillator was first coupled to the imaging array on the glass. The
coupling was achieved by placing hot melt with a lining on the surface of
the imaging array on the glass. The glass with the imaging array was then
put into an oven and kept at a temperature of 85°C for 10 minutes. After
cooling, the lining was removed to release the adhesive side of the melted
hot melt. Subsequently, the scintillating phosphor layer of the scintillator
was brought into contact with the adhesive surface of the hot melt at high
pressure and high temperature. To achieve a good sticking over the
complete area a pressure in a range of 0.8 bar was applied, at a
temperature of 115°C, for 15 min.
[0090] In the following step, the X-ray shields were coupled to the glass substrate
- imaging array - scintillator package. A polyurethane based hot melt of
maximum 25mih thickness with a lining is placed onto the substrate (2nd
substrate) of the X-ray shields SD-01 to SD-18 at the side opposite to the
X-ray absorbing layer. With the use of the comparative shield, SD-2 , the
hot melt is applied directly on one side of the metal plate. The X-ray
shields were put into the oven and kept at a temperature of 80°C for 0
minutes. After cooling, the lining was removed to release the adhesive
side of the melted hot melt. Subsequently, the glass substrate carrying the
imaging array and scintillator was brought into contact with the adhesive
surface of the hot melt at a high pressure and a high temperature. To
achieve a good sticking over the complete area a pressure of 0.8 bar was
applied, with a temperature of 15°C, for 15 min.
[0091] Following RFPDs have been prepared according to the above described
method:
a) DRGOS-01 to DRGOS-18: GOS scintillator + GOS X-ray shields SD-
0 1 to SD-18 ,
b) DRCSI-01 to DRCSI-18: Csl scintillator + GOS X-ray shields SD-01 to
SD-18 ,
c) DRGOS-19: GOS scintillator + Mo X-ray shield SD-21
d) DRCSI-19 : Csl scintillator + Mo X-ray shield SD-21
Example 2 : X-ray shielding capacity of different X-ray shields
[0092] This example illustrates the X-ray shielding capacity of X-ray shields with
different coating weights and different substrates (2nd substrate) in a
standard configuration of the RFPD with different scattering elements.
Therefore the ability of the inventive X-ray shields to reduce the
backscatter of several X-ray shields prepared according to §3.1 - 3.3 and
assembled in the standard RFPD configuration as described in
measurement method 1.1 , is demonstrated. The optical densities of the
radiographic film exposed in the standard RFPD configurations are
compared to the optical densities of the radiographic film exposed in a
RFPD configuration without scattering elements. The tests were done with
RQA X-ray beam qualities as described in the measurements method 1
and with loads for RQA3 - 12.5 mAs, RQA5 - 6.3 mAs, RQA7 - 5.6 mAs,
and RQA9 - 3mAs. Table 4 shows the measured X-ray shielding
capacities.
Table 4: Difference in optical density of the radiographic film with the X-ray
shields in a specific configuration of the RFPD with respect to the X-ray
shield in a RFPD configuration without scattering elements.
Table 4
The results show that all inventive X-ray shields in a RFPD are able to
reduce the backscatter of X-rays originating from scattering elements
which simulate the underlying electronics of the RFPD.
Claims
. A radiography flat panel detector comprising a layer configuration in the order
given,
a) a scintillator or photoconductive layer (1)
b) a single imaging array (2)
c) a first substrate (3)
d) an X-ray shield comprising a second substrate (4) and an X-ray absorbing
layer (5) on a side of the second substrate,
characterised in that the absorbing layer (5) comprises a binder and a
chemical compound having a metal element with an atomic number of 20 or
more and one or more non-metal elements.
2. The radiography flat panel detector according to claim , wherein the second
substrate (4) consists essentially of materials selected from the group
consisting of Aluminium, polyethylene terephthalate, polyethylene naphthalate,
polyimide, polyethersulphone, carbon fibre reinforced plastic, glass, cellulose
triacetate and a combination thereof or laminates thereof.
3. The radiography flat panel detector according to any of the preceding claims,
wherein the second substrate is a flexible sheet.
4 . The radiography flat panel detector according to any of the preceding claims,
wherein the layer configuration is in the order given,
a) the scintillator or photoconductive layer ( 1)
b) the imaging array (2)
c) the first substrate (3)
d) the second substrate (4)
e) the X-ray absorbing layer (5)
5. The radiography flat panel detector according to any of the preceding claims,
wherein the chemical compound is selected form the group consisting of Csl,
Gd20 2S, BaFBr, CaW0 , BaTi0 3, Gd20 3, BaCI2, BaF2, BaO, Ce20 3, Ce0 2,
CsNOs, GdF2 Pdl2, Te0 2, Snl2, SnO, BaS0 4, BaC0 3, Bal, BaFX, RFXn,
RFyOz, RFy(S0 4)z, RFySz, RFy(W0 4)z, CsBr, CsCI, CsF, CsN0 3, Cs2S0 4,
Osmium halides, Osmium oxides, Osmium sulphides, Rhenium halides,
Rhenium oxides and Rhenium sulphides or mixtures thereof, wherein:
X is a halide selected from the group of F, C , Br and I ; and
RF is a lanthanide selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb and Lu; and
n, y, z are independently an integer number higher than 1.
6. The radiography flat panel detector according to any of the preceding claims,
wherein the X-ray shield is positioned between the first substrate (3) and the
underlying electronics (6).
7 . The radiography flat panel detector according to any of the preceding claims,
wherein the binder is selected from the group of cellulose acetate butyrate,
polyalkyl (meth)acrylates, polyvinyl-n-butyral, poly(vinylacetate-covinylchloride),
poly(acrylonitrile-co-butadiene-co-styrene), polyvinyl chlorideco-
vinyl acetate-co-vinylalcohol), poly(butyl acrylate), poly(ethyl acrylate),
poly(methacrylic acid), polyvinyl butyral), trimellitic acid, butenedioic
anhydride, phthalic anhydride, polyisoprene, and mixtures thereof.
8 . The radiography flat panel detector according to any of the preceding claims,
wherein the amount of the binder in the X-ray absorbing layer is 10% by weight
or less.
9 . A method of making a radiography flat panel detector as defined in claim 1,
comprising the steps of:
a) providing a first substrate (3) with an imaging array (2) on a side of the first
substrate; and
b) gluing a scintillating phosphor (1) onto the imaging array; and
c) providing a second substrate (4); and
d) coating the X-ray absorbing layer (5) on a side of the second substrate (4);
and
c) contacting either the side of the second substrate (4) opposite to the X-ray
absorbing layer (5) or the X-ray absorbing layer (5) with the side of the first
substrate (3) opposite to the imaging array (2).
10. The method of making a radiography flat panel detector according to claim 9
wherein the coating is performed by knife coating or doctor blade coating.

Claims:
1. A radiography flat panel detector comprising a layer configuration in the
order given,
a) a scintillator or photoconductive layer (1)
b) a single imaging array (2)
c) a first substrate (3)
d) an X-ray shield comprising a second substrate (4) and an X-ray
absorbing layer (5) on a side of the second substrate,
characterised in that the absorbing layer (5) comprises a binder and a
chemical compound having a metal element with an atomic number of
20 or more and one or more non-metal elements.
2. The radiography flat panel detector according to claim 1, wherein the
second substrate (4) consists essentially of materials selected from the
group consisting of Aluminium, polyethylene terephthalate, polyethylene
naphthalate, polyimide, polyethersulphone, carbon fibre reinforced plastic,
glass, cellulose triacetate and a combination thereof or laminates thereof.
3. The radiography flat panel detector according to any of the preceding
claims, wherein the second substrate is a flexible sheet.
4. The radiography flat panel detector according to any of the preceding
claims, wherein the layer configuration is in the order given,
a) the scintillator or photoconductive layer (1)
b) the imaging array (2)
c) the first substrate (3)
d) the second substrate (4)
e) the X-ray absorbing layer (5)
5. The radiography flat panel detector according to any of the preceding
claims, wherein the chemical compound is selected form the group
consisting of CsI, Gd2O2S, BaFBr, CaWO4, BaTiO3, Gd2O3, BaCl2, BaF2,
32
BaO, Ce2O3, CeO2, CsNO3, GdF2, PdI2, TeO2, SnI2, SnO, BaSO4, BaCO3,
BaI, BaFX, RFXn, RFyOz, RFy(SO4)z, RFySz, RFy(WO4)z, CsBr, CsCl, CsF,
CsNO3, Cs2SO4, Osmium halides, Osmium oxides, Osmium sulphides,
Rhenium halides, Rhenium oxides and Rhenium sulphides or mixtures
thereof, wherein:
- X is a halide selected from the group of F, Cl, Br and I; and
- RF is a lanthanide selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd,
Tb, Dy, Ho, Er, Tm, Yb and Lu; and
- n, y, z are independently an integer number higher than 1.
6. The radiography flat panel detector according to any of the preceding
claims, wherein the X-ray shield is positioned between the first substrate
(3) and the underlying electronics (6).
7. The radiography flat panel detector according to any of the preceding
claims, wherein the binder is selected from the group of cellulose acetate
butyrate, polyalkyl (meth)acrylates, polyvinyl-n-butyral, poly(vinylacetateco-
vinylchloride), poly(acrylonitrile-co-butadiene-co-styrene), poly(vinyl
chloride-co-vinyl acetate-co-vinylalcohol), poly(butyl acrylate), poly(ethyl
acrylate), poly(methacrylic acid), poly(vinyl butyral), trimellitic acid,
butenedioic anhydride, phthalic anhydride, polyisoprene, and mixtures
thereof.
8. The radiography flat panel detector according to any of the preceding
claims, wherein the amount of the binder in the X-ray absorbing layer is
10% by weight or less.
9. A method of making a radiography flat panel detector as defined in claim 1,
comprising the steps of:
a) providing a first substrate (3) with an imaging array (2) on a side of the
first substrate; and
b) gluing a scintillating phosphor (1) onto the imaging array; and
33
c) providing a second substrate (4); and
d) coating the X-ray absorbing layer (5) on a side of the second substrate
(4); and
c) contacting either the side of the second substrate (4) opposite to the Xray
absorbing layer (5) or the X-ray absorbing layer (5) with the side of
the first substrate (3) opposite to the imaging array (2).
10. The method of making a radiography flat panel detector according to claim
9 wherein the coating is performed by knife coating or doctor blade coating.