Storage screens for medical radiography.
Field of the invention :
The present invention relates to a method for the treatment of storage screens for use in
diagnosis, more in particular for use in medical radiography. The treatment of screens
according to the present invention takes place promptly upon the production of such
screens. According to the method of the invention the so-produced screens are treated
with oxirane so as to prevent yellowing of the screens in a later stage, and the problems
related with such yellowing.
The method according to the invention is likewise applicable to semi-finisched products
in the production of screens as well as to phosphors for use in such screens.
Background of the invention
It is common knowledge that nowadays needle phosphors as well as powder phosphors
are used in the production of storage phosphors plates for computed radiography.
In the case of powderphosphors use is often made of divalent europium activated
alkaline earth metal fluoro-halide phosphors. Bariumfluorobromide/iodide compounds
are commonly used. Storage phosphors used in such plates e.g. have the following
composition Ba 0.928 Sro.07Euo.002F1.05Bro.80Io.15.
In the case of powder phosphor plates, these compounds are coated as powders in a
layer on the plate.
In contrast with such powder phosphor plates, in the case of needle phosphor plates, the
phosphor is formed on the surface of a carrier or support, made of e.g. aluminum, in the
form of needles through a vapor-deposition mechanism. In the latter case, use is often
made of cesiumbromide compounds.
In the case of phosphor plates that make use of powder compounds incorporating iodide
containing phosphors, in the course of its use, a yellowing of the plates occurs.
Adrawback occasioned by such yellowing is, that the plate thereby is characterised by
an unpleasing appearance for the user; it also gives the plate a weary appearance. A
further, more technical disadvantage is that when the plate is read out in a digitizer, an
uneven or stainy image is detected. This is the main disadvantage, since it compromises
use of the plate for (medicaljdiagnosis.
The probable cause of this phenomenon is that I-, present for e.g. 15 mol%, is available
for oxydation at the surface of the grain, and as a result herof is converted to I2 gas. This
oxydation is catalysed by UV irradiation, humidity and an acid atmosphere ( H< 7).
The vapour pressure of the so formed I 2 gas is low, and as a result hereof it remains in
the phosphor plate, and does not - or only very partially - escape herefrom.
The visual result hereof is a noticeable yellowing of the plate. As such this phenomenon
does not adversely affect the functioning of the phosphor during a radiographic
exposure. The plate stores the radiographic 'latent' image, just like before the yellowing
phenomenon. However, during the subsequent phase, this is during the reading out of
the phosphor plate in the digitizer, the I2 as formed will partly absorb the bleu emission
light of the phosphor. As a result hereof, these yellow stains will also become visual on
the digitally read-out image as stainy areas with a lower signal strength.
This clearly is unacceptable since the quality of the medical diagnosis is negatively
influenced by this event.
I is much more easily oxidized compared to Br - and certainly compared to F-. For the
latter two ions the problem as described above does not occur when storage phosphor
plates are used in conformity with usual working conditions.
This problem of yellowing of the phosphor plate and the local weakening of the emission
light caused hereby, is occurring in the case of phosphor plates that are used for general
radiography (e.g. for exposure of the lungs or the thorax], as well as for special
radiographic applications such as mammography. Also in case of the use of phosphor
plates for non-destructive testing, this is a disadvantage. The problem is in particular an
issue in the case of storage phosphor plates for use in dental applications.
The reason for the latter phenomenon is as follows : storage phosphor plates for general
or mammographic applications are usually stored in a cassette, so they are shielded
from light; storage screens for dental applications on the contrary, are either not stored
in a cassette, or in a transparant housing on e.g. the desk of a dentist, and as such are not
shielded from ambiant light.
Storage screens for dental applications are for each new exposure put in a new plastic
envelope. After exposure of a patient the phosphor plate is taken out of the enveloppe
for being read out. Only when the phosphor plate is intended to be used again, it is
placed in a new envelope. Inbetween such uses, the phosphor plate is exposed to UV
light and air humidity.
From the European Patent Publication EP 0 234 385, applicant Fuji Photo Film Co., Ltd,
published on September 2, 1987, this problem is known as such. The solution disclosed
by this patent application is as follows. Acompound containing an epoxy group is
added to the phosphor plate, containing an iodide containing phosphor, preferably a
divalent europium activated alkaline earth metal fluorohalide phosphor. This
compound may be added to the layer containing the phosphor. In this case the epoxy
containing compound can also act as binder for the phosphor. In an alternate
arrangement, such compound may be added to a neighbouring layer of the phosphor
layer.
According to this disclosure, these compounds are proposed to be added as liquid
epoxides to the phosphor layer composition; throughout the specification disclosure is
made of polymercoatings, but also the 1,2 epoxypropane compound is mentioned
(C H O .
This method however is characterised by many drawbacks.
Most of the examples cited in the text are directed to a resin, whose reactivity is
inherently limited. The reason hereof is that resinmolecules by definition are long
molecules, and as such less mobile, or have a limited diffusion velocity.
This relatively immobile resin should bridge the distance to the phosphor particle. In
quite a number of cases, this is impossible because the phosphor layer quickly dries
after coating. As from such drying, the resin molecules are in fact immobilised in the
phosphor layer, and as a result become useless for the aim as set forth above.
This phenomenon and resulting disadvantage even is stronger when the resinmolecules
are present in a layer neigbhouring the phosphor layer, and are not present in the
phosphor layer itself.
One should also take into account the thickness of the phosphor layer. In the case of
general medical radiographic applications, such thickness amounts to approx. 250 . In
the case of mammographic applications the thickness amounts to approx. 150 . In case
of dental applications the thickness is approx. 100 .
To the extent the molecular weight of the epoxy-containing compounds decreases, the
above-mentioned disadvantages decrease also in proportion herewith. However, in such
a case another drawback pops up : evaporation of low-molecular weight epoxy
containing compounds during the coating- and drying operations of the phosphor layer.
Through this evaporation such compounds do not have the time and consequently the
opportunity to react with the iodide compounds at the surface of the phosphor particles.
As a result, the gain occasioned by the use of more reactive low-molecular weight
compounds, is lost through the loss of such compounds out of the phosphor mixture, res.
the coated phosphor layer by evaporation.
The practical result of this is that the still present reactive I- at the surface of the
phosphor particle after coating and drying of the phosphor layer, cannot be 'treated' by
the still present immobile epoxides in such layer.
This will in the end give rise to yellowing of the screens, causing the problems as
described above.
Asecond disadvantage of this method is that the presence of such extra component
reduces the packing density of the phosphor screen; this packing density should be as
high as possible in case of stimulable phosphor screens.
Athird disadvantage of this method is that the presence of such extra component
negatively influences the drying characteristics of the phosphorlayer in production.
This in turn leads to screen structure and variations in packingsdensity; this latter is of
primary concern in the production of powder phosphor screens.
Finally, there still is another important disadvantage linked to this production method :
working with such resins incorporating toxic components gives rise to numerous issues
in practical working conditions, a.o. from the point of view of safety of the installations
(explosion risk), as well as from the point of view of the operating staff (carcinogenic
components). Also the final stimulable phosphor screens is not fully clean : it still
contains a certain amount of carcinogenic compounds.
For all of the abovementioned reasons there remains the need to develop a production
method devoid of any of the above mentioned problems.
Summary of the invention
The objective of the present invention is to solve the abovementioned drawbacks in an
efficient manner, by offering an improved method for the production of stimulable
phosphor screens. More in particular the invention concerns a method for treatment of
stimulable phosphor screens, that is applied promptly after the production of the
storage phosphor screens. More in particular this method provides for epoxidecontaining
gaseous compounds that operate on storage phosphor screens shortly upon
their manufacture. Oxirane (or ethylene-oxide) containing compounds are a preferred
type of such compounds.
More in particular the invention concerns the methods and storage screens as set forth
in the appended claims.
Detailed description of the invention
Methods for the production of storage phosphor screens on the basis of
bariumfluorobromide/iodide are known to the person skilled in the art. To illustrate
same, European Patent Publication EP 0 835 920 Bl discloses such method.
Generally a BaFBrl-phosphor is coated onto a rigid support, e.g. made of polyester. More
in particular a support made of polyethylene terephthalate, with a thickness of approx.
250 is often used as support. Such a support is usually supplied in the form of a roll.
This roll is derolled and a suspension of a phosphor is then coated on the roll. Hereupon
the coated roll is dried, and a protective layer is coated on the dried phosphor layer. The
finished rolls are then slit and cut to the usual commercial dimensions of phosphor
plates.
Such commercially usual formats depend on the envisaged application. In medical
diagnosis these applications can be in general radiography, in mammography or in
dental. Apart from medical diagnosis, non-destructive testing of materials is another
field of application.
According to the present invention, the above described production method of phosphor
plates can continue to be used in unaltered form and mode. According to the invention,
it suffices to apply to the phosphor plates manufactured as described above an after
treatment step, to solve the problems and drawbacks set forth supra.
To this end the phosphor plates as produced e.g. according to the method described
above are placed in an after-treatment room, an autoclave or sterilisation room. In this
isolated environment they are put into contact with gaseous epoxide-containing
compounds like e.g. oxirane.
Oxirane or ethyleneoxide is a compound known per se that may be used for the
sterilisation of objects.
According to the invention the phosphor plates, promptly after their manufacture, are
introduced in a sterilisation chamber or room, wherein they are contacted for a
predefined amount of time with an gaseous epoxide-containing compound. In a
particularly preferred mode of the present invention, use is made of oxirane, or
ethylene-oxide.
The bruto chemical formulae of oxirane is C2H4O2, and the structural formulae is as
follows :
Treating objects with oxirane is a method known as such, but with a different aim :
sterilisation. This method is also used e.g. in a hospital environment for the sterilisation
of drapes, gowns and chirurgical instruments generally.
There are also publications that describe how stimulable phosphorscreens that are
already in practical use in hospitals can be sterilized by use of this method known as
such.
An example of such public disclosure is the article entitled 'An evaluation of
microbiologic contamination on a phosphor plate system : is weekly gas sterilization
enough ?', published in Oral Surg. Oral Med Oral Pathol Oral Radiol Endod 2010; 109 :
457-462'.
This article describes how microbiological contamination of photostimulable
phosphorscreens is countered by gas-sterilization using ethylene-oxide.
To this end each friday all phosphor plates are sterilized by using a model 4XL Steri-Vac
gas sterlization chamber, produced by 3M Health Care, St. Paul, Minnesota, USA.
Asimilar publication is the US patent application nr US2008/0085228 Al, published
April 10, 2008.
This application describes how a digitizer for stimulable phosphorplates (a so-called
digitizer) comprises a sterilization or desinfection unit, that applies a sterilization or
desinfection treatment to a photostimulable phosphor plate. As desinfectant treatment
various possibilities are described, including a gas-treatment. Paragraph 59 discloses
the use of ethylene oxide as gas. The aim of the latter two disclosures however is quite
distinct from the aim of the present invention.
In the latter disclosures the sole aim of the treatment of the phosphor screens is to
desinfect these. The treatment does not go beyond this aim. The treatment as described
in these publications is quite superficial, and takes place at at time that the problem
which the method of our invention wishes to solve, has already occurred and cannot be
solved any more.
Indeed as soon as the gaseous iodide has been formed in the phosphor layer, the
problems associated herewith do or will occur, and cannot be remedied any more by a
later superficial treatment of the phosphor layer.
The method according to the present invention should take place promptly upon the
production of the phosphorscreens on roll. The phosphor screens, still on roll, are then
placed in an autoclave, and sujected to the oxirane treatment. Hereupon the final
production step of the phosphorscreens can occur, this is the slitting and cutting of the
phosphor screens on roll unto the commercially usual formats.
According to an alternate embodiment of the present invention, a socalled pre-slitting of
the phosphorrolls takes place unto strips, whereupon these strips are subjected to the
oxirane treatment. After this treatment according to the invention, the final slitting and
cutting of the phosphor plates takes place to yield the commercially usual formats of the
plates.
Another alternative embodiment of the present invention is that the final slitting and
cutting of the phosphor rolls takes place before the treatment according to the
invention. In the latter case, the slit and cut phosphor plates are subjected to the
oxirane treatment.
The invention as described supra is applied preferably on phosphor screens, promptly
upon their production, so before their being put on the market.
However, the method of our invention can also be applied to semi-finished products in
the production of the phosphor screens, as well as on the powder phosphors itself for
use in such screens.
According to the understanding of the inventors, the working mechanismm of the
oxirane treatment on the phosphorscreens as produced is as follows : a nucleophylic
substitution takes place by the strongly nucleophylic iodide on oxirane.
Because a phosphorlayer upon coating and drying can contain up to 30 %by volume of
air, the oxirane can relatively easily enter such porous phosphor plates, and exert its
function. Probably the triangle of oxirane opens up and interacts with the surface of the
phosphor particle. Probably hereupon the following compound is formed : I-[CH2-CH2-
0 - n-H with n>0.
The invention is summarized in the appended claims.
Claims :
1. Method for the treatment of powder stimulable phosphorscreens comprising
iodide containing alkaline earth metal halogenide phosphors with gaseous
epoxide-containing compounds, characterized in that such treatment occurs
promptly upon the manufacture of said screens, before first commercial use.
2. Method according to claim 1, wherein the stimulable phosphor screens comprise
divalent europium as activator.
3. Method according to claim 1 or 2, wherein the alkaline earth metal halogenide
phosphor is a bariumfluorohalogenide phosphor.
4. Method according to one of the previous claims, wherein the epoxide-containing
compound is oxirane.
5. Method according to one of the previous claims, wherein the treatment is applied
unto the powder-stimulable phosphor screens after coating and drying, before
slitting and cutting of the plates unto commercially usual formats.
6. Method according to one of the preceding claims, wherein the powder-stimulable
phosphor screens are slit and cut into formats usual for dental applications.
7. Powder stimulable phosphor screens comprising iodide containing alkaline earth
metal halogenide phosphors, characterised in that promptly following their
manufacture and before their first commercial use, a treatment with gaseous
epoxide-containing compounds has taken place.
8. Powder stimulable phosphor screens according to claim 7, wherein the gaseous
compound is oxirane.
9. Powder stimulable phosphor screens according to claim 7 or 8, wherein the
screens are slit and cut into the usual commercial formats.
AMENDED CLAIMS
received by the International Bureau on 17 September 2012 (17.09.2012)
CLAIMS
1 . Method for the production of a phosphorous screen wherein
i ) said phosphorous screen comprises a phosphorous
composition containing iodine, and
ii) a treatment with at least an epoxy group containing
compound in gaseous form is applied to the phosphorous
composition of i ) during the manufacturing process of
the phosphorous screen.
2 . Method, according to claim 1 , wherein the phosphorous
composition comprises alkaline earth metal halogenide
phosphors .
3 . Method, according to claim 1 or 2, wherein the alkaline
earth metal halogenide phosphor composition is a
bariumf luorohalogenide phosphor.
4 . Method, according to any of the claims 1 to , wherein the
phosphorous composition comprises divalent europium as
activator .
5 . Method, according to claim 1 , wherein the epoxy group
containing compound is oxirane .
6 . Method, according to any of the previous claims, wherein
the treatment with at least an epoxy group containing
compound is performed to the phosphorous composition before
it is applied onto the screen.
7 . Method according to claims 1 to 5 , wherein the treatment
with at least an epoxy group containing compound is
performed to the phosphor composition after said
composition is applied to the screen and before the screen
is slit and/or cut into the desirable formats and shapes.
8 . Method according to claims 1 to 5 , wherein when the
phosphorous composition is a powder the treatment with at
least an epoxy group containing compound is performed to
the phosphor composition after said composition is applied
to the screen and after the screen is slit and/or cut into
the desirable formats and shapes.
9 . A phosphor screen comprising a phosphorous composition
containing iodine, obtainable by the method as described in
claims 1 to 8 .
10. Use of a phosphor screen, as described in claim 9 , in
medical applications.
11. Use of a phosphor screen, as described in claim 9 , in
dental applications.
12. Use of a phosphor screen, as described in claim 9 , in non
destructive testing of materials.