Method of removing the spatial response signature of a twodimensional
computed radiography detector from a computed
radiography image .
[DESCRIPTION]
FIELD OF THE INVENTION
The present invention relates to computed radiography. The
invention more particularly relates to a method for removing the
spatial response signature of a photo- stimulable phosphor detector
from an image acquired by means of that detector.
BACKGROUND OF THE INVENTION
Computed radiography (CR) performance is tightly coupled to the
overall image quality and detection capabilities of the entire image
acquisition, processing and display chain. For diagnosis or during
technical image quality testing patient- or target (phantom) -
images are created on an intermediate storage medium, called image
plate or detector. During exposure the image plate traps the locally
impinging x-rays and stores the latent shadow image until it is
scanned and converted into a digital image by a read out device
(digitizer) .
Physical process limitations and tolerances during image plate
manufacturing generate local sensitivity variability across the
detector surface. Storage phosphor based ( amorphic or crystal ) CR
detectors are multi- layered structures composed of a substratum, an
adhesion layer, a conversion and storage layer and a protective
sealing layer. Each of these functional layers and their interfaces
may suffer from various levels of typical imperfections, blemishes
and artifacts causing locally deviating image plate sensitivity. The
medium to high spatial frequency components of the relative
sensitivity distribution across a detector's surface reflect the
12 050594
- 2 -
image plate structure (IPS), the detector's unique signature.
A CR-image should closely reflect the patient's or object's x-ray
shadow information. Since the detector's local sensitivity is the
multiplicative factor controlling the conversion of the latent dose
information into the image signal, the IPS is inevitably water
marked into each CR image acquired from it . Local image plate
sensitivity variability can by consequence lead to diagnostic image
quality loss because the relevant patient information is polluted by
the detector's IPS. Like dose-related quantum (photon) noise and
digitizer noise, the IPS is a detector-related, disturbing noise
source which diminishes the Detective Quantum Efficiency (DQE) of
the CR system. Excessive IPS thus reduces the radiologist's reading
comfort and confidence level since it becomes more difficult to
discern subtle but important image information.
Mammography, an image quality wise highly demanding CR market,
imposes tough requirements to the magnitude and spatial extent of
the detector's sensitivity variability distribution. Stringent IPS
control is key to preserve a sufficient visibility of tiny objects
like micro-calcifications and the sharp delineation of subtle,
medium to large structures inside the breast tissue. Image plate
artifacts, isolated sensitivity disturbances, also part of a
detector's characteristic IPS, are of major concern in diagnostic
image viewing since their distinct presence can potentially hide
pathology and hamper the reading of the surrounding image area.
Excessive detector sensitivity variability can easily generate
costly yield loss in detector manufacturing.
It is an object of the present invention to improve the diagnostic
image quality and expand the detection capabilities of a
radiographic image system by removing the x-ray detector' s
characteristic spatial response signature from a radiographic image.
SUMMARY OF THE INVENTION
2 050594
- 3 -
The above-mentioned aspect is realised by a method having the
specific features set out in claim 1 .
Specific features for preferred embodiments of the invention are set
out in the dependent claims .
A detector's signature is defined as the relative, medium to high
spatial frequency components of a computed radiography detector's
characteristic sensitivity.
The method of the present invention will enable the removal of an
image plate's disturbing IPS noise from diagnostic CR images to
obtain an unprecedented image quality level and increased detection
capabilities (DQE) .
The method of the present invention is generally implemented in the
form of a computer program product adapted to carry out the method
steps of the present invention when run on a computer. The computer
program product is commonly stored in a computer readable carrier
medium such as a DVD. Alternatively the computer program product
takes the form of an electric signal and can be communicated to a
user through electronic communication.
IPS removal will indirectly ( improved image quality and DQE )
weaken the need for tough IPS acceptance criteria in detector QC and
this leads to a better yield in CR image plate manufacturing.
Further advantages and embodiments of the present invention will
become apparent from the following description and drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a flow chart illustrating the different steps of the image
plate signature removal method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Below a specific embodiment of the process of removing an image
plate's (also called CR detector') spatial response signature from
a radiographic image recorded on that image plate is described.
The image plate used in computed radiography typically comprises a
photo- stimulable phosphor.
Examples of suitable detectors comprising a photo- stimulable
phosphor are described e.g. in European patent application 1 818 943
and European patent application 1 526 552.
Diagnostic Image Generation
The image generation process in its most general formulation
comprises the steps of generating a diagnostic image by exposing a
patient or object to radiation, capturing the x-ray shadow on a
computed radiography detector, preferably line-wise scanning the
exposed detector by means of light, e.g. laser light and by
digitizing the scanned image.
Examples of a scanning and digitizing method and apparatus (also
called digitizer or read out apparatus) are well known in the art.
The apparatus generally comprises means for line-wise scanning (main
scan direction) a computed radiography detector that has been
exposed to penetrating irradiation (e.g. x-rays) with stimulating
light (e.g. according to the flying spot scanning principle) and
means for transporting the detector in a second direction
substantially perpendicular to the main scanning direction (sub- scan
or slow scan transport direction) to obtain a two-dimensional scan.
Upon stimulation the radiography detector emits image-wise modulated
light. Means (such as a photomultiplier) are provided to detect
this image-wise modulated light and convert it into an electric
image signal. The electric image signal is next digitized by an
analog- to-digital convertor.
Several steps (some of which are optional) of the present invention
are described hereinafter. It will be clear to the man skilled in
the art that the numerical values which are disclosed are only given
for illustrative purposes and do not limit the present invention.
Dose-linear signal conversion
The digitizer's characteristic dose response curve is used to
convert the image signals from the native format obtained by
scanning and digitizing into dose-linearized signals (if the native
format is not dose -linear) because the intended removal of an image
plate's structure (IPS) from CR (computed radiography) images
requires a multiplicative demodulation.
This first step in image preparation is performed for the diagnostic
image .
Image Plate Signature Retrieval
The unique image plate signature (IPS) of the detector used during
diagnostic image acquisition is retrieved locally or from an IPS
repository by means of the detector's identification data,
associated with the diagnostic image.
A method of determining the spatial response signature of a two
dimensional x-ray detector comprising a photostimulable phosphor is
described in EP 2 407 106. The method comprises the step of
generating a flat field image by homogeneously exposing the detector
to radiation and scanning the homogeneously exposed detector and by
digitizing the scanned image. Next a low-pass filtered version of
said flat field image is generated. Finally the flat field image is
background demodulated by means of corresponding pixel values in
said low-pass filtered version.
Image Plate Signature Decryption
2050594
- 6 -
The IPS might require decryption (if it was stored with encryption)
to convert it into the dose-linear, normalized relative detectorsensitivity
format.
Image Plate Signature Resampling
If the retrieved IPS and the diagnostic image would have different
pixel sizes, the IPS must be resampled to match the pixel -size of
the diagnostic image first.
Image Plate Signature Adaptive Filtering ( Sharpness Matching )
After image plate assembly and before shipping the detector along
with its unique associated IPS ( IPS files can be web-distributed
too )to the hospital a task-dedicated digitizer is used during image
plate structure characterization in manufacturing.
During diagnostic imaging a different ( and possibly even a
different type of )digitizer is used in hospitals world-wide.
Due to sharpness tolerances between individual digitizers, the end
resulting sharpness ( MTF ) of a detector's spatial sensitivity
distribution as hidden in the dose-linear, diagnostic image may
differ from the sharpness representation as stored in the detectorassociated
IPS. For optimal IPS removal results the sharpness of the
IPS must be matched with the sharpness as present in the diagnostic
image. This matching-process is performed by an closed loop control
system which successively alters the characteristics of an adaptive
filter until the sharpness-filtered IPS* best correlates with the
IPS as present in the diagnostic image. The results calculated from
the in-register cross-correlation of the adaptively filtered
signature and the CR- image are used as sensitive feedback signals to
measure the pursued sharpness match. Based on these signals the
control system determines the most optimal filter settings to reach
the best possible sharpness -match . In one embodiment the feedback
signals from the various marker points in the diagnostic image are
weighed differently. The cross-correlation results from image-areas
outside the skinline in a mammography image are disregarded and more
feedback impact is given to the central and chest -wall region of the
breast -image .
High Stop Filtered Background Normalization
Next, a high-stop filtered version of the diagnostic image is
generated and the diagnostic image is background normalized by means
of corresponding pixel values in said high-stop filtered version.
A pixel -centered 5 x 5 pixels square background average kernel high
stop filter (low pass filter) demodulates the low to medium-low
spatial frequency image -components by dividing the pixel signals by
their background average signals and by scaling that result with a
fixed factor to obtain the desired normalized signal level.
The 5x5 pixels kernel size is given for illustrative purposes and
does not limit the present invention.
This background normalization step is performed for the dose-linear
diagnostic image and for its associated IPS* sharpness -matched
detector- signature .
Spatial correlation disturbing, low and medium spatial frequency
image -components are generally extensively present in diagnostic
images. These are thus effectively removed and what remains are the
higher spatial frequency components from the patient- shadow and from
the subtly present, hidden detector signature. This enables a subpixel
accurate determination of the spatial register between the
background normalized diagnostic image and its IPS* by means of
virtual marker based spatial correlation mainly steered by the fine ¬
grained, local, noisiness of the detector's relative sensitivity
distribution.
Delta Clipping
Virtual marker correlation acts on two sets of spatially associated
neighbouring pixel clusters located in the background normalized
diagnostic image and in the IPS*.
Polluting surface particles in the diagnostic image can generate
high signal contrasts and these can, if passed unsuppressed, ruin
the accuracy of the virtual marker register determination completely
if the IPS would locally contain similarly looking (size, shape,
modulation) features too.
Signal delta clipping limits the relative maximum deviation of the
local pixel signal to +/- 1% of its local background to prevent
this. This +/- 1% clip level is given for illustrative purposes and
does not limit the present invention.
Signal delta clipping is performed for the background normalized
diagnostic and IPS* images.
Virtual marker frame definition
A virtual marker grid (or mesh) spanning the majority of the
detector's surface is defined in the delta clipped diagnostic image
acting as the spatial reference image from now on.
Using a bidirectional grid-pitch of 100 pixels at a 50 micron pixel -
size a 5 mm maze- size grid of invisible but accurately detectable
landmarks is generated in the reference image.
This creates a 57 x 45 (2565) virtual marker array for a 30 x 24 cm
CR cassette format (a cassette carrying a CR detector) and enables a
tight local control of each view's spatial distortions.
The numerical data are given for illustrative purposes and are not
limitative for the present invention.
Virtual markers can also be replaced by physical markers if the
diagnostic application concerned allows the use of physical markers.
Spatial register vector calculation
Virtual marker pixel -clusters , centered about the mesh-points within
the spatial reference ( diagnostic ) image are individually subpixel
correlated with their corresponding, similarly sized, pixel
clusters arranged within their slightly larger, corresponding
register vector search regions in the delta clipped IPS* image.
This way local register vectors are detected with a sufficient
surface resolution by sub-pixel interpolated maximum correlation
based on the detector's hidden IPS between the IPS* and the
diagnostic image. The local register vectors found are arranged in
an IPS* map and by concept the register vector map of the reference
image would be filled with zero vectors.
Multiple, virtual marker based correlations are performed at various
spatially distributed locations across the detector's surface and
the set of correlation maxima obtained is used as feedback signals
to determine the parameters for the adaptive IPS filtering performed
by the embedded sharpness matching control system.
Register vector map verification and corrections
The register vector map containing the in sub-pixel spatial register
information for the IPS* with the reference image is cross-checked
for local unexpected virtual marker correlation abnormalities. This
is done by calculating the interpolated or extrapolated ( virtual
marker grid borders and corners ) average register vector based on
the immediately surrounding register vectors. The locally calculated
register vector is replaced by its surrounding vector average if its
vector-difference exceeds a certain sub-pixel distance. The entire
IPS* register vector map is subjected to this verification and
correction process.
Warp-vector map generation
Once the virtual marker register vector map has been checked and
possibly modified, the local register vectors, relating each of the
diagnostic image ( acting as register reference ) pixels to their
sub-pixel spatially associated points in the IPS* , are created.
Interpolation and or extrapolation of the available register vector
map data generates this image -wide map of sub-pixel accurate spatial
register vectors at pixel resolution.
This map uniquely defines the geometric transformation model which
is required to distort (warp) the IPS* such that its pixels
physically match (at the image plate's detector layer) with the
corresponding diagnostic image pixels.
In image register warping
Based on the sub-pixel accuracy register vector available for each
diagnostic image pixel, the in spatial register signal
reconstruction is performed by using the pixel signals from the
correlated background normalized and clipped IPS* image. Register
vector steered interpolation of the correlated image's surrounding
pixel signal computes the in register signal. This way the IPS* is
replaced by its in spatial register (with the diagnostic image)
computed image. The pixel signals of that warped IPS* image are thus
calculated at the same physical position at the surface of the
detector as the signals from their corresponding pixels in the
diagnostic image .
Sub-pixel phases controlled TF reconstruction
The background normalized and clipped IPS* has been warped according
to its image-wide register vector map based on its locally
surrounding original pixel data and its bidirectional sub-pixel
phases corresponding with the actual interpolation ( resampling )
point, indicated by the spatial register vector. A certain amount of
sharpness loss is inherent to this image resampling ( warping )
process and the resulting, bidirectional image-blur depends on the
interpolation point's bidirectional, sub-pixel phase's magnitudes.
The closer the interpolation point is located to the nearest
original image pixel in a certain image direction, the sharper the
warped image in that direction will be. The sharpness loss is at
maximum when the interpolation point is located in the center of the
four surrounding original data pixels at identical 0,5 pixel phases
in both main image directions. Modulation Transfer Function
(sharpness) reconstruction extracts the bi-directionally decoupled,
sub-pixel phases from the verified and corrected register vector map
and distils an anisotropic convolution filter kernel from it to re
establish the sharpness of the warped image at the level before
warping. The gain of this sharpness reconstruction filter process is
near unity and this reconstruction process generates the IPS**
image .
Image Plate Signature Demodulation
The in-diagnostic-image warped pixels of the IPS** contain the local
relative detector sensitivity distribution. By dividing the doselinear
diagnostic image with this near unity IPS** data a new image
signal, simulating the local pixel response for a uniform dosesensitivity
across the entire detector- surface , is created. This way
the disturbing effects of the detector's non-uniform sensitivity
distribution are effectively removed from the diagnostic image.
Saturated CR-image pixels are just passed onto the signature-removed
CR image without demodulation to avoid the inappropriate burn in of
the detector's relative sensitivity distribution in smooth,
saturated image regions. The IPS** demodulated signals are clipped
to the saturated signal level if needed.
Native Conversion
Unless the diagnostic image pixel signals were already in the doselinear
format the IPS removed image is converted into its native
signal format to finalize the process of image plate signature
removal .■
[CLAIMS]
1 . Method of removing the spatial response signature of a two
dimensional x-ray detector comprising a photostimulable phosphor
from a computed radiography (CR) image by:
- retrieving the spatial response signature of a CR-detector used
during the acquisition of said CR image,
- in-spatial-register demodulating said CR image by dividing pixel
values of said CR image by corresponding values in the retrieved and
spatially remapped signature, values of the spatially remapped
signature being obtained by warping said spatial response signature
according to a geometric transformation model characterized in that
a read out device used for reading said CR- image out of the detector
and a read out device used during determination of the spatial
response signature of the detector are different devices and that
the retrieved signature is re-sampled to match the pixel size of the
signature with that of the CR image .
2 . A method according to claim 1 wherein said CR- image and said
signature are background-normalised by pixel -wise dividing the CR
image by a low pass filtered version of said CR image and by pixelwise
dividing the signature by a low pass filtered version of said
signature .
3 . A method according to claim 1 wherein the CR- image and the
extracted signature are clipped if the signals representing the CR
image or the extracted signature deviate more than a given
threshold-percentage from the corresponding background- demodulated
signals .
. A method according to claim 1 wherein said geometric
transformation model is calculated by cross -registering a multitude
of spatially distributed markers arranged on the surface of said
detector and present in both said CR- image and in the retrieved
signature .
5 . A method according to claim 4 wherein said markers are virtual
markers which consist of a cluster of neighboring pixels arbitrarily
defined within said CR-image.
6 . A method according to claim 5 wherein for each marker an inspatial-
register location is calculated by cross-correlating pixelcluster
data values associated with said marker in said CR-image
with a multitude of physically neighboring, bi-directionally pixelshifted,
pixel -cluster data value sets in said signature to obtain a
set of cross-correlation results.
7 . A method according to claim 6 wherein each marker's in-register
location is determined with sub-pixel accuracy by interpolating said
set of cross-correlation results.
8 . A method according to claim 4 wherein a bidirectional marker-grid
is defined in said CR-image.
. A method according to claim 8 wherein interpolation or
extrapolation of marker-grid' s vector-data representing a geometric
transformation between the location of a marker grid element in said
CR image with its corresponding location in said signature is used
to compose a geometric transformation model which links each
individual CR-image pixel to its physically associated location in
the retrieved signature.
10 . A method according to claim 9 wherein the retrieved signature is
warped to correspond pixel-wise with the CR-image by interpolating
or extrapolating the signature data at the positions indicated by
said geometric transformation model.
11. A method according to claim 10 wherein bi-directionally
decoupled, sub-pixel phases defined by said vector data are
extracted and an anisotropic convolution filter kernel is distilled
from it to re-establish the sharpness of the warped signature at the
level before warping.
12 . A method according to claim 1 wherein the retrieved signature
passes an adaptive filter to compensate for modulation transfer
function differences and temporal drift of the CR-read out devices
used.
13 . A method according to claim 12 wherein the behavior of the inregister
cross-correlation feedback between the set of adaptively
filtered signatures and the CR- image determines the filter
parameters of said adaptive filter.
14. A method according to claim 13 wherein multiple, spatially
distributed regions of interest in the CR- image and in the
adaptively filtered signatures contribute to the cross-correlation
feedback.
15. A method according to claim 14 wherein feedback contributions of
said multiple regions-of -interest are weighed differently.
16. A method according to claim 15 wherein a weight attributed to
said contribution of a region of interest depends on the diagnostic
importance of said region of interest inside said CR- image.
17 . A computer program product adapted to carry out the method of
any of the preceding claims when run on a computer.
18 . A computer readable medium comprising computer executable
program code adapted to carry out the steps of any of claims 1-16.