U.S. patent application number 11/230847 was filed with the patent office on 2007-03-22 for method and apparatus for compensating non-uniform detector collimator plates.
This patent application is currently assigned to General Electric Company. Invention is credited to Erdogan O. Gurmen, Matthew Aaron Halsmer, Jiang Hsieh, Aziz Ikhlef, Bing Shen, Tyler Justin Sprenger, Gregory Scott Zeman.
Application Number | 20070064866 11/230847 |
Document ID | / |
Family ID | 37807208 |
Filed Date | 2007-03-22 |
United States Patent
Application |
20070064866 |
Kind Code |
A1 |
Hsieh; Jiang ; et
al. |
March 22, 2007 |
METHOD AND APPARATUS FOR COMPENSATING NON-UNIFORM DETECTOR
COLLIMATOR PLATES
Abstract
A method for obtaining data includes scanning an object with
radiation to collect projection data using an imaging system having
a detector array with detector cells and a post-patient collimator,
wherein the post-patient collimator has plates having non-uniform
thicknesses. The method further includes applying a correction to
the projection data to shift an effective center of at least some
of the detector cells to compensate for the non-uniform thicknesses
of the collimator plates.
Inventors: |
Hsieh; Jiang; (Brookfield,
WI) ; Gurmen; Erdogan O.; (Shorewood, WI) ;
Ikhlef; Aziz; (Waukesha, WI) ; Shen; Bing;
(Cary, NC) ; Zeman; Gregory Scott; (Waukesha,
WI) ; Halsmer; Matthew Aaron; (Waukesha, WI) ;
Sprenger; Tyler Justin; (Milwaukee, WI) |
Correspondence
Address: |
PATRICK W. RASCHE (12553 - 1000)
ARMSTRONG TEASDALE LLP
ONE METROPOLITAN SQUARE, SUITE 2600
ST. LOUIS
MO
63102-2740
US
|
Assignee: |
General Electric Company
|
Family ID: |
37807208 |
Appl. No.: |
11/230847 |
Filed: |
September 20, 2005 |
Current U.S.
Class: |
378/19 ;
378/147 |
Current CPC
Class: |
Y10S 378/901 20130101;
A61B 6/06 20130101; A61B 6/032 20130101 |
Class at
Publication: |
378/019 ;
378/147 |
International
Class: |
H05G 1/60 20060101
H05G001/60; A61B 6/00 20060101 A61B006/00; G01N 23/00 20060101
G01N023/00; G21K 1/12 20060101 G21K001/12; G21K 1/02 20060101
G21K001/02 |
Claims
1. A method for imaging an object, said method comprising: scanning
an object to collect projection data, said scanning performed using
a CT imaging system having a detector array with detector cells and
a post-patient collimator, wherein the post-patient collimator has
boundary plates between some detector cells and center plates over
other detector cells, and the boundary plates have a different
effective thickness than do the center plates; obtaining corrected
data that compensates for the non-uniform thickness between the
boundary plates and the center plates by at least one of applying a
correction to the projection data, applying a filtered
backprojection process to the projection data, and applying a fan
to parallel beam rebinning process to the projection data; and
reconstructing an image of the object using the corrected data.
2. A method in accordance with claim 1 further comprising filtering
the boundary cells prior to the reconstruction to facilitate
reducing noise in the boundary cells relative to the center
cell.
3. A method in accordance with claim 1 wherein said applying a
correction comprises applying at least one of a linear
interpolation and a non-linear interpolation.
4. A method in accordance with claim 3 wherein the non-linear
interpolation is a fourth order Lagrange interpolation.
5. A method in accordance with claim 1 wherein the detector array
comprises a plurality of modules each having a plurality of
detector cells, wherein all of the detector cells are contained in
the plurality of modules, and said scanning an object using a CT
imaging system comprises scanning the object using a CT imaging
system having a detector array in which the plurality of modules
are interchangeable with one another.
6. A method in accordance with claim 1 wherein the thicknesses of
the boundary plates are twice that of the center plates.
7. A method in accordance with claim 6 wherein said scanning an
object comprises scanning the object using a CT imaging system
having a detector array with interchangeable detector modules.
8. A method in accordance with claim 7 wherein said applying a
correction comprises applying a linear interpolation.
9. A method in accordance with claim 7 wherein said applying a
correction comprises applying a non-linear interpolation.
10. A method in accordance with claim 7 wherein the non-linear
interpolation is a Lagrange interpolation.
11. A CT imaging system comprising: an x-ray source; a detector
array having detector cells; a post-patient collimator having
boundary plates between some detector cells and center plates over
other detector cells, and the boundary plates have a different
effective thickness than do the center plates; and an image
processing system configured to obtain corrected data from
projection data collected during scanning of an object by at least
one of applying a correction to the projection data, applying a
filtered backprojection process to the projection data, and
applying a fan to parallel beam rebinning process to the projection
data to thereby shift an effective center of at least some of the
detector cells to compensate for the nonuniform thickness between
the boundary plates and the center plates, and to reconstruct an
image of the object using the corrected data.
12. A system in accordance with claim 11 wherein the image
processing system is configured to filter the boundary cells prior
to the reconstruction to facilitate reducing noise in the boundary
cells relative to the center cell.
13. A system in accordance with claim 11 wherein the image
processing system is configured to at least one of apply a linear
correction and apply a non-linear correction.
14. A system in accordance with claim 11 wherein the image
processing system is configured to apply a Lagrange
interpolation.
15. A system in accordance with claim 11 wherein the detector array
comprises a plurality of modules each having a plurality of
detector cells, wherein all of the detector cells are included in
the plurality of modules, and the plurality of modules are
interchangeable with one another and each module includes a
separate set of plates, wherein adjoining plates of different,
adjacent modules of the detector array comprise the boundary places
and non-adjoining plates of each module comprise the center
plates.
16. A system in accordance with claim 11 wherein the thicknesses of
the boundary plates are twice that of the center plates.
17. A method for correcting projection data, said method
comprising: scanning an object with radiation to collect the
projection data using an imaging system having a detector array
with detector cells and a post-patient collimator, wherein the
post-patient collimator has non-uniform plate thicknesses, and
correcting the projection data in order to compensate for the
non-uniform thickness between the boundary plates and the center
plates by at least one of applying a correction to the projection
data, applying a filtered backprojection process to the projection
data, and applying a fan to parallel beam rebinning process to the
projection data.
18. A method in accordance with claim 17 further comprising
reconstructing an image of the object using the corrected
projection data.
19. A method in accordance with claim 17 further comprising
filtering the boundary cells prior to the reconstruction to
facilitate reducing noise in the boundary cells relative to the
center cell.
20. A method in accordance with claim 17 wherein said applying a
correction to the projection data comprises applying at least one
of a linear correction and a non-linear correction.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates generally to scanned imaging systems
and methods, more particularly to methods and apparatus for
reducing artifacts in images obtained from scanning of objects.
[0002] In some known CT imaging system configurations, an x-ray
source projects a fan-shaped beam which is collimated to lie within
an X-Y plane of a Cartesian coordinate system and generally
referred to as an "imaging plane". The x-ray beam passes through an
object being imaged, such as a patient. The beam, after being
attenuated by the object, impinges upon an array of radiation
detectors. The intensity of the attenuated radiation beam received
at the detector array is dependent upon the attenuation of an x-ray
beam by the object. Each detector element of the array produces a
separate electrical signal that is a measurement of the beam
intensity at the detector location. The intensity measurements from
all the detectors are acquired separately to produce a transmission
profile.
[0003] In third generation CT systems, the x-ray source and the
detector array are rotated with a gantry within the imaging plane
and around the object to be imaged such that the angle at which the
x-ray beam intersects the object constantly changes. A group of
x-ray attenuation measurements, i.e., projection data, from the
detector array at one gantry angle is referred to as a "view". A
"scan" of the object comprises a set of views made at different
gantry angles, or view angles, during one revolution of the x-ray
source and detector.
[0004] In an axial scan, the projection data is processed to
construct an image that corresponds to a two-dimensional slice
taken through the object. One method for reconstructing an image
from a set of projection data is referred to in the art as the
filtered backprojection technique. This process converts the
attenuation measurements from a scan into integers called "CT
numbers" or "Hounsfield units" (HU), which are used to control the
brightness of a corresponding pixel on a cathode ray tube
display.
[0005] To reduce the total scan time, a "helical" scan may be
performed. To perform a "helical" scan, the patient is moved while
the data for the prescribed number of slices is acquired. Such a
system generates a single helix from a fan beam helical scan. The
helix mapped out by the fan beam yields projection data from which
images in each prescribed slice may be reconstructed.
[0006] Reconstruction algorithms for helical scanning typically use
helical weighing algorithms that weight the collected data as a
function of view angle and detector channel index. Specifically,
prior to a filtered backprojection process, the data is weighted
according to a helical weighing factor, which is a function of both
the gantry angle and detector angle. The weighted data is then
processed to generate CT numbers and to construct an image that
corresponds to a two-dimensional slice taken through the
object.
[0007] To further reduce the total acquisition time, multi-slice CT
has been introduced. In multi-slice CT, multiple rows of projection
data are acquired simultaneously at any time instant. When combined
with helical scan mode, the system generates a single helix of cone
beam projection data. Similar to the single slice helical,
weighting scheme, a method can be derived to multiply the weight
with the projection data prior to the filtered backprojection
algorithm.
[0008] In at least one known third generation CT scanner, a set of
post-patient collimator plates is positioned in front of a detector
array for scatter rejection. The collimator plates are provided as
piece separate from a scintillator pack or module of the detector
array. However, providing collimator plates separate from the
scintillator pack or modules of the detector array makes it
difficult, among other things, to repair and/or replace detector
modules. On the other hand, artifacts can occur if collimator
plates are supplied as part of each detector module in the detector
array, and it is believed that the problem of providing low
artifact images using interchangeable detector modules that each
include a set of collimator plates has not yet been addressed in
the art.
BRIEF DESCRIPTION OF THE INVENTION
[0009] One aspect of the present invention therefore provides a
method for imaging an object. The scanning is performed by a CT
imaging system having a detector array with detector cells and a
post-patient collimator, wherein the post-patient collimator has
boundary plates between some detector cells and center plates over
other detector cells, and boundary plates have a different
effective thickness than do the center plates. A correction is
applied to the projection data to shift an effective center of at
least some of the detector cells to compensate for the non-uniform
thickness between the boundary plates and the center plates. An
image of the object is then reconstructed using the corrected
projection data.
[0010] Another aspect of the present invention provides a CT
imaging system that includes an x-ray source, a detector array
having detector elements or cells, and a post-patient collimator
having boundary plates between some detector cells and center
plates over other detector cells, and the boundary plates have a
different effective thickness than do the center plates. An image
processing system is provided that is configured to apply a
correction to projection data collected during scanning of an
object to thereby shift an effective center of at least some of the
detector cells to compensate for the non-uniform thickness between
the boundary plates and the center plates, and to reconstruct an
image of object using the corrected projection data.
[0011] In yet another aspect, the present invention provides a
method for obtaining data that includes scanning an object with
radiation to collect projection data using an imaging system having
a detector array with detector cells and a post-patient collimator,
wherein the post-patient collimator has plates having non-uniform
thicknesses. The method further includes applying a correction to
the projection data to shift an effective center of at least some
of the detector cells to compensate for the non-uniform thicknesses
of the collimator plates.
[0012] In yet another embodiment, additional filtering is performed
on the projection data to compensate for the noise difference due
to non-uniform collimation plate thickness. The filter is designed
such that its parameters change as a function of the detector plate
thickness.
[0013] It will be appreciated that various configurations of the
present invention provide, among other things, an ability to use
detector arrays using fully interchangeable detector modules in
imaging systems, and that some configurations of the present
invention are useful in reducing artifacts in images resulting from
non-uniform collimator module plates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a pictorial drawing representative of some
configurations of CT imaging apparatus of the present
invention.
[0015] FIG. 2 is a functional block diagram representative of the
CT imaging apparatus of FIG. 1.
[0016] FIG. 3 is a vertical cross-sectional view of a detector
module that includes a set of collimator plates.
[0017] FIG. 4 is a schematic vertical cross-sectional view of a
detector module showing the effect of non-uniform collimator
plates.
[0018] FIG. 5 is a reconstructed image of simulated cylindrical
objects showing the effect of an ideal collimator.
[0019] FIG. 6 is a reconstructed image of the simulated cylindrical
objects of FIG. 5 showing the artifacts introduced by a collimator
having non-uniform plates.
[0020] FIG. 7 is a reconstructed image of the simulated cylindrical
objects of FIG. 5 showing how the correction applied in some
configuration
DETAILED DESCRIPTION OF THE INVENTION
[0021] A technical effect of some configurations of the present
invention is the generation of artifact-free or at least improved
images of objects resulting from the detection of radiation passing
through an object scanned using a radiation source. Another
technical effect of some configurations of the present invention is
the ability to utilize fully interchangeable detector modules in an
imaging system. These and other technical effects of the present
invention will become apparent to one of ordinary skill upon
appreciating the subject matter of the present disclosure.
[0022] As used herein, an element or step recited in the singular
and proceeded with the word "a" or "an" should be understood as not
excluding plural said elements or steps, unless such exclusion is
explicitly recited. Furthermore, references to "one embodiment" of
the present invention are not intended to be interpreted as
excluding the existence of additional embodiments that also
incorporate the recited features.
[0023] Also as used herein, the phrase "reconstructing an image" is
not intended to exclude embodiments of the present invention in
which data representing an image is generated but a viewable image
is not. However, many embodiments generate (or are configured to
generate) at least one viewable image. Some configurations of the
present invention do not necessarily reconstruct an image or
generate data representing an image, but do process projection data
of an object by compensating the projection data so that and
further processing can generate data representing an image and/or
produce a viewable image.
[0024] Referring to FIGS. 1 and 2, a multi-slice scanning imaging
system, for example, a Computed Tomography (CT) imaging system 10,
is shown as including a gantry 12 representative of a "third
generation" CT imaging system. Gantry 12 has an x-ray tube 14 (also
called x-ray source 14 herein) that projects a beam of x-rays 16
toward a detector array 18 on the opposite side of gantry 12.
Detector array 18 is formed by a plurality of detector rows (not
shown) including a plurality of detector elements or cells 20 which
together sense the projected x-rays that pass through an object,
such as a medical patient 22 between array 18 and source 14. Each
detector element 20 produces an electrical signal that represents
the intensity of an impinging x-ray beam and hence can be used to
estimate the attenuation of the beam as it passes through object or
patient 22. During a scan to acquire x-ray projection data, gantry
12 and the components mounted therein rotate about a center of
rotation 24. FIG. 2 shows only a single row of detector elements or
cells 20 (i.e., a detector row). However, multi-slice detector
array 18 includes a plurality of parallel detector rows of detector
elements or cells 20 such that projection data corresponding to a
plurality of quasi-parallel or parallel slices can be acquired
simultaneously during a scan.
[0025] Rotation of components on gantry 12 and the operation of
x-ray source 14 are governed by a control mechanism 26 of CT system
10. Control mechanism 26 includes an x-ray controller 28 that
provides power and timing signals to x-ray source 14 and a gantry
motor controller 30 that controls the rotational speed and position
of components on gantry 12. A data acquisition system (DAS) 32 in
control mechanism 26 samples analog data from detector elements or
cells 20 and converts the data to digital signals for subsequent
processing. An image reconstructor 34 receives sampled and
digitized x-ray data from DAS 32 and performs high-speed image
reconstruction. The reconstructed image is applied as an input to a
computer 36, which stores the image in a storage device 38. Image
reconstructor 34 can be specialized hardware or computer programs
executing on computer 36.
[0026] Computer 36 also receives commands and scanning parameters
from an operator via console 40 that has a keyboard. An associated
cathode ray tube, liquid crystal, plasma, or any other suitable
type of display device 42 allows the operator to observe the
reconstructed image and other data from computer 36. The operator
supplied commands and parameters are used by computer 36 to provide
control signals and information to DAS 32, x-ray controller 28, and
gantry motor controller 30. In addition, computer 36 operates a
table motor controller 44, which controls a motorized table 46 to
position patient 22 in gantry 12. Particularly, table 46 moves
portions of patient 22 through gantry opening 48.
[0027] In one embodiment, computer 36 includes a device 50, for
example, a floppy disk drive, CD-ROM drive, DVD drive, magnetic
optical disk (MOD) device, or any other digital device including a
network connecting device such as an Ethernet device for reading
instructions and/or data from a computer-readable medium 52, such
as a floppy disk, a CD-ROM, a DVD or another digital source such as
a network or the Internet, as well as yet to be developed digital
means. In another embodiment, computer 36 executes instructions
stored in firmware (not shown). Computer 36 is programmed to
perform functions described herein, and as used herein, the term
computer is not limited to just those integrated circuits referred
to in the art as computers, but broadly refers to computers,
processors, microcontrollers, microcomputers, programmable logic
controllers, application specific integrated circuits, and other
programmable circuits, and these terms are used interchangeably
herein. Although the specific embodiment mentioned above refers to
a third generation CT system, the methods described herein equally
apply to fourth generation CT systems (stationary
detector--rotating x-ray source) and fifth generation CT systems
(stationary detector and x-ray source). Additionally, it is
contemplated that the benefits of the invention accrue to imaging
modalities other than CT. Additionally, although the herein
described methods and apparatus are described in a medical setting,
it is contemplated that the benefits of the invention accrue to
non-medical imaging systems such as those systems typically
employed in an industrial setting or a transportation setting, such
as, for example, but not limited to, a baggage scanning system for
an airport or other transportation center.
[0028] In at least one known third generation CT scanner, a set of
post-patient collimator plates is positioned in front of a detector
array for scatter rejection. The collimator plates are provided as
piece separate from a scintillator pack or module of the detector
array. However, providing collimator plates separate from the
scintillator pack or modules of the detector array makes it
difficult, among other things, to repair and/or replace detector
modules.
[0029] Referring to FIG. 3, a smaller section 300 of a set of
collimator plates 302 (that is, smaller than a set of collimator
plates provided separately and placed in front of a plurality of
detector modules) can be combined with each individual scintillator
module 303 to form a stand-alone detector module 304 entity.
However, such a combination results in anomalies at module
boundaries or junctions 306 in configurations in which each module
304 includes a separate boundary plate 308 at a junction 306
between two modules 304. If boundary plates 308 are positioned
identically to other plates, referred to herein as "center plates"
310, a small gap 311 would occur between adjacent modules 304 at
junctions 306 because half the width of boundary plate 308 is
positioned over a detector cell 312 and the other half is
cantilevered over the edge of detector edge cell 312. The gap would
occur because an abutting boundary plate 308 is similarly
cantilevered, and thus the detector cell over which it is
positioned cannot be made to abut edge cell 312. The resulting gaps
would make detector sampling non-uniform and would adversely
affecting the detector quarter-quarter offset property. To avoid
this problem, boundary collimator plates 308 can be positioned
inwardly so that no gap is present between modules 304. This
configuration makes the exposure area of a detector edge cell 312
at the module 304 boundaries smaller than the non-boundary cells
314. Without compensation, image artifacts (e.g., streaking
artifacts) can result. When the boundary plate 308 thickness is
half a center plate 310 thickness, the combined "double plate" is
the same thickness as that of a center plate 310, and no artifacts
are created from this source. However, as the thicknesses of the
boundary plates 308 increase, the amount of degradation increases.
(Detector elements or cells 20 in FIG. 2 are all either edge cells
312 or non-boundary cells 314, and there may be scores of modules
304 and many hundreds or thousands of detector elements or cells 20
in a detector array, particularly in the case of a multi-row
detector array.)
[0030] To control the degradation, thinner boundary plates 308
(less than or equal to half the thickness of center plates 310)
could be used at junctions 306. However, thin boundary plates 308
can make the collimator portion of a detector module 304 fragile at
its edges. Alternately, a thicker boundary collimator plate 308 at
an edge detector cell 312 of one module 304 could be shared by an
edge cell 312 of a neighboring detector module 304, i.e., N-cell
modules alternate with N+1 plates and N-1 plates, or N-cell modules
having N plates can be used in which one edge-cell thicker plate is
shared at the boundary of two modules. However, manufacturing
complexity is increased when an attempt is made to control
degradation using these techniques. Also, more than one detector
module type is required, meaning that detector modules 304 are not
all interchangeable with one another. In addition, this design
makes the repair process of a "field replaceable" detector module
more difficult.
[0031] Therefore, some configurations of the present invention
provide and/or allow thicker boundary plates 308 to be used on all
detector modules 304, but provide compensation in image
reconstruction that reduces or eliminates the effect of these
thicker boundary plates.
[0032] Boundary plates 308 reduce the x-ray flux impinging on a
detector and make photon statistics worse for cells 312 at the
boundary than for cells 314 in the center. In some configurations
and referring to the schematic representation of FIG. 4, central
plates 310 represent collimator plates 302 that are placed at the
center (i.e., not at an edge) of a detector module 304. Portions
400 of double boundary plate(s) 308 cover portions 402 of boundary
cells 312. (Boundary plate 308 in FIG. 4 is shown schematically
because FIG. 4 can represent more than one configuration of the
present invention. For example, FIG. 4 can represent a
configuration in which a single boundary plate 308 of a first
detector module 304 is cantilevered over a second, adjacent
detector module that does not have a boundary plate on the side
adjacent to the first detector module. As another example, FIG. 4
can represent a configuration in which two boundary plates 308, one
on each of the adjacent detector modules 304, abut one another.)
Also shown in FIG. 4 are exposed areas 404 on detector cells 308
that would result from a nominal collimator plate 302 thickness
(i.e., the thickness of a center plate 310) and the smaller,
exposed areas 406 that actually result because of the extra
thickness of the double plate 308.
[0033] A factor that produces image artifacts in configurations
such as that shown in FIG. 4 is the size of the actual exposed
detector areas 406, and more particularly, a shift of detector cell
centers from 408 to 410 caused by the additional plate thickness of
double plate 308. Let us denote by t and t' the nominal plate 302
(i.e., center plate 310) thickness and the double boundary plate
308 thicknesses, respectively. The additional plate thickness,
.DELTA., is then simply .DELTA.=t'-t. The amount of shift of the
detector cell center, s, is then simply s=.DELTA./4. The shifting
direction is always towards the center of each respective detector
module 304. Therefore, the correction is an attempt to interpolate
the original samples of at least some of the detector cells 312 so
that the interpolated samples represent the detector readings as
though the cell center were located at the nominal location
408.
[0034] Let us denote by d the spacing between two nominal detector
cell centers 408, and let us denote by p(n) and p(n+1) the measured
projection readings for two adjacent boundary cells 312,
respectively. The spacing between the two adjacent boundary cells
312 (with additional collimation) is then d+.DELTA./2. Using linear
interpolation, corrected projection readings p'(n) and p'(n+1) are:
p ' .function. ( n ) = ( 1 - .alpha. ) p .function. ( n ) + .alpha.
p .function. ( n + 1 ) ; and p ' .function. ( n + 1 ) = .alpha. p
.function. ( n ) + ( 1 - .alpha. ) p .function. ( n + 1 ) ; where
.alpha. = .DELTA. / 4 d + .DELTA. / 2 = .DELTA. 4 .times. d + 2
.times. .DELTA. . ##EQU1##
[0035] Some configurations of the present invention make use of
higher order (i.e., nonlinear) interpolations. For example, in some
configurations, a fourth order Lagrange interpolation is used. In
this interpolation, the coordinates of the measured signals [x1,
x2,x3, x4], are [-3d/2, -d/2-.DELTA./4, d/2+.DELTA./4, 3d/2]. The
interpolated location of the corrected signal, [x2, x3] is [-d/2,
d/2]. An alternative approach is to keep the measured signal and
compensate for the location change in the filtering and
backprojection steps. The locations of the boundary samples are
known and are transmitted to the filtering and backprojection
process to compensate for the sample location change. Because the
amount of deviation of the boundary cell location is small, the
projection sample shift in the filtering process can be ignored and
only the deviation in the backprojection process is compensated
for.
[0036] In an alternative embodiment, compensation for the change
during the fan-to-parallel beam rebinning process can be
accomplished. That is, assumptions of the original samples being
equiangular spaced will no longer be made. The actual location of
the boundary samples are input into the fan-parallel rebinning
process so that the rebinned parallel samples incorporate the
deviation of the boundary samples.
[0037] Another impact of the double boundary plates is the
difference in the noise of the projection samples of the boundary
cells. The area exposed to the x-ray for the center detector cell
is d-t, while the boundary cell is d-(t+t')/2. If the input flux to
these detectors are the same, the variance of the detected signal
for these detectors are proportional to the exposed area. As a
result, the boundary cells have slightly higher noise level as
compared to the center cell. A method of overcoming this
shortcoming is to filter the boundary cells prior to the
reconstruction. For example, the final boundary sample can be the
weighted sum of the neighboring samples: p ' .function. ( i , j ) =
n = - 1 1 .times. k = - 1 1 .times. w .function. ( k , n ) .times.
p .function. ( i + k , j + n ) ##EQU2## where w(k, n) is the
weighting of all cells.
[0038] A technical effect of some configurations of the present
invention is thus achieved by using a CT imaging system 10 or other
scanning imaging system to scan an object 22 to collect projection
data. The scanning can be performed by a CT imaging system 10 or
other scanning imaging system having a detector array 18 with
detector cells 20 and a post-patient collimator 300, wherein the
post-patient collimator 300 has boundary plates 308 between some
detector cells 312 and center plates 310 over other detector cells
314, and boundary plates 308 have a different effective thickness
than do the center plates 310. A correction is applied to the
projection data to shift an effective center 410 of at least some
of the detector cells to compensate (e.g., by moving the effective
center to 408) for the non-uniform thickness between boundary
plates 308 and center plates 306. An image of the object is then
reconstructed using the corrected projection data. The
reconstruction of the image data can be performed using
conventional filtering and backprojection.
[0039] The collection of projection data in some configurations
utilizes, among other things, DAS 32 of CT system 10. Image
reconstructor 34, DAS 32, and/or computer 36 are used in some
configurations to correct (i.e., compensate) the projection data
and/or reconstruct an image of object 22 using the corrected
projection data. Image reconstructor 34, DAS 32, and/or computer
may be considered as together comprising an image processing
system, although this system may comprise these or any other
identifiable components that alone or in combination with other
components perform the functions required of the image processing
system. In some configurations, this image is displayed on display
42, or it may be printed or provided in some other tangible form.
Image reconstructor 34, DAS 32, and/or computer 36 may use storage
device 38 and/or computer-readable medium 52 for storage of
intermediate or final results and/or as a storage medium on which
machine-readable instructions are stored that instruct these
devices to perform steps of one or more embodiments of the
invention. In some configurations, the instructions for performing
the correction and the backprojection
[0040] In some configurations, a CT imaging system 10 is provided
that includes an x-ray source 14, a detector array 18 having
detector elements or cells 18, and a post-patient collimator 300
having boundary plates 308 between some detector cells 312 and
center plates 310 over other detector cells 314, and the boundary
plates have a different effective thickness than do the center
plates. These configurations also provide an image processing
system (for example, image reconstructor 34, DAS 32, and/or
computer 36) configured to apply a correction to projection data
collected during scanning of an object 22 to thereby shift an
effective center 410 of at least some of the detector cells to
compensate (e.g., by shifting to 408) for the non-uniform thickness
between boundary plates 308 and center plates 310, and to
reconstruct an image of object 22 using the corrected projection
data
[0041] Configurations of the present invention are not limited
solely to CT imaging systems or to detector arrays that comprise a
plurality of detector modules. Thus, some configurations of the
present invention provide a method for obtaining data that includes
scanning an object 22 with radiation 16 to collect projection data
using an imaging system 10 having a detector array 18 with a
post-patient collimator having non-uniform plate 302 thicknesses.
The method further includes applying a correction to the projection
data to shift an effective center 410 of at least some of the
detector cells 20 to compensate for the non-uniform thickness of
the collimator plates.
[0042] In a simulation, several cylindrical objects of various
densities and sizes were placed inside a scan field of view. To
simulate the worst case, most of the cylindrical objects exhibit
high-contrast to a water background. FIG. 5 is a reconstructed CT
image of these simulated cylindrical objects 500 (ww=40) using an
ideal detector array 18. FIG. 6 is a reconstructed CT image of the
same objects in which a detector array 18 having "double" boundary
plates 308 that are 197 microns thicker than center plates 314.
Note the presence of streaking artifacts 600 in FIG. 6.
[0043] FIG. 7 is a reconstructed image of the objects using the
same simulated detector array 18 as in FIG. 6, but with a
fourth-order Lagrange interpolation applied to the simulated
projection data. Essentially all artifacts are removed, and the
image is all but indistinguishable from the ideal simulation
depicted in FIG. 5.
[0044] While the invention has been described in terms of various
specific embodiments, those skilled in the art will recognize that
the invention can be practiced with modification within the spirit
and scope of the claims.
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