US20070064866A1 - Method and apparatus for compensating non-uniform detector collimator plates - Google Patents
Method and apparatus for compensating non-uniform detector collimator plates Download PDFInfo
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- US20070064866A1 US20070064866A1 US11/230,847 US23084705A US2007064866A1 US 20070064866 A1 US20070064866 A1 US 20070064866A1 US 23084705 A US23084705 A US 23084705A US 2007064866 A1 US2007064866 A1 US 2007064866A1
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/02—Devices for diagnosis sequentially in different planes; Stereoscopic radiation diagnosis
- A61B6/03—Computerised tomographs
- A61B6/032—Transmission computed tomography [CT]
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus for radiation diagnosis, e.g. combined with radiation therapy equipment
- A61B6/06—Diaphragms
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
- Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y10S378/00—X-ray or gamma ray systems or devices
- Y10S378/901—Computer tomography program or processor
Definitions
- 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.
- 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.
- 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.
- 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.
- CT numbers or “Hounsfield units” (HU)
- a “helical” scan may be performed.
- 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.
- 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.
- multi-slice CT has been introduced.
- multi-slice CT multiple rows of projection data are acquired simultaneously at any time instant.
- the system 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- FIG. 1 is a pictorial drawing representative of some configurations of CT imaging apparatus of the present invention.
- FIG. 2 is a functional block diagram representative of the CT imaging apparatus of FIG. 1 .
- FIG. 3 is a vertical cross-sectional view of a detector module that includes a set of collimator plates.
- FIG. 4 is a schematic vertical cross-sectional view of a detector module showing the effect of non-uniform collimator plates.
- FIG. 5 is a reconstructed image of simulated cylindrical objects showing the effect of an ideal collimator.
- 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.
- FIG. 7 is a reconstructed image of the simulated cylindrical objects of FIG. 5 showing how the correction applied in some configuration
- 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.
- 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.
- a multi-slice scanning imaging system for example, a Computed Tomography (CT) imaging system 10
- CT Computed Tomography
- 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 .
- 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).
- 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.
- 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 .
- 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 .
- 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 .
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 .
- 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.
- 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.
- image artifacts e.g., streaking artifacts
- 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.
- 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.
- thinner boundary plates 308 could be used at junctions 306 .
- thin boundary plates 308 can make the collimator portion of a detector module 304 fragile at its edges.
- 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.
- manufacturing complexity is increased when an attempt is made to control degradation using these techniques.
- more than one detector module type is required, meaning that detector modules 304 are not all interchangeable with one another.
- this design makes the repair process of a “field replaceable” detector module more difficult.
- 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.
- 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.
- 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.
- 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.
- 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 .
- a nominal collimator plate 302 thickness i.e., the thickness of a center plate 310
- 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 .
- t and t′ the nominal plate 302 (i.e., center plate 310 ) thickness and the double boundary plate 308 thicknesses, respectively.
- 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 .
- Some configurations of the present invention make use of higher order (i.e., nonlinear) interpolations.
- a fourth order Lagrange interpolation is used.
- the coordinates of the measured signals [x1, x2,x3, x4] are [ ⁇ 3d/2, ⁇ d/2 ⁇ /4, d/2+ ⁇ /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.
- 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.
- 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.
- 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.
- 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.
- the instructions for performing the correction and the backprojection may be used to perform steps of one or more embodiments of the invention.
- a CT imaging system 10 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
- Configurations of the present invention are not limited solely to CT imaging systems or to detector arrays that comprise a plurality of detector modules.
- 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.
- 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 .
- 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 .
Abstract
Description
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
-
FIG. 1 is a pictorial drawing representative of some configurations of CT imaging apparatus of the present invention. -
FIG. 2 is a functional block diagram representative of the CT imaging apparatus ofFIG. 1 . -
FIG. 3 is a vertical cross-sectional view of a detector module that includes a set of collimator plates. -
FIG. 4 is a schematic vertical cross-sectional view of a detector module showing the effect of non-uniform collimator plates. -
FIG. 5 is a reconstructed image of simulated cylindrical objects showing the effect of an ideal collimator. -
FIG. 6 is a reconstructed image of the simulated cylindrical objects ofFIG. 5 showing the artifacts introduced by a collimator having non-uniform plates. -
FIG. 7 is a reconstructed image of the simulated cylindrical objects ofFIG. 5 showing how the correction applied in some configuration - 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.
- 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.
- 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.
- 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 agantry 12 representative of a “third generation” CT imaging system. Gantry 12 has an x-ray tube 14 (also calledx-ray source 14 herein) that projects a beam ofx-rays 16 toward adetector array 18 on the opposite side ofgantry 12.Detector array 18 is formed by a plurality of detector rows (not shown) including a plurality of detector elements orcells 20 which together sense the projected x-rays that pass through an object, such as amedical patient 22 betweenarray 18 andsource 14. Eachdetector 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 orpatient 22. During a scan to acquire x-ray projection data,gantry 12 and the components mounted therein rotate about a center ofrotation 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 orcells 20 such that projection data corresponding to a plurality of quasi-parallel or parallel slices can be acquired simultaneously during a scan. - Rotation of components on
gantry 12 and the operation ofx-ray source 14 are governed by acontrol mechanism 26 ofCT system 10.Control mechanism 26 includes anx-ray controller 28 that provides power and timing signals to x-raysource 14 and agantry motor controller 30 that controls the rotational speed and position of components ongantry 12. A data acquisition system (DAS) 32 incontrol mechanism 26 samples analog data from detector elements orcells 20 and converts the data to digital signals for subsequent processing. Animage reconstructor 34 receives sampled and digitized x-ray data fromDAS 32 and performs high-speed image reconstruction. The reconstructed image is applied as an input to acomputer 36, which stores the image in astorage device 38.Image reconstructor 34 can be specialized hardware or computer programs executing oncomputer 36. -
Computer 36 also receives commands and scanning parameters from an operator viaconsole 40 that has a keyboard. An associated cathode ray tube, liquid crystal, plasma, or any other suitable type ofdisplay device 42 allows the operator to observe the reconstructed image and other data fromcomputer 36. The operator supplied commands and parameters are used bycomputer 36 to provide control signals and information toDAS 32,x-ray controller 28, andgantry motor controller 30. In addition,computer 36 operates atable motor controller 44, which controls a motorized table 46 to positionpatient 22 ingantry 12. Particularly, table 46 moves portions ofpatient 22 throughgantry opening 48. - In one embodiment,
computer 36 includes adevice 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. - 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.
- Referring to
FIG. 3 , asmaller 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 eachindividual scintillator module 303 to form a stand-alone detector module 304 entity. However, such a combination results in anomalies at module boundaries orjunctions 306 in configurations in which eachmodule 304 includes aseparate boundary plate 308 at ajunction 306 between twomodules 304. Ifboundary plates 308 are positioned identically to other plates, referred to herein as “center plates” 310, asmall gap 311 would occur betweenadjacent modules 304 atjunctions 306 because half the width ofboundary plate 308 is positioned over adetector cell 312 and the other half is cantilevered over the edge ofdetector edge cell 312. The gap would occur because an abuttingboundary plate 308 is similarly cantilevered, and thus the detector cell over which it is positioned cannot be made toabut 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 betweenmodules 304. This configuration makes the exposure area of adetector edge cell 312 at themodule 304 boundaries smaller than thenon-boundary cells 314. Without compensation, image artifacts (e.g., streaking artifacts) can result. When theboundary plate 308 thickness is half acenter plate 310 thickness, the combined “double plate” is the same thickness as that of acenter plate 310, and no artifacts are created from this source. However, as the thicknesses of theboundary plates 308 increase, the amount of degradation increases. (Detector elements orcells 20 inFIG. 2 are all eitheredge cells 312 ornon-boundary cells 314, and there may be scores ofmodules 304 and many hundreds or thousands of detector elements orcells 20 in a detector array, particularly in the case of a multi-row detector array.) - 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 adetector module 304 fragile at its edges. Alternately, a thickerboundary collimator plate 308 at anedge detector cell 312 of onemodule 304 could be shared by anedge cell 312 of a neighboringdetector 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 thatdetector 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. - Therefore, some configurations of the present invention provide and/or allow
thicker boundary plates 308 to be used on alldetector modules 304, but provide compensation in image reconstruction that reduces or eliminates the effect of these thicker boundary plates. -
Boundary plates 308 reduce the x-ray flux impinging on a detector and make photon statistics worse forcells 312 at the boundary than forcells 314 in the center. In some configurations and referring to the schematic representation ofFIG. 4 ,central plates 310 representcollimator plates 302 that are placed at the center (i.e., not at an edge) of adetector module 304.Portions 400 of double boundary plate(s) 308cover portions 402 ofboundary cells 312. (Boundary plate 308 inFIG. 4 is shown schematically becauseFIG. 4 can represent more than one configuration of the present invention. For example,FIG. 4 can represent a configuration in which asingle boundary plate 308 of afirst 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 twoboundary plates 308, one on each of theadjacent detector modules 304, abut one another.) Also shown inFIG. 4 are exposedareas 404 ondetector cells 308 that would result from anominal collimator plate 302 thickness (i.e., the thickness of a center plate 310) and the smaller, exposedareas 406 that actually result because of the extra thickness of thedouble plate 308. - A factor that produces image artifacts in configurations such as that shown in
FIG. 4 is the size of the actual exposeddetector areas 406, and more particularly, a shift of detector cell centers from 408 to 410 caused by the additional plate thickness ofdouble plate 308. Let us denote by t and t′ the nominal plate 302 (i.e., center plate 310) thickness and thedouble boundary plate 308 thicknesses, respectively. The additional plate thickness, Δ, is then simply Δ=t′−t. The amount of shift of the detector cell center, s, is then simply s=Δ/4. The shifting direction is always towards the center of eachrespective detector module 304. Therefore, the correction is an attempt to interpolate the original samples of at least some of thedetector cells 312 so that the interpolated samples represent the detector readings as though the cell center were located at thenominal location 408. - 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+Δ/2. Using linear interpolation, corrected projection readings p′(n) and p′(n+1) are: - 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−Δ/4, d/2+Δ/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.
- 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.
- 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:
where w(k, n) is the weighting of all cells. - 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 anobject 22 to collect projection data. The scanning can be performed by aCT imaging system 10 or other scanning imaging system having adetector array 18 withdetector cells 20 and apost-patient collimator 300, wherein thepost-patient collimator 300 hasboundary plates 308 between somedetector cells 312 andcenter plates 310 overother detector cells 314, andboundary plates 308 have a different effective thickness than do thecenter plates 310. A correction is applied to the projection data to shift aneffective 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 betweenboundary plates 308 andcenter 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. - The collection of projection data in some configurations utilizes, among other things,
DAS 32 ofCT system 10.Image reconstructor 34,DAS 32, and/orcomputer 36 are used in some configurations to correct (i.e., compensate) the projection data and/or reconstruct an image ofobject 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 ondisplay 42, or it may be printed or provided in some other tangible form.Image reconstructor 34,DAS 32, and/orcomputer 36 may usestorage 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 - In some configurations, a
CT imaging system 10 is provided that includes anx-ray source 14, adetector array 18 having detector elements orcells 18, and apost-patient collimator 300 havingboundary plates 308 between somedetector cells 312 andcenter plates 310 overother 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 anobject 22 to thereby shift aneffective center 410 of at least some of the detector cells to compensate (e.g., by shifting to 408) for the non-uniform thickness betweenboundary plates 308 andcenter plates 310, and to reconstruct an image ofobject 22 using the corrected projection data - 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 withradiation 16 to collect projection data using animaging system 10 having adetector array 18 with a post-patient collimator havingnon-uniform plate 302 thicknesses. The method further includes applying a correction to the projection data to shift aneffective center 410 of at least some of thedetector cells 20 to compensate for the non-uniform thickness of the collimator plates. - 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 anideal detector array 18.FIG. 6 is a reconstructed CT image of the same objects in which adetector array 18 having “double”boundary plates 308 that are 197 microns thicker thancenter plates 314. Note the presence of streakingartifacts 600 inFIG. 6 . -
FIG. 7 is a reconstructed image of the objects using the samesimulated detector array 18 as inFIG. 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 inFIG. 5 . - 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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