JP2014147418A - Image generating apparatus, radiation tomographic imaging apparatus, and program therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide image reconstruction, minimizing the deterioration of spatial resolution, even when a gap is formed between detector modules in a slice direction of a radiation detector.SOLUTION: A radiation tomographic imaging apparatus comprises a radiation detector 3 in which a plurality of detector modules 30, each containing a plurality of detection elements 31, are arranged in a CH direction and an SL direction, and a gap GAP is formed between the detector modules 30 in the SL direction. In the radiation tomographic imaging apparatus, storage means is provided to store positional information of each individual detection element 31 of the radiation detector 3, and image reconstruction means applies weighted summation on each real data piece Pa1, Pa2 obtained by a plurality of detection elements a1, a2 locating near an intersection point c of a radiation path p passing through a pixel g to be reconstructed from a radiation focal point f and a detection plane k of the radiation detector 3, by weighting coefficients based on positional information Lc of the intersection point c and positional information La1, La2 of the detection elements a1, a2 stored in the storage means to generate projection data P of the radiation path p, and reconstructs the pixel g by using a three-dimensional image reconstruction method on the basis of the projection data P.

Description

  The present invention relates to radiation tomography, and more particularly to an image reconstruction technique that takes into account a gap in a slice direction of a detector module constituting a radiation detector.

  In recent years, a wide range of X-ray detectors in the slice direction (usually the z-axis direction or the body axis direction of the subject) have been developed as X-ray detectors in X-ray CT (Computed Tomography) apparatuses. Also, as a wide X-ray detector in the slice direction, tile-type detector modules in which X-ray detector elements are arranged in a matrix are arranged not only in the channel direction but also in the slice direction. Is known (see, for example, Patent Document 1, Abstract).

WO2010 / 150717

  Incidentally, the X-ray CT apparatus reconstructs an image by performing a predetermined calculation based on projection data (data) obtained by each X-ray detection element of the X-ray detector. Conventionally, since the detector modules in the X-ray detector are mainly arranged only in the channel direction, the algorithm of this calculation (algorithm) is that the X-ray detector elements have an ideally constant pitch in the slice direction. (Pitch) is designed on the assumption that it is arranged.

  On the other hand, in the X-ray detector in which the detector modules are arranged in the slice direction as described above, there is a gap between the detector modules in the slice direction due to a forming error or an arrangement error of the detector module. It becomes easier to form. When the gap is formed, the arrangement pitch in the slice direction of the X-ray detection elements in the X-ray detector is not constant. For this reason, if the image reconstruction is performed by directly inputting the projection data obtained by each X-ray detection element to the above algorithm, an error occurs and the accuracy of the reconstructed image is deteriorated.

  Therefore, in general, the following measures can be taken against errors caused by the gap between the detector modules in the slice direction.

  6 and 7 are diagrams for explaining a general method for dealing with an error caused by a gap between detector modules in the slice direction. In these drawings, 2 indicates an X-ray tube, f indicates an X-ray focal point, and 3 indicates an X-ray detector. Further, 31 (a1, a2, b1) is an X-ray detection element including a positional deviation caused by a gap between detector modules, and 31 '(a1', a2 ') is a virtual position arranged at an ideal position. An X-ray detection element is shown. L0 is the center position in the slice direction SL of the X-ray detector 3, La1, La2, and Lb1 are center positions in the slice direction of the X-ray detection elements a1, a2, and b1, and La1 ′ and La2 ′ are ideal. The center positions in the slice direction of the virtual X-ray detection elements a1 ′ and a2 ′ arranged at various positions are shown. ISO represents an iso-center.

  In general, as shown in FIG. 6, the projection data to be originally obtained, that is, obtained by X-ray detection elements 31 '(a1', a2 ') arranged at ideal positions at a constant arrangement pitch. The projection data (Pa1 ′, Pa2 ′) that should be obtained is obtained by interpolation processing of the projection data (Pa1, Pa2, Pb1) obtained by the actual X detection elements 31 (a1, a2, b1) including positional deviation. cure. The projection data (Pa1 ′, Pa2 ′) recalculated in this way is applied to the above algorithm.

  However, as shown in FIG. 7, in order to reconstruct the pixel g to be reconstructed in the tomographic image G to be reconstructed, an X-ray path p that extends linearly from the X-ray focal point f through the pixel g. Projection data P is required. In many cases, the projection data P necessary for this image reconstruction is obtained by further interpolating the originally obtained projection data (Pa1 ′, Pa2 ′) obtained as described above. Therefore, in this case, the necessary projection data is obtained by performing the interpolation process twice, and the spatial resolution of the reconstructed image is greatly deteriorated depending on the photographing condition, the reconstruction condition, the pixel position, and the like. It will be.

  Under such circumstances, in radiation tomography, a technique capable of reconstructing an image without degrading the spatial resolution as much as possible even when a gap is formed between detector modules in the slice direction of the radiation detector is desired. ing.

The invention of the first aspect
A radiation source that emits radiation from a radiation focal point and a plurality of detector modules in which a plurality of detector elements are arranged in the channel direction and the slice direction are arranged in the channel direction and the slice direction, and a gap is provided between the detector modules in the slice direction. An image generation apparatus comprising: a radiation detector formed by: and a reconstruction unit configured to reconstruct an image based on projection data obtained by a scan using the radiation detector,
Comprising storage means for storing the position information of the individual detection elements;
The reconstruction means includes a plurality of detection elements located in the vicinity of an intersection between a radiation path linearly extending from the radiation focus through a pixel to be reconstructed in an image reconstruction space and a detection surface of the radiation detector. The obtained actual data is interpolated by weighted addition using a weighting factor determined based on the position information of the intersection and the position information of the plurality of detection elements stored in the storage means, and the radiation Provided is an image generation apparatus that generates projection data based on a pass and reconstructs the reconstruction target pixel by a three-dimensional image reconstruction method based on the projection data.

The invention of the second aspect is
The image generating apparatus according to the first aspect is provided in which the interpolation is linear interpolation, bi-linear interpolation, or Lagrange interpolation.

The invention of the third aspect is
The image generating apparatus according to the first aspect or the second aspect is provided, wherein the three-dimensional image reconstruction method is a three-dimensional backprojection method.

The invention of the fourth aspect is
A radiation source emitting radiation from a radiation focus;
A radiation detector in which a plurality of detector modules arranged in the channel direction and the slice direction are arranged in the channel direction and the slice direction, and a gap is formed between the detector modules in the slice direction;
Reconstructing means for reconstructing an image based on projection data obtained by scanning using the radiation source and the radiation detector, and a radiation tomography apparatus comprising:
Comprising storage means for storing the position information of the individual detection elements;
A plurality of reconstructing means positioned near an intersection of a radiation path that linearly extends from the radiation focus through a pixel to be reconstructed in an image reconstruction space and a detection surface of the radiation detector; Each real data obtained by the detection element is interpolated by weighted addition using a weighting factor determined based on the position information of the intersection and the position information of the plurality of detection elements stored in the storage means. A radiation tomography apparatus that generates projection data by the radiation path and reconstructs the pixel to be reconstructed by a three-dimensional image reconstruction method based on the projection data is provided.

The invention of the fifth aspect is
The radiation tomography apparatus according to the fourth aspect, wherein the interpolation is linear interpolation, bilinear interpolation, or Lagrangian interpolation.

The invention of the sixth aspect is
The radiation tomography apparatus according to the fourth aspect or the fifth aspect, wherein the three-dimensional image reconstruction method is a three-dimensional backprojection method.

The invention of the seventh aspect
A program for causing a computer to function as the image generation apparatus according to any one of the first to third aspects is provided.

  According to the invention of the above aspect, the object to be reconstructed from the radiation focus is obtained by interpolation of actual data obtained by the actual detection element without obtaining projection data obtained by the detection element arranged at an ideal position. Since the projection data of the radiation path passing through the pixels is obtained and the reconstruction target pixel is reconstructed by the three-dimensional image reconstruction method based on this projection data, the projection data necessary for reconstruction is only subjected to one interpolation process. In radiation tomography, even if a gap is formed between detector modules in the slice direction of the radiation detector, an image can be reconstructed without degrading the spatial resolution as much as possible.

1 is a diagram schematically showing a configuration of an X-ray CT (Computed Tomography) apparatus according to an embodiment of the invention. It is a figure which shows the structure of an X-ray detector roughly. It is a figure for demonstrating the method to measure the exact position of each X-ray detection element. It is a figure which shows an example of the geometry (geometry) with the X-ray detector in which the gap | interval is formed between the X-ray focus and the detector module of the slice direction. It is a flow figure showing the flow of processing in the X-ray CT apparatus concerning this embodiment. FIG. 10 is a diagram (No. 1) for describing a general coping method for an error caused by a gap between detector modules in a slice direction. FIG. 11 is a diagram (No. 2) for explaining a general coping method against an error caused by a gap between detector modules in the slice direction.

  Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited thereby.

  FIG. 1 is a diagram schematically showing a configuration of an X-ray CT apparatus according to the present embodiment.

  As shown in FIG. 1, the X-ray CT apparatus 1 includes a scanning gantry 10, an imaging table 11, and an operation console 12. The scanning gantry 10 further includes an X-ray tube 2, an X-ray detector 3, and an aperture 4. The operation console 12 further includes a scan condition setting unit 5, a scan control unit 6, a detection element position storage unit 7, and an image reconstruction unit 8.

  The X-ray tube 2 and the X-ray detector 3 are arranged to face each other with the imaging space B interposed therebetween. The X-ray tube 2 and the X-ray detector 3 are supported rotatably about an isocenter ISO that is the center of the imaging space B as a rotation axis. The subject 9 is placed on the imaging table 11 and arranged in the imaging space B. Here, the rotation axis direction of the X-ray tube 2 and the X-ray detector 3 is the z-axis direction, the vertical direction is the y-axis direction, and the horizontal direction perpendicular to the z-axis direction and the y-axis direction is the x-axis direction.

  The aperture 4 shapes the X-ray 2x generated from the X-ray tube 2 into a cone beam. Here, the arc-shaped spreading direction of the cone beam is defined as a channel direction CH, and the thickness direction of the cone beam is defined as a slice direction SL.

  The X-ray detector 3 detects X-rays formed by the aperture 4 and transmitted through the subject 9, and outputs a signal corresponding to the X-ray intensity at each position on the detection surface. In this example, the X-ray detector 3 is assumed to be curved so that all the normals at the respective positions of the detection surface face the X-ray tube 2, but a flat flat panel ( flat panel) detector.

  FIG. 2 is a diagram schematically showing the configuration of the X-ray detector 3.

  As shown in FIG. 2, the X-ray detector 3 includes a plurality of X-ray detector modules 30 arranged in the channel direction CH and the slice direction SL. The X-ray detector module 30 is arranged in contrast in the channel direction CH and the slice direction SL around the center of the detection surface of the X-ray detector 3. The X-ray detector module 30 includes a plurality of X-ray detection elements 31 arranged in a matrix in the channel direction CH and the slice direction SL. The detection surface of the X-ray detection element 31 is rectangular. The X-ray detection element 31 includes, for example, a scintillator and a photo-diode.

  The X-ray detection module 30 is ideally disposed in close contact with each other in the channel direction CH and the slice direction SL. In this case, the arrangement pitch of the X-ray detection elements 31 in the slice direction SL is constant. However, in practice, a gap GAP is formed between the detector modules 30 at least in the slice direction SL due to a forming error or an arrangement error of the detector modules 30. The width of the gap GAP in the slice direction SL is, for example, about 50 μm.

  The scan condition setting unit 5 sets scan conditions for performing an X-ray CT scan in accordance with an operation by the operator. The scan conditions include an imaging region, a tube voltage and tube current of the X-ray tube 2, a scan range, a slice thickness, and the like.

  The scan control unit 6 controls the opening of the aperture 4, the rotation of the X-ray tube 2 and the X-ray detector 3, the generation of X-rays from the X-ray tube 2, and the like, and performs the X-ray CT scan.

  The detection element position storage unit 7 stores information on the relative position of each X-ray detection element 31 in the X-ray detector 3. The relative positions of the respective X-ray detection elements 31 are not ideal positions when the detector modules 30 are arranged without a gap, and the width of the gap GAP between the detector modules 30 in the slice direction SL is considered. The exact position. In addition, this exact position can be measured by the following method, for example.

  FIG. 3 is a diagram for explaining a method of measuring the exact position of each X-ray detection element 31. This method is performed, for example, in an inspection process after manufacturing the X-ray detector 3. First, the X-ray detector 3 is fixed, and an X-ray source 51 and a slit 52 are installed on the detection surface k side. The X-ray 51 x emitted from the X-ray source 51 passes through the opening of the slit 52 and is irradiated to the detection surface k of the X-ray detector 3. The opening of the slit 52 is adjusted so that the width of the X-ray 51x irradiated to the detection surface k is sufficiently narrower than the width of the X-ray detection element 31 in the slice direction SL. The relative positional relationship between the X-ray source 51 and the slit 52 is fixed, and the X-ray source 51 and the slit 52 are supported so as to be movable in the slice direction SL with respect to the X-ray detector 3 as a whole. The position of the opening of the slit 52 can be controlled with high accuracy with respect to the X-ray detector 3. While the X-ray source 51 and the slit 52 are moved on the detection surface k side of the X-ray detector 3 in the slice direction SL, the X-ray 51x is irradiated, and the change in the detection signal intensity from each X-ray detection element 31 is observed. get. From the correspondence between the position of the opening of the slit 52 and the detection signal intensity of each X-ray detection element 31, the exact position of each X-ray detection element 31 can be grasped.

  The image reconstruction unit 8 performs image reconstruction based on projection data of a plurality of views obtained from the output of the X-ray detector 3 by performing an X-ray CT scan. For image reconstruction, a three-dimensional image reconstruction method is used. In this example, a three-dimensional backprojection method such as a cone beam image reconstruction method is used as the three-dimensional image reconstruction method.

  Here, the image reconstruction using the three-dimensional backprojection method by the image reconstruction unit 8 will be described.

  FIG. 4 is a diagram showing an example of the geometry of an X-ray detector in which a gap is formed between the X-ray focal point and the detector module. In FIG. 4, the center of the X-ray detector 3 in the slice direction SL is indicated by L0.

  First, an X-ray path p extending from the X-ray focal point f through the reconstruction target pixel g in the reconstruction target tomogram G is assumed. Then, an intersection c between the X-ray path p and the detection surface k of the X-ray detector 3 is obtained. Next, a plurality of X-ray detection elements 31 located in the vicinity of the intersection c are specified. In the example of FIG. 4, X-ray detection elements a1 and a2 are specified. Weights determined based on the position information Lc of the intersection c and the position information La1 and La2 of the X-ray detection elements a1 and a2 are obtained from the actual data Pa1 and Pa2 obtained by the specified X-ray detection elements a1 and a2. Interpolation by weighted addition using coefficients is performed to generate projection data P by the X-ray path p. For this interpolation, for example, linear interpolation, bilinear interpolation, Lagrange interpolation, or the like can be used. In the example of FIG. 4, the projection data P is generated by linear interpolation. Projection data P is generated for each of a number of X-ray paths p1, p2,. Then, the generated projection data P1, P2,... Are three-dimensionally backprojected to reconstruct the reconstruction target pixel g. Such processing is performed for each pixel g1, g2,... (Not shown) constituting the tomographic image G to be reconstructed, and the tomographic image G is reconstructed.

  In the example of FIG. 4, only the slice direction SL is described for simplicity, but in practice, interpolation and the like are performed in consideration of the channel direction CH. That is, the intersection c is specified by two-dimensional coordinates in the channel direction CH and the slice direction SL on the detection surface k of the X-ray detector 3. In addition, the X-ray detection element located in the vicinity of the intersection c is also specified in two dimensions of the channel direction CH and the slice direction SL, and X-ray detection is performed by interpolation using projection data obtained by these X-ray detection elements. Projection data P of the line path p is generated.

  As described above, the image reconstruction unit 8 in this example does not use an image reconstruction algorithm that assumes a state in which the arrangement pitch of the X-ray detection elements in the slice direction SL is ideally constant as in the past. That is, based on the projection data obtained by the ideally arranged X-ray detection element, the projection data obtained by the actual X-ray detection element and its X-ray detection are not an algorithm for obtaining projection data necessary for reconstruction. Based on the element position information, an algorithm for obtaining projection data necessary for reconstruction is used. Therefore, in the case of this example, it is not necessary to recreate the projection data obtained by the ideally arranged X-ray detection elements from the actually obtained projection data, and the interpolation processing for this can be omitted. As a result, it is possible to reconstruct a highly accurate image with little degradation in spatial resolution.

  The scan condition setting unit 5, the scan control unit 6, the detection element position storage unit 7, and the image reconstruction unit 8 are realized by causing a computer to execute a predetermined program, for example.

  Hereafter, the flow of processing in the X-ray CT apparatus according to the present embodiment will be described.

  FIG. 5 is a flowchart showing the flow of processing in the X-ray CT apparatus according to the present embodiment.

  In step S1, scanning conditions are set according to the operation of the operator.

  In step S2, the subject 9 is scanned according to the set scan conditions, and projection data of a plurality of views is collected.

  In step S3, one of the tomographic images to be reconstructed is determined as a tomographic image G to be reconstructed.

  In step S4, one of the pixels constituting the tomographic image G to be reconstructed is selected as the pixel g to be reconstructed.

  In step S5, the view v from which the projection data of the X-ray path passing through the pixel g to be reconstructed or its vicinity is identified from among the multiple views from which the projection data has been collected.

  In step S6, the position of the X-ray focal point fv at the time corresponding to the specified view v is specified.

  In step S7, an X-ray path p that linearly extends from the identified X-ray focal point fv through the pixel g to be reconstructed is assumed.

  In step S8, an intersection c between the assumed X-ray path p and the three detection surfaces k of the X-ray detector is obtained.

  In step S9, among the X-ray detection elements 31 constituting the X-ray detector 3, a plurality of X-ray detection elements 31 located in the vicinity of the obtained intersection c are specified. The X-ray detection element 31 in the vicinity of the intersection c is identified by the rotation angle position θ of the data acquisition system in the specified view v, the geometry of the data acquisition system including the X-ray tube 2 and the X-ray detector 3, and the data This is performed using the relative position of each X-ray detection element 31 in the collection system. The relative position of each X-ray detection element 31 used here is information stored in the detection element position storage unit 7.

  In step S10, the projection data (actual data) actually obtained by the X-ray detection element 31 near the intersection c in the specified view v is interpolated to generate projection data P of the X-ray path p. .

  In step S11, it is determined whether there is another view from which projection data of an X-ray path passing through the vicinity of the pixel g to be reconstructed is obtained. If there is, the process returns to step S5. If not, the process proceeds to step S12.

  In step S12, image reconstruction processing is performed based on the projection data P1, P2,... Generated by repeating the steps S5 to S11 a plurality of times, and the reconstruction target pixel g is reconstructed. A cone beam image reconstruction method is used for image reconstruction.

  In step S13, it is determined whether or not there is another pixel to be reconstructed in the tomographic image G to be reconstructed. If there is, the process returns to step S4. If not, the process proceeds to step S14.

  In step S14, it is determined whether there is another tomographic image to be reconstructed. If there is, the process proceeds to step S3. If not, the process proceeds to step S15.

  In step S15, the reconstructed tomographic images G1, G2,... Obtained by repeating a plurality of times from step S3 to S14 are appropriately stored, output, or displayed.

  As described above, according to the present embodiment, without obtaining projection data obtained by the X-ray detection elements arranged at ideal positions, interpolation of actual data obtained by the actual X-ray detection elements can be performed. X-ray path projection data passing through the pixel to be reconstructed from the line focus and reconstructing the pixel to be reconstructed by the three-dimensional image reconstruction method based on this projection data, projection data necessary for reconstruction Can be obtained by only one interpolation process, and even if a gap is formed between detector modules in the slice direction of the X-ray detector, an image can be reconstructed without degrading the spatial resolution as much as possible.

  The invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

  For example, although the above embodiment is an X-ray CT apparatus, an image generation apparatus that performs image reconstruction as described above is also an embodiment of the invention.

  A program for causing a computer to function as such an image generation apparatus is also an embodiment of the invention.

  Moreover, although the said embodiment is an X-ray CT apparatus, invention is applicable also to the PET-CT apparatus, SPECT-CT apparatus, etc. which combined X-ray CT apparatus and PET or SPECT.

DESCRIPTION OF SYMBOLS 1 X-ray CT apparatus 2 X-ray tube 2x X-ray 3 X-ray detector 4 Aperture 6 Scan condition setting part 7 Scan control part 8 Image reconstruction part 9 Subject 10 Scanning gantry 11 Imaging table 12 Operation console 31 X-ray detection element 51 X-ray source 51x X-ray 52 Slit f X-ray focal point G Reconstruction target tomographic image g Reconstruction target pixel p X-ray path P Projection data required for reconstruction c Intersections a1, a2 Located near the intersection c X-ray detection element k Detection surface ISO Isocenter

Claims (7)

A radiation source that emits radiation from a radiation focal point and a plurality of detector modules in which a plurality of detector elements are arranged in the channel direction and the slice direction are arranged in the channel direction and the slice direction, and a gap is provided between the detector modules in the slice direction. An image generating apparatus comprising: a radiation detector formed by: and a reconstruction unit configured to reconstruct an image based on projection data obtained by scanning using
Comprising storage means for storing the position information of the individual detection elements;
The reconstruction means includes a plurality of detection elements located in the vicinity of an intersection between a radiation path that linearly extends from the radiation focus through a pixel to be reconstructed in an image reconstruction space and a detection surface of the radiation detector. The obtained actual data is interpolated by weighted addition using a weighting factor determined based on the position information of the intersection and the position information of the plurality of detection elements stored in the storage means, and the radiation An image generation apparatus that generates projection data based on a pass and reconstructs the reconstruction target pixel by a three-dimensional image reconstruction method based on the projection data.   The image generation apparatus according to claim 1, wherein the interpolation is linear interpolation, bilinear interpolation, or Lagrangian interpolation.   The image generation apparatus according to claim 1, wherein the three-dimensional image reconstruction method is a three-dimensional backprojection method. A radiation source emitting radiation from a radiation focus;
A radiation detector in which a plurality of detector modules arranged in the channel direction and the slice direction are arranged in the channel direction and the slice direction, and a gap is formed between the detector modules in the slice direction;
Reconstructing means for reconstructing an image based on projection data obtained by scanning using the radiation source and the radiation detector, and a radiation tomography apparatus comprising:
Comprising storage means for storing the position information of the individual detection elements;
The reconstruction means includes a plurality of detection elements located in the vicinity of an intersection between a radiation path that linearly extends from the radiation focus through a pixel to be reconstructed in an image reconstruction space and a detection surface of the radiation detector. The obtained actual data is interpolated by weighted addition using a weighting factor determined based on the position information of the intersection and the position information of the plurality of detection elements stored in the storage means, and the radiation A radiation tomography apparatus that generates projection data based on a path and reconstructs the reconstruction target pixel by a three-dimensional image reconstruction method based on the projection data.   The radiation tomography apparatus according to claim 4, wherein the interpolation is linear interpolation, bilinear interpolation, or Lagrangian interpolation.   The radiation tomography apparatus according to claim 4, wherein the three-dimensional image reconstruction method is a three-dimensional backprojection method.   The program for functioning a computer as an image generation apparatus as described in any one of Claims 1-3.

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