WO2016017402A1 - データ処理方法、データ処理装置、及びx線ct装置 - Google Patents
データ処理方法、データ処理装置、及びx線ct装置 Download PDFInfo
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Definitions
- the present invention relates to a data processing method, a data processing apparatus, and an X-ray CT apparatus, and more particularly to forward projection and backprojection processing in image reconstruction processing.
- Line bundle drive type forward projection and backprojection processing is a method that considers a beam as a reference, and scans the beam and sequentially embeds projection values into pixels that contribute to each beam.
- Pixel-driven forward projection and backprojection processing is a method that considers pixels as a reference, and scans the pixels and sequentially embeds projection values related to each pixel.
- the beam is handled as a line segment, and a projection data value (projection value) is assigned to the pixel through which the line segment passes (back projection). Therefore, when the pixel interval is narrow, a pixel to which no projection value is assigned is generated, resulting in sampling unevenness. Sampling unevenness causes problems such as moire appearing in an image.
- the pixel drive type is adopted, the value of the beam (projection data) passing through the pixel center of the target pixel is assigned with attention to the pixel. Therefore, there is projection data that is not used when the pixel is rough. As a result, the utilization efficiency of the projection data decreases, and the image noise increases.
- Patent Document 1 dynamically adjusts the size of the square window for one of the pixel and the detector bin so that adjacent windows form a continuous shadow on one of the detector bin and the pixel, A projection and backprojection method is described that determines the effect of each pixel on each bin of the detector and vice versa. According to the method of Patent Document 1, when the pixel size is relatively large compared to the detector element size, noise is reduced and uniform back projection is possible. Accordingly, there is an advantage that no high frequency error such as moire occurs.
- Patent Document 1 described above, adjacent windows are arranged so as to be continuous. That is, the pixel size is set equal to the pixel interval. In this case, when performing 3D display such as volume rendering, image quality degradation may occur due to aliasing. In addition, when a structure having a size equal to or smaller than the pixel size is located between the pixels, there is a problem that the rendering ability is deteriorated due to the partial volume effect. In addition, when back projection as described in Patent Document 1 is employed in the successive approximation reconstruction process, if the slice thickness is reduced, noise may increase due to insufficient number of X-ray photons, and a desired image quality may not be obtained. is there.
- the smoothing process (regularization process) based on the similarity of neighboring pixels in the image space, which is performed in the successive approximation reconstruction process, does not work well, and the rendering of minute structures deteriorates.
- the slice thickness can be increased according to the imaging dose, and the inter-pixel distance should not be excessively increased.
- the present invention has been made in view of the above-described problems, and its object is to consider that adjacent pixels and beams overlap in back projection processing or forward projection processing performed in image reconstruction processing.
- a data processing method, a data processing apparatus, and an X-ray CT apparatus capable of suppressing the occurrence of high-frequency errors such as moire by performing calculation in consideration of overlapping of pixels and beams and uniformly using data. Is to provide.
- the present invention sets the beam size to be set wider than the beam interval or sets the pixel size wider than the pixel interval in the forward projection processing or back projection processing executed by the data processing apparatus.
- the data processing method is characterized in that an interpolation value assigned to a beam or a pixel is calculated using a size-dependent weight according to the amount of overlap of adjacent beams or the amount of overlap of adjacent pixels.
- an X-ray CT apparatus having a data processing device, and a data processing device comprising: a calculation unit that calculates an interpolation value to be assigned to a beam or a pixel using a size-dependent weight.
- an X-ray source that emits X-rays from a focal point having an area
- an X-ray detector that is disposed opposite to the X-ray source and detects X-rays transmitted through the subject, and detected by the X-ray detector
- a data collection device that collects transmitted X-rays, and a forward projection process or backprojection process that is performed when acquiring the transmitted X-rays and reconstructing an image based on the acquired transmitted X-rays, the beam size is determined from the beam interval.
- An image processing apparatus that performs image reconstruction processing that includes processing for calculating an interpolation value to be assigned to a beam or a pixel using a size-dependent weight according to an overlap amount of adjacent beams that are set widely.
- X-ray CT apparatus that performs image reconstruction processing that includes processing for calculating an interpolation value to be assigned to a beam or a pixel using a size-dependent weight according to an overlap amount of adjacent beams that are set widely.
- the present invention in back projection processing or forward projection processing when reconstructing an image, it is assumed that adjacent pixels or beams overlap, and a value assigned to the pixel or beam is determined in consideration of the overlap of pixels or beams. Therefore, it is possible to provide a data processing method, a data processing apparatus, and an X-ray CT apparatus capable of uniformly using data and suppressing generation of high frequency errors such as moire.
- FIGS. A flowchart showing a procedure for calculating a value (pixel interpolation value bv) to be assigned to the beam bc in the forward projection process using the pixel window shown in FIGS.
- Diagram explaining a general beam window The figure explaining the beam window according to the relationship between the beam interval and the beam width in the present invention (beam interval ⁇ beam width), and the distance from the radiation source.
- FIGS. 8 and 9 are flowcharts showing a procedure for calculating a value (beam interpolation value pv) assigned to the pixel pc in the back projection processing using the beam window shown in FIGS.
- FIGS. 8 is a flowchart showing a procedure for calculating a value (pixel interpolation value bv) to be assigned to the beam bc in the forward projection process using the beam window shown in FIGS.
- the X-ray CT apparatus 1 includes a scan gantry unit 100, a bed 105, and a console 120.
- the scan gantry unit 100 is an apparatus that irradiates a subject with X-rays and detects X-rays transmitted through the subject.
- the console 120 is a device that controls each part of the scan gantry unit 100, acquires transmission X-ray data measured by the scan gantry unit 100, and generates an image.
- the bed 105 is a device that places a subject on the bed and carries the subject in and out of the X-ray irradiation range of the scan gantry unit 100.
- the scan gantry unit 100 includes an X-ray source 101, a turntable 102, a collimator 103, an X-ray detector 106, a data collection device 107, a gantry control device 108, a bed control device 109, and an X-ray control device 110.
- the console 120 includes an input device 121, an image processing device (data processing device) 122, a storage device 123, a system control device 124, and a display device 125.
- the rotating plate 102 of the scan gantry unit 100 is provided with an opening 104, and the X-ray source 101 and the X-ray detector 106 are arranged to face each other through the opening 104.
- the subject placed on the bed 105 is inserted into the opening 104.
- the turntable 102 rotates around the subject by a driving force transmitted from the turntable drive device through a drive transmission system.
- the turntable driving device is controlled by a gantry control device.
- the X-ray source 101 is controlled by the X-ray control device 110 to irradiate X-rays having a predetermined intensity continuously or intermittently.
- the X-ray controller 110 controls the X-ray tube voltage and the X-ray tube current applied or supplied to the X-ray source 101 according to the X-ray tube voltage and the X-ray tube current determined by the system controller 124 of the console 120. To do.
- a collimator 103 is provided at the X-ray irradiation port of the X-ray source 101.
- the collimator 103 limits the irradiation range of the X-rays emitted from the X-ray source 101. For example, it is formed into a cone beam (conical or pyramidal beam).
- the opening width of the collimator 103 is controlled by the system controller 124.
- the X-ray detector 106 is a two-dimensional array of X-ray detection element groups configured by, for example, a combination of a scintillator and a photodiode, in the channel direction (circumferential direction) and the column direction (body axis direction).
- the X-ray detector 106 is disposed so as to face the X-ray source 101 through the subject.
- the X-ray detector 106 detects the X-ray dose irradiated from the X-ray source 101 and transmitted through the subject, and outputs it to the data collection device 107.
- the data collection device 107 collects the X-ray dose detected by each X-ray detection element of the X-ray detector 106 at a predetermined sampling interval, converts it into a digital signal, and performs image processing of the console 120 as transmitted X-ray data. The data is sequentially output to the device 122.
- the image processing device (data processing device) 122 acquires transmission X-ray data input from the data collection device 107, and performs preprocessing such as logarithmic conversion and sensitivity correction to create projection data necessary for reconstruction. Further, the image processing apparatus 122 reconstructs a subject image such as a tomographic image using the generated projection data.
- the system control device 124 stores the subject image data reconstructed by the image processing device 122 in the storage device 123 and displays it on the display device 125.
- the pixel size is set wider than the pixel interval, and a size-dependent weight (pixel window) corresponding to the amount of overlap between adjacent pixels is used.
- Back projection processing including processing for calculating an interpolation value to be assigned to the pixel is performed. Details of the backprojection processing will be described later (see FIGS. 2 to 4).
- the system control device 124 is a computer having a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like.
- the storage device 123 is a data recording device such as a hard disk, and stores programs, data, and the like for realizing the functions of the X-ray CT apparatus 1 in advance.
- the display device 125 includes a display device such as a liquid crystal panel and a CRT monitor, and a logic circuit for executing display processing in cooperation with the display device, and is connected to the system control device 124.
- the display device 125 displays the subject image output from the image processing device 122 and various information handled by the system control device 124.
- the input device 121 includes, for example, a keyboard, a pointing device such as a mouse, a numeric keypad, and various switch buttons, and outputs various instructions and information input by the operator to the system control device 124.
- the operator operates the X-ray CT apparatus 1 interactively using the display device 125 and the input device 121.
- the input device 121 may be a touch panel type input device configured integrally with the display screen of the display device 125.
- the couch 105 includes a couch for placing a subject, a vertical movement device, and a couch drive device.
- the couch control device 109 controls the couch height to move up and down and move back and forth in the body axis direction. Or move left and right in the direction perpendicular to the body axis and parallel to the floor (left and right direction).
- the couch controller 109 moves the couch at the couch moving speed and moving direction determined by the system controller 124.
- the image processing device 122 sets the pixel size wider than the pixel interval. This causes an overlap between adjacent pixels.
- the image processing device 122 calculates a size-dependent weight (pixel window) according to the amount of overlap between adjacent pixels, and calculates an interpolation value to be assigned to the pixel using the size-dependent weight (pixel window).
- imaging conditions and reconstruction conditions are input from the input device 121 of the X-ray CT apparatus 1, and the subject is imaged.
- the imaging conditions are set such that the beam pitch is “1.1”, the tube voltage is “120 kV”, the tube current is “300 mA”, and the scan speed is “0.5 s / rotation”.
- the reconstruction FOV (Field Of View) included in the reconstruction condition and the reconstruction center position are determined according to the imaging region so that the disease can be easily diagnosed. For example, in cardiac imaging, the reconstruction FOV is set to “250 mm”, and the reconstruction center position is set to “heart is the center”.
- the reconstructed image matrix size is usually fixed, 512 pixels (number of pixels on one side of the square reconstructed image), the number of reconstructed image slices, the interval between slices, and the slice thickness are the imaging range, the size of the disease to be diagnosed, and the imaging It is set according to the dose. For example, the number of slices is “200”, the slice interval is “1.25 mm”, and the slice thickness is “2.5 mm”.
- the reconstruction filter is selected according to the imaging region. For example, “abdominal standard filter” may be selected for photographing the abdomen, and “standard filter for head” may be selected for photographing the head.
- the image processing device 122 acquires projection data obtained by shooting, executes image reconstruction processing based on the above-described reconstruction conditions, and creates a reconstructed image.
- image reconstruction process for example, a filter-corrected 3D backprojection method is used.
- the image processing device 122 performs backprojection processing in consideration of overlap between adjacent pixels.
- the backprojection processing in consideration of the overlap between adjacent pixels will be described with reference to FIGS.
- FIG. 2 and 3 are diagrams showing size-dependent weights (pixel windows 2a to 2g and beam window 3) used for back projection processing in the present invention.
- FIG. 4 is a flowchart showing a processing procedure for calculating a value pv to be assigned to the pixel pc in the backprojection processing.
- the pixel windows 2a to 2g are collectively referred to as a pixel window 2.
- the pixel window 2 is a weight (size-dependent weight) used when calculating an interpolation value assigned to a pixel in back projection processing.
- the pixel window 2 to be used is determined according to the overlapping amount of adjacent pixels.
- the shape of the pixel window 2 is defined by the width of the pixel window 2 (pixel window width pww) and the weight size (pixel size-dependent weight value pwt k ) at each position (pixel area) in the width direction.
- the vertical lengths of the pixel windows 2a to 2g shown in FIGS. 2 and 3 indicate the pixel size dependent weight value pwt k .
- k is an index in the pixel window 2 (number indicating the pixel area from the left end, “0”, “1”, “2”,).
- the pixel region is each region obtained by dividing the pixel by the pixel interval.
- the image processing device 122 determines the pixel window width pww from the pixel size (pixel size psx) and the pixel interval ppx. Furthermore, the sum of the weight values (pixel size-dependent weight values pwt k ) when adjacent pixel windows 2 are arranged in an overlapping manner is equal at each pixel position, and the half width of the pixel window 2 is equal to the pixel size. A pixel size dependent weight value pwt k is determined.
- FIG. 2 is a diagram showing the arrangement of the pixel window 2 and the beam window 3 when the pixel size psx is wider than the pixel interval and the beam window width bww> the pixel window width pww.
- FIG. 2A shows the pixel window width pww. Shows the shape of the pixel window 2a when equal to the interval, (b) shows the shape of the pixel window 2b when the pixel window width pww is twice the pixel interval, and (c) shows the pixel window width pww as the pixel interval (D) shows the shape of the pixel window 2d when the pixel window width pww is three times the pixel interval.
- FIG. 3 is a diagram showing the arrangement of the pixel window 2 and the beam window 3 when the pixel size psx is wider than the pixel interval and the beam window width bww ⁇ the pixel window width pww, and (a) is the pixel window width pww. Shows the shape of the pixel window 2e when is equal to the pixel interval, (b) shows the shape of the pixel window 2f when the pixel window width pww is twice the pixel interval, and (c) shows the pixel window width pww. The shape of the pixel window 2g when the pixel interval is 3 times is shown.
- the pixel window 2 as shown in FIG. 2 or FIG. 3 is used when obtaining the beam interpolation value pv to be assigned to each pixel in the back projection process.
- Which pixel window 2 is used is determined according to the amount of overlap between pixels. For example, the amount of overlap between pixels is determined according to the relationship between the slice thickness and the slice interval.
- step S101 when the pixel size psx [mm] is determined by a reconstruction condition or the like set by the operator via the input device 121 or the like (step S101), the image processing device 122 Ask for 2. That is, the pixel window width pww and the pixel size dependent weight value pwt k are calculated (steps S102 and S103).
- the pixel interval ppx is expressed by the following equation (1).
- the pixel size psx described above is, for example, the slice thickness of the reconstructed image, and the pixel interval ppx is the slice interval of the reconstructed image.
- step S101 If the pixel size determined in step S101 is psx [mm], the image processing apparatus 122 calculates the pixel window width pww [pixel] by the following equation (2) (step S102).
- the pixel window center position pwc in the pixel window width pww is expressed by Expression (3), and the pixel window end position pwe with respect to the pixel window center position pwc is expressed by Expression (4).
- the leading pixel position psc of the pixel window 2 is expressed by the following equation (6).
- the image processing apparatus 122 calculates an interpolation kernel f (step S104), and calculates a beam interpolation value pv (step S105).
- an interpolation kernel f step S104
- a beam interpolation value pv step S105
- calculation of the interpolation kernel f and calculation of the beam interpolation value pv will be described.
- the position of the pixel boundary ps j and pe j on the common axis 4 at a certain pixel pc j (j is the pixel index) is P (ps j ), P (pe j), the beam boundary bs i at a beam bc i (i is the index of the beam), a position on the common shaft 4 P (bs i) the be i, P (be i) , pixels on a common shaft 4 Bc js , bc je , the beam where the boundary P (ps j ) and P (pe j ) are located, the interpolation kernel, that is, the ratio of the beam bc i on the common axis 4 occupying the pixel pc j (the length on the common axis 4 the proportion of) the f i, j, raw i projection values located at position i on a common axis 4, and when the interpolation kernel, that is, the ratio of the beam bc i
- the image processing device 122 assigns the above-described beam interpolation value pv j to the pixel pc j (step S106).
- the image processing device 122 reconstructs an image by an analytical method such as the filter-corrected 3D backprojection method
- the pixel size is set wider than the pixel interval, and the pixel The back projection process is performed in consideration of the overlap of.
- the backprojection process can be performed with a slice thickness wider than the slice interval. Since the slice thickness can be made wider than the slice interval of the reconstructed image, aliasing artifacts in 3D display can be suppressed.
- the size dependent weight is determined from the pixel size, the pixel is divided by the pixel interval, the size dependent weight value is determined for the divided pixel region, and the interpolation value is calculated from the size dependent weight and the interpolation kernel.
- Imaging conditions and reconstruction conditions are input from the input device 121 of the X-ray CT apparatus 1, and the subject is imaged. Imaging conditions and reconstruction conditions are the same as those in the first embodiment.
- the image processing device 122 acquires projection data obtained by shooting, executes image reconstruction processing based on the above-described reconstruction conditions, and creates a reconstructed image.
- the image processing apparatus 122 first executes a filter-corrected 3D backprojection method including backprojection in consideration of an overlap between adjacent pixels according to the present invention (the method of the first embodiment) ).
- the image processing apparatus 122 receives an instruction input as to whether or not to execute the successive approximation process.
- the operator confirms the reconstructed image by the above-described filter correction 3D back projection method or the like, and determines that there are many noises and artifacts and causes a problem in diagnosis, the operator executes the successive approximation process via the input device 121. select.
- the image processing apparatus 122 accepts setting of parameters for the successive approximation process by the operator.
- the parameters of the successive approximation process are the maximum number of iterations, convergence condition (end condition), prior probability weight (coefficient that determines the degree of smoothing), and the like.
- the image processing apparatus 122 first creates an initial image.
- the initial image may be an image reconstructed by using a filter-corrected 3D backprojection method including backprojection in consideration of an overlap between adjacent pixels, or other reconstruction The method may be used. Note that a constant value image can be used as the initial image without using a reconstructed image.
- the number of iterations until convergence in the successive approximation process varies depending on the reconstruction method and reconstruction filter used to create the initial image. If there is a large discrepancy between the forward projection data obtained by forward projecting the initial image and the projection data, for example, the initial image has a lot of artifacts, distortion, and noise, and the discrepancy between the projection data and the forward projection data is large. In some cases, the number of repetitions until convergence increases. Therefore, it is desirable to use a reconstruction filter and a reconstruction method that can obtain forward projection data with as little contradiction as possible with respect to projection data.
- the image processing apparatus 122 performs successive approximation processing (sequential approximation reconstruction) using forward projection and backprojection in consideration of overlap between adjacent pixels based on the obtained initial image. Thereby, a successive approximation reconstructed image is obtained.
- the portions other than forward projection and backprojection in the successive approximation reconstruction are the same as those in the conventional successive approximation reconstruction method.
- the successive approximation methods include ML (Maximum Likelihood Estimation: Maximum likelihood) method, MAP (Maximum A posteriori probability: Maximum a Posterior) method, WLS (Weighted Least Squares) and PWLS (Weighted Least Squares with penalties): Known successive approximation reconstruction methods such as Penalized (Weighted (Least) Squares) method and SIRT (Simultaneous Reconstruction (Technique) method) can be used.
- ML Maximum Likelihood Estimation: Maximum likelihood
- MAP Maximum A posteriori probability: Maximum a Posterior
- WLS Weighted Least Squares
- PWLS Weighted Least Squares with penalties
- Known successive approximation reconstruction methods such as Penalized (Weighted (Least) Squares) method and SIRT (Simultaneous Reconstruction (Technique) method) can be used.
- Acceleration methods such as OS (Ordered Subset), SPS (Separable Paraboloidal Surrogate), and RAMLA (Row-Action Maximum Likelihood Algorithm) may be applied to these successive approximation methods.
- the image processing device 122 sets the pixel size wider than the pixel interval, as in the case of back projection. This causes an overlap between adjacent pixels.
- the image processing device 122 calculates size-dependent weights (pixel windows 2a to 2g; see FIGS. 2 and 3) according to the amount of overlap between adjacent pixels, and uses the size-dependent weights (pixel windows 2a to 2g) to generate a beam.
- the interpolation value assigned to is calculated.
- step S201 to step S203 in the flowchart of FIG. 5 is the same as that in the case of back projection considering the overlapping of pixels in the first embodiment (step S101 to step S103 in FIG. 4).
- Step S201 when the pixel size psx [mm] is determined by the reconstruction conditions set by the operator via the input device 121 or the like (step S201), the image processing device 122, the effective field size FOV, Calculate the pixel interval ppx using the matrix size MATRIX, and calculate the pixel window width pww and the pixel size dependent weight value pwt k from the pixel size psx and the pixel interval ppx using the above equations (1) to (5). (Step S202, Step S203).
- the image processing device 122 calculates an interpolation kernel g (step S204), and calculates a pixel interpolation value bv (step S205).
- an interpolation kernel g step S204
- a pixel interpolation value bv step S205
- calculation of the interpolation kernel g and calculation of the pixel interpolation value bv will be described.
- the position of the pixel boundary ps j and pe j on the common axis 4 at a certain pixel pc j (j is the pixel index) is P (ps j ), P (pe j), the beam boundary bs i at a beam bc i (i is the index of the beam), a position on the common shaft 4 P (bs i) the be i, P (be i) , the beam on a common axis 4 boundary P (bs i), P pixels (bE i) is positioned pc js, pc ie, interpolation kernel, i.e.
- the proportion pixel pc j on a common shaft 4 occupies the beam bc i (length on a common shaft 4 percentage) of g i, j, the pixel value located at position j on the common shaft 4 img j, to the beam bc pixel interpolation value assigned to i bv i the following equations (9) and (10 ).
- the image processing device 122 assigns the above-described pixel interpolation value bv i to the beam bc i (step S206).
- the pixel size is set wider than the pixel interval, and forward projection is performed in consideration of pixel overlap. Processing and back projection processing are performed. In addition, it is desirable to perform back projection processing in consideration of pixel overlap when creating an initial image in successive approximation processing.
- the beam intervals and beam widths of the beams 30a, 30b, and 30c emitted from the X-ray source 101 are matched, and the pixel position 41, Beam windows 38 and 39 having a size corresponding to the distance up to 42 are set. Adjacent beams 30a, 30b, 30c are continuously arranged without overlapping.
- the beam irradiated from the X-ray source 101 is actually irradiated with an area. This is because the focal point of the X-ray source 101 is not actually a point but a certain size (area) as shown in FIG. Therefore, as shown in FIG. 7 (a), beams 31a, 31b, 31c having an area are emitted from the radiation source, and adjacent beams 31a, 31b, 31c are overlapped at pixel positions 41,.
- set a plurality of points as the source It is necessary to calculate back projection or forward projection at all points (ray sources) and take the average value of the calculation results. Therefore, the calculation amount increases.
- the beam is irradiated from the X-ray source 101 with an area as shown in FIG.
- the beam width is wider than the beam interval
- the image processing device 122 performs back projection in consideration of the overlap between adjacent beams.
- beam windows 3a to 3g (see FIG. 8 and FIG. 9) having a beam width wider than the beam interval and having weight values corresponding to the amount of overlap of the adjacent beams 31a, 31b, and 31c are used.
- a back projection process for calculating a beam interpolation value assigned to each pixel is performed.
- the beam window 3 to be applied is changed according to the overlapping amount of the beam. To do. For example, at a pixel position 41 close to the X-ray source 101, a beam window 3A having a beam width that is twice the beam interval is used as shown in the upper part of FIG. As shown in the lower part of FIG. 7 (b), the beam overlap amount is small, so a beam window 3B having the same width as the beam interval is used.
- the beam windows 3a to 3g, 3A, and 3B are collectively referred to as a beam window 3.
- the beam window width bww is determined from the beam size (beam width bsx) and the beam interval bpx. Further, the sum of the weight values (beam size-dependent weight values bwt k ) when the adjacent beam windows 3 are arranged in an overlapping manner is equal at each pixel position, and the half width of the beam window 3 is equal to the beam width.
- the beam size dependent weight value bwt k is determined. For example, the beam windows 3a to 3g and the pixel window 2 as shown in FIGS. 8 and 9 are set, and the value pv to be assigned to the pixel is calculated according to the procedure shown in the flowchart of FIG.
- the beam window 3 is a weight (size-dependent weight) used when calculating an interpolation value assigned to a pixel in back projection processing or calculating an interpolation value assigned to a projection (beam) in forward projection processing.
- the beam window 3 to be used is determined according to the overlapping amount of adjacent beams. For example, the beam window to be used is changed according to the distance between the radiation source and the pixel position.
- the shape of the beam window 3 is defined by the width of the beam window 3 (beam window width bww) and the weight (beam size-dependent weight value bwt k ) at each position (pixel region) in the width direction.
- the beam region is a region obtained by dividing a beam by a beam interval.
- FIG. 8 is a diagram illustrating an example of a diagram illustrating an arrangement of the beam window 3 and the pixel window 2 when the beam size (beam width bsx) is wider than the beam interval bpx and the beam window width bww ⁇ the pixel window width pww.
- (A) shows the shape of the beam window 3a when the beam window width bww is equal to the beam interval bpx
- (b) shows the shape of the beam window 3b when the beam window width bww is twice the beam interval bpx.
- (C) shows the shape of the beam window 3c when the beam window width bww is four times the beam interval bpx
- (d) shows the beam window when the beam window width bww is three times the beam interval bpx. Shows 3d shape.
- FIG. 9 is a diagram showing the arrangement of the pixel window 2 and the beam window 3 when the beam size (beam width bsx) is wider than the beam interval bpx and the beam window width bww> the pixel window width pww. Shows the shape of the beam window 3e when the beam window width bww is equal to the beam interval bpx, (b) shows the shape of the beam window 3f when the beam window width bww is twice the beam interval bpx, and (c ) Shows the shape of the beam window 3g when the beam window width bww is three times the beam interval bpx.
- the beam windows 3a, 3b,... are collectively referred to as a beam window 3.
- the image processing apparatus 122 calculates a beam size (beam width) bsx [mm] and a beam interval bpx [mm] (step S301).
- the source size is fsx [mm]
- the detector element size is dsx [mm]
- the source-detector distance is SID [mm]
- the source-pixel distance is SPD [mm]
- the beam at the pixel position The size (beam width) bsx [mm] is expressed by the following formula (11), and the beam interval bpx [mm] is expressed by the following formula (12).
- the image processing apparatus 122 sets the beam window width bww [channel] at the pixel position by the following equation (13). Obtained (step S302).
- the beam window center position bwc in the beam window width bww is expressed by Expression (14), and the beam window end position bwe with respect to the beam window center position bwc is expressed by Expression (15).
- the leading pixel position bsc of the beam window 3 is expressed by the following equation (17).
- the image processing device 122 calculates an interpolation kernel f (step S304), and calculates a beam interpolation value pv (step S305).
- an interpolation kernel f calculates a beam interpolation value pv (step S305).
- the position of the pixel boundary ps j and pe j on the common axis 4 at a certain pixel pc j (j is the pixel index) is P (ps j ), P (pe j ),
- Interpolation kernel that is, the ratio of the beam bc i on the common axis 4 to the pixel pc j (the ratio of the length on the common axis 4) f i, j ,
- the projection value located at position i on common axis 4 is raw i , Then, the
- the image processing device 122 assigns the beam interpolation value pv j described above to the pixel pc j (step S306).
- back projection processing is performed in consideration of overlapping of adjacent beams in back projection processing or the like in the filter correction 3D back projection method or the successive approximation method. As a result, it is possible to obtain a good image quality with good data utilization efficiency without image quality deterioration caused by data utilization unevenness.
- the relationship between the beam size and the beam interval (the degree of beam overlap) is changed according to the distance from the radiation source to the target pixel, the radiation source size, the detection element size, and the distance between the radiation source and the detection element. As a result, it is possible to obtain a result in consideration of the source size and the detection element size at a high speed by a series of calculations.
- the back projection process of the third embodiment is applied to a beam having a size (area) instead of a point as a source, the back projection process is performed at a high speed while improving the accuracy of the model during successive approximation reconstruction. It can be performed.
- the beam window width bww is determined from the beam size (beam width bsx) and the beam interval bpx, as in the third embodiment (in the case of back projection). . Further, the beam size dependent weights are set so that the sum of the beam size dependent weight values bwt k when the adjacent beam windows 3 are arranged in an overlapping manner is equal at each pixel position, and the half width of the beam window 3 is equal to the beam width. Determine the value bwt k . For example, the beam window 3 and the pixel window 2 as shown in FIGS. 8 and 9 are set, and the value bv assigned to the beam is calculated according to the procedure shown in the flowchart of FIG.
- step S401 to step S403 in the flowchart in FIG. 11 is the same as that in the case of back projection in the third embodiment (step S301 to step S303 in FIG. 10).
- the image processing apparatus 122 calculates the beam size (beam width) bsx at the pixel position from the radiation source size fsx, the detector element size dsx, the radiation source-detector distance SID, and the radiation source-pixel distance SPD, and the beam.
- the interval bpx is calculated using the above equations (11) and (12).
- the beam window width bww is calculated based on the beam interval bpx and the beam size bsx (Formula (13)).
- the image processing device 122 calculates the beam size dependent weight value bwt k in the same manner as the above equation (16).
- the image processing apparatus 122 calculates an interpolation kernel g (step S404), and calculates a pixel interpolation value bv (step S405).
- an interpolation kernel g step S404
- a pixel interpolation value bv step S405.
- the position of the pixel boundary ps j and pe j on the common axis 4 at a certain pixel pc j (j is the pixel index) is P (ps j ), P (pe j ),
- There beam bc i (i is the index of the beam) the beam boundary bs i at a position on a common axis 4 of be i P (bs i), P (be i),
- the pixels where the beam boundaries P (bs i ) and P (be i ) on the common axis 4 are located are pc is , pc ie , Interpolation kernel, i.e., the ratio of the pixel pc j on the common axis 4 to the beam bc i (the ratio of the length on the common axis 4)
- g i, j , Img j the pixel value located at position j on the common
- the image processing device 122 assigns the above-described pixel interpolation value bv i to the beam bc i (step S406).
- forward projection processing is performed in consideration of overlapping of adjacent beams in forward projection processing at the time of image reconstruction by the successive approximation method.
- forward projection can be performed in consideration of the size of the radiation source, and a good image quality with high data utilization efficiency without image quality deterioration due to data utilization unevenness can be obtained.
- the relationship between the beam size and the beam interval (the degree of overlap of the beams) is changed according to the distance from the radiation source to the target pixel, the radiation source size, the detection element size, and the distance between the radiation source and the detection element. As a result, it is possible to obtain a result in consideration of the source size and the detection element size at a high speed by a series of calculations.
- the forward projection processing of the fourth embodiment is applied to a beam whose size is not the point but the size (area) of the radiation source, the model accuracy at the time of successive approximation reconstruction is improved and high-speed sequential processing is performed. Projection processing can be performed.
- the pixel window 2 and the beam window 3 shown in FIG. 2, FIG. 3, FIG. 8, FIG. 9, etc. are used as in the first and third embodiments.
- the calculation procedure of the beam interpolation value pv in back projection taking into account the overlap between adjacent beams and the overlap between adjacent pixels will be described with reference to the flowchart of FIG.
- the image processing apparatus 122 calculates the beam size (beam width) bsx and beam interval bpx at the pixel position from the source size fsx, detector element size dsx, source-detector distance SID, and source-pixel distance SPD. Is calculated from the above equations (11) and (12).
- the beam window width bww is calculated based on the beam interval bpx and the beam size bsx (Formula (13)). Further, the image processing apparatus 122 calculates the beam size dependent weight value bwt k in the same manner as the above equations (14) to (16).
- the image processing device 122 as in the case of back projection considering the overlap of pixels in the first embodiment (step S101 to step S103 in FIG. 4), the pixel size psx, the pixel interval ppx, and the pixel window width pww Then, the pixel size dependent weight value pwt k is calculated (steps S504 to S506).
- the pixel size psx [mm] is determined by a reconstruction condition or the like set by the operator via the input device 121 or the like.
- the pixel interval ppx and the pixel window width pww are the effective field size FOV and the reconstructed image matrix size MATRIX. Are calculated from the equations (1) and (2), respectively.
- the pixel size dependent weight value pwt k is calculated using the above equations (3) to (5).
- the image processing apparatus 122 calculates an interpolation kernel f (step S507), and calculates a beam interpolation value pv (step S508).
- an interpolation kernel f step S507
- a beam interpolation value pv step S508
- the positions of the pixel boundaries ps j and pe j on the common axis 4 at a certain pixel pc j (j is a pixel index) are P (ps j ), P ( pe j), there beam bc i (i is beam boundary bs i in the index) of the beam, the position on a common axis 4 of be i P (bs i), P (be i), pixel boundary on a common shaft 4 P (ps i ), P (pe i ) are located at the beam bc js , bc je , the interpolation kernel, that is, the ratio of the beam bc i on the common axis 4 to the pixel pc j (the length on the common axis 4 ratio) and f i, j, when the raw i, the projection value located at position i on a common shaft 4, the value assigned to the pixel
- the image processing device 122 assigns the above-described beam interpolation value pv j to the pixel pc j (step S509).
- both the overlap between adjacent beams and the overlap between adjacent pixels are considered in the back projection process.
- the data can be used uniformly, and a good image quality with high data use efficiency without image quality deterioration caused by data use unevenness can be obtained. Generation of high frequency errors such as moire can be suppressed.
- the backprojection process of the fifth embodiment is performed when the image is reconstructed by the filter-corrected 3D backprojection method, or whether the image is reconstructed by the successive approximation method as described in the second embodiment. It can be applied when reconstructing an image for determination, or when creating an image by a successive approximation method.
- the pixel window 2 and the beam window 3 shown in FIG. 2, FIG. 3, FIG. 8, FIG. 9, etc. are used as in the second and fourth embodiments.
- the procedure for calculating the pixel interpolation value bv in forward projection in consideration of the overlap between adjacent beams and the overlap between adjacent pixels will be described with reference to the flowchart of FIG.
- the image processing device 122 has a beam size (beam width) bsx at the pixel position.
- a beam interval bpx, a beam window width bww, and a beam size dependent weight value bwt k are calculated (steps S601 to S603). That is, the image processing apparatus 122 calculates the beam size (beam width) bsx and beam interval bpx at the pixel position from the source size fsx, detector element size dsx, source-detector distance SID, and source-pixel distance SPD. Is calculated from the above equations (11) and (12).
- the beam window width bww is calculated based on the beam interval bpx and the beam size bsx (Formula (13)). Further, the image processing apparatus 122 calculates the beam size dependent weight value bwt k in the same manner as the above equations (14) to (16).
- the image processing device 122 has a pixel size psx, a pixel interval ppx, and a pixel window width pww.
- the pixel size dependent weight value pwt k is calculated (steps S604 to S606).
- the pixel size psx [mm] is determined by a reconstruction condition or the like set by the operator via the input device 121 or the like.
- the pixel interval ppx and the pixel window width pww are the effective field size FOV and the reconstructed image matrix size MATRIX. Are calculated from the equations (1) and (2), respectively.
- the pixel size dependent weight value pwt k is calculated using the above equations (3) to (5).
- the image processing device 122 calculates an interpolation kernel g (step S607), and calculates a pixel interpolation value bv (step S608).
- an interpolation kernel g step S607
- a pixel interpolation value bv step S608
- the positions of the pixel boundaries ps j and pe j on the common axis 4 at a certain pixel pc j (j is a pixel index) are P (ps j ), P ( pe j), there beam bc i (i is beam boundary bs i in the index) of the beam, the position on a common axis 4 of be i P (bs i), P (be i), a beam boundary on the common shaft 4 P (bs i), P ( be i) is a pixel positioned pc iS, pc ie, interpolation kernel, i.e.
- the image processing device 122 assigns the pixel interpolation value bv i described above to the beam bc i (step S609).
- forward projection processing is performed in consideration of both overlapping of adjacent beams and overlapping of pixels.
- the data can be used uniformly, and a good image quality with high data use efficiency without image quality deterioration due to data use unevenness can be obtained. Generation of high frequency errors such as moire can be suppressed.
- the backprojection processing of the sixth embodiment can be applied when creating an image by the successive approximation method.
- FIG. 14 (a) is a diagram showing the dose distribution (electron density distribution) in the X-ray source 101
- FIG. 14 (b) is a diagram showing the sensitivity distribution of the X-ray detector 106.
- the focal point of the X-ray source 101 is not strictly a point, but actually has a size (area).
- the magnitude (electron density) of the beam irradiated from the surface has a characteristic that it varies depending on the focal position as shown in FIG. 14 (a). Further, as shown in FIG. 14 (b), the sensitivity of the X-ray detector 106 also differs depending on the detector position.
- the image processing apparatus 122 uses the beam window 3 illustrated in the third and fourth embodiments as a dose distribution function or a detector sensitivity distribution function as shown in FIG. Superimpose on 9). Then, the image processing device 122 normalizes the beam window 3 after superimposing the dose distribution function or the detector sensitivity distribution function so that the sum of the weight values when adding the adjacent beams becomes equal at each pixel position. Get a modified beam window. The image processing apparatus 122 performs forward projection or backprojection according to any of the third to sixth embodiments using the above-described modified beam window at the time of image reconstruction.
- the present invention is not limited to the above-described embodiment.
- one-dimensional processing is exemplified, but the present invention may be applied when calculating an interpolation value for projection data obtained by a two-dimensional detector.
- the final interpolation value can be obtained by first calculating the interpolation value in the channel direction and then calculating the interpolation value in the column direction.
- the present invention can be applied to fan beam back projection, forward projection, parallel beam back projection, and forward projection.
- the data processing method of the present invention can be applied to image reconstruction in various X-ray CT apparatuses using a single slice detector, a multi-slice detector, and a flat panel detector.
- both forward projection and backprojection considering the pixel size and the source size are performed when the successive approximation reconstruction is performed.
- the reverse projection considering the pixel size and the source size is performed. Only one of projection and forward projection may be used.
- 1 X-ray CT device 100 scan gantry section, 101 X-ray source, 102 turntable, 106 X-ray detector, 120 console, 121 input device, 122 image processing device (data processing device), 123 storage device, 124 system Control device, 125 display device, 2, 2a-2g pixel window (pixel size dependent weight), 3, 3a-3g, 3A, 3B beam window (beam size dependent weight), 4 common axis, 41, 42 pixel position, 5 Pixel
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Abstract
Description
まず、図1を参照してX線CT装置1の全体構成について説明する。
次に、本発明の第2の実施の形態について図5を参照して説明する。第2の実施の形態では、近接する画素間の重なりを考慮した順投影処理を含む逐次近似再構成処理により画像を生成する例について説明する。なお、以下の説明において近接する画素間の重なりを考慮した逆投影の詳細に関しては第1実施形態と同様であるため重複する説明を省略する。
上述のフィルタ補正3D逆投影法等による再構成画像を操作者が確認し、ノイズやアーチファクトが多く診断上問題となると判断した場合は、操作者は入力装置121を介して逐次近似処理の実行を選択する。画像処理装置122は、操作者による逐次近似処理のパラメータの設定を受け付ける。
次に、本発明の第3の実施の形態について図6~図10を参照して説明する。
隣接するビーム30a、30b、30cは重なりを持たずに連続的に配置される。
ある画素pcj(jは画素のインデックス)における画素境界psj、pejの共通軸4上の位置をP(psj)、P(pej)、
あるビームbci(iはビームのインデックス)におけるビーム境界bsi、beiの共通軸4上の位置をP(bsi)、P(bei)、
共通軸4上のビーム境界P(bsi)、P(bei)が位置するビームをbcjs、bcje、
補間カーネル、すなわち共通軸4上のビームbciが画素pcjを占める割合(共通軸4上の長さの割合)をfi,j、
共通軸4上の位置iに位置する投影値をrawi、
とすると、画素pcjに割り当てられるビーム補間値pvjは以下の式(18)及び式(19)のように算出される。
これによりデータ利用ムラに起因した画質劣化のない、データ利用効率のよい良好な画質を得ることができる。
次に、本発明の第4の実施の形態について図11を参照して説明する。第4の実施の形態では、近接するビーム間の重なりを考慮した順投影方法について説明する。近接するビーム間の重なりについては、第3の実施の形態と同様であるので(図7参照)、説明を省略する。
例えば、図8、図9に示すようなビームウィンドウ3及び画素ウィンドウ2を設定し、図11のフローチャートに示す手順でビームに割り当てる値bvを算出する。
ある画素pcj(jは画素のインデックス)における画素境界psj、pejの共通軸4上の位置をP(psj)、P(pej)、
あるビームbci(iはビームのインデックス)におけるビーム境界bsi、beiの共通軸4上の位置をP(bsi)、P(bei)、
共通軸4上のビーム境界P(bsi)、P(bei)が位置する画素をpcis、pcie、
補間カーネル、すなわち共通軸4上の画素pcjがビームbciを占める割合(共通軸4上の長さの割合)をgi,j、
共通軸4上の位置jに位置する画素値をimgj、
とすると、ビームbciに割り当てられる画素補間値bviは以下の式(20)、式(21)のように算出される。
次に本発明の第5の実施の形態として、近接するビーム間の重なり及び近接する画素間の重なりの双方を考慮した逆投影方法について説明する。
次に、本発明の第6の実施の形態として、近接するビーム間の重なり及び近接する画素間の重なりの双方を考慮した順投影方法について説明する。
第7の実施の形態では、ビームの線量分布(電子密度分布)やX線検出器106の感度を考慮した逆投影、順投影の方法について説明する。
Claims (10)
- データ処理装置が実行する順投影処理または逆投影処理において、設定されるビームサイズをビーム間隔より広く設定し、または画素サイズを画素間隔より広く設定し、近接するビームの重なり量または近接する画素の重なり量に応じたサイズ依存重みを用いてビームまたは画素に割り当てる補間値を算出することを特徴とするデータ処理方法。
- データ処理装置が、
前記画素サイズ及び前記画素間隔に基づいて前記サイズ依存重みの幅を算出するステップと、
画素を前記画素間隔で区分し、区分した各画素領域における前記サイズ依存重みの重み値を算出するステップと、
前記サイズ依存重みと補間カーネルとに基づきビームまたは画素に割り当てる補間値を算出するステップと、
算出した補間値をビームまたは画素へ割り当てるステップと、
を含む順投影処理または逆投影処理を行うことを特徴とする請求項1に記載のデータ処理方法。 - 前記画素サイズは再構成画像のスライス厚であり、前記画素間隔は再構成画像のスライス間隔であることを特徴とする請求項2に記載のデータ処理方法。
- データ処理装置が、
前記ビームサイズ及び前記ビーム間隔に基づいて前記サイズ依存重みの幅を算出するステップと、
ビームを前記ビーム間隔で区分し、区分した各ビーム領域における前記サイズ依存重みの重み値を算出するステップと、
前記サイズ依存重みと補間カーネルとに基づきビームまたは画素に割り当てる補間値を算出するステップと、
算出した補間値をビームまたは画素へ割り当てるステップと、
を含む順投影処理または逆投影処理を行うことを特徴とする請求項1に記載のデータ処理方法。 - 前記ビームサイズと前記ビーム間隔との関係を、線源から対象画素までの距離、線源サイズ、検出器素子サイズ、及び線源と検出器間の距離に応じて変更するステップを更に含むことを特徴とする請求項4に記載のデータ処理方法。
- 前記サイズ依存重みの和が各画素で等しくなるように前記サイズ依存重みの重み値が算出されることを特徴とする請求項1に記載のデータ処理方法。
- 前記サイズ依存重みに対し、線源における線量分布を示す関数または検出器感度分布を示す関数を重畳し、重畳後の前記サイズ依存重みの和が近接ビーム間で等しくなるように規格化することにより修正サイズ依存重みを得るステップを更に含み、
前記修正サイズ依存重みを用いて、ビームまたは画素に割り当てる補間値を算出することを特徴とする請求項4に記載にデータ処理方法。 - 順投影処理または逆投影処理において、ビームサイズをビーム間隔より広く設定し、または画素サイズを画素間隔より広く設定する設定部と、
近接するビームの重なり量または近接する画素の重なり量に応じたサイズ依存重みを用いてビームまたは画素に割り当てる補間値を算出する算出部と、
を備えることを特徴とするデータ処理装置。 - 請求項8に記載のデータ処理装置を有するX線CT装置。
- 面積を持つ焦点からX線を照射するX線源と、
前記X線源に対向配置され被検者を透過したX線を検出するX線検出器と、
前記X線検出器により検出した透過X線を収集するデータ収集装置と、
前記透過X線を取得し、取得した透過X線に基づいて画像を再構成する際に行う順投影処理または逆投影処理において、ビームサイズをビーム間隔より広く設定し、近接するビームの重なり量に応じたサイズ依存重みを用いてビームまたは画素に割り当てる補間値を算出する処理を含む画像再構成処理を実行する画像処理装置と、
を備えることを特徴とするX線CT装置。
Priority Applications (3)
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CN201580034343.2A CN106572832A (zh) | 2014-07-30 | 2015-07-10 | 数据处理方法、数据处理装置以及x射线ct装置 |
US15/321,401 US20170202532A1 (en) | 2014-07-30 | 2015-07-10 | Data processing method, data processing device, and x-ray ct apparatus |
JP2016538250A JPWO2016017402A1 (ja) | 2014-07-30 | 2015-07-10 | データ処理方法、データ処理装置、及びx線ct装置 |
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US10145968B2 (en) * | 2014-05-12 | 2018-12-04 | Purdue Research Foundation | Linear fitting of multi-threshold counting data |
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US10223813B2 (en) * | 2015-08-13 | 2019-03-05 | InstaRecon | Method and system for reprojection and backprojection for tomography reconstruction |
WO2019191891A1 (zh) * | 2018-04-02 | 2019-10-10 | 北京大学 | 用于视频处理的方法和设备 |
CN108652656B (zh) * | 2018-05-21 | 2024-04-12 | 北京达影科技有限公司 | 复合探测器、体层成像系统及方法 |
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CN109636874B (zh) * | 2018-12-17 | 2023-05-26 | 浙江科澜信息技术有限公司 | 一种三维模型透视投影方法、系统及相关装置 |
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US8456469B2 (en) * | 2009-12-10 | 2013-06-04 | Satpal Singh | 3D reconstruction from oversampled 2D projections |
US10223813B2 (en) * | 2015-08-13 | 2019-03-05 | InstaRecon | Method and system for reprojection and backprojection for tomography reconstruction |
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