PCCT device and control method thereof

Abstract

The invention provides a PCCT device and a control method thereof, which can improve the correction accuracy of a projection value even in a smaller range of the projection value. This PCCT device is provided with: a scanner that rotates, around a subject, an X-ray source that irradiates X-rays to the subject and a photon counting detector that detects X-rays that have passed through the subject for each of a plurality of energy intervals; an image generation unit that generates a tomographic image using projection data calculated from the output of the photon counting detector; and a control unit for controlling each unit, the PCCT device being characterized in that: the control unit calculates a conversion formula for converting a projection value of the subject into a transmission length using a plurality of projection values and a plurality of transmission lengths of a phantom; the plurality of projection values are acquired by rotational imaging of the phantom which is arranged so as not to overlap the center of rotation of the scanner and which has a known material, shape, and size.

Description

Technical Field

[0001] The present invention relates to a PCCT (Photon Counting Computed Tomography) apparatus that generates a tomographic image of a subject using projection data divided into a plurality of energy intervals, and more particularly to a technology for correcting nonlinearity of projection values. Background Art

[0002] X-ray CT systems generate tomographic images of a subject using projection data from multiple directions. This data is obtained by rotating an X-ray source, which irradiates the subject with X-rays, and an X-ray detector, which detects the X-rays that have passed through the subject. The generated tomographic images depict the shapes of organs in the subject and are used for diagnostic imaging.

[0003] Generally, the higher the X-ray energy, the smaller the X-ray absorption coefficient of substances contained in the subject. Therefore, when irradiating X-rays with a predetermined energy range, beam hardening occurs, where the average energy of the X-rays increases as the X-rays pass through the subject. Beam hardening causes a nonlinear relationship between the projection value (derived from the amount of X-rays transmitted through the subject) and the transmission length of the subject, degrading the quality of tomographic images. This necessitates correction of the projection value.

[0004] Patent Document 1 discloses correcting projection values ​​using a correction function obtained by performing beam hardening correction and multiple function fitting on a sinogram obtained by rotationally imaging a phantom disposed offset from the center of an imaging region.

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-180808

[0006] However, Patent Document 1 does not adequately consider correction accuracy in a range with smaller projection values, that is, a range with a high transmitted X-ray dose. In PCCT devices equipped with photon-counting detectors that count incident X-ray photons one by one, pile-up, etc., occurs when more X-ray photons are detected, i.e., when the X-ray dose is higher, disrupting the proportional relationship between input and output, reduces the correction accuracy of the projection values. This reduction in projection correction accuracy degrades the quality of tomographic images. Summary of the Invention

[0007] Therefore, an object of the present invention is to provide a PCCT apparatus and a control method thereof that can improve the correction accuracy of projection values ​​even in a smaller range of projection values.

[0008] In order to achieve the above-mentioned object, the present invention is a PCCT device comprising: a scanner that rotates around the subject an X-ray source that irradiates X-rays toward the subject and a photon counting detector that detects X-rays that have passed through the subject in each of a plurality of energy intervals; an image generating unit that generates a tomographic image using projection data calculated based on an output of the photon counting detector; and a control unit that controls each unit, wherein the PCCT device is characterized in that the control unit calculates a conversion formula for converting the projection value of the subject into a transmission length using a plurality of projection values ​​and a plurality of transmission lengths of a phantom, wherein the plurality of projection values ​​are obtained by rotationally photographing the phantom of a known material, shape, and size that is arranged so as not to overlap with the rotation center of the scanner.

[0009] Furthermore, the present invention is a control method for a PCCT device, the PCCT device comprising: a scanner that rotates around the subject an X-ray source that irradiates X-rays toward the subject and a photon counting detector that detects X-rays that have passed through the subject in each of a plurality of energy intervals; an image generating unit that generates a tomographic image using projection data calculated based on an output of the photon counting detector; and a control unit that controls each unit, the control method being characterized in that a conversion formula for converting the projection value of the subject into a transmission length is calculated using a plurality of projection values ​​and a plurality of transmission lengths of a phantom, the plurality of projection values ​​being obtained by rotationally photographing the phantom of a known material, shape, and size that is arranged so as not to overlap with the rotation center of the scanner.

[0010] Effects of the Invention

[0011] According to the present invention, it is possible to provide a PCCT apparatus and a control method thereof that can improve the correction accuracy of projection values ​​even in a smaller range of projection values. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 It is a diagram showing the overall structure of a PCCT apparatus.

[0013] Figure 2 This is a diagram showing an example of X-rays divided into a plurality of energy intervals.

[0014] Figure 3 This is a diagram showing an example of the processing flow of Example 1.

[0015] Figure 4 This is a diagram showing an example of projection values ​​of a phantom arranged so as not to overlap with the rotation center of the scanner.

[0016] Figure 5 This is a diagram illustrating a jig that supports a phantom.

[0017] Figure 6 This is a diagram explaining how the position of a phantom is calculated based on projection data.

[0018] Figure 7 This is a diagram explaining how the position of a phantom is calculated from a tomographic image generated using projection data.

[0019] Figure 8 This is a diagram explaining the calculation of the transmission length of a phantom.

[0020] Figure 9 This is a diagram for explaining how to calculate a formula for converting a projection value into a transmission length.

[0021] Figure 10 This is a diagram explaining a case where a phantom is arranged at the rotation center of a scanner.

[0022] Figure 11 This is a diagram for explaining a state where a phantom is placed on a bed.

[0023] Figure 12 This is a diagram for explaining the case of using a phantom having a polygonal cross-sectional shape.

[0024] Explanation of symbols

[0025] 10- Subject, 100- Scanner, 101- X-ray Source, 102- Rotating Plate, 103- Collimator, 104- Opening, 105- Bed, 105A- Lay Plate, 105B- Base, 106- Photon Counting Detector, 107- Data Collection Unit, 108- Rotating Plate Controller, 109- Bed Controller, 110- X-ray Controller, 111- High Voltage Generator, 120- Operation Unit, 121- Input Unit, 12 2-Image generation unit, 123-Storage unit, 124-System control unit, 125-Display unit, 400-Phantom, 401-Rotation center, 402-Projection data, 403-Projection profile, 500-Fixture, 700-Projection field of view, 701-Phantom area, 702-Ring artifact, 801-Transmission length, 802-Transmission length, 900-Measurement point, 901-Approximate curve, 1101-Projection profile, 1102-Area. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments of a PCCT (photon counting computed tomography) apparatus according to the present invention will be described with reference to the accompanying drawings. In the following description and drawings, components having the same functional structure are denoted by the same reference numerals to avoid duplication of description.

[0027] [Example 1]

[0028] use Figure 1The overall structure of the PCCT apparatus of Example 1 will be described. The PCCT apparatus includes a scanner 100 and an operating unit 120. The scanner 100 is installed in an imaging room surrounded by a shielding material that shields X-rays, and the operating unit 120 is installed in an operating room located outside the imaging room. The rotation axis of the scanner 100 is defined as the Z-axis, the horizontal direction perpendicular to the Z-axis is defined as the X-axis, and the vertical direction perpendicular to the Z-axis is defined as the Y-axis.

[0029] The scanner 100 includes an X-ray source 101, a rotating plate 102, a collimator 103, a photon-counting detector 106, a data acquisition unit 107, a bed 105, a rotating plate controller 108, a bed controller 109, an X-ray controller 110, and a high-voltage generator 111. The X-ray source 101 is a device, such as an X-ray tube, that irradiates a subject 10 placed on the bed 105 with X-rays. The collimator 103 is a device that limits the irradiation range of the X-rays. The rotating plate 102 includes an opening 104 for receiving the subject 10 placed on the bed 105. The rotating plate 102 carries the X-ray source 101 and the photon-counting detector 106 and rotates the X-ray source 101 and the photon-counting detector 106 around the subject 10.

[0030] The photon counting detector 106 is a device that detects X-rays by having multiple detection elements arranged opposite to the X-ray source 101. The detection elements of the photon counting detector 106 are arranged two-dimensionally in the rotation direction and the rotation axis direction of the rotating plate 102, and detect the incident X-ray photons one by one and distinguish each X-ray photon into multiple energy ranges. Figure 2 In FIG, X-rays detected in three energy bins, T1 to T2, T2 to T3, and T3 to T3, are represented as bin 1, bin 2, and bin 3. The data acquisition unit 107 is a device that collects the spatial distribution of X-rays in each energy bin acquired by the photon counting detector 106 as digital data.

[0031] The rotating plate controller 108 controls the rotation and tilt of the rotating plate 102. The bed controller 109 controls the vertical, forward, backward, and left and right movement of the bed 105. The high-voltage generator 111 is a power supply that generates the tube voltage (voltage applied to the X-ray source 101) and the tube current (current supplied to the X-ray source 101). The X-ray controller 110 controls the output of the high-voltage generator 111. The rotating plate controller 108, bed controller 109, and X-ray controller 110 are, for example, microprocessing units (MPUs).

[0032] The operation unit 120 includes an input unit 121, an image generator 122, a display unit 125, a storage unit 123, and a system control unit 124. The input unit 121 is a device used to input examination data such as the subject 10's name, examination date and time, and imaging conditions, and is, for example, a keyboard, pointing device, or touch panel. The image generator 122 is a device that generates tomographic images using digital data collected by the data acquisition unit 107, and is, for example, an MPU or a GPU (Graphics Processing Unit). The display unit 125 is a device that displays tomographic images generated by the image generator 122, and is, for example, a liquid crystal display or touch panel. The storage unit 123 is a device that stores digital data collected by the data acquisition unit 107, tomographic images generated by the image generator 122, programs executed by the system control unit 124, and data used by the programs, and is, for example, an HDD (Hard Disk Drive) or an SSD (Solid State Drive). The system control unit 124 is a device that controls each unit such as the rotating plate control unit 108 , the bed control unit 109 , and the X-ray control unit 110 , and is, for example, a CPU (Central Processing Unit).

[0033] The high-voltage generator 111 generates a tube voltage and tube current based on the imaging conditions set via the input unit 121, thereby irradiating the subject 10 with X-rays corresponding to the imaging conditions from the X-ray source 101. The photon-counting detector 106 uses multiple detection elements to detect X-rays emitted from the X-ray source 101 and transmitted through the subject 10, thereby acquiring the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate controller 108 and rotates based on the imaging conditions input from the input unit 121, particularly the rotation speed. The bed 105 is controlled by the bed controller 109 and moves relative to the rotating plate 102 to move the imaging position set for the subject 10 to the imaging field of view, which is the range for detecting transmitted X-rays.

[0034] By repeatedly irradiating the X-rays from the X-ray source 101 and detecting them with the photon-counting detector 106 as the rotating plate 102 rotates, projection data of the subject 10 is measured at various projection angles. The projection data is associated with a view (View) representing each projection angle, and a channel (ch) number and column number representing the number of detection elements in the photon-counting detector 106. The measured projection data is transmitted to the image generator 122. The image generator 122 generates tomographic images by back-projecting the plurality of projection data. The generated tomographic images are displayed as medical images on the display 125 or stored in the storage 123.

[0035] Generally, the higher the X-ray energy, the smaller the X-ray absorption coefficient of the material contained in the subject 10. Therefore, when the X-rays irradiated from the X-ray source 101 have a predetermined energy bandwidth, the X-rays transmitted through the subject 10 undergo so-called beam hardening. Beam hardening causes the relationship between the projection value calculated based on the X-ray dose transmitted through the subject 10 and the transmission length of the subject 10 to become nonlinear, necessitating correction of the projection value of the subject 10. In particular, in a PCCT apparatus equipped with a photon-counting detector 106, in addition to the aforementioned beam hardening, the relationship further becomes nonlinear due to the effects of pile-up and other factors that occur when the X-ray dose is even higher. Therefore, correction accuracy within a range of smaller projection values ​​is crucial. Therefore, in Example 1, a conversion formula is calculated that can accurately convert the projection value of the subject 10 to the transmission length even within a range of smaller projection values.

[0036] use Figure 3 An example of the processing flow of Example 1 will be described step by step.

[0037] (S301)

[0038] The system control unit 124 operates the scanner 100 in a state where nothing is placed in the opening 104 to image the air. For example, the imaged data of the air is used to calculate the sensitivity of each detection element of the photon counting detector 106 .

[0039] (S302)

[0040] like Figure 4 As shown in (a) of FIG. 1 , the phantom 400 is placed in the opening 104 so as not to overlap with the rotation center 401 of the scanner 100 . The material, shape, and size of the phantom 400 are known. Figure 4 (a) shows an example of a phantom 400 having a circular cross-sectional shape and a diameter of the circle smaller than the radius of the opening 104 .

[0041] In addition, the phantom 400 is formed by Figure 5 The jig 500 shown is supported from the direction of the rotation axis of the scanner 100 . Figure 5 The clamp 500 shown in the example is attached to the top of a top plate 105A included in the bed 105. The phantom 400 is positioned in the opening 104 by sliding the top plate 105A along the rotation axis of the scanner 100. Furthermore, the position of the phantom 400 is adjusted by moving the base 105B supporting the top plate 105A from below vertically and horizontally.

[0042] (S303)

[0043] The system control unit 124 operates the scanner 100 in a state where the phantom 400 is placed in the opening 104 , thereby performing rotational imaging of the phantom 400 .

[0044] (S304)

[0045] The system control unit 124 calculates projection data of the phantom 400 using the air imaging data obtained in S301 and the imaging data of the phantom 400 obtained in S303. Before calculating the projection data, pre-processing such as offset correction, logarithmic conversion, and reference correction may be performed on each imaging data.

[0046] exist Figure 4 (b) shows an example of projection data 402 of a phantom 400 that is set so as not to overlap with the rotation center 401. The projection data 402 represents the projection value of each coordinate represented by the projection angle θ and the channel number ch of the detection element. Figure 4 (c) shows a projection profile 403 showing projection values ​​for each projection angle θ at the center ch. In the projection data 402 of the phantom 400 set so as not to overlap with the rotation center 401, most of the projection profiles 403 of ch include projection values ​​ranging from zero to a maximum value.

[0047] (S305)

[0048] The system control unit 124 calculates the position of the phantom 400. For example, the position of the phantom 400 is calculated based on the vertical and horizontal movement of the base 105B of the bed 105. Alternatively, the position of the phantom 400 may be calculated based on the projection data 402 calculated in S304.

[0049] use Figure 6 An example of calculating the position of the phantom 400 based on the projection data 402 will be described. First, the range of ch, which is the projection value of air, is extracted at each projection angle θ of the projection data 402. Then, the position of the phantom 400 is calculated by determining the projection angle at which the range of ch, which is the projection value of air, is widest and the ch that is the boundary of the projection value of air at this projection angle. Figure 6 In (a), arrows indicate the range of ch, which represents the projection value for air. The widest projection angle of the extracted ch range is θ0, and the boundary of the projection value for air at projection angle θ0 is ch0. Since the shape and size of phantom 400 are known, the position of phantom 400 is calculated based on the determined θ0 and ch0.

[0050] Alternatively, the position of the phantom 400 can be calculated by fitting a sine curve to the boundaries of the air projection values ​​in the projection data 402 of the phantom 400. Calculating the position of the phantom 400 based on the projection data 402 improves positional accuracy compared to calculating the position of the phantom 400 based on the amount of movement of the base 105B. Alternatively, the position of the phantom 400 can be calculated based on a tomographic image generated using the projection data 402.

[0051] use Figure 7 An example of calculating the position of the phantom 400 based on the tomographic image generated using the projection data 402 will be described. Figure 7 , an image including a phantom region 701 in an imaging field of view 700 is shown as an example of a tomographic image generated using projection data 402. Since projection data 402 is not subjected to nonlinear correction, ring artifacts 702 are included in phantom region 701. However, the boundary of phantom region 701 can be determined, so the position of phantom 400 can be calculated. Furthermore, by calculating the position of phantom 400 based on the tomographic image, the position accuracy of phantom 400 is improved compared to calculating the position of phantom 400 based on the movement amount of base 105B. Return to Figure 3 Description.

[0052] (S306)

[0053] The system control unit 124 calculates the transmission length of the phantom 400 located at the position calculated in S305 for each projection angle θ and each channel number ch of the detection element.

[0054] use Figure 8 The calculation of the transmission length of the phantom 400 will be described. Since the position of the phantom 400, whose shape and size are known, has been determined, the length of the portion where the straight line connecting the X-ray source 101 and the detection elements of the photon counting detector 106 overlaps with the phantom 400 is calculated as the transmission length of the phantom 400. Figure 8 The middle diagram shows a transmission length 801 in the detection element of ch1 and a transmission length 802 in the detection element of ch2 when the projection angle is θ1. The transmission length in the detection element of ch3 is zero.

[0055] (S307)

[0056] The system control unit 124 calculates a conversion formula for converting the projection value of the subject 10 into the transmission length using the projection data 402 calculated in S304 and the transmission length calculated in S305 .

[0057] use Figure 9The calculation of the conversion formula is explained. The transmission length calculated in S305 and the projection value obtained from the projection data 402 are obtained for each projection angle θ and each channel number ch of the detection element. Therefore, the projection angle θ and the channel number ch are associated. Therefore, Figure 9 As shown in the example, a pair of transmission length and projection value associated with the projection angle θ and the channel number ch is plotted as one measurement point 900 on a coordinate plane with the projection value as the horizontal axis and the transmission length as the vertical axis. Figure 9 9 is a graph in which measurement points 900 for each of a plurality of projection angles θ are plotted under a predetermined channel number ch. Figure 9 The measurement points 900 shown are sets of transmission lengths and projection values ​​obtained at different projection angles θ under the same channel number ch.

[0058] Then, by fitting the approximate curve 901 represented by a high-order function or the like Figure 9 A conversion formula for converting projection values ​​into transmission lengths for a given channel number ch is calculated based on the plurality of measurement points 900 illustrated in FIG. The calculated conversion formula is used to convert projection values ​​obtained from projection data 402 of the subject 10 into transmission lengths. Furthermore, the conversion formula may be calculated for each channel number ch, or a common conversion formula may be calculated for all channel numbers ch.

[0059] pass Figure 3 The processing flow illustrated in calculates a conversion formula that can convert projection values ​​into transmission lengths with high accuracy even in a smaller range of projection values, thereby suppressing degradation in the image quality of tomographic images generated by the PCCT apparatus.

[0060] use Figure 10 A comparative example with Example 1 is described. Figure 10 As shown in (a), when the center of the phantom 400 overlaps with the rotation center 401 of the scanner 100, the projection data 402 of the phantom 400 becomes as follows. Figure 10 As shown in (b), the projection profile 403 is as follows Figure 10 As shown in (c), the projection value is constant regardless of the projection angle θ. Figure 10 As shown in (d), there is only one measurement point 900 plotted on the coordinate plane with the projection value as the horizontal axis and the transmission length as the vertical axis. Therefore, the approximate curve 901 cannot be fitted, and the conversion formula for converting the projection value of the subject 10 to the transmission length cannot be calculated.

[0061] In addition, the configuration of the phantom 400 is not limited to the configuration based on Figure 5 The configuration of the clamp 500 is shown in FIG. Figure 11The following describes a case where the phantom 400 is placed on the top plate 105A of the bed 105. Figure 11 As shown in (a) of FIG. 1 , when the phantom 400 is placed on the top plate 105A, the projection data 402 of the phantom 400 includes the projection value of the top plate 105A. Figure 11 As in (b), only the top plate 105A is placed in the opening 104 and rotation photography is performed to obtain Figure 11 The projection data 402 of the top plate 105A illustrated in (c) is obtained, and the acquired projection data 402 is used for calculation of the conversion formula.

[0062] For example, by Figure 11 The projection profile 1101 of the reclining board 105A obtained from the projection data 402 of (c) overlaps with the projection profile 403 of the phantom 400 placed on the reclining board 105A to determine the region 1102 including the reclining board 105A. Then, the projection value of the determined region 1102 is not used in the calculation of the conversion formula. In addition, the projection value of only the phantom 400 can be calculated by subtracting the projection profile 1101 of the reclining board 105A from the projection profile 403 of the phantom 400 placed on the reclining board 105A. By using the projection data 402 of the reclining board 105A in the calculation of the conversion formula, it is possible to suppress the deterioration of the correction accuracy of the conversion formula. In addition, by placing the phantom 400 on the reclining board 105A, it is possible to eliminate the need for the projection data 402 of the reclining board 105A. Figure 5 The clamp 500 is shown.

[0063] In addition, the cross-sectional shape of the phantom 400 is not limited to a circle. Figure 12 The following describes the case where a phantom 400 having a polygonal cross-sectional shape is used. Figure 12 The phantom 400 having a right triangle cross-sectional shape as in (a) is rotated and photographed in a state where it is arranged so as not to overlap with the rotation center 401. Figure 12 (b) shows the projection data 402. Figure 12 As exemplified in (c), according to Figure 12 (b) The projection profile 403 obtained from the projection data 402 includes the slope ratio of the projection value with respect to the projection angle θ Figure 4 As a result, the sampling interval of the projection value becomes smaller in the range close to zero, as shown in FIG. Figure 12 As shown in (d), measurement points 900 are also densely plotted within the range of projection values ​​close to zero, further improving the correction accuracy of projection values ​​within a range with smaller projection values. Furthermore, the smaller the internal angles of the vertices of the polygon arranged near the rotation center 401, the finer the sampling interval of the projection values ​​within the range of projection values ​​close to zero. Therefore, the cross-sectional shape of the phantom 400 is preferably a polygon with acute internal angles.

[0064] The embodiments of the present invention have been described above. The present invention is not limited to the above embodiments, and the components can be modified and embodied without departing from the main purpose of the invention. Furthermore, multiple components disclosed in the above embodiments can be appropriately combined. Furthermore, some components can be deleted from all the components shown in the above embodiments.

Claims

1. A PCCT apparatus comprising: a scanner that rotates around the subject an X-ray source that irradiates an object with X-rays; and a photon counting detector that detects X-rays that have passed through the subject in each of a plurality of energy intervals; an image generator that generates a tomographic image using projection data calculated based on output from the photon counting detector; and a control unit that controls each unit, wherein the PCCT apparatus is characterized in that: The control unit calculates a conversion formula for converting the projection value of the subject into the transmission length using a plurality of projection values ​​and a plurality of transmission lengths of the phantom, wherein the plurality of projection values ​​and the plurality of transmission lengths of the phantom are obtained by performing rotational photography on the phantom of known material, shape and size, which is configured in a manner not overlapping with the rotation center of the scanner.

2. The PCCT device according to claim 1, characterized in that The cross-sectional shape of the phantom is a polygon with acute internal angles.

3. The PCCT device according to claim 2, characterized in that The vertex of the acute inner angle of the phantom is arranged near the rotation center of the scanner.

4. The PCCT device according to claim 1, characterized in that When the phantom is placed on a top plate, the projection value including the top plate is not used in the calculation of the conversion formula.

5. The PCCT device according to claim 1, characterized in that: The control unit calculates the position of the phantom using the projection data of the phantom before obtaining the transmission length of the phantom.

6. The PCCT device according to claim 1, characterized in that The control unit calculates the position of the phantom using a tomographic image generated based on projection data of the phantom before obtaining the transmission length of the phantom.

7. A method for controlling a PCCT apparatus comprising: a scanner that rotates around the subject an X-ray source that irradiates an object with X-rays and a photon counting detector that detects X-rays that have passed through the object in each of a plurality of energy intervals; and an image generator that generates a tomographic image using projection data calculated based on output from the photon counting detector, the method comprising: A conversion formula for converting the projection value of the subject into the transmission length is calculated using multiple projection values ​​and multiple transmission lengths of the phantom, wherein the multiple projection values ​​are obtained by rotationally photographing the phantom of known material, shape and size, which is configured in a manner not overlapping with the rotation center of the scanner.

Images