PCCT device and control method thereof

Abstract

The invention provides a PCCT device and a control method thereof, which can improve interpolation precision related to substance discrimination and shorten processing time. 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; and an image generation unit that generates a tomographic image using projection data calculated from the output of the photon counting detector, the PCCT device being characterized by being further provided with: a map creation unit that creates a tomographic image using projection data calculated from the output of the photon counting detector; creating a calibration data map on the basis of the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first base material and a second base material of known material and thickness; a region division unit that divides the calibration data map into a non-linear region and a linear region; and an interpolation unit that performs a non-linear interpolation in the non-linear region and a linear interpolation in the linear region.

Description

Technical Field

[0001] The present invention relates to a PCCT (Photon Counting Computed Tomography) apparatus which is an X-ray CT apparatus equipped with a photon counting detector, and more particularly to an interpolation process of calibration data used in calibration of the photon counting detector. Background Art

[0002] An X-ray CT apparatus generates a tomographic image of a subject using projection data from multiple directions, and the projection data from multiple directions is obtained by rotating an X-ray source that irradiates the subject with X-rays and an X-ray detector that detects the X-rays transmitted through the subject around the subject. The generated tomographic image is used as a medical image for image diagnosis of the subject.

[0003] In a PCCT apparatus using a photon counting detector as an X-ray detector, it is possible to count the number of X-ray photons for each energy, and thus it is possible to distinguish substances in the subject using the projection values obtained in each of multiple energy bins. However, if the amount of X-rays incident on the photon counting detector is large, non-linear phenomena such as pile up, in which multiple X-ray signals are erroneously counted as an X-ray with an added energy, or missed counts, in which other X-rays are incident during the process of processing an X-ray signal by a certain photon counting detector and are not processed and thus not counted, may occur, and sometimes the discrimination accuracy of substances may be reduced.

[0004] Patent Document 1 discloses the following: In an X-ray CT apparatus that counts the number of X-ray photons for each energy bin, substances in the subject are distinguished based on the amount of X-rays transmitted through the subject and the amount of X-rays transmitted through a specified substance. Also disclosed is the following: In order to improve the discrimination accuracy, the same signal processing is performed multiple times on the two amounts of X-rays, the condition under which the error between the two after the signal processing becomes smaller is selected, and substances are distinguished based on the result of the signal processing under the selected condition. Further disclosed is the following: When the interval of the transmission distance of the specified substance is wide, the transmission distance interval is interpolated with high precision by parabolic fitting, linear approximation, polynomial approximation, spline, bicubic, etc.

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2019-58488

[0006] In Patent Document 1, considerations regarding non-linear or linear interpolation are not sufficient. That is, if linear interpolation is performed in a region with strong non-linearity, the interpolation accuracy decreases, and if non-linear interpolation is performed in a region with strong linearity, the amount of calculation increases and the processing time becomes longer. Summary of the Invention

[0007] Accordingly, an object of the present invention is to provide a PCCT apparatus and a control method thereof that can improve interpolation accuracy related to material discrimination and shorten processing time.

[0008] To achieve the above object, a PCCT apparatus according to the present invention includes: a scanner that rotates an X-ray source that irradiates an object to be examined with X-rays and a photon counting detector that detects X-rays transmitted through the object to be examined for each of a plurality of energy ranges around the object to be examined; and an image generation unit that generates a tomographic image using projection data calculated from the output of the photon counting detector. The PCCT apparatus is characterized by further including: a map creation unit that creates a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first base material and a second base material with known material and thickness; a region segmentation unit that divides the calibration data map into a non-linear region and a linear region; and an interpolation unit that performs non-linear interpolation in the non-linear region and linear interpolation in the linear region.

[0009] Moreover, a control method of a PCCT apparatus according to the present invention, the PCCT apparatus including: a scanner that rotates an X-ray source that irradiates an object to be examined with X-rays and a photon counting detector that detects X-rays transmitted through the object to be examined for each of a plurality of energy ranges around the object to be examined; and an image generation unit that generates a tomographic image using projection data calculated from the output of the photon counting detector. The control method is characterized by including: a map creation step of creating a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first base material and a second base material with known material and thickness; a region segmentation step of dividing the calibration data map into a non-linear region and a linear region; and an interpolation step of performing non-linear interpolation in the non-linear region and linear interpolation in the linear region.

[0010] Advantages 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 interpolation accuracy related to material discrimination and shorten processing time. Brief Description of the Drawings

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

[0013] Figure 2 It is a diagram for explaining calibration data of a photon counting detector.

[0014] Figure 3 It is a diagram showing a structural example of a calibration phantom.

[0015] Figure 4 This is a diagram showing an example of the process of the treatment in Example 1.

[0016] Figure 5 This is a diagram showing an example of a calibration data graph.

[0017] Figure 6 This is a diagram for explaining the regional segmentation of the calibration data graph.

[0018] Figure 7 This is a diagram showing an example of the process of generating a tomographic image in which substances have been discriminated.

[0019] Symbol Explanation

[0020] 10 - Subject, 100 - Scanner, 101 - X-ray source, 102 - Rotating plate, 103 - Collimator, 104 - Opening, 105 - Bed, 106 - Photon counting detector, 107 - Data collection unit, 108 - Rotating plate control unit, 109 - Bed control unit, 110 - X-ray control unit, 111 - High voltage generation unit, 120 - Operation unit, 121 - Input unit, 122 - Image generation unit, 123 - Storage unit, 124 - System control unit, 125 - Display unit, 201 - Calibration phantom, 202 - First base substance, 203 - Second base substance, 210 - Calibration data, 500 - Calibration data graph, 601 - Nonlinear region, 602 - Linear region, 603 - Reference line. Detailed Embodiment

[0021] Hereinafter, embodiments of a PCCT (Photon Counting Computed Tomography) apparatus and a control method thereof according to the present invention will be described with reference to the accompanying drawings. In addition, in the following description and drawings, repeated descriptions are omitted by assigning the same reference numerals to constituent elements having the same functional structure.

[0022] [Example 1]

[0023] Use Figure 1 The overall structure of the PCCT apparatus of Example 1 will be described. The PCCT apparatus includes a scanner 100 and an operation unit 120. The scanner 100 is installed in a photography room surrounded by a shielding material that shields X-rays, and the operation unit 120 is installed in an operation room located outside the photography room. In addition, the direction of the rotation axis of the scanner 100 is set as the Z-axis, the horizontal direction orthogonal to the Z-axis is set as the X-axis, and the vertical direction orthogonal to the Z-axis is set as the Y-axis.

[0024] The scanner 100 includes an X-ray source 101, a rotating plate 102, a collimator 103, a photon-counting detector 106, a data collection unit 107, a table 105, a rotating plate control unit 108, a table control unit 109, an X-ray control unit 110, and a high-voltage generation unit 111. The X-ray source 101 is a device that irradiates an object 10 placed on the table 105 with X-rays, for example, an X-ray tube device. The collimator 103 is a device that limits the irradiation range of the X-rays. The rotating plate 102 has an opening 104 for accommodating the object 10 placed on the table 105, and mounts 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 object 10.

[0025] The photon-counting detector 106 is a device that obtains the spatial distribution of X-rays by including a plurality of detection elements that are arranged opposite to the X-ray source 101 and detect X-rays. The detection elements of the photon-counting detector 106 are two-dimensionally arranged in the rotation direction and the rotation axis direction of the rotating plate 102, count the incident X-ray photons respectively, and discriminate each X-ray photon into a plurality of energy ranges.

[0026] The rotating plate control unit 108 is a device that controls the rotation and tilt of the rotating plate 102. The table control unit 109 is a device that controls the vertical, front-back, left-right movement of the table 105. The high-voltage generation unit 111 is a power source that generates the voltage applied to the X-ray source 101, i.e., the tube voltage, and the current supplied to the X-ray source 101, i.e., the tube current. The X-ray control unit 110 is a device that controls the output of the high-voltage generation unit 111. The rotating plate control unit 108, the table control unit 109, and the X-ray control unit 110 are, for example, an MPU (Micro-Processing Unit).

[0027] The operation unit 120 includes an input unit 121, an image generation unit 122, a display unit 125, a storage unit 123, and a system control unit 124. The input unit 121 is a device used for inputting examination data such as the name of the subject 10, the examination date and time, and the imaging conditions. For example, it is a keyboard, a pointing device, a touch panel, or the like. The image generation unit 122 is a device that generates tomographic images using the digital data collected by the data collection unit 107. For example, it is an MPU or a GPU (Graphics Processing Unit). The display unit 125 is a device that displays tomographic images and the like generated by the image generation unit 122. For example, it is a liquid crystal display and a touch panel. The storage unit 123 is a device that stores the digital data collected by the data collection unit 107, the tomographic images generated by the image generation unit 122, the programs executed by the system control unit 124, the data used by the programs, and the like. For example, it is an HDD (Hard Disk Drive) or an SSD (Solid State Drive). The system control unit 124 is a device that controls various units such as the rotating plate control unit 108, the bed control unit 109, and the X-ray control unit 110. For example, it is a CPU (Central Processing Unit).

[0028] The high-voltage generation unit 111 generates tube voltage and tube current according to the imaging conditions set via the input unit 121, and irradiates the subject 10 with X-rays corresponding to the imaging conditions from the X-ray source 101. The photon-counting detector 106 uses a plurality of detection elements to detect the X-rays irradiated from the X-ray source 101 and transmitted through the subject 10, and obtains the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate control unit 108, and rotates according to the imaging conditions input from the input unit 121, particularly according to the rotation speed and the like. The bed 105 is controlled by the bed control unit 109, and moves the imaging position set for the subject 10 to the imaging field, which is the range for detecting the transmitted X-rays, by moving relative to the rotating plate 102.

[0029] By repeatedly performing the irradiation of X-rays from the X-ray source 101 and the detection of X-rays by the photon-counting detector 106 along with the rotation of the rotating plate 102, the projection data of the subject 10 is measured at various projection angles. The projection data is associated with the view representing each projection angle, the channel (ch) number, which is the detection element number of the photon-counting detector 106, and the column number. The measured projection data is sent to the image generation unit 122. The image generation unit 122 generates tomographic images by performing back-projection processing on a plurality of projection data. The generated tomographic images are displayed on the display unit 125 as medical images, or stored in the storage unit 123.

[0030] A PCCT apparatus equipped with a photon-counting detector 106 can acquire projection data of a subject 10 for each energy range, and thus can generate medical images for each energy range or medical images distinguishable as being composed of different substances. In addition, in order to obtain medical images distinguishable as being composed of different substances, etc., it is necessary to previously acquire calibration data of the photon-counting detector 106.

[0031] Use Figure 2 To describe the calibration data 210 of the photon-counting detector 106. The calibration data 210 is acquired using a calibration phantom 201 having a first base material 202 and a second base material 203 with known composition and thickness. That is, by counting X-ray photons transmitted through the calibration phantom 201 in each of a plurality of energy ranges by the detection element P of the photon-counting detector 106, calibration data 210 corresponding to Figure 2 the photon energy spectrum exemplified in the lower part of is obtained. In Figure 2 are shown the numbers of X-ray photons counted by dividing into three energy range bins 1, 2, and 3 from T1 to T2, T2 to T3, T3 to. The calibration data 210 is previously acquired using each of a plurality of calibration phantoms 201 having different thicknesses of the first base material 202 and the second base material 203.

[0032] Use Figure 3 To describe an example of the plurality of calibration phantoms 201. In Figure 3 are exemplified 16 types of calibration phantoms 201 having different numbers of the first base material 202 and the second base material 203. That is, the calibration phantom 201 exemplified in Figure 3 is a combination of 0 to 3 sheets of the first base material 202 and 0 to 3 sheets of the second base material 203. As the first base material 202, for example, an acrylic plate is used as a material equivalent to the soft tissue of the subject 10. And, as the second base material 203, for example, a tin plate is used as a material equivalent to the iodine contrast agent used in angiography. In addition, as long as the first base material 202 is a material equivalent to water or soft tissue, it may be a material other than acrylic. And, as long as the second base material 203 is a material equivalent to the iodine contrast agent, in addition to tin, it may be silver, or may be aluminum or titanium equivalent to the bone part of the subject 10, rhodium, palladium, lead, gold, platinum, tungsten or their alloys as metals other than these.

[0033] The calibration data 210 obtained using a plurality of calibration phantoms 201 is compared with the projection data of the subject 10, and the calibration data 210 having a photon energy spectrum similar to that of the projection data of the subject 10 is extracted from the plurality of calibration data 210. Then, based on the thicknesses of the first base material 202 and the second base material 203 of the calibration phantom 201 corresponding to the extracted calibration data 210, material discrimination of the subject 10 is performed.

[0034] In addition, since the calibration data 210 is obtained using a calibration phantom 201 having a prescribed thickness, appropriate interpolation processing is required among the plurality of calibration data 210. That is, if linear interpolation is performed in a region with strong non-linearity, the interpolation accuracy decreases, and if non-linear interpolation is performed in a region with strong linearity, the amount of calculation increases and the processing time becomes longer. Therefore, in the first embodiment, the calibration data map, which is an aggregate of the calibration data 210, is divided into a non-linear region and a linear region, non-linear interpolation is performed in the non-linear region, and linear interpolation is performed in the linear region.

[0035] Using Figure 4 , an example of the process of dividing the calibration data map, which is an aggregate of the calibration data 210, into a non-linear region and a linear region is described for each step.

[0036] (S401)

[0037] The calibration phantom 201 is disposed between the X-ray source 101 and the photon counting type detector 106.

[0038] (S402)

[0039] The system control unit 124 counts the X-ray photons transmitted through the calibration phantom 201 while changing the amount of X-rays irradiated from the X-ray source 101. In addition, in S402, the rotating plate 102 is not rotated.

[0040] (S403)

[0041] It is determined whether the counting for all the calibration phantoms 201 has been completed. If the counting for all the calibration phantoms 201 has been completed, the process proceeds to S404. If there is an uncounted calibration phantom 201, the process returns to S401, and the uncounted calibration phantom 201 is disposed between the X-ray source 101 and the photon counting type detector 106.

[0042] (S404)

[0043] The system control unit 124 creates based on the counting result in S402 Figure 5The calibration data graph 500 illustrated in []. The calibration data graph 500 is an aggregate of calibration data 210 obtained for each of the calibration phantoms 201. Figure 5 The calibration data graph 500 illustrated in [is a graph created for each energy range, having the number of acrylic plates as the first base material 202 on the horizontal axis and the number of tin plates as the second base material 203 on the vertical axis. Figure 5 One of the white circles in [] corresponds to Figure 3 one of the calibration phantoms 201 illustrated in [], and includes calibration data 210 for each of a plurality of irradiation doses.

[0044] (S405)

[0045] The system control unit 124 divides the calibration data graph 500 created in S404 into Figure 6 the two regions of the non-linear region 601 and the linear region 602 illustrated in []. The region division of the calibration data graph 500 is performed based on the linearity of the Log value of the count value of the transmitted photons with respect to each of the plurality of irradiation doses. That is, in a graph with the Log value of the count value on the vertical axis and the irradiation dose on the horizontal axis, the region containing non-linear data with a difference between the reference line 603 and the Log value of the count value greater than a predetermined threshold is set as the non-linear region 601, and the region not containing it is set as the linear region 602. In Figure 6 [], the non-linear data is represented by black circles, and the non-linear region 601 is represented by diagonal lines. In addition, the reference line 603 is calculated based on the thicknesses of the first base material 202 and the second base material 203 of the calibration phantom 201. And the threshold is determined according to the discrimination accuracy.

[0046] Through Figure 4 the processing flow illustrated in [], the calibration data graph 500 is divided into the non-linear region 601 and the linear region 602. The calibration data graph 500 divided into the non-linear region 601 and the linear region 602 is stored in the storage unit and used for material discrimination of the subject 10. In addition, the system control unit 124 that executes S404 functions as a graph creation unit that creates the calibration data graph 500 based on the number of X-ray photons transmitted through each of the plurality of calibration phantoms 201 having the first base material 202 and the second base material 203 with known material and thickness. And the system control unit 124 that executes S405 functions as a region division unit that divides the calibration data graph 500 into the non-linear region 601 and the linear region 602.

[0047] Using Figure 7 [], an example of the processing flow for generating a tomographic image of the identified material based on the calibration data graph 500 is described for each step.

[0048] (S701)

[0049] Projection data of the subject 10 is acquired under the control of the system control unit 124. More specifically, with the rotating plate 102 rotating, X-rays are irradiated from the X-ray source 101 to the subject 10, and the X-rays transmitted through the subject 10 are detected by the photon counting type detector 106, thereby acquiring projection data at various projection angles. In addition, the projection data of the subject 10 is acquired by dividing it into multiple energy intervals.

[0050] (S702)

[0051] The system control unit 124 discriminates the projection data of the subject 10 acquired in S701 into multiple substances, such as the first base substance 202 and the second base substance 203, according to the calibration data map 500 read from the storage unit. In addition, when performing substance discrimination, non-linear interpolation is performed in the non-linear region 601, and linear interpolation is performed in the linear region 602. In the non-linear interpolation in the non-linear region 601, for example, a high-order function is preferably used, and the order is increased according to the intensity of the non-linearity. The stronger the non-linearity, the higher the order is increased, thereby further improving the interpolation accuracy. Also, in the linear region 602, linear interpolation is preferably performed according to the statistical error of the Log value of the count value. By performing linear interpolation according to the statistical error, the interpolation accuracy can be further improved.

[0052] (S703)

[0053] The system control unit 124 uses the projection data whose substances have been discriminated in S702 to reconstruct tomographic images of each substance. That is, tomographic images of the first base substance 202 and the second base substance 203 are reconstructed.

[0054] According to the processing flow described Figure 7 Using the projection data whose substances have been discriminated according to the calibration data map 500, tomographic images of the subject 10 are generated for each substance. The calibration data map 500 divided into the non-linear region 601 and the linear region 602 is used for substance discrimination. Non-linear interpolation is performed in the non-linear region 601, and linear interpolation is performed in the linear region 602. Therefore, the interpolation accuracy related to substance discrimination can be improved, and the processing time can be shortened. In addition, the system control unit 124 that executes S702 functions as an interpolation unit that performs non-linear interpolation in the non-linear region 601 and linear interpolation in the linear region 602.

[0055] As described above, embodiments of the present invention have been described. In addition, the present invention is not limited to the above-described embodiments, and constituent elements can be modified within the scope not departing from the gist of the invention and embodied. Also, a plurality of constituent elements disclosed in the above embodiments can be appropriately combined. Moreover, some constituent elements can be deleted from all the constituent elements shown in the above embodiments.

Claims

1. A PCCT device, comprising: a scanner that rotates an X-ray source that irradiates an object to be examined with X-rays and a photon-counting detector that detects X-rays transmitted through the object to be examined for each of a plurality of energy ranges around the object to be examined; and an image generation unit that generates a tomographic image using projection data calculated from the output of the photon-counting detector. The PCCT device is characterized by further comprising: a map creation unit that creates a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first base material and a second base material with known material and thickness; a region segmentation unit that divides the calibration data map into a non-linear region and a linear region; and an interpolation unit that performs non-linear interpolation in the non-linear region and linear interpolation in the linear region.

2. The PCCT device according to claim 1, wherein the region segmentation unit performs region segmentation based on the linearity of the Log value of the count value of the photons transmitted through the calibration phantom with respect to the irradiation dose to the calibration phantom.

3. The PCCT device according to claim 2, wherein the region segmentation unit sets, as the non-linear region, a region including non-linear data in which the difference between a reference line calculated based on the thickness of the first base material and the thickness of the second base material and the Log value is greater than a preset threshold value, and sets, as the linear region, a region not including the non-linear data.

4. The PCCT device according to claim 1, wherein the interpolation unit uses a high-order function in the non-linear interpolation, and the higher the non-linearity of the non-linear region, the higher the order of the high-order function is increased.

5. The PCCT device according to claim 1, wherein the interpolation unit performs linear interpolation based on the statistical error of the Log value of the count value of the photons transmitted through the calibration phantom.

6. A control method for a PCCT device, the PCCT device comprising: a scanner that rotates an X-ray source that irradiates an object to be examined with X-rays and a photon-counting detector that detects X-rays transmitted through the object to be examined for each of a plurality of energy ranges around the object to be examined; and an image generation unit that generates a tomographic image using projection data calculated from the output of the photon-counting detector. The control method is characterized by including: a map creation step of creating a calibration data map based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms having a first base material and a second base material with known material and thickness; a region segmentation step of dividing the calibration data map into a non-linear region and a linear region; and an interpolation step of performing non-linear interpolation in the non-linear region and linear interpolation in the linear region.

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