Photon-counting x-ray CT device and calibration data acquisition method

Abstract

To enable acquisition of calibration data by minimizing additional mechanisms to a device and reducing user work.SOLUTION: A second phantom 303 of a second base material having different thickness portions is installed on a filter 105 of a photon-counting X-ray CT device, the filter is moved by a filter drive mechanism 304 so that the X-rays 104 from an X-ray tube 103 pass through a desired thickness portion of the second phantom, and an arithmetic device 111 causes the X-ray tube to emit X-rays in a state where a first phantom 301 of a first base material having a linear attenuation coefficient smaller than that of the second base material is inserted into an opening 109 so that the X-rays can pass through, and acquires projection data based on the detection value output by a detector 108.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a photon-counting X-ray CT apparatus and a method for acquiring calibration data therefor. [Background technology]

[0002] Photon-counting X-ray computed tomography (CT) devices use photon-counting detectors. Photon-counting detectors can discriminate the energy of incident radiation photons and count the number of incident radiation photons for each energy range (bin). This allows photon-counting X-ray CT devices to obtain medical images in which materials with different compositions are differentiated, such as medical images in which iodine contrast agents used in angiography are differentiated from calcified plaques in blood vessels. To obtain such material-discriminated medical images, it is necessary to obtain calibration data for each detector element, which indicates the relationship between the output and photon energy when a phantom composed of a combination of multiple base materials with known compositions and thicknesses is measured using the photon-counting detector. For example, Patent Document 1 discloses a phantom, a radiation imaging device, and a method for calibrating a photon-counting detector that can shorten the time required to acquire configuration data even for a large irradiation field. Patent document 1 discloses that a phantom is used in combination with a first basis material and a second basis material, and the phantom of the first basis material and the phantom of the second basis material are driven separately, so that the phantom of the first basis material is driven by a bed and the phantom of the second basis material is driven by a dedicated driving unit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2022-190306 Summary of the Invention [Problem to be solved by the invention]

[0004] The phantom used to acquire calibration data is made of a combination of multiple basis materials with known compositions and thicknesses, for example, a combination of two basis materials, a first basis material and a second basis material. To acquire calibration data using multiple phantoms with different thicknesses of the combined basis materials, it is necessary to acquire projection data by switching between phantoms. On the other hand, if multiple phantoms are combined into a single phantom, such as the stepped phantom disclosed as a conventional technique in Patent Document 1, the phantom becomes huge and difficult to handle.

[0005] The technology disclosed in Patent Document 1 makes it possible to reduce the user's workload for acquiring calibration data by driving the phantom of the first basis material and the phantom of the second basis material separately, but this requires a new driving unit for moving the phantom of the second basis material.

[0006] The present invention enables the acquisition of calibration data by using the mechanism of a photon-counting X-ray CT apparatus to also move the phantom, minimizing the need for additional mechanisms in the apparatus and reducing the user's workload. [Means for solving the problem]

[0007] A photon-counting X-ray CT apparatus according to one embodiment of the present invention comprises an X-ray tube, a filter, a filter drive mechanism for driving the filter, a detector equipped with a photon-counting detection element, a gantry mounted so that the X-ray tube, filter drive mechanism, and detector are arranged facing each other across an opening, and a computing device, wherein a second phantom made of a second basis material and having a different thickness portion is placed on the filter, and the computing device moves the filter using the filter drive mechanism so that X-rays from the X-ray tube pass through a desired thickness portion of the second phantom, and the computing device inserts a first phantom made of a first basis material having a smaller linear attenuation coefficient than the second basis material into the opening so that X-rays from the X-ray tube pass through, and then emits X-rays from the X-ray tube and acquires projection data based on the detection values ​​output by the detector. [Effects of the Invention]

[0008] The present invention provides a photon-counting X-ray CT apparatus and a calibration data acquisition method that enable acquisition of calibration data with minimal additional mechanisms added to the apparatus and reduced user workload. Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram illustrating the overall configuration of a photon-counting X-ray CT apparatus. [Figure 2A] FIG. 10 is a diagram for explaining calibration of a photon counting detector. [Figure 2B] FIG. 10 is a diagram for explaining calibration of a photon counting detector. [Figure 3] FIG. 4 is a diagram for explaining a method for acquiring calibration data in the first embodiment. [Figure 4] FIG. 10 is a diagram schematically illustrating the configuration of a bowtie filter in which a second phantom is placed. [Figure 5] FIG. 10 is a diagram for explaining a method for acquiring calibration data according to the second embodiment. [Figure 6A] FIG. 10 is a diagram for explaining a method for acquiring calibration data according to the third embodiment. [Figure 6B] FIG. 10 is a diagram for explaining a method for acquiring calibration data according to the third embodiment. [Figure 7] FIG. 10 is a diagram for explaining a method for acquiring calibration data according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] 1 shows the overall configuration of a photon-counting X-ray CT system 101. The horizontal direction of the page is the X-axis, the vertical direction is the Y-axis, and the direction perpendicular to the XY plane is the Z-axis. The X-ray CT system 101 mainly comprises a gantry 102, an X-ray tube 103, a bowtie filter 105, a bed 106, a detector 108, and a console 110.

[0011] An object 107 is placed on a bed 106 and placed in an opening 109 provided in the gantry 102. X-rays 104 emitted from an X-ray tube 103 are shaped into a beam suitable for the size of the object 107 by a bowtie filter 105, and then irradiated onto the object 107. After passing through the object 107, the X-ray tube 103 and the detector 108 are detected by a detector 108. The X-ray tube 103 and the detector 108 are attached to the gantry 102 so as to face each other with the object 107 in between, and are rotated around the object 107 by a rotary drive unit of the gantry 102. X-ray irradiation from the X-ray tube 103 and X-ray measurement by the detector 108 are repeated together with rotation by the rotary drive unit, thereby acquiring projection data at various projection angles.

[0012] The console 110 includes a calculation device 111, a display device 112, and an input device 113. The calculation device 111 performs image reconstruction processing on the acquired projection data to generate a tomographic image of the subject 107, which is then displayed on the display device 112. When projection data is acquired while the bed 106 on which the subject 107 is placed and the gantry 102 are moving relatively in the Z-axis direction, a volume image of the subject 107 is generated. The amount of X-rays irradiated from the X-ray tube 103, the rotation speed of the gantry 102, and the relative movement speed between the gantry 102 and the bed 106 are set based on scan conditions input by the operator via the input device 113.

[0013] The detector 108 is composed of a plurality of detection elements arranged in an arc shape centered on the X-ray focal point of the X-ray tube 103. The detection elements are photon-counting detection elements that measure X-ray energy, which is the energy of incident X-ray photons, and output a detection value corresponding to the X-ray energy. The X-ray CT device 101 equipped with such a photon-counting detector can acquire an X-ray energy spectrum related to projection data of the subject 107, and can therefore generate medical images in which materials with different compositions are differentiated or medical images separated into multiple energy components.

[0014] The arithmetic device 111 has the same hardware configuration as a general computer device, and includes a CPU (Central Processing Unit), memory, and non-volatile storage devices such as an HDD (Hard Disk Drive), and controls each part and performs correction processing on projection data, etc.

[0015] Calibration of a photon-counting detector will be described with reference to FIGS. 2A and 2B. To calibrate a photon-counting detector, as shown in FIG. 2A, a phantom 201 is placed between the X-ray tube 103 and the detector 108, and projection data is acquired to provide calibration data. The phantom 201 uses a combination of multiple basis materials with known compositions and thicknesses, such as a first basis material 202 and a second basis material 203. While the basis materials are not particularly limited, when a human subject is considered, acrylic or polyethylene can be selected as the first basis material 202, as a material with a linear attenuation coefficient similar to that of water, and tin or titanium can be selected as the second basis material 203, as a material with a linear attenuation coefficient similar to that of bone or a contrast agent. Furthermore, multiple combinations of the first basis material 202 and the second basis material 203, each with a different thickness, are prepared for the phantom 201. For example, if the first basis material 202 has J different thicknesses and the second basis material 203 has K different thicknesses, a phantom that is a combination of J×K different basis materials is used, and a photon energy spectrum is obtained for each combination for each detection element.

[0016] In the example of FIG. 2B, J=3 and K=3, so nine types of photon energy spectra are shown as the calibration data 204. The acquired calibration data 204 is stored in the memory of the calculation device 111 and used to calibrate the projection data of the subject 107. In this example, basis materials of different thicknesses are obtained by combining multiple first basis materials 202 and multiple second basis materials 203 of the same thickness, but basis materials of different thicknesses may also be prepared. In either case, it is desirable that the thickness of each basis material constituting the phantom 201 be an integer multiple of the thinnest thicknesses d1 and d2. This reduces the computational load for calibrating the projection data. [Example]

[0017] A method for acquiring calibration data for the photon-counting detector in Example 1 will be described with reference to Figure 3. In this example, the phantom used to calibrate the photon-counting detector is a combination of a first phantom 301 made of a first basis material with a small linear attenuation coefficient and a second phantom 303 made of a second basis material with a large linear attenuation coefficient, and the second phantom 303 is placed on the bowtie filter 105. When acquiring calibration data, the plate-shaped first phantom 301 having a desired thickness is inserted into the opening 109 by the dolly 302, and the second phantom 303 placed on the bowtie filter 105 is moved by the filter drive mechanism 304 so that the X-rays 104 from the X-ray tube 103 pass through the second phantom 303 having the desired thickness.

[0018] FIG. 4 shows a schematic configuration of the bowtie filter 105 in which the second phantom 303 is disposed. The bowtie filter is provided to uniformize the dose of X-rays 104 reaching the detector 108 and to suppress unnecessary radiation exposure due to X-rays reaching areas outside the FOV (Field of View). For this reason, it is desirable to change the shape of the bowtie filter according to the FOV. Therefore, the bowtie filter 105 has portions with different shapes as shown in the AA and BB cross sections, and a filter driving mechanism 304 makes it possible to adjust the position of the bowtie filter 105 through which the X-rays 104 pass. In this embodiment, this mechanism is also used to adjust the thickness of the second phantom 303.

[0019] A second phantom 303 having portions of different thicknesses is placed on the bowtie filter 105. In FIG. 4, the second phantom 303 is a stack of second basis materials of the same thickness stacked at offset positions. That is, the CC cross section uses one layer of second basis material, and the DD cross section uses three layers of second basis material, thereby varying the thickness of the second phantom 303. The means for fixing the second phantom 303 to the bowtie filter 105 is not limited as long as it does not interfere with the area through which X-rays 104 pass through the bowtie filter 105 and / or the second phantom 303. When the second basis material is tin, the thickness of one layer of the second basis material can be on the order of 0.01 mm. Therefore, fixing the second phantom 303 to the filter drive mechanism 304 does not adversely affect the drive of the bowtie filter 105 by the filter drive mechanism 304. Further, although an example in which second phantom 303 is placed on the X-ray tube 103 side of bowtie filter 105 has been shown here, second phantom 303 may also be placed on the opening 109 side of bowtie filter 105 .

[0020] In the example of FIG. 4 , the CC and DD planes have the same filter shape as the AA plane. The filter drive mechanism 304 simultaneously moves both the bowtie filter 105 and the second phantom 303, thereby changing the thickness of the first phantom 301 and the thickness of the second phantom 303 relative to the bowtie filter 105 in the AA plane and obtaining calibration data. Meanwhile, calibration data obtained by changing the thickness of the first phantom 301 and the thickness of the second phantom 303 relative to the bowtie filter 105 in the BB plane can be calculated as follows. First, the change in X-ray radiation due to differences in filter shape is calculated from the projection data obtained by transmitting X-rays to change the thickness of the first phantom 301 in the BB plane and the projection data obtained by transmitting X-rays to change the thickness of the first phantom 301 in the AA plane. Using the change in X-ray radiation due to the calculated filter shape and the projection data for each thickness of the second phantom 303 relative to the bowtie filter 105 in the AA plane, the projection data obtained by changing the thickness of the first phantom 301 and the thickness of the second phantom 303 relative to the bowtie filter 105 in the BB plane can be calculated. In addition, calibration data for the BB cross section is obtained taking into consideration the nonlinear response of the detector 108 that depends on the X-ray dose.

[0021] In the first embodiment, the thickness of the second phantom 303 can be changed by the filter driving mechanism 304, and the user only needs to replace the first phantom 301 with the opening 109. This reduces the amount of work required by the user to obtain calibration data.

[0022] Although an example in which the second phantom 303 is placed on the bowtie filter 105 has been shown here, the second phantom 303 may be placed on a filter other than the bowtie filter 105. For example, the second phantom 303 may be placed on a low-energy X-ray removal filter. The low-energy X-ray removal filter is a filter for changing the spectrum of the X-rays 104, and the same effect can be obtained by placing the second phantom 303 on the low-energy X-ray removal filter and changing the thickness of the second phantom 303 using its drive mechanism. The same applies to the following examples.

[0023] 4 shows an example in which second phantom 303 and bowtie filter 105 are moved together by filter driving mechanism 304, and calibration data for the BB cross section where second phantom 303 does not overlap is obtained by calculation, but second phantom 303 and bowtie filter 105 may also be driven separately. In this case, calibration data for the BB cross section can be obtained by actual measurement. Furthermore, second phantom 303 of different thicknesses may be configured to overlap the AA cross section shape portion and the BB cross section shape portion of bowtie filter 105, respectively, so that calibration data for the BB cross section can be measured actually. [Example]

[0024] A method for acquiring calibration data for a photon counting detector in Example 2 will be described with reference to Fig. 5. In Example 1, in order to change the thickness of first phantom 301, work is required to replace first phantoms 301 of different thicknesses into opening 109. In Example 2, a dolly 302b equipped with a phantom driving mechanism (not shown) is used to insert first phantoms 301b having portions of different thicknesses into opening 109, thereby further reducing the workload of the user.

[0025] 5, first phantom 303b is a laminate formed by stacking plate-shaped first basis materials of the same thickness at offset positions. The user controls filter drive mechanism 304 and the phantom drive mechanism of cart 302b in conjunction with each other to change the thicknesses of first phantom 301b and second phantom 303, through which X-rays 104 pass, thereby changing the combination of the thickness of first phantom 301b and the thickness of second phantom 303 and acquiring calibration data. This further reduces the amount of work required by the user to acquire calibration data. [Example]

[0026] 6A, 6B, and 7, a method for acquiring calibration data for a photon counting detector in Example 3 will be described. FIG. 6B shows an EE cross section (a cross section parallel to the XY plane) with first phantom 401 inserted into opening 109. First phantom 401 is inserted into opening 109 by dolly 302 so as not to overlap with the center of rotation of gantry 102. First phantom 401 has a cylindrical shape, and the diameter of the cross section of first phantom 401 is less than the radius of opening 109.

[0027] In the third embodiment, calibration data is acquired while rotating the gantry 102. Fig. 7 shows the state of the X-ray tube 103, the second phantom 303, the first phantom 401, and the detector 108 when the calibration data is acquired. Since the shape and size of the first phantom 401 and its position in the opening 109 are known, the length of the portion where the line connecting the X-ray tube 103 and each detection element of the detector 108 overlaps with the first phantom 401 is calculated as the transmission length of the first phantom 401. Fig. 7 also shows the transmission length 501 of the detection element of ch1 and the transmission length 502 of the detection element of ch2 when the projection angle is θ1. The transmission length of the detection element of ch3, where the X-rays reach without passing through the first phantom 401, is 0 (zero).

[0028] In this case, the user can acquire calibration data by changing the combination of the transmission length of the first phantom 401 and the thickness of the second phantom 303 by changing the thickness of the second phantom 303 using the filter drive mechanism 304 and repeating the process of acquiring calibration data by rotating the gantry 102. This further reduces the amount of work required by the user to acquire calibration data.

[0029] Although an example of a cylindrical first phantom is shown here, the cross section of the first phantom is not limited to a circle and may be a polygon, in which case a polygon with an acute interior angle is preferable. In this case as well, the circumscribing circle of the polygon that is the cross section of the first phantom has a diameter that is less than the radius of opening 109, and is inserted into opening 109 so that the center of the circumscribing circle does not overlap with the center of rotation of gantry 102.

[0030] The present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations. [Explanation of symbols]

[0031] 101: Photon counting X-ray CT device, 102: Gantry, 103: X-ray tube, 104: X-ray, 105: Bowtie filter, 106: Bed, 107: Subject, 108: Detector, 109: Opening, 110: Console, 111: Computing unit, 112: Display unit, 113: Input unit, 201: Phantom, 202: First basis material, 203: Second basis material, 204: Calibration data, 301, 401: First phantom, 302: Cart, 303: Second phantom, 304: Filter drive mechanism, 501, 502: Transmission length.

Claims

1. An X-ray tube; A filter, a filter driving mechanism that drives the filter; a detector having a photon counting type detection element; a gantry on which the X-ray tube and the filter drive mechanism and the detector are mounted so as to be disposed opposite each other with an opening therebetween; a computing device; A second phantom of a second basis material having a different thickness is placed on the filter; the arithmetic unit moves the filter by the filter drive mechanism so that the X-rays from the X-ray tube pass through a portion of the second phantom with a desired thickness; the computing device is a photon-counting X-ray CT device that, while inserting a first phantom made of a first basis material having a smaller linear attenuation coefficient than the second basis material into the opening so that X-rays from the X-ray tube can pass through, radiates X-rays from the X-ray tube, and acquires projection data based on detection values ​​output by the detector.

2. In claim 1, The calculation device acquires the projection data for each combination of a plurality of thicknesses of the first basis material and a plurality of thicknesses of the second basis material, and obtains calibration data for the detector.

3. In claim 2, the second phantom is a laminate of the second basis material of the same thickness; The difference in thickness of the second phantom is due to the number of layers of the second base material.

4. In claim 2, the first phantom is a laminate of the first base material of the same thickness; The difference in thickness of the first phantom is due to the number of layers of the first base material.

5. In claim 2, the first phantom is the first basis material in a columnar shape, and is inserted into the opening so as not to overlap with the rotation center of the gantry; The calculation device is a photon counting X-ray CT device that acquires the projection data while rotating the gantry.

6. In claim 5, A photon counting X-ray CT device, wherein the cross section of the first phantom is circular or polygonal.

7. In claim 1, The photon counting X-ray CT device, wherein the filter is a bowtie filter.

8. A method for acquiring calibration data for a photon-counting X-ray CT apparatus including an X-ray tube, a filter, a filter drive mechanism for driving the filter, a detector having a photon-counting detection element, a gantry mounted so that the X-ray tube, the filter drive mechanism, and the detector are disposed opposite each other with an opening therebetween, and a computing device, comprising: placing a second phantom of a second basis material having portions of different thickness on the filter; the arithmetic unit moves the filter by the filter drive mechanism so that the X-rays from the X-ray tube pass through a portion of the second phantom with a desired thickness; the calculation device inserts a first phantom made of a first basis material having a linear attenuation coefficient smaller than that of the second basis material into the opening so that the X-rays from the X-ray tube can pass through, causes the X-ray tube to emit X-rays, and acquires projection data based on detection values ​​output by the detector.

9. In claim 8, the first phantom is the first basis material in a columnar shape, and is inserted into the opening so as not to overlap with the rotation center of the gantry; The calculation device acquires the projection data while rotating the gantry.

10. In claim 9, A calibration data acquisition method, wherein the cross section of the first phantom is circular or polygonal.

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