Photon counting ct device and material decomposition method

Abstract

A photon counting CT device and a material decomposition method capable of reducing the influence of variation in the amount of accumulation due to differences in the line quality of X-rays. A photon counting CT device characterized by comprising: a storage unit that, while changing the thickness of a first base material and the thickness of a second base material, acquires the energy spectrum of X-rays that have transmitted a calibration member composed of the first base material and the second base material as known materials to become a reference, and stores the X-ray amount as calibration data in advance; a correction table creation unit that, while changing the thickness of the first base material, the thickness of the second base material, and the X-ray amount, acquires the energy spectrum of X-rays that have transmitted the calibration member, and creates a correction table that indicates the relationship between the number of X-ray photons per energy bin and the X-ray amount; and an image generation unit that generates a tomographic image that has been decomposed by each material based on the calibration data and the correction table.

Description

Technical Field

[0001] This invention relates to a photon-counting CT device as an X-ray CT (Computed Tomography) device equipped with a photon-counting detector and a method for material decomposition. Background Technology

[0002] X-ray CT equipment is a device that generates tomographic images of a subject by acquiring multiple projection data through the irradiation of the subject with X-rays at multiple projection angles and the detection of X-rays transmitted through the subject. In photon-counting CT equipment that uses a photon-counting detector in the X-ray detection, tomographic images are obtained that are decomposed into tomographic images of different constituent substances, such as tomographic images of iodine contrast agents used for angiography and soft tissues.

[0003] The linear dose of X-rays irradiating the subject varies over time, and this variation causes artifacts in the tomographic images generated by an X-ray CT scanner. Therefore, artifacts in the tomographic images are suppressed by using reference data to correct the projection data. This reference data includes the temporal variation of the X-ray dose obtained by a reference detector that detects X-rays that do not penetrate the subject. In photon-counting CT scanners, as the linear dose rate of X-rays increases, the amount of accumulation also increases. Therefore, in the correction of projection data based on reference data, it is necessary to understand (calibrate) the spectral changes caused by accumulation.

[0004] Patent Document 1 discloses a photon counting CT apparatus that: pre-observes how the detection signal of a reference detector changes when the linear rate of X-rays is changed, and corrects for detection errors caused by accumulation based on the acquired data. Specifically, if the linear rate of X-rays increases, the amount of accumulation increases, thereby reducing the detection signal of the reference detector; therefore, the degree of reduction in the detection signal relative to the increase in linear rate is used to correct for detection errors caused by accumulation.

[0005] Patent Document 1: Japanese Patent No. 6564330

[0006] However, Patent Document 1 does not take into account the variation in the amount of X-ray accumulation due to differences in X-ray quality. The X-ray linearity of each channel of a photon-counting detector varies depending on the transmission path through the analyte, and the linear quality also varies. The amount of accumulation increases or decreases not only according to the linearity but also according to the variation in linear quality; therefore, if the influence of linear quality variation is not included, the accuracy of matter decomposition decreases. Summary of the Invention

[0007] Therefore, the object of the present invention is to provide a photon counting CT device and a material decomposition method that can reduce the effects of accumulation caused by differences in X-ray quality.

[0008] To achieve the above objectives, the present invention provides a photon-counting CT apparatus comprising: an X-ray source for irradiating a subject with X-rays; a photon-counting detector for detecting the X-rays and outputting a signal corresponding to the photon energy of the X-rays; and an image generation unit for generating a tomographic image of the subject based on the signal output by the photon-counting detector. The photon-counting CT apparatus is characterized by comprising: a storage unit for acquiring, while changing the thickness of a first substrate material and the thickness of a second substrate material, the energy spectrum of X-rays transmitted through a calibration component composed of the first substrate material and the second substrate material (which are known materials), using a reference X-ray dose, and pre-storing it as calibration data; a calibration table generation unit for acquiring, while changing the thickness of the first substrate material, the thickness of the second substrate material, and the X-ray dose, the energy spectrum of X-rays transmitted through the calibration component, and generating a calibration table representing the relationship between the number of X-ray photons and the X-ray dose in each energy chamber; and an image generation unit for generating a tomographic image decomposed according to each material, based on the calibration data and the calibration table.

[0009] Furthermore, the present invention is a material decomposition method, which is a material decomposition method based on a photon-counting CT device, the photon-counting CT device comprising: an X-ray source for irradiating a subject with X-rays; a photon-counting detector for detecting the X-rays and outputting a signal corresponding to the photon energy of the X-rays; and an image generation unit for generating a tomographic image of the subject based on the signal output by the photon-counting detector. The material decomposition method is characterized by comprising: a step of obtaining, while changing the thickness of a first substrate material and the thickness of a second substrate material, the energy spectrum of X-rays transmitted through a calibration component composed of the first substrate material and the second substrate material (which are known materials) with a reference X-ray amount, and pre-storing it as calibration data; a step of obtaining, while changing the thickness of the first substrate material, the thickness of the second substrate material, and the X-ray amount, the energy spectrum of X-rays transmitted through the calibration component, and creating a calibration table representing the relationship between the number of X-ray photons and the X-ray amount in each energy chamber; and a step of generating a tomographic image decomposed according to each material based on the calibration data and the calibration table.

[0010] Invention Effects

[0011] According to the present invention, a photon counting CT apparatus and a material decomposition method are provided that can reduce the effects of accumulation caused by variations in the quality of X-rays. Attached Figure Description

[0012] Figure 1 This is a diagram showing the overall structure of a PCCT device.

[0013] Figure 2 This is a diagram illustrating an example of X-rays divided into multiple energy chambers.

[0014] Figure 3 This diagram illustrates the calibration of a photon counting detector.

[0015] Figure 4 This is a diagram illustrating an example of the process for creating a calibration table.

[0016] Figure 5 This is a diagram illustrating an example of a calibration table.

[0017] Figure 6 This is a diagram illustrating an example of the processing flow for generating tomographic images broken down into individual substances.

[0018] Figure 7 This is a diagram illustrating an example of a material decomposition process.

[0019] Figure 8 This is a diagram illustrating the correction of calibration data based on reference data.

[0020] Figure 9 This is another diagram illustrating the process of material decomposition.

[0021] Figure 10 This diagram illustrates the process of determining the estimated thickness of the specimen.

[0022] Symbol Explanation

[0023] 101-Photon counting CT device, 102-Gantry, 103-X-ray tube, 104-Bow-tie filter, 105-Bed, 106-Subject, 107-Detector panel, 107R-Reference detector, 108-Arithmetic unit, 109-Input device, 110-Display device, 111-X-ray, 112-Opening, 300-Calibration component, 301-First substrate material, 302-Second substrate material, 303-Calibration data, P-Detection element. Detailed Implementation

[0024] Hereinafter, embodiments of the photon counting CT apparatus and matter decomposition method of the present invention will be described with reference to the accompanying drawings. Furthermore, in the following description and drawings, components having the same functional structure are labeled with the same symbols, thereby omitting repeated descriptions. [Example 1]

[0025] Figure 1The diagram shows the overall structure of the photon counting CT apparatus 101 of this embodiment. The horizontal plane of the paper is designated as the X-axis, the vertical plane as the Y-axis, and the direction orthogonal to the XY plane as the Z-axis. The photon counting CT apparatus 101 includes a gantry 102, an X-ray tube 103, a bow filter 104, a bed 105, a detector panel 107, a processing unit 108, an input device 109, and a display device 110.

[0026] Subject 106 is placed on bed 105 and positioned in opening 112 provided in frame 102. X-rays 111 emitted from X-ray tube 103 are shaped into a beam shape suitable for the size of subject 106 by bow filter 104 and irradiate subject 106. After passing through subject 106, they are detected by detector panel 107. X-ray tube 103 and detector panel 107 are mounted on frame 102 in an opposing configuration that clamps subject 106, and rotate around subject 106 by rotation drive of frame 102. X-ray irradiation from X-ray tube 103 and X-ray measurement in detector panel 107 are repeated along with the rotation of both, thereby acquiring projection data at various projection angles.

[0027] By performing image reconstruction processing on the acquired projection data using the computing device 108, a tomographic image of the subject 106 is generated and displayed on the display device 110. Furthermore, if projection data is acquired while the bed 105 carrying the subject 106 and the gantry 102 move relative to each other in the Z-axis direction, a volumetric image of the subject 106 is generated. Additionally, the amount of X-rays irradiated from the X-ray tube 103, the rotational speed of the gantry 102, and the relative movement speed between the gantry 102 and the bed 105 are set according to the scanning conditions input by the operator via the input device 109. Moreover, the computing device 108 has the same hardware structure as a general computer device, including a CPU (Central Processing Unit), memory, HDD (Hard Disk Drive), etc., and performs correction processing on the projection data and controls various components.

[0028] Regarding the detector panel 107, a plurality of detection elements P are configured in an arc shape centered on the X-ray focal point of the X-ray tube 103. Furthermore, at the end of the detector panel 107, a reference detector 107R is provided for detecting X-rays that do not penetrate the subject 106. Since the reference data obtained by the reference detector 107R includes the time-varying amount of X-rays irradiating the subject 106, the reference data is used for correction of the time-varying amount of X-rays.

[0029] When the detection element P is a photon counting detector, the incident X-ray photons are counted and the energy of the X-ray photons is measured. Figure 2 The figure shows the number of X-ray photons counted by dividing the energy into three energy chambers bin1, bin2, and bin3: T1~T2, T2~T3, and T3~.

[0030] In the photon-counting CT apparatus 101 equipped with a photon-counting detector, the photon energy spectrum related to the projection data of the subject 106 can be acquired, thus enabling the generation of medical images decomposed into different constituent substances or medical images divided into multiple energy components. Furthermore, when obtaining medical images decomposed into different constituent substances, it is necessary to pre-calibrate the relationship between the output and photon energy of the photon-counting detector when measuring a combination of multiple substrate materials with known composition and thickness using a photon-counting detector in each detection element.

[0031] use Figure 3 The calibration of a photon-counting detector is described below. In the calibration of the photon-counting detector, a calibration component 300 is used, which combines multiple substrate materials with known composition and thickness. The calibration component 300 may, for example, combine a first substrate material 301 and a second substrate material 302, and multiple plates of different thicknesses are used for each substrate material. If, for example, the thickness of the first substrate material 301 is type J and the thickness of the second substrate material 302 is type K, then a J×K type calibration component 300 is used, and the energy spectrum of X-rays transmitted through each calibration component 300 is obtained as calibration data 303 in each detection element. Figure 3 Since J=3 and K=3, nine energy spectra are shown as calibration data 303. The acquired calibration data 303 is stored in the storage unit of the computing device 108 and used for calibration of the projection data of the test subject 106.

[0032] Furthermore, since an increase in the amount of X-rays irradiating the calibration component 300 results in a greater accumulation, the energy spectrum of the X-rays transmitted through the calibration component 300 also changes depending on the amount of X-rays. Here, in this invention, a correction table is created based on the energy spectrum obtained while changing the amount of X-rays irradiating the calibration component 300, for correcting the calibration data 303 or the projection data of the subject 106.

[0033] use Figure 4 An example of the process for creating a calibration table is explained step by step.

[0034] (S401)

[0035] A calibration component 300, which is a combination of a first substrate material 301 and a second substrate material 302, is provided in the opening 112. The composition and thickness of the first substrate material 301 and the second substrate material 302 are known, for example, acrylic acid is used in the first substrate material 301 and aluminum is used in the second substrate material 302.

[0036] (S402)

[0037] By controlling the computing device 108, the energy spectrum of X-rays that have transmitted through the calibration component 300 set in S401 is acquired with a reference X-ray dose. More specifically, with the gantry 102 stationary, X-rays are irradiated onto the calibration component 300 from an X-ray tube 103 with a tube current set as a reference value. The X-rays that have transmitted through the calibration component 300 are detected by the detector panel 107, thereby acquiring the energy spectrum for each detection element. The acquired energy spectrum is stored in the storage unit as calibration data 303. In addition, X-rays that do not transmit through the calibration component 300 are detected in the reference detector 107R, and a corresponding association is established with the calibration data 303 and stored in the storage unit.

[0038] (S403)

[0039] By controlling the computational device 108, the energy spectrum of X-rays that have transmitted through the calibration component 300 set in S401 is obtained from the X-ray quantity that varies from the reference. More specifically, the energy spectrum is obtained in each detection element by X-ray irradiation from an X-ray tube 103 with a tube current set to a different value than the reference value and X-ray detection based on the detector panel 107. In the reference detector 107R, X-rays that do not transmit through the calibration component 300 are detected in the same manner as in S402. The tube current set in S403 is preferably in the range of 295 mA to 305 mA when the reference value of the tube current is 300 mA. By setting the tube current to a range that corresponds to the variation of the X-ray quantity over time, the variation of the X-ray quantity over time can be corrected more accurately.

[0040] (S404)

[0041] The processing unit 108 uses the energy spectrum acquired in S402 and S403 to create a calibration table. The calibration table represents the relationship between the number of X-ray photons and the amount of X-rays in each energy chamber. The created calibration table is stored in the storage unit.

[0042] use Figure 5 An example of a correction table will be explained. Figure 5The calibration table illustrated in the figure has a calibration coefficient on the vertical axis and a count realization value on the horizontal axis. The count realization value is the measured value of the number of X-ray photons during X-ray irradiation at the tube current set in S402 or S403. Furthermore, the calibration coefficient is obtained by dividing the number of X-ray photons counted in each count realization value by the number of X-ray photons when the count realization value is the reference value, and is calculated for each energy chamber. That is, when the count realization value is the reference value, the calibration coefficient represents 1; when the count realization value is a slight increase, the calibration coefficient represents a value greater than 1; and when the count realization value is a slight decrease, the calibration coefficient represents a value less than 1.

[0043] In addition, a calibration table is made in a corresponding quantity to the number of calibration components 300 for each type of detection element. Furthermore, the calibration table can be a linear approximation of the number of X-ray photons counted by different tube currents or an approximation using a quadratic curve.

[0044] (S405)

[0045] The processing unit 108 determines whether the energy spectrum has been acquired for all calibration components 300. If the energy spectrum has been acquired for all calibration components 300, the processing flow ends; otherwise, the processing returns to S401, and another calibration component 300 is set.

[0046] By using Figure 4 The described processing flow involves changing the thickness of the first substrate material 301 (a known material), the thickness of the second substrate material 302, and the amount of X-rays irradiated onto the calibration component 300, while acquiring the energy spectrum of the X-rays transmitted through the calibration component 300. Furthermore, based on the acquired energy spectrum, calibration data 303 and a calibration table are stored in a storage unit and used for material decomposition of the test subject 106.

[0047] use Figure 6 An example of the processing flow for generating tomographic images of each substance based on calibration data 303 and the correction table is described step by step.

[0048] (S601)

[0049] Projection data of the subject 106 is acquired through control based on the computing device 108. More specifically, with the gantry 102 rotated, X-rays are irradiated onto the subject 106 through the X-ray tube 103, and the X-rays transmitted through the subject 106 are detected by the detector panel 107, thereby acquiring projection data at various projection angles. Furthermore, the projection data of the subject 106 is acquired by dividing it into multiple energy chambers. Reference data is also acquired along with the projection data of the subject 106. The reference data obtained by detecting X-rays that do not transmit through the subject 106 includes the variation of the amount of X-rays irradiating the subject 106 over time.

[0050] (S602)

[0051] The processing unit 108 decomposes the projection data of the test subject 106 acquired in S601 into multiple substances, such as the first substrate material 301 and the second substrate material 302, based on the calibration data 303 and the calibration table read from the storage unit.

[0052] use Figure 7 An example of the processing flow of S602 is explained step by step.

[0053] (S701)

[0054] The processing unit 108 corrects the calibration data 303 based on the reference data and calibration table obtained in S601. For example, as Figure 8 As shown, with reference data of 303 mA, the correction coefficient C_A for energy chamber A and the correction coefficient C_B for energy chamber B are obtained from the correction table, and the calibration data 303 is corrected according to the correction coefficient C_A or C_B. That is, the calibration data to be corrected is calculated by multiplying the number of X-ray photons in energy chamber A (obtained with reference X-ray dose) by C_A, and the number of X-ray photons in energy chamber B by C_B.

[0055] (S702)

[0056] The processing unit 108 uses the calibration data corrected in S701 to perform material decomposition on the projection data of the test subject 106. Specifically, firstly, it selects the energy spectrum that is closest to the projection data of the test subject 106 from the corrected calibration data. Then, it performs material decomposition by measuring the thickness of the first substrate material 301 and the thickness of the second substrate material 302 corresponding to the selected energy spectrum, generating projection data of the first substrate material 301 and projection data of the second substrate material 302.

[0057] By using Figure 7 The described processing flow uses calibration data corrected according to reference data and a calibration table to decompose the projection data of the subject 106. Since the calibration data corrected according to reference data and a calibration table is used for decomposition, the effects of variations in X-ray dose over time or accumulation due to differences in X-ray quality are reduced, thus improving the accuracy of decomposition. Furthermore, the decomposition of matter based on calibration data 303 and a calibration table in S602 is not limited to... Figure 7 The processing flow.

[0058] use Figure 9 Here is another example of the S602 processing flow, step by step.

[0059] (S901)

[0060] The processing unit 108 corrects the projection data of the test object 106 based on the reference data and calibration table obtained in S601. Specifically, firstly, using the data that the thickness of the second substrate material 302 is zero in the calibration data 303, it calculates the projection data of the test object 106. Figure 10 The relationship between the thickness of the first substrate material 301 and the total number of X-ray photons is shown. Next, by... Figure 10 The relationship illustrated in the diagram is compared with the projection data of the subject 106 to calculate the estimated thickness T_est of the subject 106. Furthermore, from the correction table in the estimated thickness T_est, correction factors for the reference data are calculated for each energy chamber, and the projection data of the subject 106 is corrected based on these correction factors. That is, by dividing the projection data of the subject 106, which includes the time-varying X-ray dose, by the correction factor for each energy chamber, projection data of the subject 106 equivalent to the reference X-ray dose is calculated.

[0061] (S902)

[0062] The processing unit 108 uses calibration data 303 to perform material decomposition on the projection data corrected in S901. Specifically, firstly, it selects the energy spectrum from the calibration data 303 that is closest to the energy spectrum of the projection data corrected in S901. Then, it performs material decomposition by measuring the thickness of the first substrate material 301 and the thickness of the second substrate material 302 corresponding to the selected energy spectrum, thereby generating projection data of the first substrate material 301 and projection data of the second substrate material 302.

[0063] By using Figure 9 The described processing flow involves decomposing the substance using the projection data of the test subject 106, corrected according to reference data and a calibration table, based on calibration data 303. Since the projection data of the test subject 106, corrected according to reference data and a calibration table, is used for substance decomposition, the effects of variations in X-ray dose over time or accumulation due to differences in X-ray quality are reduced, thus improving the accuracy of substance decomposition. Furthermore, compared to using… Figure 7 Compared to the described processing flow, the computing device 108 uses less memory and can shorten the computation time. Return to Figure 6 Explanation.

[0064] (S603)

[0065] The processing unit 108 uses the projection data obtained from the material decomposition in S602 to reconstruct the tomographic image of each material. That is, it reconstructs the tomographic images of the first substrate material 301 and the second substrate material 302.

[0066] By using Figure 6The described processing flow uses projection data derived from substance decomposition based on calibration data 303 and a calibration table to generate tomographic images of the subject 106 for each substance. Because calibration data 303 and a calibration table are used in the substance decomposition, the effects of accumulation due to variations in X-ray quality are reduced, enabling the generation of high-precision tomographic images derived from substance decomposition.

[0067] The embodiments of the photon counting CT device and matter decomposition method of the present invention have been described above. Furthermore, the present invention is not limited to the above embodiments, and can be embodied by modifying the constituent elements without departing from the spirit of the invention. Moreover, multiple constituent elements disclosed in the above embodiments can be appropriately combined. Furthermore, some constituent elements can be deleted from all the constituent elements shown in the above embodiments.

Claims

1. A photon-counting CT apparatus, comprising: an X-ray source for irradiating a subject with X-rays; a photon-counting detector for detecting the X-rays and outputting a signal corresponding to the photon energy of the X-rays; and an image generation unit for generating a tomographic image of the subject based on the signal output by the photon-counting detector. The photon counting CT device is characterized by having: The storage unit, while changing the thickness of the first substrate material and the thickness of the second substrate material, acquires the energy spectrum of X-rays transmitted through the calibration component, which is composed of the first substrate material and the second substrate material, which are known materials, at a reference X-ray amount, and stores it in advance as calibration data. The calibration table production unit, while changing the thickness of the first substrate material, the thickness of the second substrate material, and the X-ray dose, acquires the energy spectrum of the X-rays transmitted through the calibration component, and produces a calibration table showing the relationship between the number of X-ray photons and the X-ray dose in each energy chamber; and The image generation unit generates tomographic images broken down into individual substances based on the calibration data and the correction table. The calibration table fabrication unit acquires the energy spectrum of X-rays transmitted through the calibration component within the range of X-ray quantity variation over time.

2. The photon counting CT device according to claim 1, characterized in that, The image generation unit corrects the calibration data according to the calibration table and reference data, and uses the corrected calibration data to perform material decomposition on the projection data of the subject.

3. The photon counting CT device according to claim 1, characterized in that, The image generation unit corrects the projection data of the subject according to the calibration table and reference data, and uses the calibration data to perform material decomposition on the corrected projection data.

4. A method for decomposing matter, which is a method for decomposing matter based on a photon-counting CT device, the photon-counting CT device comprising: an X-ray source for irradiating a subject with X-rays; a photon-counting detector for detecting the X-rays and outputting a signal corresponding to the photon energy of the X-rays; and an image generation unit for generating a tomographic image of the subject based on the signal output by the photon-counting detector. The method for decomposing substances is characterized by having: While changing the thickness of the first substrate material and the thickness of the second substrate material, the energy spectrum of X-rays transmitted through the calibration component, which is composed of the first substrate material and the second substrate material, which are known materials, is obtained with a reference X-ray amount, and is stored in advance as calibration data. The steps include: simultaneously changing the thickness of the first substrate material, the thickness of the second substrate material, and the X-ray intensity, acquiring the energy spectrum of X-rays transmitted through the calibration component, and creating a calibration table showing the relationship between the number of X-ray photons and the X-ray intensity in each energy chamber; and The step of generating tomographic images based on the calibration data and the correction table, broken down into individual substances. In the step of creating the calibration table, the energy spectrum of the X-rays transmitted through the calibration component is obtained within the range of changes in X-ray amount over time.

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