PCCT device and control method for the same

Abstract

To provide a PCCT device capable of obtaining a spectrum that passes through a base material with negative thickness without measuring the spectrum of any material other than the base material, and a control method for the device.SOLUTION: A PCCT device including an X-ray source irradiating a subject with X rays, a scanner turning a photon counting type detector that detects the X-rays passing through the subject for each of a plurality of energy bins around the subject, and an image generator using projection data calculated on the basis of an output of the photon counting type detector to generate a tomographic image is characterized by further having: a map generation unit generating a calibrated data map on the basis of the number of X-ray photons passing through each of a plurality of calibration phantoms having a first base material and a second base material with known materials and thickness; an expansion unit performing extrapolation processing on the number of photons of the X-rays to generate an expanded data map which is a calibrated data map expanded in a negative area; and a material discrimination part discriminating a material on the basis of the expanded data map.SELECTED DRAWING: Figure 4

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 particularly relates to an extension process of calibration data used for 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 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 diagnosing the subject.

[0003] In a PCCT apparatus using a photon counting detector as the X-ray detector, since the number of X-ray photons can be counted for each energy, substances in the subject can be discriminated using the projection values obtained in each of a plurality of energy bins. In order to obtain a tomographic image in which substances are discriminated, it is necessary to previously acquire calibration data for substance discrimination using a plurality of basis substances that are substances with known composition and thickness.

[0004] Patent Document 1 discloses estimating the thickness of basis substances in a sample by comparing a calibration spectrum, which is an energy spectrum of X-ray photons transmitted through a combination of basis substances with known composition and thickness, with the spectrum transmitted through the sample. That is, the one most similar to the spectrum of the sample is searched for among a plurality of calibration spectra measured for each combination of basis substances with different thicknesses, and the thickness of the basis substance corresponding to the searched calibration spectrum is estimated as the thickness of the basis substances in the sample. It is also disclosed that the number of calibration spectra is limited by interpolating the calibration spectra. Further, in order to estimate the thickness of a sample having an effective atomic number outside the range of the effective atomic numbers of the basis substances, it is disclosed that a spectrum transmitted through a basis substance with a negative thickness is obtained by measuring the spectra of substances other than the basis substances.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in Patent Document 1, in order to obtain a spectrum that has passed through a substrate with a negative thickness, it is necessary to measure the spectrum of a substance other than the substrate.

[0007] Therefore, an object of the present invention is to provide a PCCT apparatus and a control method thereof that can obtain a spectrum that has passed through a substrate with a negative thickness without measuring the spectrum of a substance other than the substrate.

Means for Solving the Problems

[0008] To achieve the above object, the present invention provides a PCCT apparatus including an X-ray source that irradiates a subject with X-rays, a scanner that rotates around the subject and a photon counting type detector that detects the X-rays transmitted through the subject for each of a plurality of energy bins, and an image generation unit that generates a tomographic image using projection data calculated based on the output of the photon counting type detector. The PCCT apparatus further includes a map creation unit that creates a calibration data map based on the number of X-ray photons passing through each of a plurality of calibration phantoms having a first substrate and a second substrate with known material and thickness, an expansion unit that creates an expanded data map by extrapolating the number of X-ray photons and expanding the calibration data map into a negative region, and a substance discrimination unit that performs substance discrimination based on the expanded data map.

[0009] The present invention also relates to a control method for a PCCT apparatus including an X-ray source that irradiates a subject with X-rays, a scanner that rotates around the subject a photon counting type detector that detects the X-rays transmitted through the subject for each of a plurality of energy bins, and an image generation unit that generates a tomographic image using projection data calculated based on the output of the photon counting type detector. The control method includes 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, an expansion step of creating an expanded data map in which the calibration data map is expanded to a negative region by extrapolating the number of X-ray photons, and a material discrimination step of performing material discrimination based on the expanded data map.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide a PCCT apparatus and a control method thereof capable of obtaining a spectrum transmitted through a base material with a negative thickness without measuring the spectrum of a material other than the base material.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

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Figure 8

Embodiments for Carrying Out the Invention

[0012] Hereinafter, an embodiment of a PCCT (Photon Counting Computed Tomography) apparatus and its control method according to the present invention will be described with reference to the accompanying drawings. In the following description and the accompanying drawings, components having the same functional configuration are denoted by the same reference numerals, and redundant description is omitted.

Embodiment

[0013] The overall configuration of the PCCT apparatus according to Embodiment 1 will be described with reference to FIG. 1. The PCCT apparatus includes a scanner 100 and an operation unit 120. The scanner 100 is installed in an imaging room surrounded by a shielding material that shields X-rays, and the operation unit 120 is installed in an operation room outside the imaging room. The rotation axis direction of the scanner 100 is defined as the Z-axis, the horizontal direction orthogonal to the Z-axis is defined as the X-axis, and the vertical direction orthogonal to the Z-axis is defined as the Y-axis.

[0014] 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 bed 105, a rotating plate control unit 108, a bed 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 bed 105 with X-rays, and is, for example, an X-ray tube device. The collimator 103 is a device that limits the irradiation range of X-rays. The rotating plate 102 includes an opening 104 into which the object 10 placed on the bed 105 enters, 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.

[0015] The photon counting detector 106 is a device that is disposed opposite to the X-ray source 101 and includes a plurality of detection elements for detecting X-rays, thereby acquiring the spatial distribution of 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, individually count incident X-ray photons, and discriminate each X-ray photon into a plurality of energy bins.

[0016] The rotating plate control unit 108 is a device that controls the rotation and tilt of the rotating plate 102. The bed control unit 109 is a device that controls the up / down, front / back, left / right movement of the bed 105. The high voltage generation unit 111 is a power source that generates the tube voltage, which is the voltage applied to the X-ray source 101, and the tube current, which is the current supplied to the X-ray source 101. 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 bed control unit 109, and the X-ray control unit 110 are, for example, an MPU (Micro-Processing Unit) or the like.

[0017] 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 imaging conditions, and is, for example, a keyboard, a pointing device, a touch panel, or the like. The image generation unit 122 is a device that generates a tomographic image using the digital data collected by the data collection unit 107, and is, for example, an MPU, a GPU (Graphics Processing Unit), or the like. The display unit 125 is a device that displays the tomographic image and the like generated by the image generation unit 122, and is, for example, a liquid crystal display, a touch panel, or the like. The storage unit 123 is a device that stores the digital data collected by the data collection unit 107, the tomographic image generated by the image generation unit 122, the program executed by the system control unit 124, the data used by the program, and the like, and is, for example, an HDD (Hard Disk Drive), an SSD (Solid State Drive), or the like. 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).

[0018] Based on the imaging conditions set via the input unit 121, the high voltage generator 111 generates tube voltage and tube current, and X-rays corresponding to the imaging conditions are irradiated from the X-ray source 101 to the subject 10. The photon counting detector 106 detects the X-rays irradiated from the X-ray source 101 and transmitted through the subject 10 with a number of detection elements, and acquires the spatial distribution of the transmitted X-rays. The rotating plate 102 is controlled by the rotating plate control unit 108 and rotates based on the imaging conditions input from the input unit 121, particularly the rotation speed and the like. The examination table 105 is controlled by the examination table control unit 109 and relatively moves with respect to the rotating plate 102, thereby moving the imaging position designated for the subject 10 within the imaging field, which is the range where the transmitted X-rays are detected.

[0019] By repeating the irradiation of X-rays by 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 a view representing each projection angle, and the channel (ch) number and column number, which are the detection element numbers of the photon counting detector 106. The measured projection data is transmitted to the image generation unit 122. The image generation unit 122 generates a tomographic image by performing back-projection processing on a plurality of projection data. The generated tomographic image is displayed on the display unit 125 as a medical image or stored in the storage unit 123.

[0020] Since the PCCT apparatus equipped with the photon counting detector 106 can acquire the projection data of the subject 10 for each energy bin, it is possible to generate medical images for each energy bin and medical images discriminated for substances with different compositions. In order to obtain medical images discriminated for substances with different compositions, etc., it is necessary to acquire the calibration data of the photon counting detector 106 in advance.

[0021] With reference to FIG. 2, calibration data 210 of the photon counting detector 106 will be described. The calibration data 210 is obtained using a calibration phantom 201 having a first substrate 202 and a second substrate 203 with known composition and thickness. That is, by counting X-ray photons transmitted through the calibration phantom 201 at each of a plurality of energy bins by the detection element P of the photon counting detector 106, calibration data 210 corresponding to the photon energy spectrum illustrated in the lower part of FIG. 2 is obtained. In FIG. 2, the number of X-ray photons counted separately in three energy bins bin1, bin2, and bin3 from T1 to T2, T2 to T3, and T3 to are shown. The calibration data 210 is obtained in advance using each of a plurality of calibration phantoms 201 with different thicknesses of the first substrate 202 and the second substrate 203.

[0022] With reference to FIG. 3, an example of a plurality of calibration phantoms 201 will be described. FIG. 3 illustrates nine types of calibration phantoms 201 with different numbers of the first substrate 202 and the second substrate 203. That is, the calibration phantom 201 illustrated in FIG. 3 is a combination of 0 to 2 first substrates 202 and 0 to 2 second substrates 203. As the first substrate 202, for example, an acrylic plate is used as a substance corresponding to the soft tissue of the subject 10. Also, as the second substrate 203, for example, a tin plate is used as a substance corresponding to the iodine contrast agent used in angiography. Note that the first substrate 202 may be other than acrylic as long as it is a substance corresponding to water or soft tissue. Also, the second substrate 203 may be silver other than tin as long as it is a substance corresponding to the iodine contrast agent, or aluminum, titanium, rhodium, palladium, lead, gold, platinum, tungsten, or their alloys, which are metals corresponding to the bone of the subject 10, or other metals.

[0023] Calibration data 210 obtained using a plurality of calibration phantoms 201 is compared with the projection data of the subject 10, and calibration data 210 similar to the photon energy spectrum of the projection data of the subject 10 is selected from among 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 selected calibration data 210, the subject 10 is subjected to material discrimination.

[0024] Note that if the subject 10 contains a material having an effective atomic number outside the range Zeff1 < Z < Zeff2 defined by the effective atomic number Zeff1 of the first base material 202 and the effective atomic number Zeff2 of the second base material 203, the thickness of one of the first base material 202 and the second base material 203 may become a negative value. On the other hand, since the thicknesses of the first base material 202 and the second base material 203 included in the calibration phantom 201 used for obtaining the calibration data 210 are not negative values, it becomes necessary to obtain a negative thickness by extrapolation processing. When obtaining a negative thickness by extrapolation processing using the calibration data 210, the amount of calculation increases and the processing time becomes longer.

[0025] Therefore, in the first embodiment, a calibration data map, which is an aggregate of calibration data 210, is extended to a negative region where the thickness of the base material is a negative value, and material discrimination is performed based on an extended data map, which is the calibration data map extended to the negative region. By discriminating the material based on the extended data map, extrapolation processing becomes unnecessary, so the amount of calculation does not increase and the processing time is also maintained.

[0026] Using FIG. 4, an example of the flow of the process of extending the calibration data map, which is an aggregate of calibration data 210, to the negative region will be described step by step.

[0027] (S401) The calibration phantom 201 is set between the X-ray source 101 and the photon counting type detector 106.

[0028] (S402) The system control unit 124 counts the X-ray photons transmitted through the calibration phantom 201 by X-ray irradiation from the X-ray source 101 and X-ray detection by the photon-counting detector 106. In S402, the rotating plate 102 is not rotated.

[0029] (S403) 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 set between the X-ray source 101 and the photon-counting detector 106.

[0030] (S404) The system control unit 124 creates a calibration data map 500 illustrated in FIG. 5 based on the counting results in S402. The calibration data map 500 is an aggregate of calibration data 210 obtained for each of the calibration phantoms 201. The calibration data map 500 illustrated in FIG. 5 has the number of acrylic plates, which is the first base material 202, on the horizontal axis and the number of tin plates, which is the second base material 203, on the vertical axis, and is created for each energy bin. One white circle in FIG. 5 corresponds to one of the calibration phantoms 201 illustrated in FIG. 3.

[0031] (S405) The system control unit 124 expands the calibration data map 500 created in S404 to the negative region 601 illustrated in FIG. 6, and creates an extended data map 600. The expansion of the calibration data map 500 to the negative region 601 is performed based on the number of photons counted for each energy bin using each of the calibration phantoms 201. That is, an extrapolation point 603, which is the number of photons when the thickness is a negative value, is obtained by an extrapolation process using an approximate curve 602 calculated in a graph with the Log value of the count value on the vertical axis and the thickness of the first base material 202 or the second base material 203 of the calibration phantom 201 on the horizontal axis. The approximate curve 602 is obtained based on the number of photons counted using the calibration phantom 201 and is shown by a dotted line in FIG. 6. The extrapolation point 603 is obtained by inputting a negative thickness value into the approximate curve 602 and is shown by a black circle in FIG. 6.

[0032] When the thickness of the base material is extremely thin and the number of X-ray photons incident on the photon counting type detector 106 increases, the count value may saturate and the Log value of the count value with respect to the thickness of the base material may become non-linear. Therefore, as illustrated in FIG. 7, in the vicinity where the thickness of the base material becomes zero, the count value may be acquired while changing the thickness more finely, and the extrapolation point 603 may be obtained. In FIG. 7, the point where there is 0.5 acrylic plate or tin plate is the point where the thickness of one plate is halved.

[0033] By the processing flow illustrated in FIG. 4, the calibration data map 500 is expanded to the negative region 601, and the extended data map 600 can be created without measuring the spectrum of substances other than the base material. The extended data map 600 is stored in the storage unit and used for material discrimination of the subject 10.

[0034] The system control unit 124 that executes S404 functions as a map creation unit that creates a calibration data map 500 based on the number of X-ray photons transmitted through each of a plurality of calibration phantoms 201 having a first base material 202 and a second base material 203 with known material and thickness. Further, the system control unit 124 that executes S405 functions as an expansion unit that creates an expanded data map 600 obtained by expanding the calibration data map 500 to a negative region 601 by extrapolating the number of X-ray photons transmitted through each of the plurality of calibration phantoms 201.

[0035] Using FIG. 8, an example of the flow of processing for generating a tomographic image in which substances are discriminated based on the expanded data map 600 will be described step by step.

[0036] (S801) Under the control of the system control unit 124, projection data of the subject 10 is acquired. More specifically, with the rotating plate 102 rotated, 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, whereby projection data at various projection angles is acquired. Note that the projection data of the subject 10 is acquired by being divided into a plurality of energy bins.

[0037] (S802) Based on the expanded data map 600 read from the storage unit, the system control unit 124 discriminates the projection data of the subject 10 acquired in S801 into a plurality of substances, for example, the first base material 202 and the second base material 203. Note that when discriminating substances, the thickness of either the first base material 202 or the second base material 203 may be a negative value. In the negative region 601, since the relationship between the thickness of the base material and the count value is non-linear, it is preferable to perform non-linear interpolation using a higher-order function. Further, depending on the strength of the non-linearity of the expanded data map 600, it is preferable to increase the order of the higher-order function used for non-linear interpolation. The higher the non-linearity, the higher the order, and the more the interpolation accuracy can be improved.

[0038] (S803) The system control unit 124 reconstructs tomographic images for each substance using the projection data discriminated by substance in S802. That is, tomographic images of the first base substance 202 and the second base substance 203 are reconstructed.

[0039] Based on the processing flow described with reference to FIG. 8, tomographic images of the subject 10 are generated for each substance using the projection data discriminated by substance based on the extended data map 600. Since the extended data map 600, which is the calibrated data map 500 extended to the negative region 601, is used for substance discrimination, the extrapolation process for obtaining the thickness of negative values is not required, so the amount of calculation does not increase and the processing time can be maintained. Note that the system control unit 124 that executes S802 functions as a substance discrimination unit that performs substance discrimination based on the extended data map 600.

[0040] The embodiments of the present invention have been described above. Note that the present invention is not limited to the above embodiments, and components can be modified and embodied without departing from the gist of the invention. Also, a plurality of components disclosed in the above embodiments may be appropriately combined. Furthermore, some components may be deleted from all the components shown in the above embodiments.

Explanation of Reference Numerals

[0041] 10: Subject, 100: Scanner, 101: X-ray source, 102: Rotating plate, 103: Collimator, 104: Aperture, 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 map, 600: Extended data map, 601: Negative region, 602: Approximation curve, 603: Extrapolation point.

Claims

1. An XCT device comprising: an X-ray source that irradiates a subject with X-rays; a scanner that rotates around the subject a photon-counting detector that detects X-rays transmitted through the subject for each of a plurality of energy bins; and an image generation unit that generates a tomographic image using projection data calculated based on an output of the photon-counting detector, 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 basis material and a second basis material with known material and thickness, an expansion unit that creates an expanded data map by extrapolating the number of X-ray photons to expand the calibration data map into a negative region, and further comprising a material discrimination unit that performs material discrimination based on the expanded data map.

2. The XCT device according to Claim 1, wherein the expansion unit creates the expanded data map while changing the thickness more finely in the vicinity where the thickness of at least one of the first basis material and the second basis material becomes zero.

3. The XCT device according to Claim 1, wherein the material discrimination unit performs non-linear interpolation in the negative region.

4. The XCT device according to Claim 3, wherein the material discrimination unit uses a higher-order function for the non-linear interpolation and increases the order of the higher-order function as the non-linearity of the expanded data map becomes stronger.

5. A control method for an XCT device comprising: an X-ray source that irradiates a subject with X-rays; a scanner that rotates around the subject a photon-counting detector that detects X-rays transmitted through the subject for each of a plurality of energy bins; and an image generation unit that generates a tomographic image using projection data calculated based on an output of the photon-counting detector, the control method comprising: 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 basis material and a second basis material with known material and thickness, an expansion step of creating an expanded data map by extrapolating the number of X-ray photons to expand the calibration data map into a negative region, and a material discrimination step of performing material discrimination based on the expanded data map.

Images