PCCT device and control method for the same

Abstract

To provide a PCCT device capable of improving interpolation precision regarding material discrimination and shortening the processing time, 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 area division unit dividing the calibrated data map into a non-linear area and a linear area; and an interpolation unit performing non-linear interpolation in the non-linear area and linear interpolation in the linear area.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 interpolation 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 that uses a photon counting detector as an 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. However, when the amount of X-rays incident on the photon counting detector is large, non-linear phenomena such as pile-up in which a plurality of X-ray signals are erroneously counted as X-rays of one added energy, and missed counts in which another X-ray is incident while an X-ray signal is being processed by a certain photon counting detector and as a result is not processed and not counted, may occur, and the discrimination accuracy of substances may decrease.

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

Prior Art Documents

Patent Document

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in Patent Document 1, considerations regarding interpolation based on non-linearity and linearity are insufficient. That is, when linear interpolation is performed in a region with strong non-linearity, the interpolation accuracy decreases, and when non-linear interpolation is performed in a region with strong linearity, the amount of calculation increases and the processing time becomes long.

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

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 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 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 division 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] 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 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 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 basis material and a second basis material with known material and thickness, a region division 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.

Advantages of the Invention

[0010] According to the present invention, it is possible to provide a PCCT apparatus and a control method thereof that can improve the interpolation accuracy related to material discrimination and shorten the processing time.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

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Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0012] Hereinafter, an example 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.

Example

[0013] The overall configuration of the PCCT apparatus according to Example 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 direction of the rotation axis 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 couch 105, a rotating plate control unit 108, a couch 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 couch 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 couch 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 arranged to face the X-ray source 101 and acquires the spatial distribution of X-rays by including a plurality of detection elements that 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, 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 the 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 large 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 moves relative to the rotating plate 102 to move the subject 10 to the imaging position designated for the subject 10 within the imaging field, which is the range where the transmitted X-rays are detected.

[0019] The irradiation of X-rays by the X-ray source 101 and the detection of X-rays by the photon-counting detector 106 are repeated along with the rotation of the rotating plate 102, so that the projection data of the subject 10 are measured at various projection angles. The projection data are 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 are 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, it is necessary to acquire the calibration data of the photon-counting detector 106 in advance.

[0021] Using 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 by being divided into three energy bins bin1, bin2, and bin3 of T1 to T2, T2 to T3, and T3 to is 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] Using FIG. 3, an example of a plurality of calibration phantoms 201 will be described. FIG. 3 illustrates 16 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 3 first substrates 202 and 0 to 3 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 corresponding to the bone of the subject 10, rhodium, palladium, lead, gold, platinum, tungsten, or their alloys, which are other metals.

[0023] The 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 extracted 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 extracted calibration data 210, the subject 10 is discriminated in terms of substance.

[0024] Since the calibration data 210 is obtained using a calibration phantom 201 having a predetermined thickness, appropriate interpolation processing is necessary 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, a calibration data map, which is an aggregate of 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.

[0025] Using FIG. 4, an example of the flow of the process of dividing a calibration data map, which is an aggregate of calibration data 210, into a non-linear region and a linear region will be described step by step.

[0026] (S401) The calibration phantom 201 is set between the X-ray source 101 and the photon counting type detector 106.

[0027] (S402) The system control unit 124 counts the X-ray photons transmitted through the calibration phantom 201 while changing the X-ray dose irradiated from the X-ray source 101. In S402, the rotating plate 102 is not rotated.

[0028] (S403) It is determined whether the counting for all calibration phantoms 201 is completed. If the counting for all calibration phantoms 201 is 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.

[0029] (S404) Based on the counting results in S402, the system control unit 124 creates a calibration data map 500 illustrated in FIG. 5. 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 substrate material 202, on the horizontal axis and the number of tin plates, which is the second substrate 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 and includes calibration data 210 for each of a plurality of irradiation doses.

[0030] (S405) The system control unit 124 divides the calibration data map 500 created in S404 into two regions: a non-linear region 601 and a linear region 602 illustrated in FIG. 6. The region division of the calibration data map 500 is performed based on the linearity of the Log value of the count value of transmitted photons for each of a 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, a region including non-linear data where the difference between the reference line 603 and the Log value of the count value is greater than a predetermined threshold is defined as the non-linear region 601, and a region not including such data is defined as the linear region 602. In FIG. 6, the non-linear data is indicated by black circles, and the non-linear region 601 is indicated by hatching. The reference line 603 is calculated based on the respective thicknesses of the first substrate material 202 and the second substrate material 203 of the calibration phantom 201. The threshold is determined based on the discrimination accuracy.

[0031] According to the processing flow illustrated in FIG. 4, the calibration data map 500 is divided into a non-linear region 601 and a linear region 602. The calibration data map 500 divided into the non-linear region 601 and the linear region 602 is stored in the storage unit and used for substance discrimination of the subject 10. The system control unit 124 that executes S404 functions as a map creation unit that creates the 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 substance 202 and a second base substance 203 with known material and thickness. Also, the system control unit 124 that executes S405 functions as a region division unit that divides the calibration data map 500 into a non-linear region 601 and a linear region 602.

[0032] Using FIG. 7, an example of the processing flow for generating a tomographic image in which substances are discriminated based on the calibration data map 500 will be described step by step.

[0033] (S701) 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. The projection data of the subject 10 is acquired by being divided into a plurality of energy bins.

[0034] (S702) Based on the calibration data map 500 read from the storage unit, the system control unit 124 discriminates the projection data of the subject 10 acquired in S701 into a plurality of substances, for example, a first base substance 202 and a second base substance 203. When discriminating substances, non-linear interpolation is performed in the non-linear region 601, and linear interpolation is performed in the linear region 602. For non-linear interpolation in the non-linear region 601, for example, a higher-order function is used, and it is preferable that the order is increased according to the strength of the non-linearity. The higher the non-linearity, the higher the order, and the more the interpolation accuracy can be improved. In the linear region 602, it is preferable that linear interpolation is performed based on the statistical error of the Log value of the count value. By performing linear interpolation based on the statistical error, the interpolation accuracy can be further improved.

[0035] (S703) Using the projection data discriminated by substance in S702, the system control unit 124 reconstructs tomographic images for each substance. That is, a tomographic image of the first base substance 202 and a tomographic image of the second base substance 203 are reconstructed.

[0036] According to the processing flow described with reference to FIG. 7, tomographic images of the subject 10 are generated for each substance using the projection data discriminated by substance based on the calibration data map 500. 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. 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.

[0037] The embodiments of the present invention have been described above. It should be noted that the present invention is not limited to the above embodiments, and the 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

[0038] 10: Subject, 100: Scanner, 101: X-ray source, 102: Rotating plate, 103: Collimator, 104: Aperture, 105: Bed, 106: Photon counting detector, 107: Data acquisition unit, 108: Rotating plate control unit, 109: Bed control unit, 110: X-ray control unit, 111: High voltage generator, 120: Operation unit, 121: Input unit, 122: Image generation unit, 123: Memory unit, 124: System control unit, 125: Display unit, 201: Calibration phantom, 202: First base material, 203: Second base material, 210: Calibration data, 500: Calibration data map, 601: Nonlinear region, 602: Linear region, 603: Reference line.

Claims

1. An XCT apparatus 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 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 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 base material and a second base material with 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 non-linear interpolation in the non-linear region and linear interpolation in the linear region. The XCT apparatus is characterized by further comprising the interpolation unit.

2. The XCT apparatus according to claim 1, wherein the region division unit divides regions based on the linearity of the Log value of the count value of photons transmitted through the calibration phantom with respect to the irradiation dose to the calibration phantom. The XCT apparatus is characterized by this.

3. The XCT apparatus according to claim 2, wherein the region division unit sets, as the non-linear region, a region including non-linear data in which the difference between a reference straight 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 predetermined threshold value, and sets, as the linear region, a region not including the non-linear data. The XCT apparatus is characterized by this.

4. The XCT apparatus according to claim 1, wherein the interpolation 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 non-linear region is stronger. The XCT apparatus is characterized by this.

5. The XCT apparatus 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 photons transmitted through the calibration phantom. The XCT apparatus is characterized by this.

6. A control method for an XCT apparatus 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 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 detector, the 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 base material and a second base material with known material and thickness, An area division step of dividing the calibration data map into a non-linear area and a linear area; A control method characterized by further comprising an interpolation step of performing non-linear interpolation in the non-linear area and performing linear interpolation in the linear area.

Images