Calibration phantom

Abstract

To provide a calibration phantom that is hard to deform and is easy to handle.SOLUTION: The calibration phantom is used in a case of acquiring calibration data of a photon-counting type detector that outputs an electrical signal corresponding to photon energy of an incident X-ray. The calibration phantom includes first base substances and a second base substance which are known substances, the second base substance having an atomic number larger than an atomic number of the first base substances and being sandwiched between the first base substances.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to an X-ray CT (Computed Tomography) apparatus equipped with a photon counting detector, and to a calibration phantom used for calibrating the photon counting detector. [Background technology]

[0002] An X-ray CT scanner is a device that generates a tomographic image of a subject using multiple projection data acquired by irradiating the subject with X-rays and detecting the X-rays that pass through the subject at multiple projection angles. The generated tomographic image is used as a medical image for diagnosing the subject.

[0003] In photon-counting CT systems that use photon-counting detectors to detect X-rays, cross-sectional images decomposed into materials with different compositions, for example, cross-sectional images decomposed into iodine contrast agents used in angiography and soft tissue, can be obtained as medical images. Note that in order to obtain cross-sectional images with materials resolved, it is necessary to obtain in advance for each detector element as calibration data the relationship between the output and photon energy when a phantom consisting of a combination of multiple base materials with known compositions and thicknesses is measured by a photon-counting detector.

[0004] Patent Document 1 discloses that a two-dimensional map of acrylic thickness-iodine thickness is obtained as calibration data using a stepped phantom made of acrylic and iodine. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-146480 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, Patent Document 1 does not give sufficient consideration to the handling of the phantom. In a phantom composed of a combination of multiple basis materials, the thickness of a basis material having a relatively large atomic number may be made extremely thin, for example, about 0.1 mm. A basis material having a thickness of about 0.1 mm is easily deformed, and calibration data obtained using a phantom composed of a deformed basis material reduces the accuracy of discriminating between materials.

[0007] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a calibration phantom that is less likely to deform and is easy to handle. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the present invention provides a calibration phantom used when acquiring calibration data for a photon counting detector that outputs an electrical signal corresponding to the photon energy of an incident X-ray, the phantom comprising a first basis material and a second basis material which are known substances, the second basis material having an atomic number larger than that of the first basis material and being sandwiched between the first basis material. Effect of the Invention

[0009] According to the present invention, it is possible to provide a calibration phantom that is less likely to deform and is easy to handle. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing the overall configuration of a photon counting CT apparatus. [Diagram 2] FIG. 13 is a diagram illustrating calibration data of a photon counting detector. [Diagram 3] 1A and 1B are diagrams illustrating an example of the configuration of a calibration phantom and an example of calibration data acquired using the calibration phantom. [Figure 4] FIG. 13 is a diagram showing another example of the configuration of the calibration phantom. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the calibration phantom of the present invention will be described with reference to the drawings. The calibration phantom of the present invention is used in a photon-counting CT apparatus equipped with a photon-counting detector. In the following description and the accompanying drawings, components having the same functional configuration are designated by the same reference numerals, and duplicated explanations will be omitted. EXAMPLES

[0012] 1 shows the overall configuration of a photon-counting CT device 101. The horizontal direction on 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 photon-counting CT device 101 includes a gantry 102, an X-ray tube 103, a bowtie filter 104, a bed 105, a detector panel 107, a computing device 108, an input device 109, and a display device 110.

[0013] The subject 106 is placed on a bed 105 and placed in an opening 112 provided in the gantry 102. X-rays 111 emitted from the X-ray tube 103 are irradiated onto the subject 106 via a bowtie filter 104, and are detected by a detector panel 107 after passing through the subject 106. The X-rays are shaped into a beam suitable for the size of the subject 106 by passing through the bowtie filter 104. The X-ray tube 103 and the detector panel 107 are attached to the gantry 102 so as to be disposed opposite each other with the subject 106 in between, and are rotated around the subject 106 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 panel 107 are repeated along with the rotation of both, thereby acquiring projection data at various projection angles.

[0014] The acquired projection data is subjected to image reconstruction processing in the arithmetic device 108, whereby a tomographic image of the subject 106 is generated and displayed as a medical image on the display device 110. When projection data is acquired while the bed 105 on which the subject 106 is placed and the gantry 102 are moving relatively in the Z-axis direction, a volume image of the subject 106 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 105 are set based on scan conditions input by an operator via an input device 109. The arithmetic device 108 has the same hardware configuration as a general computer device, and includes a CPU (Central Processing Unit), memory, HDD (Hard Disk Drive), etc., and performs correction processing for the projection data, etc., and controls each part.

[0015] The detector panel 107 is configured by arranging a plurality of detection elements P in an arc shape centered on the X-ray focal point of the X-ray tube 103. When the detection elements P are photon counting detectors, the incident X-ray photons are counted individually and the energy of the X-ray photons is measured.

[0016] The photon-counting CT apparatus 101 equipped with a photon-counting detector can obtain a photon energy spectrum related to the projection data of the subject 106, and can generate medical images in which materials with different compositions are decomposed or medical images separated into multiple energy components. Note that in order to obtain medical images etc. in which materials with different compositions are decomposed, it is necessary to obtain calibration data for the photon-counting detector in advance.

[0017] The calibration data of the photon counting detector will be described with reference to FIG. 2. A calibration phantom 201 made of a plurality of base materials with known compositions and thicknesses is used to obtain the calibration data of the photon counting detector. That is, a photon energy spectrum 210, which is the number of X-ray photons for each photon energy illustrated in the lower part of FIG. 2, is obtained in advance by counting X-rays transmitted through the calibration phantom 201 with a detection element P of the photon counting detector. FIG. 2 shows the number of X-ray photons counted in three energy bins bin1, bin2, and bin3 of T1-T2, T2-T3, and T3-. The calibration data is a group of photon energy spectra 210 obtained using each of a plurality of calibration phantoms 201 with different thicknesses of base materials, and is compared with the photon energy spectrum related to the projection data of the subject 106. Then, the object 106 is decomposed into a plurality of basis materials based on the thickness of the basis material of the calibration phantom 201 used to acquire a photon energy spectrum 210 similar to the photon energy spectrum of the object 106 from the calibration data.

[0018] The calibration phantom 201 is composed of a first basis material 202 corresponding to the soft tissue of the subject 106, and a second basis material 203 corresponding to the iodine contrast agent used in angiography. Specifically, the first basis material 202 corresponding to the soft tissue is an acrylic plate, and the second basis material 203 corresponding to the iodine contrast agent is a tin plate. The first basis material 202 may be a material other than acrylic as long as it corresponds to water or soft tissue. The second basis material 203 may be silver other than tin as long as it corresponds to the iodine contrast agent, or may be aluminum or titanium corresponding to the bone of the subject 106, or other metals such as rhodium, palladium, lead, gold, platinum, tungsten, or alloys thereof. That is, the second basis material 203 has a larger atomic number than the first basis material 202.

[0019] Incidentally, the thickness of the tin plate, which corresponds to the iodine contrast agent flowing in blood vessels, may be set to, for example, about 0.1 mm based on the concentration of the iodine contrast agent in blood. A tin plate with a thickness of about 0.1 mm is easily deformed, and the number of X-ray photons passing through the calibration phantom 201 changes due to the deformation of the tin plate, so that the error included in the acquired calibration data becomes large. In other words, the accuracy of discriminating materials decreases due to the deformation of the calibration phantom 201. Therefore, in the first embodiment, the calibration phantom 201 is configured so that the deformation of the base material is suppressed.

[0020] The calibration phantom 201 of the first embodiment will be described with reference to Fig. 3. Nine types of calibration phantoms 201-1 to 9 each composed of a first basis material 202 and a second basis material 203 are illustrated in Fig. 3. All of the nine types of calibration phantoms 201-1 to 9 are composed of the second basis material 203 sandwiched between the first basis materials 202. With this configuration, even if the second basis material 203 is a very thin plate with a thickness of 1 mm or less and easily deformed, it is restrained by the first basis material 202, so that deformation of the second basis material 203 is suppressed, and errors included in the acquired calibration data can be reduced.

[0021] Since the first basis material 202 has a smaller atomic number than the second basis material 203, it is a flat plate that is thicker and less likely to deform than the second basis material 203. For example, the first basis material 202 is an acrylic plate having a thickness of 10 mm, and the second basis material 203 is a tin plate having a thickness of 0.1 mm.

[0022] More specifically, the calibration phantom 201-1 is configured by sandwiching one 0.1 mm thick tin plate between two 10 mm thick acrylic plates, the calibration phantom 201-2 is configured by sandwiching two 0.1 mm thick tin plates between two 10 mm thick acrylic plates, and the calibration phantom 201-3 is configured by sandwiching three 0.1 mm thick tin plates between two 10 mm thick acrylic plates.

[0023] Similarly, calibration phantom 201-4 is composed of one tin plate having a thickness of 0.1 mm, calibration phantom 201-5 is composed of two tin plates having a thickness of 0.1 mm, calibration phantom 201-6 is composed of three tin plates having a thickness of 0.1 mm, each sandwiched between four acrylic plates having a thickness of 10 mm. Furthermore, calibration phantom 201-7 is composed of one tin plate having a thickness of 0.1 mm, calibration phantom 201-8 is composed of two tin plates having a thickness of 0.1 mm, and calibration phantom 201-9 is composed of three tin plates having a thickness of 0.1 mm, each sandwiched between six acrylic plates having a thickness of 10 mm.

[0024] 3 illustrates an example of calibration data 300 acquired using calibration phantoms 201-1 to 201-9. That is, photon energy spectra 210-1 to 210-9 acquired using calibration phantoms 201-1 to 201-9, respectively, constitute the calibration data 300. Note that the calibration phantom 201-1 corresponds to the photon energy spectrum 210-1, the calibration phantom 201-2 corresponds to the photon energy spectrum 210-2, ..., the calibration phantom 201-9 corresponds to the photon energy spectrum 210-9, respectively. Note that the calibration data 300 may include a photon energy spectrum acquired without using the calibration phantom 201, that is, a photon energy spectrum of an X-ray transmitted through air.

[0025] Calibration phantoms 201-1 to 201-9 in which the thickness of the first basis material 202 sandwiching the second basis material 203 is equal above and below as illustrated in Fig. 3 have the advantage of being less likely to deform. However, the calibration phantom 201 is not limited to Fig. 3, and the thickness of the first basis material 202 may be different above and below. In other words, if the thickness through which X-rays pass through the first basis material 202 is known, the acquired photon energy spectrum 210 can be used as calibration data 300.

[0026] Another example of the calibration phantom 201 will be described with reference to FIG. 4. FIG. 4 shows a perspective view of the calibration phantom 201 before the second basis material 203 is sandwiched between the first basis material 202. The calibration phantom 201 shown in FIG. 4 is composed of two first basis materials 202-1 to 2, a second basis material 203, and an outer frame portion 204. The outer frame portion 204 has a frame shape, and the second basis material 203 is fitted into the outer frame portion 204. That is, the side surface of the second basis material 203 is covered by the outer frame portion 204. The thickness of the outer frame portion 204 is equal to or greater than that of the second basis material 203. The first basis material 202-1 and the first basis material 202-2 are disposed above and below the outer frame portion 204 into which the second basis material 203 is fitted, and the second basis material 203 and the outer frame portion 204 are sandwiched between the first basis material 202-1 and the first basis material 202-2. The outer frame portion 204 may be formed of the first basis material 202.

[0027] In the calibration phantom 201 configured as shown in Figure 4, the second basis material 203 is not exposed, so even if the second basis material 203 is extremely thin, deformation of the second basis material 203 is more suppressed, and the errors contained in the acquired calibration data can be more reduced.

[0028] The above describes the embodiments of the present invention. Note that the present invention is not limited to the above embodiments, and the components can be modified and embodied without departing from the scope of the invention. In addition, multiple 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 symbols]

[0029] 101: photon counting CT device, 102: gantry, 103: X-ray tube, 104: bowtie filter, 105: bed, 106: subject, 107: detector panel, 108: computing device, 109: input device, 110: display device, 111: X-ray, 112: aperture, 201: calibration phantom, 202: first basis material, 203: second basis material, outer frame 204, 210: photon energy spectrum, 300: calibration data.

Claims

1. A calibration phantom used when acquiring calibration data of a photon counting detector that outputs an electrical signal corresponding to the photon energy of an incident X-ray, A calibration phantom comprising a first basis material and a second basis material, each of which is a known material, the second basis material having an atomic number greater than that of the first basis material and being sandwiched between the first basis material.

2. 2. The calibration phantom of claim 1, A calibration phantom, characterized in that the thicknesses of the first base material sandwiching the second base material are equal above and below.

3. 2. The calibration phantom of claim 1, A calibration phantom, characterized in that the second base material is sandwiched between the first base material and has its sides covered.

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