Phantom for calibration

Abstract

The invention provides a phantom for calibration, which is not easy to deform and easy to operate. A phantom for calibration, which is used when acquiring calibration data for a photon counting detector that outputs an electric signal corresponding to the photon energy of incident X-rays, is characterized by containing, as known substances, a first substrate substance and a second substrate substance having a larger atomic number than the first substrate substance, the first electrode is sandwiched between the first substrate material and the second substrate material.

Description

Technical Field

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

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

[0003] In a photon counting CT apparatus that uses a photon counting detector for detecting X-rays, a tomographic image decomposed into materials having different compositions, for example, a tomographic image decomposed into an iodine contrast agent and soft tissue for angiography is obtained as a medical image. In addition, in order to obtain a tomographic image decomposed into materials, it is necessary to obtain in advance, for each detection element, the relationship between the output and the photon energy when measuring a phantom composed of a combination of a plurality of base materials as materials with known compositions and thicknesses using a photon counting detector as calibration data.

[0004] Patent Document 1 discloses using a step-shaped phantom made of acrylic acid and iodine to obtain a two-dimensional map of acrylic acid thickness and iodine thickness as calibration data.

[0005] Patent Document 1: Japanese Patent Application Publication No. 2013-146480

[0006] However, Patent Document 1 does not adequately consider the operation of the phantom. In a phantom composed of a combination of multiple base materials, the thickness of a base material with a relatively large atomic number is sometimes set to be extremely thin, for example, to about 0.1 mm. A base material with a thickness of about 0.1 mm is easily deformed, and the accuracy of distinguishing materials is reduced in the calibration data obtained using a phantom composed of a deformed base material. Summary of the invention

[0007] Therefore, an object of the present invention is to provide a calibration phantom that is not easily deformed and is easy to handle.

[0008] In order to achieve the above-mentioned purpose, the present invention is a calibration phantom, which is used when obtaining calibration data of a photon counting detector that outputs an electrical signal corresponding to the photon energy of an incident X-ray, and is characterized in that it includes a first base material and a second base material as known substances, and the second base material has an atomic number greater than that of the first base material and is clamped by the first base material.

[0009] Effects of the Invention

[0010] According to the present invention, it is possible to provide a calibration phantom that is not easily deformed and is easy to handle. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a diagram showing the overall structure of a photon counting CT apparatus.

[0012] Figure 2 This is a diagram for explaining calibration data of a photon counting detector.

[0013] Figure 3 1 is a diagram showing a configuration example of a calibration phantom and an example of calibration data acquired using the calibration phantom.

[0014] Figure 4 This is a diagram showing another structural example of a calibration phantom.

[0015] Explanation of symbols

[0016] 101-photon counting CT device, 102-gantry, 103-X-ray tube, 104-bow-tie filter, 105-bed, 106-subject, 107-detector panel, 108-calculation device, 109-input device, 110-display device, 111-X-ray, 112-opening, 201-calibration phantom, 202-first base material, 203-second base material, 204-outer frame, 210-photon energy spectrum, 300-calibration data. DETAILED DESCRIPTION

[0017] Hereinafter, an embodiment of the calibration phantom of the present invention will be described with reference to the accompanying drawings. The calibration phantom of the present invention is used in a photon counting CT device having a photon counting detector. In addition, in the following description and the accompanying drawings, the same symbols are marked for components having the same functional structure, thereby omitting repeated descriptions.

[0018] [Example 1]

[0019] Figure 1 , the entire structure of the photon counting CT apparatus 101 is shown in FIG. In addition, the horizontal direction of the paper is set as the X-axis, the vertical direction is set as the Y-axis, and the direction perpendicular to the XY plane is set as the Z-axis. The photon counting CT apparatus 101 includes a gantry 102, an X-ray tube 103, a bow-tie filter 104, a bed 105, a detector panel 107, a computing device 108, an input device 109, and a display device 110.

[0020] The subject 106 is placed on a bed 105 and arranged in an opening 112 provided in the gantry 102. The X-rays 111 emitted from the X-ray tube 103 are irradiated to the subject 106 via the bow-tie filter 104, and are detected by the detector panel 107 after passing through the subject 106. In addition, the X-rays are formed into a beam shape suitable for the size of the subject 106 by passing through the bow-tie filter 104. The X-ray tube 103 and the detector panel 107 are mounted on the gantry 102 in a manner of being arranged opposite to each other with the subject 106 sandwiched therebetween, and are rotated around the subject 106 by the rotation drive unit of the gantry 102. The X-ray irradiation from the X-ray tube 103 and the X-ray measurement in the detector panel 107 are repeated together with the rotation of the two, thereby acquiring projection data at various projection angles.

[0021] The computing device 108 performs image reconstruction processing on the acquired projection data to generate a cross-sectional image of the subject 106, and displays it as a medical image on the display device 110. Furthermore, if the projection data is acquired while the bed 105 on which the subject 106 is placed and the gantry 102 are relatively moved in the Z-axis direction, a volume image of the subject 106 is generated. In addition, the amount of X-rays irradiated from the X-ray tube 103, the rotation speed of the gantry 102, and the relative movement speed of the gantry 102 and the bed 105 are set according to the scanning conditions input by the operator via the input device 109. Furthermore, the computing device 108 has the same hardware structure as a general computer device, and is equipped with a CPU (Central Processing Unit), a memory, an HDD (Hard Disk Drive), etc., and performs correction processing of projection data, etc. or controls each part.

[0022] The detector panel 107 is composed of a plurality of detection elements P arranged in an arc shape centered on the X-ray focus of the X-ray tube 103. When the detection element P is a photon counting detector, each incident X-ray photon is counted and the energy of the X-ray photon is measured.

[0023] In the photon counting CT apparatus 101 equipped with a photon counting detector, it is possible to obtain a photon energy spectrum related to projection data of the subject 106, and thus it is possible to generate a medical image decomposed into substances having different components or a medical image divided into a plurality of energy components. In addition, in order to obtain a medical image decomposed into substances having different components, it is necessary to obtain calibration data of the photon counting detector in advance.

[0024] use Figure 2, the calibration data of the photon counting detector is explained. When obtaining the calibration data of the photon counting detector, a calibration phantom 201 composed of a plurality of base materials with known composition and thickness is used. That is, by counting the X-rays transmitted through the calibration phantom 201 with the detection element P of the photon counting detector, the calibration data is obtained in advance. Figure 2 The number of X-ray photons for each photon energy shown in the lower section is the photon energy spectrum 210. Figure 2 The number of X-ray photons counted in three energy bins bin1, bin2, and bin3 of T1 to T2, T2 to T3, and T3 to T3 is shown in FIG. The calibration data is a group of photon energy spectra 210 obtained using each of a plurality of calibration phantoms 201 having different thicknesses of the base material, and is compared with the photon energy spectrum of the projection data of the subject 106. Then, the subject 106 is decomposed into a plurality of base materials according to the thickness of the base material of the calibration phantom 201 used to obtain the photon energy spectrum 210 similar to the photon energy spectrum of the subject 106 from the calibration data.

[0025] The calibration phantom 201 is composed of, for example, a first base material 202 corresponding to the soft tissue of the subject 106 and a second base material 203 corresponding to the iodine contrast agent used in angiography. Specifically, the first base material 202 corresponding to the soft tissue is an acrylic plate, and the second base material 203 corresponding to the iodine contrast agent is a tin plate. In addition, the first base material 202 may be a substance other than acrylic acid as long as it is a substance equivalent to water or soft tissue. Furthermore, the second base material 203 may be silver other than tin, aluminum or titanium corresponding to the bone of the subject 106, or metals other than these, namely rhodium, palladium, lead, gold, platinum, tungsten, or alloys thereof, as long as it is a substance equivalent to the iodine contrast agent. That is, the atomic number of the second base material 203 is greater than that of the first base material 202.

[0026] However, the thickness of the tin plate corresponding to the iodine contrast agent flowing in the blood vessel is sometimes set to about 0.1 mm, for example, depending on the concentration of the iodine contrast agent in the blood. The tin plate with a thickness of about 0.1 mm is easy to deform, and the number of X-ray photons transmitted through the calibration phantom 201 changes due to the deformation of the tin plate, so the error contained in the calibration data obtained becomes larger. That is, the accuracy of distinguishing the substance decreases due to the deformation of the calibration phantom 201. Therefore, in Example 1, the calibration phantom 201 is constructed in a manner to suppress the deformation of the base material.

[0027] use Figure 3 , the calibration phantom 201 of Example 1 is described. Figure 3, nine types of calibration phantoms 201-1 to 9 composed of a first base material 202 and a second base material 203 are illustrated. The nine types of calibration phantoms 201-1 to 9 are all composed of a first base material 202 sandwiching a second base material 203. With this structure, even if the second base material 203 is an extremely thin flat plate with a thickness of less than 1 mm and easily deformed, the deformation of the second base material 203 can be suppressed due to the restriction of the first base material 202, and the error contained in the acquired calibration data can be reduced.

[0028] In addition, the first base material 202 has a smaller atomic number than the second base material 203, and is therefore a thicker and less deformable flat plate than the second base material 203. For example, the first base material 202 is a 10 mm thick acrylic plate, and the second base material 203 is a 0.1 mm thick tin plate.

[0029] More specifically, the calibration phantom 201-1 is composed of two 10 mm thick acrylic plates sandwiching one 0.1 mm thick tin plate, and the calibration phantom 201-2 is composed of two 10 mm thick acrylic plates sandwiching two 0.1 mm thick tin plates, and the calibration phantom 201-3 is composed of two 10 mm thick acrylic plates sandwiching three 0.1 mm thick tin plates.

[0030] Similarly, the calibration phantom 201-4 is composed of 1 tin plate of 0.1 mm thickness sandwiched between 4 acrylic plates of 10 mm thickness, the calibration phantom 201-5 is composed of 2 tin plates of 0.1 mm thickness sandwiched between 4 acrylic plates of 10 mm thickness, and the calibration phantom 201-6 is composed of 3 tin plates of 0.1 mm thickness sandwiched between 4 acrylic plates of 10 mm thickness. Furthermore, the calibration phantom 201-7 is composed of 1 tin plate of 0.1 mm thickness sandwiched between 6 acrylic plates of 10 mm thickness, the calibration phantom 201-8 is composed of 2 tin plates of 0.1 mm thickness sandwiched between 6 acrylic plates of 10 mm thickness, and the calibration phantom 201-9 is composed of 3 tin plates of 0.1 mm thickness sandwiched between 6 acrylic plates of 10 mm thickness.

[0031] exist Figure 3 The lower section illustrates calibration data 300 obtained using calibration phantoms 201-1 to 9. That is, photon spectra 210-1 to 9 obtained using each of calibration phantoms 201-1 to 9 are calibration data 300. In addition, calibration phantom 201-1 corresponds to photon spectrum 210-1, calibration phantom 201-2 corresponds to photon spectrum 210-2, ..., calibration phantom 201-9 corresponds to photon spectrum 210-9, respectively. In addition, calibration data 300 may also include a photon spectrum obtained without using calibration phantom 201, that is, a photon spectrum of X-rays that have passed through air.

[0032] like Figure 3 The calibration phantoms 201-1 to 9 in which the thickness of the first base material 202 sandwiching the second base material 203 is equal in height have the advantage of being less likely to deform. However, the calibration phantom 201 is not limited to Figure 3 The thickness of the first base material 202 may also be different at the top and bottom. That is, if the thickness of the first base material 202 through which X-rays are transmitted is known, the acquired photon spectrum 210 can be used as the calibration data 300 .

[0033] use Figure 4 , another example of the calibration phantom 201 is described. Figure 4 2 is a perspective view of the calibration phantom 201 in a state before the second base material 203 is sandwiched by the first base material 202 . Figure 4 The calibration phantom 201 illustrated in the figure is composed of two first substrates 202-1 and 2, a second substrate 203 and an outer frame 204. The outer frame 204 has a frame shape, and the second substrate 203 is embedded in the outer frame 204. That is, the side surface of the second substrate 203 is covered by the outer frame 204. In addition, the thickness of the outer frame 204 is equal to or greater than the second substrate 203. The first substrate 202-1 and the first substrate 202-2 are arranged on the upper side and the lower side of the outer frame 204 in which the second substrate 203 is embedded, and the second substrate 203 and the outer frame 204 are clamped by the first substrate 202-1 and the first substrate 202-2. In addition, the outer frame 204 can also be composed of the first substrate 202.

[0034] In such Figure 4 In the thus constructed calibration phantom 201 , since the second base material 203 is not exposed, even when the second base material 203 is extremely thin, deformation of the second base material 203 can be further suppressed, and errors included in the acquired calibration data can be further reduced.

[0035] The embodiments of the present invention are described above. In addition, the present invention is not limited to the above embodiments, and the constituent elements can be modified and concretized within the scope of the main purpose of the invention. In addition, multiple constituent elements disclosed in the above embodiments can be appropriately combined. Furthermore, some constituent elements can also be deleted from all the constituent elements shown in the above embodiments.

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 incident X-rays, characterized in that: The invention comprises a first base material which is a known material and a second base material, wherein the second base material has an atomic number greater than that of the first base material and is sandwiched by the first base material.

2. The calibration phantom according to claim 1, characterized in that: The thickness of the first base material sandwiching the second base material is equal in the upper and lower parts.

3. The calibration phantom according to claim 1, wherein: The second base material is sandwiched by the first base material and covers the side surface.

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