PCCT equipment

Abstract

To provide a PCCT device capable of correcting band artifacts of one substance discrimination image and band artifacts of the other substance discrimination image.SOLUTION: A PCCT device for acquiring projection data divided into a plurality of energy bins by irradiating a subject with an X-ray includes: a first correction unit for correcting band artifacts of a first substance discrimination image of a plurality of substance discrimination images created on the basis of the projection data, and calculating a first correction amount, which is a correction amount of the band artifacts; an energy calculation unit for calculating average energy of the X-ray transmitted through the subject; and a second correction unit for correcting band artifacts of a second substance discrimination image using a second correction amount, which is a correction amount calculated on the basis of the first correction amount and the average energy.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to correction of artifacts in a material decomposition image, in which materials within a subject are differentiated, created based on projection data divided into a plurality of energy bins acquired by a photon counting detector or the like. [Background technology]

[0002] Photon Counting Computed Tomography (PCCT) devices equipped with photon counting detectors that use the photon counting method can display medical images that contain more information than conventional CT devices. For example, they can display energy bin images, which are images created using projection data divided into multiple energy bins, and material decomposition images, which are images decomposed into multiple materials.

[0003] However, in X-ray CT images, artifacts due to beam hardening may occur in areas with high X-ray absorption coefficients, such as bone tissue in the subject or metal parts used for treatment, such as dark bands that reduce the CT value between areas with high X-ray absorption coefficients. Artifacts due to beam hardening can be corrected by a method such as that disclosed in Patent Document 1. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2009-50413 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in Patent Document 1, no consideration is given to band artifacts such as dark bands that occur in a material decomposition image. When a dark band occurs in one material decomposition image, a bright band occurs in the other material decomposition image, and correction of only one band artifact is insufficient.

[0006] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a PCCT apparatus capable of correcting band artifacts in one material decomposition image as well as band artifacts in the other material decomposition image. [Means for solving the problem]

[0007] In order to achieve the above object, the present invention provides a PCCT device that acquires projection data divided into a plurality of energy bins by irradiating a subject with X-rays, and is characterized by comprising a first correction unit that corrects band artifacts in a first material decomposition image among a plurality of material decomposition images created based on the projection data and calculates a first correction amount that is a correction amount for the band artifacts, an energy calculation unit that calculates an average energy of X-rays transmitted through the subject, and a second correction unit that corrects band artifacts in a second material decomposition image using a second correction amount that is a correction amount calculated based on the first correction amount and the average energy. Effect of the Invention

[0008] According to the present invention, it is possible to provide a PCCT apparatus capable of correcting band artifacts in one material decomposition image as well as band artifacts in the other material decomposition image. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing the overall configuration of a PCCT device. [Diagram 2] FIG. 1 shows an example of X-rays divided into multiple energy bins. [Diagram 3] FIG. 13 is a diagram illustrating material decomposition data. [Figure 4]FIG. 13 is a diagram showing an example of a material decomposition map. [Diagram 5] 1A and 1B are diagrams illustrating band artifacts in a material decomposition image. [Figure 6] FIG. 2 is a diagram showing functional blocks of the first embodiment. [Figure 7] FIG. 2 is a diagram illustrating an example of a processing flow according to the first embodiment. [Figure 8] FIG. 11 is a diagram showing an example of a flow of correction processing for a material decomposition image. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of a PCCT (Photon Counting Computed Tomography) device 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 designated by the same reference numerals, and duplicated explanations will be omitted. EXAMPLES

[0011] The overall configuration of the PCCT device 100 will be described with reference to Fig. 1. The PCCT device 100 includes an input / output unit 200, an imaging unit 300, and a general control unit 400.

[0012] The input / output unit 200 has an input device 210 and a monitor 220. The input device 210 is a device used when an operator inputs imaging conditions, etc., and is, for example, a mouse or a keyboard. The monitor 220 is a display device that outputs the input imaging conditions, etc., and is also used as the input device 210 if it has a touch panel function.

[0013] The imaging unit 300 includes an X-ray source 310, an X-ray detector 320, a gantry 330, a table 102, and an imaging control unit 340 in order to acquire projection data of the subject 101 at various projection angles. The acquired projection data is divided into a number of energy bins.

[0014] The X-ray source 310 is a device that irradiates the subject 101 with X-rays. A collimator 311 is provided between the X-ray source 310 and the subject 101. The collimator 311 is a device that adjusts the length of the X-rays irradiated to the subject 101 in the z direction.

[0015] The X-ray detector 320 is a device that detects direct rays, which are X-rays that pass through the subject 101 without being scattered, and has multiple detection elements 321. The detection elements 321 are arranged at equal distances, for example, at a distance of 1000 mm, from the X-ray generation point of the X-ray source 310, and approximately 1000 elements are arranged. The detection elements 321 are elements that detect X-rays, and output an electrical signal according to the amount of X-rays incident on each element. The detection elements 321 are arranged on the xy plane, and have a size of, for example, 0.5 mm square. The detection elements 321 may be indirect detection elements that combine scintillator elements and photodiode elements, or may be semiconductor detection elements represented by CdTe. In the indirect detection elements, the scintillator elements emit fluorescence due to the incident X-rays, and the fluorescence is converted into an electrical signal by the photodiode elements.

[0016] The detection element 321 detects the incident X-rays by dividing them into a plurality of energy bins as shown in Fig. 2. In Fig. 2, the X-rays detected by dividing them into three energy bins, T1-T2, T2-T3, and T3-, are shown as bin1, bin2, and bin3. Note that X-rays of various combinations of energy bins may be calculated from the X-rays detected by dividing them into a plurality of energy bins. For example, the X-ray intensity of bin1+bin2 or the X-ray intensity of bin2+bin3 may be calculated from the X-ray intensities of bin1, bin2, and bin3.

[0017] Returning to the explanation of Fig. 1, a circular opening 331 is provided in the center of the gantry 330 for arranging the table 102 on which the subject 101 is placed. The diameter of the opening 331 is, for example, 700 mm. A rotating plate 332 is provided inside the gantry 330, which mounts the X-ray source 310 and the X-ray detector 320 and rotates the X-ray source 310 and the X-ray detector 320 around the subject 101. The table 102 moves in the z direction to adjust the position of the subject 101 with respect to the gantry 330.

[0018] The imaging control unit 340 has an X-ray control unit 341, a gantry control unit 342, a table control unit 343, and a detector control unit 344. The X-ray control unit 341 controls the voltage applied to the X-ray source 310, etc. The gantry control unit 342 controls the rotation of the rotating plate 332, for example, rotating the rotating plate 332 at 1.0 s / turn. The detector control unit 344 controls the detection of X-rays by the X-ray detector 320, for example, causing the X-ray detector 320 to detect X-rays at 0.4 degrees / turn. The table control unit 343 controls the movement of the table 102.

[0019] The general control unit 400 has a CPU (Central Processing Unit) 401, a memory 402, and a storage device 403, and controls the imaging control unit 340, and performs various processes on the projection data acquired by the X-ray detector 320. For example, the general control unit 400 executes a process of reconstructing a tomographic image from the projection data acquired according to the imaging conditions set by the input device 210. The tomographic image may be reconstructed for each energy bin. The reconstructed tomographic image and the projection data used in the reconstruction process may be displayed on the monitor 220 or stored in the storage device 403, and may be handled for each energy bin. Furthermore, material decomposition data used for creating a material decomposition image is obtained by imaging a combination of a plurality of basis materials whose compositions and thicknesses are known.

[0020] The material decomposition data 520 used to create a material decomposition image will be described with reference to FIG. 3. The material decomposition data 520 is a plurality of types of X-ray energy spectra acquired using a calibration member 510 made of a plurality of basis materials whose compositions and thicknesses are known. The calibration member 510 is, for example, a combination of a first basis material 511 and a second basis material 512, each of which has an arc shape centered on the X-ray focal point 501 and a uniform thickness. The first basis material 511 is a material having a relatively small effective atomic number, such as acrylic or polyethylene, and the second basis material 512 is a material having a relatively large effective atomic number, such as aluminum or a calcium mixture, an iodine mixture, or tin. The first basis material 511 and the second basis material 512 each have a plurality of different thicknesses. For example, if the first basis material 511 has J types of thicknesses and the second basis material 512 has K types of thicknesses, J×K types of calibration members 510 are used, and an X-ray energy spectrum is acquired for each combination for each detection element 321. 3, since J=3 and K=3, nine types of X-ray energy spectra are shown as the material decomposition data 520. Note that the material decomposition data 520 is acquired in advance, prior to imaging of the subject 101.

[0021] The acquired material decomposition data 520 may be stored in the storage device 403 as, for example, a material decomposition map 521 shown in Fig. 4. The material decomposition map 521 shown in Fig. 4 is a result of detecting X-rays that have passed through a calibration member 510, which is a combination of acrylic having a thickness of 0 mm to 200 mm and aluminum having a thickness of 0 mm to 8 mm, and are divided into three energy bins. Note that the material decomposition map 521 is not limited to the table shown in Fig. 4, and may be expressed by a graph, a formula, or the like.

[0022] A procedure for creating a material decomposition image using the material decomposition data 520 will be described. The projection data of the subject 101 acquired at various projection angles by the imaging unit 300 has an X-ray energy spectrum for each detection element 321. Then, from the X-ray energy spectra of the material decomposition data 520, one having a shape closest to the X-ray energy spectrum for each detection element 321 is searched for, and a combination of thicknesses of the basis materials corresponding to the searched X-ray energy spectrum is obtained. That is, if the first basis material 511 is acrylic and the second basis material 512 is aluminum, the thicknesses of the acrylic and the aluminum are obtained for each detection element 321 at various projection angles, and the projection data of the acrylic and the projection data of the aluminum are obtained. Then, a tomographic image for each basis material is reconstructed from the projection data for each basis material thus obtained, thereby creating a material decomposition image.

[0023] The band artifacts occurring in material decomposition images will be described with reference to Fig. 5. Fig. 5 shows an aluminum image 540 and an acrylic image 550 as material decomposition images created using projection data of a phantom 530 simulating a subject including a region with a high X-ray absorption coefficient. The phantom 530 includes rod-shaped aluminum 531 in cylindrical acrylic 532. The aluminum 531 simulates the region with a high X-ray absorption coefficient. The acrylic 532 simulates the region other than the region with a high X-ray absorption coefficient.

[0024] In an aluminum image 540 which is an image showing the distribution of aluminum, only an aluminum region 541 corresponding to aluminum 531 in a phantom 530 should be displayed. However, between the aluminum regions 541 which correspond to areas with a high X-ray absorption coefficient, a dark band 542 which is a band artifact having a lower pixel value than the surroundings occurs. Also, in an acrylic image 550 which is an image showing the distribution of acrylic, the pixel value of an acrylic region 551 which corresponds to acrylic 532 should be uniform. However, between the regions corresponding to aluminum 531, a bright band 552 which is a band artifact having a higher pixel value than the surroundings occurs. A medical image including a dark band 542 or a bright band 552 reduces the accuracy of diagnosis.

[0025] The dark band 542 and the bright band 552 are generated because the ratio of scattered rays contained in the X-rays incident on the detection element 321a located directly under the aluminum 531 is higher than the ratio of scattered rays contained in the material decomposition data 520. That is, in Fig. 5, most of the scattered rays generated by the acrylic 532 are incident on the detection element 321a, whereas in Fig. 3, the scattered rays generated by the acrylic, which is the first basis material 511, are absorbed by the aluminum, which is the second basis material 512, and the amount of scattered rays incident on the detection element 321a is relatively small. As a result, even if the thickness combinations of the aluminum 531 and the acrylic 532 and the first basis material 511 and the second basis material 512 are the same, differences occur in the X-ray energy spectrum at the detection element 321a, and errors occur in the thicknesses obtained by material decomposition. More specifically, since the X-ray energy spectrum in Fig. 5 has a slightly larger X-ray intensity on the low energy side than in Fig. 3, thinner aluminum and thicker acrylic are required than in reality, and a dark band 542 and a bright band 552 occur due to thickness errors. Note that the difference between the average energy of the X-rays after passing through the phantom 530 and the average energy of the X-rays after passing through the calibration member 510 is within the allowable range.

[0026] Although one of the dark band 542 and the bright band 552 can be dealt with by beam hardening correction, the other artifact cannot be dealt with. Therefore, in the first embodiment, the band artifact of one material decomposition image is corrected using a first correction amount that is a correction amount used when correcting the band artifact of the other material decomposition image, and a second correction amount that is a correction amount calculated based on the average energy of X-rays transmitted through the subject.

[0027] The functional blocks of the first embodiment will be described with reference to Fig. 6. These functional blocks may be configured with dedicated hardware, or may be configured with software that runs on the CPU 401. In the following description, the functional blocks of the first embodiment will be configured with software. In the first embodiment, a first correction unit 601, an energy calculation unit 602, and a second correction unit 603 are provided. Each component will be described below.

[0028] The first correction unit 601 corrects band artifacts in the first material decomposition image and calculates a first correction amount, which is a correction amount of the band artifacts in the first material decomposition image. The band artifacts in the first material decomposition image may be corrected by any method, and may be, for example, the method described in Patent Document 1. The first correction amount is calculated by subtracting the material decomposition image before correction from the material decomposition image after the band artifacts are corrected.

[0029] The energy calculation unit 602 calculates the average energy of the X-rays transmitted through the subject 101. The average energy may be calculated based on a virtual monochromatic image, which is an image at a specific X-ray energy, created using a material decomposition image including band artifacts. That is, a virtual monochromatic image with the smallest band artifacts is extracted from a plurality of virtual monochromatic images, and the X-ray energy corresponding to the extracted virtual monochromatic image is calculated as the average energy. Alternatively, the average energy may be calculated from the X-ray energy spectrum of the projection data of the subject 101.

[0030] The second correction unit 603 calculates a second correction amount based on the first correction amount and the average energy, and corrects band artifacts in the other material decomposition image by using the second correction amount. The second correction amount COR2 is calculated by using, for example, the following equation.

[0031] COR2=μ1(E)·ρ1·COR1 / (μ2(E)·ρ2) … (Formula 1) where μ(E) is the X-ray attenuation coefficient of the first material at X-ray energy E, ρ is the density of the first material, COR is the first correction quantity, μ(E) is the X-ray attenuation coefficient of the second material at X-ray energy E, ρ is the density of the second material, and E is the average energy.

[0032] An example of the process flow of the first embodiment will be described step by step with reference to FIG.

[0033] (S701) The general control unit 400 controls the imaging control unit 340 to obtain projection data of the subject 101 .

[0034] (S702) The general control unit 400 performs material decomposition on the projection data acquired in S701 to acquire projection data of the first material and projection data of the second material. For the material decomposition, for example, material decomposition data 520 acquired in advance is used.

[0035] (S703) The general control unit 400 creates a first material decomposition image and a second material decomposition image by reconstructing the projection data of the first material and the projection data of the second material acquired in S702.

[0036] (S704) The general control unit 400 creates a plurality of virtual monochromatic images using the first material decomposition image and the second material decomposition image acquired in S703. The pixel value V of the virtual monochromatic image at the X-ray energy E is calculated, for example, using the following equation.

[0037] V=μ1(E)·ρ1·M1+μ2(E)·ρ2·M2 … (Formula 2) Here, μ(E) is the X-ray attenuation coefficient of the first material at X-ray energy E, ρ is the density of the first material, M is the pixel value of the first material decomposition image, μ(E) is the X-ray attenuation coefficient of the second material at X-ray energy E, ρ is the density of the second material, and M is the pixel value of the second material decomposition image.

[0038] (S705) The integrated control unit 400 determines whether or not a band artifact is included in the first material decomposition image or the second material decomposition image acquired in S703, or the virtual monochromatic image created in S704. If a band artifact is present, the process proceeds to S706, and if not, the process proceeds to S708. The presence or absence of a band artifact may be determined by a determiner created in advance by machine learning, or may be determined by the operator.

[0039] (S706) The general control unit 400 corrects band artifacts in the material decomposition image.

[0040] An example of the flow of the material decomposition image correction process in S706 will be described step by step with reference to FIG.

[0041] (S801) The first correction unit 601 corrects band artifacts in one of the material decomposition images, for example, the dark bands 542 in the aluminum image 540. The band artifacts are corrected by, for example, beam hardening correction.

[0042] (S802) The first correction unit 601 calculates a first correction amount, which is a correction amount of the band artifact of one of the material decomposition images. The first correction amount is calculated by subtracting the aluminum image 540, which is an image before correction, from the image after correction obtained in S801.

[0043] (S803) The energy calculation unit 602 calculates the average energy of the X-rays transmitted through the subject 101. For example, a virtual monochromatic image with the smallest band artifact is extracted from the multiple virtual monochromatic images created in S704, and the X-ray energy corresponding to the extracted virtual monochromatic image is calculated as the average energy. Alternatively, the average energy may be calculated from the X-ray energy spectrum of the projection data acquired in S701.

[0044] (S804) The second correction unit 603 calculates a second correction amount based on the first correction amount calculated in S802 and the average energy calculated in S803. The second correction amount is calculated using, for example, (Equation 1).

[0045] (S805) The second correction unit 603 uses the second correction amount calculated in S804 to correct a band artifact in the other material decomposition image, for example, the bright band 552 in the acrylic image 550. That is, the second correction amount is added to the other material decomposition image, for example, the acrylic image 550, thereby correcting the bright band 552, which is a band artifact in the other material decomposition image.

[0046] 8, band artifacts in one material decomposition image and band artifacts in the other material decomposition image are corrected. Returning to the description of FIG.

[0047] (S707) The general control unit 400 creates a virtual monochromatic image using the first material decomposition image and the second material decomposition image corrected in S706. The pixel values ​​of the virtual monochromatic image are calculated using, for example, (Equation 2).

[0048] (S708) The integrated control unit 400 stores the first material decomposition image or the second material decomposition image corrected in S706, or the virtual monochromatic image created in S705 or S707, in the storage device 403. The material decomposition image or the virtual monochromatic image stored in the storage device 403 is displayed on the monitor 220 as necessary and used for diagnosis.

[0049] According to the processing flow exemplified in FIG. 7, a material decomposition image and a virtual monochromatic image in which band artifacts have been corrected are generated, and diagnostic accuracy can be improved.

[0050] 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]

[0051] 100: PCCT device, 101: subject, 102: table, 200: input / output unit, 210: input device, 220: monitor, 300: imaging unit, 310: X-ray source, 311: collimator, 320: X-ray detector, 321: detection element, 330: gantry, 331: opening, 332: rotating plate, 340: imaging control unit, 341: X-ray control unit, 342: gantry control unit, 343: table control unit, 344: detector control unit, 400: general control unit, 401: CPU, 402 : memory, 403: storage device, 501: X-ray focus, 510: calibration member, 511: first basis material, 512: second basis material, 520: material decomposition data, 521: material decomposition map, 531: aluminum, 532: acrylic, 540: aluminum image, 541: aluminum region, 542: dark band, 550: acrylic image, 551: acrylic region, 552: bright band, 601: first correction unit, 602: energy calculation unit, 603: second correction unit

Claims

1. A PCCT apparatus for acquiring projection data divided into a plurality of energy bins by irradiating an object with X-rays, comprising: a first correction unit that corrects a band artifact in a first material decomposition image among a plurality of material decomposition images created based on the projection data, and calculates a first correction amount that is a correction amount of the band artifact; an energy calculation unit that calculates an average energy of the X-rays transmitted through the subject; A PCCT apparatus comprising: a second correction unit that corrects band artifacts in a second material decomposition image by using a second correction amount that is a correction amount calculated based on the first correction amount and the average energy.

2. 2. The PCCT apparatus according to claim 1, The PCCT apparatus according to claim 1, wherein the energy calculation unit calculates the average energy based on a plurality of virtual monochromatic images created using the first material decomposition image and the second material decomposition image.

3. 3. The PCCT apparatus according to claim 2, The energy calculation unit extracts a virtual monochromatic image having the smallest band artifact from the plurality of virtual monochromatic images, and sets the X-ray energy corresponding to the extracted virtual monochromatic image as the average energy.

4. 2. The PCCT apparatus according to claim 1, The PCCT apparatus is characterized in that the energy calculation unit calculates the average energy from an X-ray energy spectrum of the projection data.

5. 2. The PCCT apparatus according to claim 1, The PCCT apparatus is characterized in that, when the X-ray attenuation coefficient of a first material at mean energy E is μ1(E), the density of the first material is ρ1, the X-ray attenuation coefficient of a second material at mean energy E is μ2(E), the density of the second material is ρ2, and the first correction amount is COR1, the second correction amount COR2 is calculated by COR2 = μ1(E) ρ1 COR1 / (μ2(E) ρ2).

6. 2. The PCCT apparatus according to claim 1, The PCCT apparatus, wherein the first correction unit corrects band artifacts in the first material decomposition image by beam hardening correction.

Images