Positioning device, method and program, and radiation image processing device, method and program

Abstract

To execute reproducible positioning easily when imaging a subject in a positioning device, method, and program, and a radiation image processing device, method, and program.SOLUTION: A processor acquires an image for positioning obtained by imaging a subject with a radiation image pick-up device, specifies a criterion position in the image for positioning and a reference position in an object structure included in the subject in the image for positioning, and derives a relative amount of movement of a top plate, a radiation source, and a radiation detector on the basis of an amount of deviation from the criterion position of the reference position.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a positioning device, method and program, as well as a radiation image processing device, method and program.

Background Art

[0002] In bone diseases such as osteoporosis, various methods for deriving bone mineral content used for diagnosing bone density have been proposed. For example, in Patent Document 1, pixel values of a bone region of a subject are obtained from two radiation images obtained by irradiating the subject with two types of radiation having different energy distributions, and based on the imaging conditions, the thickness of the subject, and the pixel values of the bone region, a method for deriving the bone mineral content in the bone region for each pixel in the bone region has also been proposed.

[0003] On the other hand, when obtaining a radiation image of a subject lying supine on an imaging table, the subject is positioned so that the target site is located at a desired position in the radiation image. As such a positioning method, a method has been proposed in which a positioning error between reference image data for specifying and diagnosing an affected part of a subject imaged in advance and positioning image data obtained by reading the inside of the subject as an image is obtained, and the imaging table is horizontally moved based on the positioning error (see Patent Document 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] By the way, when diagnosing osteoporosis, it is important to observe the change in bone mineral density over time. Therefore, when deriving bone mineral density using a radiographic image, it is necessary to position the subject so that the optical axis of the radiation in the imaging device coincides with the target bone for which the bone mineral density is to be derived, so that the target bone can be imaged reproducibly under the same conditions at all times.

[0006] However, the positioning of the subject is left to the skill of the radiologic technologist, and in some cases, it may be difficult for the radiologic technologist to position the target bone so that it coincides with the optical axis of the radiation. In such cases, during imaging, the radiologic technologist repeats trial and error to align the target bone with the optical axis of the radiation. Therefore, the burden on the radiologic technologist who performs the positioning operation is large.

[0007] The present disclosure has been made in view of the above circumstances, and an object thereof is to enable easy and reproducible positioning during imaging of a subject.

Means for Solving the Problems

[0008] The positioning device according to the present disclosure includes a radiation source that irradiates a subject with radiation, a top plate on which the subject is placed, at least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting the radiation that has passed through the subject, a positioning device for a subject in a radiation image capturing device including a moving mechanism that relatively moves the top plate, the radiation source, and the radiation detector in a direction intersecting the optical axis of the radiation, including at least one processor, the processor acquires a positioning image acquired by imaging the subject with the radiation image capturing device, identifies a reference position in the positioning image and a reference position within a target structure included in the subject in the positioning image, and derives a relative movement amount of the top plate, the radiation source, and the radiation detector based on the deviation amount of the reference position from the reference position.

[0009] In the positioning device according to the present disclosure, the processor may identify a target structure in the positioning image and identify the reference position by deriving the reference position within the identified target structure. In the positioning device according to the present disclosure, the processor may also identify the reference position by displaying the positioning image and receiving the designation of the reference position in the displayed positioning image.

[0010] In the positioning device according to the present disclosure, the processor may also derive the magnification of the target structure on the positioning image based on the shooting distance when the positioning image is acquired and the distance between the radiation detector and the target structure, and derive the movement amount by correcting the deviation amount of the reference position from the reference position of the reference position by the magnification.

[0011] In the positioning device according to the present disclosure, the processor may also derive the magnification of the target structure on the positioning image based on the shooting distance when the positioning image is acquired, the thickness of the subject, and the ratio of the position where the target structure exists to the thickness, and derive the movement amount by correcting the deviation amount of the reference position from the reference position of the reference position by the magnification.

[0012] In the positioning device according to the present disclosure, the processor may also control the movement mechanism based on the movement amount to relatively move the top plate, the radiation source, and the radiation detector. In the positioning device according to the present disclosure, the processor may also display the movement amount.

[0013] In the positioning device according to the present disclosure, the processor may also control the movement mechanism based on the movement amount to relatively move the top plate, the radiation source, and the radiation detector.

[0014] In the positioning device according to the present disclosure, the processor may also display the movement amount.

[0015] Further, in the positioning device according to the present disclosure, the processor may update the amount of movement according to the relative movement of the top plate, the radiation source, and the radiation detector, and display the updated amount of movement.

[0016] Further, in the positioning device according to the present disclosure, the reference position in the positioning image is the center of the irradiation field in the positioning image, the object structure is the target bone, and the reference position within the object structure may be the center of gravity of the target bone.

[0017] The radiation image processing apparatus according to the present disclosure includes at least one processor, and the processor acquires a first radiation image and a second radiation image based on radiations having different energy distributions respectively, which are acquired by photographing a subject in which the target bone is positioned by the positioning device according to the present disclosure, derives a bone part image by extracting the bone part of the subject from the first radiation image and the second radiation image, and derives at least the bone mineral content of the target bone based on the pixel values of at least the region of the target bone in the bone part image.

[0018] Note that, in the radiation image processing apparatus according to the present disclosure, the processor removes the scattered ray component from the first radiation image and the second radiation image, and may derive the bone part image and derive the bone mineral content using the first radiation image and the second radiation image from which the scattered ray component has been removed.

[0019] The positioning method according to the present disclosure includes a radiation source that irradiates a subject with radiation, a top plate on which the subject is placed, and at least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting the radiation that has passed through the subject. A method for positioning a subject in a radiation imaging apparatus including a top plate, a radiation source, and a radiation detector, and a moving mechanism that relatively moves the radiation source and the radiation detector in a direction intersecting the optical axis of the radiation, Obtain a positioning image acquired by photographing a subject with a radiation imaging apparatus, Specify a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image, Based on the deviation amount of the reference position from the reference position, derive the relative movement amount between the top plate, the radiation source, and the radiation detector.

[0020] The radiation image processing method according to the present disclosure acquires a first radiation image and a second radiation image based on radiations having different energy distributions from each other, which are acquired by photographing a subject in which a target bone is positioned by the positioning apparatus according to the present disclosure, Derive a bone part image obtained by extracting the bone part of the subject from the first radiation image and the second radiation image, Based on the pixel values of at least the region of the target bone in the bone part image, derive at least the bone mineral content of the target bone.

[0021] Note that the positioning method and the radiation image processing method according to the present disclosure may be provided as a radiation image processing program to be executed by a computer.

Advantages of the Invention

[0022] According to the present disclosure, reproducible positioning can be performed when photographing a subject.

Brief Description of the Drawings

[0023]

Figure 1

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Figure 15

Mode for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. FIG. 1 is a schematic block diagram showing the configuration of a radiation image imaging system to which the positioning device and the radiation image processing device according to the embodiment of the present disclosure are applied. As shown in FIG. 1, the radiation image imaging system according to the present embodiment includes an imaging device 1 and a positioning device and a radiation image processing device according to the present embodiment (hereinafter, may be represented by the radiation image processing device in some cases) 10.

[0025] The imaging device 1 is an imaging device for performing energy subtraction by a so-called one-shot method in which the first radiation detector 5 and the second radiation detector 6 irradiate radiation such as X-rays emitted from the radiation source 2 and transmitted through the subject H lying supine on the top plate 3A of the imaging table 3, respectively, after changing the energy. At the time of imaging, as shown in FIG. 1, in order from the side close to the radiation source 2, a scatter radiation removal grid (hereinafter simply referred to as a grid) 4, the first radiation detector 5, a radiation energy conversion filter 7 made of a copper plate or the like, and the second radiation detector 6 are arranged, and the radiation source 2 is driven. The first and second radiation detectors 5 and 6 are in close contact with the radiation energy conversion filter 7. The grid 4, the first radiation detector 5, the radiation energy conversion filter 7, and the second radiation detector 6 are arranged at positions facing the radiation source 2 with the top plate 3A of the imaging table 3 sandwiched therebetween below the top plate 3A.

[0026] As a result, in the first radiation detector 5, a first radiation image G1 of the subject H is obtained by low-energy radiation including so-called soft rays. In the second radiation detector 6, a second radiation image G2 of the subject H is obtained by high-energy radiation from which soft rays are removed. The first and second radiation images G1 and G2 are input to the radiation image processing device 10.

[0027] The first and second radiation detectors 5 and 6 can repeatedly record and read out radiation images, and a so-called direct-type radiation detector that directly receives radiation irradiation and generates charges may be used, or a so-called indirect-type radiation detector that once converts radiation into visible light and then converts the visible light into a charge signal may be used. As a method for reading out the radiation image signal, it is desirable to use a so-called TFT readout method in which the radiation image signal is read out by turning on and off a TFT (thin film transistor) switch, or a so-called optical readout method in which the radiation image signal is read out by irradiating reading light, but it is not limited to this and other methods may be used.

[0028] In the imaging device 1, in order to acquire the positioning image GP described later, the subject H may be imaged using only one radiation detector. In this case, the first radiation detector 5 on the side closer to the radiation source 2 is used.

[0029] In the imaging device 1, the radiation emitted from the radiation source 2 is positioned such that its optical axis irradiates the centers of the first radiation detector 5 and the second radiation detector 6. Also, the imaging table 3 is positioned such that the top plate 3A is orthogonal to the optical axis of the radiation.

[0030] The grid 4 is configured by alternately arranging, for example, a material such as lead that does not transmit radiation and an interspace material such as aluminum or fiber that easily transmits radiation at a fine grid density of about 4.0 lines / mm. By using the grid 4, the scattered ray component of the radiation transmitted through the subject H can be removed, but it cannot be completely removed. For this reason, the first and second radiation images G1, G2 include not only the primary ray component of the radiation transmitted through the subject H but also the scattered ray component.

[0031] Note that the primary ray component is the signal component of the pixel value represented by the radiation that has reached the radiation detector without being scattered by the subject H among the radiation transmitted through the subject H. On the other hand, the scattered ray component is the signal component of the pixel value represented by the radiation that has been scattered by the subject H and has reached the radiation detector among the radiation transmitted through the subject H.

[0032] The imaging device 1 also includes a moving mechanism 8 that two-dimensionally moves the top plate 3A in a direction intersecting the optical axis of the radiation emitted from the radiation source 2. In this embodiment, the direction intersecting the optical axis of the radiation is the direction orthogonal to the optical axis (the left-right direction and the depth direction of the paper surface in FIG. 1, hereinafter referred to as the horizontal direction). The moving mechanism 8 can move the top plate 3A two-dimensionally in the horizontal direction and can stop at a determined stop position after the movement. Although not shown, the moving mechanism 8 can use known reciprocating means such as belt moving means for moving the top plate 3A by a motor and a belt connected thereto, moving means by a motor, a drive screw rotated thereby, and a traveling nut, or moving means by a motor, a gear and a pinion rotated thereby, and a rack, etc., or known parallel moving means. Note that the moving mechanism 8 may be configured to manually move the top plate 3A without including a motor.

[0033] The moving mechanism 8 is also provided with a sensor (not shown) for measuring the movement amount of the top plate 3A. As the sensor, one that detects the rotation speed of the motor to detect the movement amount in each of the left-right direction and the depth direction of the paper surface in FIG. 1 can be used. Alternatively, a sensor for measuring the movement amount may be used by providing a scale on the top plate 3A and counting the number of scales corresponding to the movement of the top plate 3A.

[0034] The radiation image processing device 10 is connected to an image storage system 9 via a network (not shown). The image storage system 9 is a system that stores the image data of the radiation image captured by the imaging device 1. The image storage system 9 extracts an image corresponding to a request from the radiation image processing device 10 from the stored radiation images and transmits it to the requesting device. As a specific example of the image storage system 9, PACS (Picture Archiving and Communication Systems) can be mentioned.

[0035] Next, the positioning device and the radiation image processing device according to the present embodiment will be described. First, with reference to FIG. 2, the hardware configuration of the positioning device and the radiation image processing device according to the present embodiment will be described. As shown in FIG. 2, the positioning device and the radiation image processing device 10 are computers such as a workstation, a server computer, and a personal computer, and include a CPU (Central Processing Unit) 11, a nonvolatile storage 13, and a memory 16 as a temporary storage area. The positioning device and the radiation image processing device 10 also include a display 14 such as a liquid crystal display, an input device 15 such as a keyboard and a mouse, and a network I / F (InterFace) 17 connected to a network (not shown). The CPU 11, the storage 13, the display 14, the input device 15, the memory 16, and the network I / F 17 are connected to a bus 18. Note that the CPU 11 is an example of the processor in the present disclosure.

[0036] The storage 13 is realized by an HDD (Hard Disk Drive), an SSD (Solid State Drive), a flash memory, or the like. The storage 13 as a storage medium stores a positioning program 12A and a radiation image processing program 12B installed in the positioning device and the radiation image processing device 10. The CPU 11 reads the positioning program 12A and the radiation image processing program 12B from the storage 13, expands them in the memory 16, and executes the expanded positioning program 12A and radiation image processing program 12B.

[0037] The positioning program 12A and the radiation image processing program 12B are stored in a storage device of a server computer connected to a network or in a network storage in a state accessible from the outside, and are downloaded and installed on a computer constituting the positioning device and the radiation image processing device 10 in response to a request. Alternatively, they are recorded on a recording medium such as a DVD (Digital Versatile Disc) or a CD-ROM (Compact Disc Read Only Memory), distributed, and installed on a computer constituting the positioning device and the radiation image processing device 10 from the recording medium.

[0038] Next, the functional configuration of the positioning device and the radiation image processing device according to this embodiment will be described. FIG. 3 is a diagram showing the functional configuration of the positioning device and the radiation image processing device according to this embodiment. As shown in FIG. 3, the positioning device and the radiation image processing device 10 include an image acquisition unit 21, a position identification unit 22, a movement amount derivation unit 23, a movement control unit 24, a scattered ray removal unit 25, a subtraction unit 26, a bone mineral content derivation unit 27, and a display control unit 28. Then, the CPU 11 functions as the image acquisition unit 21, the position identification unit 22, the movement amount derivation unit 23, the movement control unit 24, and the display control unit 28 by executing the positioning program 12A. Further, the CPU 11 functions as the image acquisition unit 21, the scattered ray removal unit 25, the subtraction unit 26, the bone mineral content derivation unit 27, and the display control unit 28 by executing the radiation image processing program 12B.

[0039] When performing positioning, the image acquisition unit 21 drives the radiation source 2 to irradiate the subject H with radiation, detects the radiation that has passed through the subject H using the first radiation detector 5, and acquires a positioning image GP. The positioning image GP is only used for positioning the subject H and is not used for diagnosis. Therefore, when acquiring the positioning image GP, the radiation irradiated to the subject H is set to a lower dose than when acquiring a radiation image used for diagnosis. Note that the positioning image GP may be acquired using radiation at the same dose as when acquiring a radiation image used for diagnosis. When acquiring the positioning image GP, imaging conditions such as the imaging dose, tube voltage, SID (Source Image receptor Distance), which is the distance between the surface of the radiation source 2 and the first radiation detector 5, and SOD (Source Object Distance), which is the distance between the surface of the radiation source 2 and the subject H, are set.

[0040] The SOD is preferably acquired using, for example, a TOF (Time Of Flight) camera. The SID is preferably acquired using, for example, a potentiometer, an ultrasonic distance meter, a laser distance meter, or the like. The other imaging conditions may be set by input from the input device 15 by the radiation technologist who is the operator. The set imaging conditions are stored in the storage 13.

[0041] Also, when deriving the bone mineral content, the image acquisition unit 21 drives the radiation source 2 to irradiate the subject H with radiation, detects the radiation that has passed through the subject H using the first and second radiation detectors 5 and 6, and acquires the first and second radiation images G1 and G2. Also in this case, imaging conditions are set and stored in the storage 13.

[0042] In this embodiment, the positioning image GP, the first radiographic image G1, and the second radiographic image G2 may be acquired by a program separate from the positioning program 12A and the radiographic image processing program 12B and stored in the storage 13. Further, when the positioning image GP, the first radiographic image G1, and the second radiographic image G2 have already been acquired and stored in the image storage system 9, the image acquisition unit 21 may acquire these images from the image storage system 9. In this case, the image acquisition unit 21 reads out the positioning image GP, the first radiographic image G1, and the second radiographic image G2 stored in the storage 13 from the storage 13 for processing.

[0043] The position specifying unit 22 specifies a reference position P1 in the positioning image GP and a reference position P2 of a target structure included in the subject H in the positioning image GP. In this embodiment, the reference position P1 in the positioning image GP is the center of the irradiation field in the positioning image GP. When the positioning image GP of the subject H is acquired without using an irradiation field diaphragm, the center of the irradiation field is the center of gravity of the positioning image GP. When the positioning image GP is acquired using an irradiation field diaphragm, the center of the irradiation field is the center of gravity of the irradiation field region in the positioning image GP.

[0044] In this embodiment, the target structure is the fifth lumbar vertebra, and the reference position of the target structure is the center of gravity of the fifth lumbar vertebra. The fifth lumbar vertebra is an example of a target bone. Therefore, the position specifying unit 22 first specifies the fifth lumbar vertebra from the positioning image GP. The specification of the fifth lumbar vertebra may be performed by using a learned neural network that has been trained to extract the fifth lumbar vertebra from the positioning image GP. Further, a template representing the shape of the fifth lumbar vertebra may be stored in the storage 13, and the fifth lumbar vertebra may be specified by template matching between the template and the positioning image GP.

[0045] Note that the positioning image GP may be displayed on the display 14, and the fifth lumbar vertebra and the center of gravity of the fifth lumbar vertebra may be specified by receiving a specific instruction for the fifth lumbar vertebra and the center of gravity of the fifth lumbar vertebra using the input device 15 by the operator. In this case, the position specifying unit 22 specifies the center of gravity of the specified fifth lumbar vertebra as the reference position P2. FIG. 4 is a diagram showing the positioning image GP. Note that FIG. 4 shows the specified reference position P1 and reference position P2. As shown in FIG. 4, it can be seen that the reference position P2 is shifted from the reference position P1. In FIG. 4, the left-right direction is the x direction, and the up-down direction is the y direction. The x direction and the y direction respectively correspond to the depth direction and the left-right direction in FIG. 1.

[0046] The movement amount derivation unit 23 derives the relative movement amount between the top plate 3A, the radiation source 2, and the radiation detectors 5 and 6 based on the deviation amount of the reference position P2 from the reference position P1. FIG. 5 is a diagram for explaining the deviation between the reference position P2 and the reference position P1. As shown in FIG. 5, when the fifth lumbar vertebra L5, which is the target bone of the subject H, is not located on the optical axis X0 of the radiation emitted from the radiation source 2, on the detection surface 5A of the radiation detector 5, the position of the optical axis X0 and the position of the fifth lumbar vertebra L5 are deviated. As a result, in the positioning image GP, as shown in FIG. 4, the reference position P2 is deviated from the reference position P1.

[0047] In the present embodiment, in order to match the position of the optical axis X0 and the center of gravity of the fifth lumbar vertebra L5, the movement amount derivation unit 23 first derives the deviation amount (Δx0, Δy0) between the reference position P1 and the reference position P2 on the positioning image GP. The deviation amount is the distance in the x direction and the distance in the y direction on the positioning image GP for matching the reference position P2 with the reference position P1.

[0048] On the one hand, since the target bone (the fifth lumbar vertebra L5) is present within the subject H, and further the subject H is placed on the top plate 3A, the target bone is located at a position away from the detection surface 5A of the radiation detector 5. For this reason, the amount of deviation between the center of gravity of the target bone within the subject H and the optical axis X0 of the radiation will be magnified on the positioning image GP. As a result, even if the top plate 3A is moved using the amount of positional deviation (Δx0, Δy0) derived on the positioning image GP as it is, it is impossible to align the target bone with the optical axis X0 of the radiation. In the present embodiment, the movement amount derivation unit 23 derives the movement amount of the top plate 3A in consideration of the magnification of the deviation amount on the positioning image GP.

[0049] Here, the positional relationship among the detection surface 5A of the radiation detector 5, the SID, and the fifth lumbar vertebra L5 which is the target bone when the subject H is viewed from the side is as shown in FIG. 6. Therefore, the movement amount derivation unit 23 derives the horizontal movement amounts (Δx1, Δy1) of the top plate 3A by the following formula (1) using the SID included in the imaging conditions and the distance z0 from the detection surface 5A of the radiation detector 5 to the center of gravity of the target bone.

[0050] Δx1 = Δx0 × (SID - z0) / SID Δy1 = Δy0 × (SID - z0) / SID (1)

[0051] The SID is included in the imaging conditions. On the other hand, depending on the type of the target bone, the distance from the back side within the human body is anatomically substantially constant. For example, if the target bone is the fifth lumbar vertebra, the center of gravity of the fifth lumbar vertebra is at a distance of about 3 cm from the back of the subject H, and if it is the femur, the center of gravity of the joint part of the femur is at a distance of about 10 cm from the back side of the subject H. For this reason, in the present embodiment, the average distance z0 from the back of the human body of the center of gravity of the target bone is derived in advance according to the type of the target bone and stored in the storage 13. The movement amount derivation unit 23 reads out the distance z0 corresponding to the type of the target bone from the storage 13 and uses it for deriving the movement amount.

[0052] Note that the distance z0 may be prepared according to the gender, age, height, weight, etc. of the subject H and stored in the storage 13, and the movement amount may be derived using the distance z0 corresponding to the gender, age, height, weight, etc. of the subject H. Further, when a three-dimensional image such as a CT image or an MRI image of the subject H to be photographed is stored in the image storage system 9, the distance from the center of gravity of the target bone to the back may be measured using the three-dimensional image, and the movement amount may be derived using the measured value.

[0053] Also, as shown in FIG. 7, the distance d1 from the back of the target bone is at a position of a substantially constant ratio with respect to the body thickness T0 of the subject H. For example, it is at a position of a substantially constant ratio, such as 0.3 for the lumbar vertebra and 0.5 for the femur. Therefore, the movement amount derivation unit 23 may derive the body thickness T0 of the subject H using the SID and SOD included in the imaging conditions, and multiply the derived body thickness T0 by the ratio R corresponding to the target bone to derive the distance d1 (T0 × R). Here, the distance between the surface of the top plate 3A and the detection surface 5A of the radiation detector 5 is a value unique to the imaging apparatus 1. Therefore, if the distance between the surface of the top plate 3A and the detection surface 5A of the radiation detector 5 is d2, the body thickness T0 = SID - SOD - d2, and the distance z0 shown in FIG. 6 can be derived by z0 = d1 + d2. Note that the ratio R may be stored in the storage 13 according to the type of the target bone.

[0054] The movement control unit 24 controls the driving of the movement mechanism 8 based on the movement amount derived by the movement amount derivation unit 23 to move the top plate 3A in the horizontal direction. That is, the movement control unit 24 moves the top plate 3A by Δx1 in the x direction and Δy1 in the y direction. Thereby, the optical axis X0 of the radiation emitted from the radiation source 2 coincides with the center of gravity of the target bone of the subject H. Note that the movement may be started by an instruction from the operator using the input device 15.

[0055] The display control unit 28 displays the movement amount derived by the movement amount derivation unit 23 on the display 14. FIG. 8 is a diagram showing a display screen of the movement amount. As shown in FIG. 8, the display screen 30 includes an image display area 31 and a movement amount display area 32. In the image display area 31, a positioning image GP is displayed. In FIG. 8, the reference position P1 and the reference position P2 in the positioning image GP are also displayed. In the movement amount display area 32, the movement amount derived by the movement amount derivation unit 23 is displayed. In the present embodiment, the output of the sensor of the movement mechanism 8 for detecting the movement amount of the top plate 3A is input to the display control unit 28. Thereby, the movement amount displayed in the movement amount display area 32 is updated so as to decrease as the top plate 3A moves and the center of gravity of the target bone approaches the optical axis X0 of the radiation. Then, when the top plate 3A moves by the derived movement amount, both the x-direction and the y-direction become 0.

[0056] Thereby, the positioning of the subject H is completed. When the subject H is photographed in the state where the positioning is completed, as shown in the radiation image G0 of FIG. 9, the reference position P1 of the radiation image G0 and the reference position P2 which is the center of gravity of the fifth lumbar vertebra L5 coincide, and the fifth lumbar vertebra L5 is positioned near the center of the radiation image G0.

[0057] When the positioning is completed as described above, the operator gives an instruction for photographing to derive the bone mineral content of the subject H using the input device 15. Thereby, the image acquisition unit 21 causes the imaging device 1 to image the subject H, and acquires a first radiation image G1 and a second radiation image G2 which are front images of the subject H from the first and second radiation detectors 5 and 6. The imaging conditions set at this time are stored in the storage 13.

[0058] Here, each of the first radiographic image G1 and the second radiographic image G2 includes a scattered ray component based on radiation scattered within the subject H, in addition to the primary ray component of the radiation that has passed through the subject H. For this reason, the scattered ray removal unit 25 removes the scattered ray component from the first radiographic image G1 and the second radiographic image G2. For example, the scattered ray removal unit 25 may apply the method described in Japanese Patent Application Laid-Open No. 2015-043959 to remove the scattered ray component from the first radiographic image G1 and the second radiographic image G2. When using the method described in Japanese Patent Application Laid-Open No. 2015-043959 or the like, the derivation of the thickness distribution of the subject H and the derivation of the scattered ray component for removing the scattered ray component are performed simultaneously.

[0059] Hereinafter, the removal of the scattered ray component from the first radiographic image G1 will be described, but the removal of the scattered ray component from the second radiographic image G2 can be performed in the same manner. First, the scattered ray removal unit 25 acquires a virtual model K of the subject H having an initial thickness distribution T0(x, y). The virtual model K is data that virtually represents the subject H, in which the thickness according to the initial thickness distribution T0(x, y) is associated with the coordinate position of each pixel of the first radiographic image G1. Note that the virtual model K of the subject H having the initial thickness distribution T0(x, y) may be stored in advance in the storage 13. Also, based on the SID and SOD included in the imaging conditions, the thickness distribution T(x, y) of the subject H may be calculated. In this case, the thickness distribution can be obtained by subtracting the distance between the SOD and the surface of the top plate 3A and the detection surface 5A of the radiation detector 5 from the SID.

[0060] Next, based on the virtual model K, the scattered ray removal unit 25 generates, as an estimated image of the first radiographic image G1 obtained by imaging the subject H, an image obtained by synthesizing an estimated primary ray image estimated by imaging the virtual model K and an estimated scattered ray image estimated by imaging the virtual model K.

[0061] Next, the scattered ray removal unit 25 corrects the initial thickness distribution T0(x, y) of the virtual model K so that the difference between the estimated image and the first radiation image G1 becomes small. The scattered ray removal unit 25 repeatedly generates the estimated image and corrects the thickness distribution until the difference between the estimated image and the first radiation image G1 satisfies a predetermined end condition. The scattered ray removal unit 25 derives the thickness distribution at the time when the end condition is satisfied as the thickness distribution T(x, y) of the subject H. Further, the scattered ray removal unit 25 removes the scattered ray component included in the first radiation image G1 by subtracting the scattered ray component at the time when the end condition is satisfied from the first radiation image G1. On the other hand, the scattered ray removal unit 25 performs the same scattered ray removal process on the second radiation image G2 as on the first radiation image G1.

[0062] The subtraction unit 26 performs energy subtraction processing to derive a bone image Gb in which the bone part of the subject H is extracted from the first and second radiation images G1 and G2 that have been subjected to the scattered ray removal process. Note that the first and second radiation images G1 and G2 in the subsequent processing have had the scattered ray components removed. When deriving the bone image Gb, the subtraction unit 26 performs weighted subtraction between the corresponding pixels of the first and second radiation images G1 and G2 as shown in the following formula (2) to generate a bone image Gb in which the bone part of the subject H included in each radiation image G1 and G2 is extracted, as shown in FIG. 10. In formula (2), α is a weighting coefficient. Note that the pixel value of each pixel in the bone part region in the bone image Gb becomes the bone part pixel value. Gb(x, y)=α·G2(x, y)-G1(x, y) (2)

[0063] In the present embodiment, since the positioning is performed as described above, the center of gravity of the fifth lumbar vertebra L5, which is the target bone, coincides with the optical axis X0 of the radiation. Therefore, as shown in FIG. 10, the fifth lumbar vertebra L5 in the bone image Gb is located near the center of the bone image Gb.

[0064] The bone mineral density derivation unit 27 derives the bone mineral density for each pixel of the bone region image Gb. In the present embodiment, the bone mineral density derivation unit 27 derives the bone mineral density B by converting each pixel value of the bone region image Gb into the pixel value of the bone image when acquired under the reference imaging conditions. Specifically, the bone mineral density derivation unit 27 derives the bone mineral density by correcting each pixel value of the bone region image Gb using a correction coefficient obtained from a lookup table described later.

[0065] Here, the higher the tube voltage in the radiation source 2 and the higher the energy of the radiation emitted from the radiation source 2, the smaller the contrast between the soft tissue and the bone tissue in the radiation image. Also, in the process of the radiation passing through the subject H, the low-energy component of the radiation is absorbed by the subject H, resulting in beam hardening where the radiation becomes higher in energy. The higher energy of the radiation due to beam hardening increases as the thickness of the subject H increases.

[0066] FIG. 11 is a diagram showing the relationship between the contrast of the bone part and the soft part with respect to the thickness of the subject H. In FIG. 11, the relationship between the contrast of the bone part and the soft part with respect to the thickness of the subject H at three tube voltages of 80 kV, 90 kV, and 100 kV is shown. As shown in FIG. 11, the higher the tube voltage, the lower the contrast. Also, when the thickness of the subject H exceeds a certain value, the larger the thickness, the lower the contrast. Note that the larger the pixel value of the bone region in the bone region image Gb, the larger the contrast between the bone part and the soft part. Therefore, the relationship shown in FIG. 11 will shift to the high-contrast side as the pixel value of the bone region in the bone region image Gb increases.

[0067] In the present embodiment, a lookup table for obtaining a correction coefficient for correcting the difference in contrast according to the tube voltage at the time of imaging and the decrease in contrast due to the influence of beam hardening in the bone region image Gb is stored in the storage 13 of the radiation image processing apparatus 10. The correction coefficient is a coefficient for correcting each pixel value of the bone region image Gb.

[0068] FIG. 12 is a diagram showing an example of a look-up table for obtaining a correction factor. In FIG. 12, a look-up table LUT1 (hereinafter simply referred to as a table) in which the reference imaging conditions are set to a tube voltage of 90 kV is illustrated. As shown in FIG. 12, in the table LUT1, the larger the tube voltage and the larger the thickness of the subject H, the larger the correction factor is set. In the example shown in FIG. 11, since the reference imaging conditions are a tube voltage of 90 kV, the correction factor is 1 when the tube voltage is 90 kV and the thickness is 0. In FIG. 12, although the table LUT1 is shown two-dimensionally, the correction factor varies according to the pixel value of the bone region. For this reason, the table LUT1 is actually a three-dimensional table with an axis representing the pixel value of the bone region added.

[0069] The bone mineral content derivation unit 27 extracts the correction factor C0(x, y) for each pixel according to the imaging conditions including the thickness distribution T(x, y) of the subject H and the set value of the tube voltage stored in the storage 13 from the table LUT1. Then, as shown in the following formula (3), the bone mineral content derivation unit 27 multiplies each pixel (x, y) in the bone region of the bone image Gb by the correction factor C0(x, y) to obtain the bone mineral content B(x, y) (g / cm 2 ) of each pixel of the bone image Gb. The bone mineral content B(x, y) derived in this way represents the pixel value of the bone region included in the radiation image obtained by imaging the subject H at a tube voltage of 90 kV, which is the reference imaging condition, and from which the influence of beam hardening has been removed. B(x,y)=C0(x,y)×Gb(x,y) (3)

[0070] The display control unit 28 displays the bone mineral content derived by the bone mineral content derivation unit 27 on the display 14. FIG. 13 is a diagram showing a display screen of the bone mineral content. As shown in FIG. 13, the display screen 40 has a bone mineral content display area 41.

[0071] In the bone mineral density display area 41, a bone part image Gb is displayed. In the bone part image Gb, a pattern is applied to the bone part area according to the bone mineral density. In FIG. 13, for the sake of simplicity of explanation, a pattern representing the bone mineral density is applied only to the vertebrae including the fifth lumbar vertebra L5. Below the bone mineral density display area 41, a reference 42 indicating the magnitude of the bone mineral density of the applied pattern is displayed. By reading the bone part image Gb while referring to the reference 42, the operator can easily recognize the bone mineral density. Instead of the pattern, different colors may be applied to the bone part image Gb according to the bone mineral density.

[0072] In addition, the identification of the fifth lumbar vertebra L5 and other vertebrae in the bone part image Gb may be performed by using a trained neural network that has been machine-learned to extract the fifth lumbar vertebra L5 etc. from the bone part image Gb. Also, a template representing the fifth lumbar vertebra L5 etc. may be stored in the storage 13, and the template matching between this template and the bone part image Gb may be performed.

[0073] Next, the processing performed in this embodiment will be described. FIG. 14 is a flowchart of the positioning process performed in this embodiment. It is assumed that the positioning image GP is acquired by the image acquisition unit 21 and stored in the storage 13. When an instruction to start the process is given, the position specifying unit 22 specifies the reference position P1 in the positioning image GP and the reference position P2 of the target structure included in the subject H in the positioning image GP (position specification: step ST1).

[0074] Furthermore, the movement amount derivation unit 23 derives the relative movement amount between the top plate 3A, the radiation source 2, and the radiation detectors 5 and 6 based on the deviation amount of the reference position P2 from the reference position P1 (step ST2). Then, the movement control unit 24 controls the driving of the movement mechanism 8 based on the movement amount derived by the movement amount derivation unit 23, moves the top plate 3A in the horizontal direction (step ST3), and ends the positioning process.

[0075] Next, the radiation image processing in this embodiment will be described. FIG. 15 is a flowchart of the radiation image processing performed in this embodiment. After the positioning is performed as described above, the process is started by giving an instruction to take a picture, and the image acquisition unit 21 performs energy subtraction imaging of the subject H to acquire the first and second radiation images G1 and G2 (radiation image acquisition; step ST11). Next, the scattered ray removal unit 25 removes the scattered ray components from the first and second radiation images G1 and G2 (step ST12). Subsequently, the subtraction unit 26 derives a bone image Gb from which the bone part of the subject H has been extracted from the first and second radiation images G1 and G2 (step ST13).

[0076] Subsequently, the bone mineral density derivation unit 27 derives the bone mineral density for each pixel of the bone image Gb (step ST14). Then, the display control unit 28 displays the bone mineral density on the display 14 (step ST15), and the process ends.

[0077] As described above, in this embodiment, the reference position P1 in the positioning image GP and the reference position P2 in the target bone included in the subject H in the positioning image GP are specified, and based on the deviation amount of the reference position P2 from the reference position P1, the relative movement amount of the top plate 3A, the radiation source 2, and the radiation detector 5 is derived. For this reason, based on the derived movement amount, by relatively moving the top plate 3A, the radiation source 2, and the radiation detector 5, the target bone can be made to coincide with the optical axis X0 of the radiation. Therefore, the radiological technologist, who is the operator, can position the subject H so that the target bone is at the desired position without repeating trial and error. Therefore, regardless of the skill of the radiological technologist, who is the operator, it becomes possible to easily perform reproducible positioning at the time of imaging the subject H.

[0078] In addition, by deriving the magnification of the target structure on the positioning image and correcting the deviation amount of the reference position P2 from the reference position P1 by the magnification to derive the movement amount, the reference position P2 of the target structure part can be made to coincide more accurately with the optical axis X0 of the radiation.

[0079] Also, in the present embodiment, after positioning the subject H, first and second radiographic images G1 and G2 are acquired by performing energy subtraction imaging, a bone image Gb is derived from the first and second radiographic images G1 and G2, and a bone mineral density is derived based on the bone image Gb. For this reason, the bone image Gb can be acquired in a state where the target bone is always positioned at the center position. Therefore, especially when performing follow-up observation, it is possible to derive a bone mineral density with reduced error due to different positioning of the target bone. Thus, the follow-up observation of the bone mineral density can be performed with higher accuracy.

[0080] In the above embodiment, the movement control unit 24 controls the driving of the movement mechanism 8 based on the movement amount derived by the movement amount derivation unit 23 to move the top plate 3A in the horizontal direction, but it is not limited to this. By receiving an operation by an operator using the input device 15, the movement mechanism 8 may be controlled to move the top plate 3A in the horizontal direction. For example, if the input device 15 is a keyboard, the movement mechanism 8 may be operated by an instruction using the arrow keys in the up, down, left, and right directions to move the top plate 3A in the horizontal direction. In this case, the movement amount displayed on the display 14 is updated according to the movement of the top plate 3A, and when the top plate 3A moves by the derived movement amount, it becomes 0 in both the x direction and the y direction. For this reason, the operator only needs to move the top plate 3A while looking at the displayed movement amount until the movement amount becomes 0.

[0081] Alternatively, without providing the movement mechanism 8, a radiologic technologist as an operator may horizontally move the top plate 3A. Also in this case, by displaying the movement amount on the display 14, the operator can easily position the subject H.

[0082] In the above-described embodiment, the movement amount derivation unit 23 derives the movement amount of the top plate 3A in consideration of the fact that the deviation amount is enlarged on the positioning image GP. However, the present invention is not limited to this. Without considering the enlargement of the deviation amount, the deviation amount (Δx0, Δy0) between the reference position P1 and the reference position P2 on the positioning image GP may be derived as the movement amount. In this case, even if the top plate 3A is moved in the horizontal direction based on the derived movement amount, there is a possibility that the center of gravity of the target bone and the optical axis X0 of the radiation do not exactly coincide. However, in the case of a bone with a relatively large size such as the lumbar vertebra, even if the center of gravity of the target bone and the optical axis X0 of the radiation do not exactly coincide, the optical axis X0 of the radiation will be present within the target bone. Therefore, it is possible to generally align the target bone with the optical axis X0 of the radiation.

[0083] In the above-described embodiment, the center of gravity of the target bone and the optical axis X0 of the radiation are made to coincide by moving the top plate 3A in the horizontal direction. However, the present invention is not limited to this. Instead of the movement mechanism 8, another movement mechanism for integrally moving the radiation source 2 and the radiation detectors 5 and 6 in parallel with respect to the top plate 3A may be provided in the imaging apparatus 1, and the radiation source 2 and the radiation detectors 5 and 6 may be integrally moved in parallel with respect to the top plate 3A using the other movement mechanism so that the center of gravity of the target bone and the optical axis X0 of the radiation coincide. In addition, another movement mechanism may be provided in addition to the movement mechanism 8. In this case, since the top plate 3A, the radiation source 2, and the radiation detectors 5 and 6 can be moved simultaneously, the center of gravity of the target bone and the optical axis X0 of the radiation can be quickly made to coincide.

[0084] In the above-described embodiment, the fifth lumbar vertebra L5 of the subject H is used as the target bone. However, the present invention is not limited to this. Any bone such as other vertebrae, femur, and calcaneus other than the fifth lumbar vertebra L5 can be used as the target bone according to the purpose of diagnosis. In addition, the target structure to be made to coincide with the optical axis X0 of the radiation is not limited to bone. Organs such as the lungs, liver, and heart according to the purpose of diagnosis may be used as the target structure.

[0085] Further, in the above-described embodiment, positioning processing and radiation image processing are performed using radiation images obtained in a system for capturing first and second radiation images G1 and G2 of a subject H using first and second radiation detectors 5 and 6. However, it goes without saying that the present embodiment can also be applied to a system for obtaining a radiation image of a subject using a storage phosphor sheet instead of a radiation detector. In this case, to obtain the positioning image GP, radiation transmitted through the subject H is irradiated onto a single storage phosphor sheet, the radiation image information of the subject H is stored and recorded in the storage phosphor sheet, and the radiation image information can be obtained by photoelectrically reading it from the storage phosphor sheet. Further, to obtain the first and second radiation images G1 and G2, radiation transmitted through the subject H is irradiated onto two stacked storage phosphor sheets, the radiation image information of the subject H is stored and recorded in each storage phosphor sheet, and the radiation image information can be obtained by photoelectrically reading it from each storage phosphor sheet. Note that, even when obtaining the first and second radiation images G1 and G2 using a storage phosphor sheet, the two-shot method may be used. In this case, the storage phosphor sheet is an example of a radiation detector.

[0086] Further, the radiation in the above-described embodiment is not particularly limited, and in addition to X-rays, α-rays or γ-rays, etc. can be used.

[0087] Also, in the above-described embodiment, as the hardware structure of a processing unit (Processing Unit) that executes various processes such as the image acquisition unit 21, the position identification unit 22, the movement amount derivation unit 23, the movement control unit 24, the scattered ray removal unit 25, the subtraction unit 26, the bone mineral density derivation unit 27, and the display control unit 28, the following various processors (Processor) can be used. As described above, in addition to the CPU, which is a general-purpose processor that executes software (program) and functions as various processing units, the above various processors include a programmable logic device (Programmable Logic Device: PLD), which is a processor whose circuit configuration can be changed after manufacturing, such as an FPGA (Field Programmable Gate Array), and a dedicated electric circuit, which is a processor having a circuit configuration specifically designed to execute specific processes, such as an ASIC (Application Specific Integrated Circuit).

[0088] One processing unit may be composed of one of these various processors, or may be composed of a combination of two or more processors of the same type or different types (for example, a combination of multiple FPGAs or a combination of a CPU and an FPGA). Also, a plurality of processing units may be composed of one processor.

[0089] As an example of configuring a plurality of processing units with one processor, first, as represented by computers such as clients and servers, there is a form in which one processor is configured by a combination of one or more CPUs and software, and this processor functions as a plurality of processing units. Second, as represented by a system on chip (System On Chip: SoC), there is a form in which a processor that realizes the functions of the entire system including a plurality of processing units with one IC (Integrated Circuit) chip is used. Thus, as a hardware structure, the various processing units are configured using one or more of the above various processors.

[0090] Furthermore, as the hardware structure of these various processors, more specifically, an electric circuit (Circuitry) combined with circuit elements such as semiconductor elements can be used.

Explanation of Signs

[0091] 1 Imaging device 2 Radiation source 3 Imaging table 3A Top plate 4 Grid 5,6 Radiation detector 7 Radiation energy conversion filter 8 Moving mechanism 10 Positioning device and radiation image processing device 11 CPU 12 Memory 13 Storage 14 Display 15 Input device 16 Memory 17 Network I / F 18 Bus 21 Image acquisition unit 22 Position identification unit 23 Movement amount derivation unit 24 Movement control unit 25 Scattered ray removal unit 26 Subtraction unit 27 Bone mineral content derivation unit 28 Display control unit 30,40 Display screen 31,41 Image display area 32 Movement amount display area 42 Reference d1 Distance G1 First radiation image G2 Second radiation image Gb Bone image GP Positioning image H Subject LUT1 Look-up table P1 Reference position P2 Reference position T0 Body thickness X0 optical axis z0 distance

Claims

Claim 1 A radiation source that irradiates a subject with radiation, A top plate on which the subject is placed, At least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting the radiation that has passed through the subject, A positioning device for the subject in a radiation image capturing apparatus including the top plate, a moving mechanism that relatively moves the radiation source and the radiation detector in a direction intersecting the optical axis of the radiation, Comprising at least one processor, The processor, Acquires a positioning image obtained by capturing the subject with the radiation image capturing apparatus, Identifies a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image, Based on the shooting distance when the positioning image is acquired and the distance corresponding to the type of the target structure or the gender, age, height, or weight of the subject between the radiation detector and the target structure, derives the magnification of the target structure on the positioning image, A positioning device that derives the relative movement amount between the top plate, the radiation source, and the radiation detector by correcting the deviation amount of the reference position from the reference position by the magnification. Claim 2 A radiation source that irradiates a subject with radiation, A top plate on which the subject is placed, At least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting the radiation that has passed through the subject, A positioning device for the subject in a radiation image capturing apparatus including the top plate, a moving mechanism that relatively moves the radiation source and the radiation detector in a direction intersecting the optical axis of the radiation, Comprising at least one processor, The processor, Acquires a positioning image obtained by capturing the subject with the radiation image capturing apparatus, Identifies a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image, Based on the shooting distance when the positioning image is acquired, the body thickness of the subject, and the ratio corresponding to the type of the target structure at the position where the target structure exists with respect to the body thickness, derives the magnification of the target structure on the positioning image, A positioning device that derives the relative movement amount between the top plate, the radiation source, and the radiation detector by correcting the deviation amount of the reference position from the reference position according to the magnification factor.

3. The processor identifies the target structure in the positioning image, The positioning device according to claim 1 or 2, wherein the reference position is identified by deriving the reference position within the identified target structure.

4. The processor displays the positioning image, The positioning device according to claim 1 or 2, wherein the reference position is identified by receiving a designation of the reference position in the displayed positioning image.

5. The positioning device according to any one of claims 1 to 4, wherein the processor controls the movement mechanism based on the movement amount to relatively move the top plate, the radiation source, and the radiation detector.

6. The positioning device according to any one of claims 1 to 5, wherein the processor displays the movement amount.

7. The positioning device according to claim 6, wherein the processor updates the movement amount according to the relative movement between the top plate, the radiation source, and the radiation detector, and displays the updated movement amount.

8. The reference position in the positioning image is the center of the irradiation field in the positioning image, The target structure is a target bone, The positioning device according to any one of claims 1 to 7, wherein the reference position within the target structure is the center of gravity of the target bone.

9. A radiation source that irradiates a subject with radiation, A top plate on which the subject is placed, At least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting the radiation that has passed through the subject, A method for positioning a subject in a radiation image capturing apparatus including a moving mechanism that relatively moves the top plate, the radiation source, and the radiation detector in a direction intersecting the optical axis of the radiation, Obtaining a positioning image obtained by photographing the subject with the radiation image capturing apparatus, Identifying a reference position in the positioning image and a reference position within a target structure included in the subject in the positioning image. Based on the shooting distance when acquiring the positioning image, the type of the target structure between the radiation detector and the target structure, or the distance corresponding to the gender, age, height or weight of the subject, derive the magnification of the target structure on the positioning image. A positioning method for deriving the relative movement amount between the top plate, the radiation source and the radiation detector by correcting the deviation amount of the reference position from the reference position by the magnification.

10. A radiation source for irradiating a subject with radiation. A top plate on which the subject is placed. At least one radiation detector disposed at a position facing the radiation source with the top plate interposed therebetween, and deriving a radiation image of the subject by detecting the radiation transmitted through the subject. A positioning method for a subject in a radiation image capturing apparatus including a moving mechanism that relatively moves the top plate, the radiation source, and the radiation detector in a direction intersecting the optical axis of the radiation, Acquire a positioning image obtained by photographing the subject with the radiation image capturing apparatus. Specify a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image. Based on the shooting distance when acquiring the positioning image, the body thickness of the subject, and the ratio corresponding to the type of the target structure at the position where the target structure exists with respect to the body thickness, derive the magnification of the target structure on the positioning image. A positioning method for deriving the relative movement amount between the top plate, the radiation source and the radiation detector by correcting the deviation amount of the reference position from the reference position by the magnification.

11. A radiation source for irradiating a subject with radiation. A top plate on which the subject is placed. At least one radiation detector disposed at a position facing the radiation source with the top plate interposed therebetween, and deriving a radiation image of the subject by detecting the radiation transmitted through the subject. A positioning program for causing a computer to execute a positioning method for a subject in a radiation image capturing apparatus including a moving mechanism that relatively moves the top plate, the radiation source, and the radiation detector in a direction intersecting the optical axis of the radiation, A procedure for acquiring a positioning image obtained by photographing the subject with the radiation image capturing apparatus. A procedure for specifying a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image; Based on the shooting distance when the positioning image was acquired, the type of the target structure of the target structure between the radiation detector and the target structure, or the distance according to the gender, age, height or weight of the subject, a procedure for deriving the magnification of the target structure on the positioning image; A positioning program that causes a computer to execute a procedure for deriving the relative movement amount between the top plate, the radiation source, and the radiation detector by correcting the deviation amount of the reference position from the reference position by the magnification.

12. A radiation source that irradiates a subject with radiation; A top plate on which the subject is placed; At least one radiation detector that is disposed at a position facing the radiation source with the top plate interposed therebetween and derives a radiation image of the subject by detecting radiation that has passed through the subject; A moving mechanism that relatively moves the top plate, the radiation source, and the radiation detector in a direction intersecting the optical axis of the radiation; A positioning program that causes a computer to execute a method for positioning a subject in a radiation image capturing apparatus including: A procedure for acquiring a positioning image acquired by photographing the subject with the radiation image capturing apparatus; A procedure for specifying a reference position in the positioning image and a reference position in a target structure included in the subject in the positioning image; Based on the shooting distance when the positioning image was acquired, the body thickness of the subject, and the ratio according to the type of the target structure at the position where the target structure exists with respect to the body thickness, derive the magnification of the target structure on the positioning image, A positioning program that causes a computer to execute a procedure for deriving the relative movement amount between the top plate, the radiation source, and the radiation detector by correcting the deviation amount of the reference position from the reference position by the magnification.

Images