Image processing device, radiation imaging system, image processing method, and program

Abstract

To provide a technique capable of appropriately correcting an afterimage of a first radiation image in the first radiation imaging in a second radiation image in the second radiation imaging.SOLUTION: An image processing device includes: a radiation image acquisition unit 151 which acquires a first radiation image obtained by irradiating a subject 20 with radiation of first energy and a second radiation image that is obtained later than the first radiation image and obtained by irradiating the subject 20 with radiation of second energy different from the first energy; an afterimage amount estimation unit 154 which estimates an afterimage amount of the first radiation image in the second radiation image on the basis of information about a soft tissue region of the subject 20 in the first radiation image and the second radiation image; and an afterimage correction unit 155 which corrects the second radiation image using the afterimage amount estimated by the afterimage amount estimation unit 154.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an image processing device, a radiation imaging system, an image processing method, and a program. [Background technology]

[0002] Flat panel detectors (Fla) are used as imaging devices for medical imaging diagnosis using radiation such as X-rays. Radiography devices (radiography systems) using flat panel detectors (FPDs) are becoming more and more common. FPDs can digitally process the images obtained by imaging, so various applications have been developed and are being put to practical use.

[0003] Patent Document 1 describes a method in which a subject is continuously irradiated with radiation having different energy distributions to obtain multiple radiographic images and then energy subtraction processing is performed, in which offset images corresponding to each radiographic image are obtained and each radiographic image is corrected. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-31961 A Summary of the Invention [Problem to be solved by the invention]

[0005] There is a technology called DXA (Dual Energy X-ray Absorptiometry) that performs energy subtraction processing on multiple radiographic images obtained by radiography to generate an image of only the bones (bone image) with the soft tissue of the subject removed, and measures bone density from the bone image. As the population ages, the number of osteoporosis patients is on the rise, and the demand for bone density measurement using the DXA method is increasing year by year.

[0006] An FPD is a radiation detection device that converts incident radiation into visible light using a phosphor, converts the visible light into electric charges using a photodiode, and detects the electric signals. In this case, the photodiode and the TFT (thin film transistor) constituting the FPD are made of, for example, amorphous silicon or IGZO, but in principle, the existence of many defects in the material traps electric charges. For this reason, it is difficult to read out all electric signals based on electric charges accumulated in the FPD by one radiation imaging. Electric signals based on electric charges remaining in the FPD by the first radiation imaging may be read out during the second or subsequent radiation imaging. For this reason, an afterimage of a first radiation image acquired based on electric signals read out from the FPD by the first radiation imaging may be superimposed on a second radiation image acquired based on electric signals read out from the FPD by the second radiation imaging.

[0007] Bone density measurement by DXA method needs to be independent of the subject's body thickness, and the approval standard for X-ray bone density measurement devices (Yakushoku Notification No. 0401050 dated April 1, 2005) requires that the coefficient of variation of the subject's body thickness dependency be 2% or less. Here, if the residual image of the first radiographic image from the above-mentioned first radiographic imaging is superimposed on the second radiographic image from the second radiographic imaging, the bone density measured from the bone image obtained by energy subtraction processing of these images will be highly dependent on the subject's body thickness.

[0008] In this regard, in Patent Document 1, since the timing of capturing the radiographic image and the offset image is not simultaneous, there is a problem in that it is difficult to appropriately correct the residual image of the first radiographic image captured in the first radiographic capture in the second radiographic capture.

[0009] The present disclosure has been made in consideration of such problems, and aims to provide a technique capable of appropriately correcting an afterimage of a first radiographic image obtained during a first radiographic imaging in a second radiographic image obtained during a second radiographic imaging. [Means for solving the problem]

[0010] The image processing device of the present disclosure includes an image acquisition means for acquiring a first radiographic image obtained by irradiating a subject with radiation of a first energy and a second radiographic image obtained after the first radiographic image, the second radiographic image being obtained by irradiating the subject with radiation of a second energy different from the first energy; an estimation means for estimating information regarding an afterimage of the first radiographic image in the second radiographic image based on information regarding a soft tissue region of the subject in the first radiographic image and the second radiographic image; and a correction means for correcting the second radiographic image using the information regarding the afterimage. The present disclosure also includes a radiation imaging system including the image processing device described above, an image processing method using the image processing device described above, and a program for causing a computer to execute each step of the image processing method. Effect of the Invention

[0011] According to the present disclosure, it is possible to appropriately correct an afterimage of a first radiographic image obtained through a first radiographic imaging operation in a second radiographic image obtained through a second radiographic imaging operation. [Brief description of the drawings]

[0012] [Figure 1] 1 is a diagram illustrating an example of a schematic configuration of a radiation imaging system according to an embodiment of the present disclosure. [Diagram 2] 1 is a flowchart showing an example of a processing procedure of an image processing method by an image processing unit (image processing device) according to an embodiment of the present disclosure. [Diagram 3] 1A to 1C are diagrams illustrating examples of various images processed by an image processing unit (image processing device) according to an embodiment of the present disclosure. [Figure 4] FIG. 1 illustrates an embodiment of the present disclosure, showing an example of a form in which a user specifies an ROI as a soft tissue region of a subject on a display screen. [Diagram 5] FIG. 11 is a diagram for explaining experimental results showing the effects of the embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a mode (embodiment) for carrying out the present disclosure will be described with reference to the drawings. Note that, although X-rays are preferably applied as radiation in the present disclosure, the radiation is not limited to X-rays and includes, for example, α-rays, β-rays, γ-rays, particle rays, cosmic rays, and the like.

[0014] 1 is a diagram showing an example of a schematic configuration of a radiation imaging system 10 according to an embodiment of the present disclosure. In this embodiment, the radiation imaging system 10 is a system for performing radiation imaging of a subject 20, and may be referred to as a radiation imaging device. Here, in this embodiment, an example in which a human body is applied as the subject 20 will be described, but the present disclosure is not limited to a human body, and for example, the body of an animal other than a human may be applied.

[0015] As shown in FIG. 1, the radiation imaging system 10 includes an information processing device 100, a radiation generating device 200, an exposure switch 210, a radiation tube 220, a radiation detecting device 230, and an anti-scatter grid 240.

[0016] When the exposure switch 210 is operated by, for example, a user, the radiation generating device 200 applies a voltage pulse to the radiation tube 220, causing the radiation tube 220 to generate radiation 221. In the example shown in FIG. 1, the radiation tube 220 irradiates the subject 20 with radiation 221. Also in FIG. 1, the body thickness of the subject 20 is illustrated as body thickness 21. A portion of the radiation 221 irradiated to the subject 20 passes through the subject 20 and enters the radiation detection device 230 via the anti-scattering grid 240. The anti-scattering grid 240 has a function of suppressing scattered radiation generated by the subject 20 from entering the radiation detection device 230.

[0017] The radiation detection device 230 is a device that detects the radiation 221 transmitted through the subject 20 as an electrical signal (image signal). For example, the above-mentioned FPD is preferably applied as the radiation detection device 230. Specifically, the radiation detection device 230 includes a radiation detection unit in which pixels that output signals (charges) according to the incident radiation 221 are arranged in an array (two-dimensional area). For example, each pixel in the radiation detection unit of the radiation detection device 230 converts the incident radiation 221 into visible light using a phosphor, and converts the visible light into charges using a photodiode to detect the radiation 221 as an electrical signal. Then, the radiation detection device 230 transmits the detected electrical signal to the information processing device 100. At this time, for example, a drive unit (not shown) of the radiation detection device 230 outputs the detected electrical signal to the general control unit 110 according to an instruction from the general control unit 110 of the information processing device 100.

[0018] The information processing device 100 processes various types of information including an electrical signal (image signal) obtained by a radiation detection device 230 by imaging a subject 20 using radiation 221. The information processing device 100 includes an overall control unit 110, an operation unit 120, a monitor 130, a storage unit 140, and an image processing unit 150, as shown in FIG.

[0019] The central control unit 110 includes one or more processors (not shown), and executes programs stored in the storage unit 140 to realize various controls and various processes of the information processing device 100.

[0020] The operation unit 120 inputs, for example, information input by a user operation to the overall control unit 110 or the monitor 130. For example, the operation unit 120 can input instructions for the image processing unit 150 or the radiation detection device 230 to the overall control unit 110. For example, the operation unit 120 accepts input of instructions for the radiation detection device 230 via a user interface (not shown).

[0021] The monitor 130 (display unit) displays various images including the radiation image obtained by the image processing unit 150, and various information obtained by the control and processing by the general control unit 110.

[0022] The storage unit 140 stores programs, information, etc. required when the general control unit 110 performs various controls and various processes. The storage unit 140 also stores information, etc. obtained by the general control unit 110 performing various controls and various processes, and radiation images and information, etc. obtained by the image processing unit 150. The storage unit 140 is configured to include, for example, a ROM (Read Only Memory), a RAM (Random Access Memory), etc.

[0023] The image processing unit 150 processes the electric signal (image signal) output from the radiation detection device 230 via the general control unit 110 to obtain a radiographic image, and performs various processes on the obtained radiographic image. In this embodiment, the image processing unit 150 may be referred to as an image processing device. As shown in FIG. 1, the image processing unit 150 has functional configurations including a radiographic image obtaining unit 151, a region specifying unit 152, a subject information estimating unit 153, an afterimage amount estimating unit 154, an afterimage correcting unit 155, a bone density measuring unit 156, and a display control unit 157.

[0024] Each functional configuration (151 to 157) of the image processing unit 150 may be realized by one or more processors included in the general control unit 110 executing a program read from the storage unit 140. Also, each functional configuration (151 to 157) of the image processing unit 150 may be realized by one or more processors included in the image processing unit 150 executing a program read from the storage unit 140. In this case, the processor of the general control unit 110 and the image processing unit 150 may be configured as a CPU (Central Processing Unit). Also, each functional configuration (151 to 157) of the image processing unit 150 may be configured as an integrated circuit or the like as long as it performs the same function. Also, the internal configuration of the information processing device 100 may include a graphics control unit such as a GPU (Graphics Processing Unit), a communication unit such as a network card, an input / output control unit such as a keyboard, a display, or a touch panel, and the like.

[0025] In this embodiment, when the exposure switch 210 is operated by a user, the radiation generating device 200 applies two different voltages to the radiation tube 220 in a short time of 0.1 to 1.0 seconds (kV switching). In this embodiment, the voltage applied the second time (for example, about 140 kV) is set to be higher than the voltage applied the first time (for example, about 80 kV). As a result, the radiation tube 220 sequentially irradiates the subject 20 with low-energy radiation 221 by applying a low voltage and high-energy radiation 221 by applying a high voltage at a predetermined time interval. That is, in this embodiment, the subject 20 is sequentially irradiated with low-energy (first energy) radiation 221 and high-energy (second energy) radiation 221 different from the low energy (higher than the low energy) at a predetermined time interval. Then, the radiation detection device 230 sequentially detects the low-energy radiation 221 transmitted through the subject 20 as an electrical signal (image signal) and detects the high-energy radiation 221 transmitted through the subject 20 as an electrical signal (image signal) at predetermined time intervals. In this way, a first radiography of the subject 20 is performed using the low-energy radiation 221, and a second radiography of the subject 20 is performed using the high-energy radiation 221. Thereafter, the integrated control unit 110 sequentially receives the electrical signal (image signal) obtained by the first radiography and the electrical signal (image signal) obtained by the second radiography from the radiation detection device 230.

[0026] The radiation image acquisition section 151 first sequentially acquires an electrical signal (image signal) obtained by the first radiation imaging and an electrical signal (image signal) obtained by the second radiation imaging from the radiation detection device 230. The radiation image acquisition section 151 is an image acquisition means that processes the electrical signal (image signal) obtained by the first radiation imaging to acquire a first radiation image, and processes the electrical signal (image signal) obtained by the second radiation imaging to acquire a second radiation image. In this embodiment, the first radiation image is a low-energy radiation image I obtained by the first radiation imaging in which low-energy (first energy) radiation 221 is irradiated to the subject 20. LThe second radiation image is obtained after the first radiation image is obtained, and is a high-energy radiation image I obtained by a second radiation imaging in which the subject 20 is irradiated with radiation 221 having a high energy (second energy) higher than the low energy. H The low-energy radiation image I, which is the first radiation image acquired by the radiation image acquisition unit 151, is L The high energy radiation image I, which is the second radiation image acquired by the radiation image acquisition unit 151, is also assumed to simply include data in which the signal value (pixel value) of the electric signal (image signal) acquired by the first radiation imaging is associated with the position information. H It is assumed that the second radiography data also includes data in which the signal values ​​(pixel values) of the electrical signals (image signals) obtained by the second radiography are associated with the position information.

[0027] The region specifying unit 152 detects the low-energy radiation image I acquired by the radiation image acquiring unit 151. L and High Energy Radiation Imaging I H In this embodiment, the subject 20 is irradiated with radiation 221 of two different energies to produce a low-energy radiation image I L and High Energy Radiation Imaging I H Since the low-energy radiation image I is acquired, the bones and soft tissues of the subject 20 can be separated by performing energy subtraction processing on the low-energy radiation image I. In other words, since the bone image and the soft tissue image of the subject 20 can be separated, it is easy to identify the soft tissue region of the subject 20. Note that, in this embodiment, the soft tissue region of the subject 20 is identified by the region identifying section 152 based on the low-energy radiation image I. L and High Energy Radiation Imaging I H It is not necessary to use bone images or soft tissue images obtained by image processing of low-energy radiation images I. L and High Energy Radiation Imaging I HOn a display screen on which these processed images are displayed, a ROI (region of interest) designated by a user for the image displayed on the display screen may be identified as a soft tissue region of the subject 20. In this case, it is preferable to use a graphical user interface (GUI) for the display screen. Also, if a large number of radiation images of the subject 20 can be prepared in advance, the soft tissue region of the subject 20 may be identified by using, for example, semantic segmentation by machine learning.

[0028] The subject information estimation unit 153 estimates the low-energy radiation image I L and High Energy Radiation Imaging I H The subject information estimation unit 153 estimates information about the soft tissue region of the subject 20 identified by the region identification unit 152 using the low-energy radiation image I L and High Energy Radiation Imaging I H By logarithmically transforming the measured value, information indicating a quantity proportional to the body thickness 21 of the subject 20 in the soft tissue region is estimated.

[0029] The residual image amount estimation unit 154 estimates a high-energy radiation image I, which is a second radiation image, based on the information about the soft tissue region of the subject 20 estimated by the subject information estimation unit 153. H The first radiation image, low-energy radiation image I L In the present disclosure, the "amount of residual image" of the first radiographic image in the second radiographic image is an example of "information regarding the residual image." Specifically, the residual image amount estimating unit 154 estimates the amount of residual image of the low-energy radiographic image I L and High Energy Radiation Imaging I HBy dividing the information indicating the amount proportional to the body thickness 21 of the subject 20 in each soft tissue region, information indicating the amount independent of the body thickness 21 of the subject 20 is obtained. As will be described later, if the effect of beam hardening is small, the amount should be almost independent of the body thickness 21 of the subject 20. However, if the amount is affected by an afterimage, the amount will depend on the body thickness of the subject 20. Therefore, by adjusting the amount of afterimage removal so as to reduce the dependency of the amount on the body thickness of the subject 20, the afterimage amount estimating unit 154 can estimate the amount of afterimage removal by the high-energy radiation image I H Low Energy Radiation Imaging in I L In this embodiment, the subject information estimating section 153 and the residual image amount estimating section 154 constitute an estimating means.

[0030] The residual image correction unit 155 uses the amount of residual image estimated by the residual image amount estimation unit 154 to correct the high-energy radiation image I H Specifically, the residual image correction unit 155 corrects the amount of residual image estimated by the residual image amount estimation unit 154 based on the high energy radiation image I H Afterimage correction is performed by subtracting from the

[0031] In this embodiment, the first radiation image is a low-energy radiation image I L In order to obtain the first high-energy radiation image, the second radiography is performed. H Low Energy Radiography I L When a portion of the electrical signals accumulated in the radiation detection device 230 during the first radiography is not read out, and this portion of the electrical signals is read out together with the electrical signals accumulated in the radiation detection device 230 during the subsequent second radiography, this is superimposed on the second radiographic image as an amount of residual image. Note that in this embodiment, the first radiographic image obtained in the first radiography is converted into a low-energy radiographic image I L The second radiation image obtained by the subsequent second radiation photography is called a high-energy radiation image I HHowever, the present disclosure is not limited to this embodiment. In the present disclosure, the first radiation image obtained by the first radiation shooting is called a high-energy radiation image I H The second radiation image obtained by the subsequent second radiation photography is called a low-energy radiation image I L In this case, a low-energy radiation image I L A second radiograph is taken to obtain a high-energy radiographic image I. H This embodiment, which acquires the above-mentioned image lag amount can be reduced because low-energy radiation 221 is used in the first radiation imaging, and is therefore more preferable.

[0032] The bone density measurement unit 156 first measures a low-energy radiation image I L and a high-energy radiation image I, which is a second radiation image corrected by the residual image correction unit 155. H and the bone density measuring unit 156 is a measuring means for measuring the bone density of a specific bone in the subject 20 by using the acquired bone image. Specifically, the bone density measuring unit 156 measures the bone density of the specific bone by using the pixel value of the specific bone in the acquired bone image. The bone density measuring unit 156 can also measure the bone density of a plurality of bones at a plurality of different body thicknesses 21 of the subject 20.

[0033] The display control unit 157 is a display control means for displaying the bone density measured by the bone density measuring unit 156 on the monitor 130, which is a display unit, via the general control unit 110. In this embodiment, the display control unit 157 also displays the low-energy radiation image I, which is the first radiation image, L , the second radiation image, high-energy radiation image I H The processed images obtained by subjecting these to energy subtraction processing are also displayed on the monitor 130.

[0034] 2 is a flowchart showing an example of a processing procedure of an image processing method by the image processing unit 150 (image processing device) according to an embodiment of the present disclosure. Note that in this embodiment, a low-energy radiation image I obtained by radiographing the subject 20 is L and High Energy Radiation Imaging I H is normalized by division using a radiation image obtained by radiography under the same shooting conditions without placing the subject 20. This normalization by division is not essential, but is desirable for ease of calculation.

[0035] <Step S201> When the process of the flowchart shown in FIG. 2 starts, the process proceeds to step S201. When proceeding to step S201, the radiation image acquisition unit 151 first sequentially acquires an electrical signal (image signal) obtained by the first radiation imaging and an electrical signal (image signal) obtained by the second radiation imaging from the radiation detection device 230. Then, the radiation image acquisition unit 151 processes the electrical signal (image signal) obtained by the first radiation imaging to obtain a low-energy radiation image I, which is the first radiation image. L Next, the radiation image acquisition unit 151 processes the electrical signal (image signal) obtained by the second radiation imaging to obtain a high-energy radiation image I H Get the.

[0036] <Step S202> When the process of step S201 is completed, the process proceeds to step S202. In step S202, the region specifying unit 152 extracts the low-energy radiation image I L and High Energy Radiation Imaging I H From this, a soft tissue region is identified that is a portion made of soft tissue (muscle, internal organs, fat, etc.) that does not include bones of the subject 20. A detailed processing example of this step S202 will be described below.

[0037] FIG. 3 is a diagram showing an example of various images processed by the image processing unit 150 (image processing device) according to the embodiment of the present disclosure.

[0038] In step S202, the region specifying unit 152 first extracts the high-energy radiation image I shown in FIG. 3(a) acquired in step S201. H and the low-energy radiation image I shown in Fig. 3(b). L Using the above, the bone image d shown in Fig. 3(c) is obtained by the following equation (1). B Generate.

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[0039] The bone image shown in Fig. 3(c) B The lumbar vertebrae 301 of the subject 20 included in the image are designated L1, L2, L3, L4, and L5 from the top, and are the main sites for bone density measurement by the DXA method. The lumbar vertebrae 301 are a site where changes in bone density are prominent, and are suitable for diagnosing osteoporosis and confirming the effects of drugs. In addition, the femur 303 shown in FIG. 3(d) is also a main site for bone density measurement by the DXA method. The femur 303 is a clinically useful site because a fracture of the femur 303 not only requires invasive treatment such as an implant, but may also cause the patient to become bedridden and may lead to a decrease in healthy life expectancy. In this disclosure, the site for measuring bone density can be either the lumbar vertebrae 301 or the femur 303, but since the method is common, the following description will be given for the case where the lumbar vertebrae 301 is applied.

[0040] The region specifying unit 152 specifies, for example, ROI 302 shown in Fig. 3(d) as a soft tissue region consisting of soft tissue at a position at a predetermined distance from the lumbar vertebrae 301, which is a site for measuring bone density. The position and size of ROI 302, which is this soft tissue region, do not vary much depending on the person who is the subject 20, so they may be set to predetermined values.

[0041] For example, the region specifying unit 152 may use the bone image d shown in FIG. B The lumbar vertebrae 301 are extracted by performing image processing for segmenting the lumbar vertebrae 301. In this case, a well-known method of segmentation processing is, for example, a method of specifying the periphery of the lumbar vertebrae 301 and performing threshold processing. In this case, the threshold processing may be performed using Otsu's binarization method or the like. Similarly, the lumbar vertebrae 301 may be extracted using the well-known techniques such as the Kmeans method, the snake method, the region growing method, the edge detection method, the graph cut method, or the like. Furthermore, when radiographic images of a large number of subjects 20 are available, the lumbar vertebrae 301 may be extracted using segmentation processing by machine learning.

[0042] In this embodiment, in order to easily identify the soft tissue region, the bone image d shown in FIG. B However, the present disclosure is not limited to this form. For example, in the present disclosure, high-energy radiation image I H or low energy radiography I L For the above, the known technique may be used to specify ROI302, which is a soft tissue region made of the soft tissue of subject 20. Also, the known technique may be used to specify ROI302, which is a soft tissue region made of the soft tissue of subject 20, directly without extracting bones, which are sites for measuring bone density. Note that ROI302 is not limited to a rectangular shape and may be any shape as long as it does not include bones of subject 20.

[0043] In the above-described embodiment, the region specifying unit 152 detects the low-energy radiation image I L , High Energy Radiation Imaging I H, or a processed image obtained by performing energy subtraction processing on these images is image-processed to identify the soft tissue region of the subject 20. However, the present disclosure is not limited to the above-described embodiment. For example, the region identifying unit 152 may identify the soft tissue region of the subject 20 by processing the low-energy radiation image I L , High Energy Radiation Imaging I H Alternatively, the ROI 302 designated by the user on the image on the display screen on which these energy subtraction processed images are displayed may be specified as the soft tissue region. Hereinafter, a form in which the user designates the ROI 302 on this display screen (a display screen using a GUI) will be described with reference to FIG. 4.

[0044] FIG. 4 illustrates an embodiment of the present disclosure, and is a diagram showing an example of a form in which a user specifies an ROI 302 as a soft tissue region of a subject 20 using a display screen 400. The display screen 400 shown in FIG. 4 is a GUI screen displayed on the monitor 130 shown in FIG. 1, and can be operated using the operation unit 120. The display screen 400 is provided with an image display area 410, an ROI manual drawing button 420, an ROI automatic designation button 430, an ROI designation button 440, a height designation section 441, a width designation section 442, a distance designation section 443, and a bone density analysis button 450. The image display area 410 of the display screen 400 displays, for example, a low-energy radiation image I L , High Energy Radiation Imaging I H , or a processed image obtained by energy subtraction of these images, a bone image d Bis displayed. The ROI manual drawing button 420 is a button operated when manually drawing and specifying the ROI 302 using, for example, the pointer 411. The ROI automatic designation button 430 is a button operated when automatically designating the ROI 302 as a soft tissue region of the subject 20 by image processing the image displayed in the image display area 410 as described in the above-mentioned embodiment. The ROI designation button 440 is a button operated when designating the ROI 302 by setting numerical values ​​in a height designation section 441 for designating the height of the ROI 302, a width designation section 442 for designating the width of the ROI 302, and a distance designation section 443 for designating the distance of the ROI 302 from the lumbar vertebrae. The bone density analysis button 450 is a button operated when analyzing the bone density of the lumbar vertebrae 301 of the subject 20. In addition, the position of ROI 302 may be fine-tuned by combining the operations of ROI manual drawing button 420, ROI automatic designation button 430, and ROI designation button 440 (related to height designation section 441, width designation section 442, and distance designation section 443).

[0045] <Step S203> When the process of step S202 is completed, the process proceeds to step S203. In step S203, the subject information estimation unit 153 estimates the low-energy radiation image I L and High Energy Radiation Imaging I H To be more specific, the subject information estimation unit 153 estimates information about the soft tissue region of the subject 20 identified in step S202 using the low-energy radiation image I L and High Energy Radiation Imaging I H By logarithmically transforming, information indicating an amount proportional to the body thickness 21 of the subject 20 in the soft tissue region is estimated. More specifically, the subject information estimation unit 153 estimates information indicating an amount proportional to the body thickness 21 of the subject 20 in the soft tissue region by the following equations (2) and (3). H and Low Energy Radiography I LUsing the above equations, information indicating a quantity proportional to the body thickness of the subject 20 in the soft tissue region identified in step S202 is estimated. In equations (2) and (3), the body thickness 21 of the subject 20 in the xy coordinate system shown in FIG. 1 is calculated using d A (x, y). Specifically, the left side of the following equation (2) is the high-energy radiation image I H The body thickness d of the subject 20 in the soft tissue region A This corresponds to information that indicates a quantity proportional to (x, y). In addition, the left side of the following equation (3) corresponds to the low-energy radiation image I L The body thickness d of the subject 20 in the soft tissue region A It corresponds to information indicating a quantity proportional to (x, y).

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[0046] Since it is known that the soft tissue region of the subject 20 is composed only of soft tissue, simple calculations such as the above-mentioned formulas (2) and (3) are valid. In reality, the spectrum of the radiation 221 has a broad width, and therefore is affected by beam hardening (radiation quality effect), but the above-mentioned monochromatic approximation is valid with a certain degree of accuracy. In addition, since the radiation detection device 230 shown in FIG. 1 is in direct contact with the anti-scatter grid 240, it is considered that scattered radiation is sufficiently removed. Therefore, it is considered that the effect of scattered radiation is small. In reality, for high-precision diagnosis such as bone density measurement by the DXA method, it is preferable to set the grid ratio of the anti-scatter grid 240 to 10 or more. More preferably, it is preferable to use a cross grid formed so that the grid is in a lattice shape as the anti-scatter grid 240. In this case, the cross grid can remove scattered radiation in the vertical and horizontal directions, so that it can remove scattered radiation isotropically and has a high absolute ability to remove scattered radiation.

[0047] <Step S204> When the process of step S203 ends, the process proceeds to step S204. In step S204, the residual image amount estimation unit 154 estimates the high-energy radiation image I based on the information about the soft tissue region of the subject 20 estimated in step S203. H Low Energy Radiation Imaging I L Specifically, the residual image amount estimation unit 154 estimates the amount of residual image of the low-energy radiation image I L Subject 20 body thickness d A Information showing quantities proportional to (x,y) and high-energy radiation image I H Subject 20 body thickness d A The amount of residual image is estimated using the ratio to information indicating a quantity proportional to (x, y). The detailed process of step S204 will be described in detail below.

[0048] The residual image amount estimation unit 154 estimates the high-energy radiation image I H Low Energy Radiation Imaging in I L The image retention amount coefficient α is estimated as the amount of image retention by the following procedure. First, the image retention amount estimation unit 154 calculates the ratio (logarithmic ratio) γ given by the following equation (4) by dividing the above equation (3) by equation (2).

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[0049] On the other hand, high energy radiation images I H Low Energy Radiography I L When the residual images of the high-energy radiation image I are superimposed at a rate of the residual image amount coefficient α, the following formula (5) is obtained. H and Low Energy Radiography I L The ratio (logarithmic ratio) of body thickness d A will depend on.

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[0050] For example, the ratio (logarithmic ratio) γ is previously calculated based on the low-energy radiation image IL and High Energy Radiation Imaging I H The amount of residual image can be calculated from the logarithmic ratio of the above-mentioned formula (4) by making the interval between the images extremely long (several tens of seconds) so that the residual image is almost completely eliminated. In this case, the amount of residual image can be calculated from the logarithmic ratio of the low-energy radiation image I used to estimate the amount of residual image in S204. L and High Energy Radiation Imaging I H The first acquisition time interval (several tens of seconds) is longer than the second acquisition time interval (0.1 to 1.0 seconds) of these images used when measuring bone density.

[0051] In addition, when taking the image to obtain the ratio (logarithmic ratio) γ, the low-energy radiation image I L and High Energy Radiation Imaging I H It is also possible to take several images without irradiating radiation 221 between the images. Also, it is possible to take a low-energy radiation image I without actually taking an image. L and High Energy Radiation Imaging I H During the acquisition time interval between the low-energy radiation image I and the low-energy radiation image I, the electrical signal may be read out multiple times from the radiation detection device 230. L and High Energy Radiation Imaging I H During the acquisition time interval, the electrical signal is read out from the radiation detection device 230 multiple times.

[0052] By carrying out the above-mentioned processing, electrical signals based on charges trapped in defects in the amorphous silicon or the like of the radiation detection device 230 can be efficiently eliminated, and the influence of image lag can be reduced.

[0053] Then, as shown in the following formula (6), the low-energy radiation image I L and High Energy Radiation Imaging I H From αI L If the residual image amount coefficient α is determined so that the logarithmic ratio obtained by subtracting

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[0054] There are cases where it is difficult to measure the ratio (logarithmic ratio) γ in advance for the subject 20, which is an actual human body. This is because it is difficult to keep the human body still for the above-mentioned several tens of seconds, and such radiography cannot be used for diagnosis, resulting in ineffective exposure to radiation 221. In such cases, the ratio (logarithmic ratio) γ may be obtained in advance using a phantom, which is a model of the subject 20, made of a material similar to human soft tissue, such as polyurethane or acrylic.

[0055] In addition, when using a phantom made of a material similar to human soft tissue, such as polyurethane or acrylic, the residual image coefficient α can be estimated in advance without using the ratio (logarithmic ratio) γ. For example, when a low-energy radiation image I is generated using a phantom made of thin polyurethane, L1 and High Energy Radiation Imaging I H1 A low-energy radiation image I was taken using a phantom made of thick polyurethane. L1 and High Energy Radiation Imaging I H1 In this case, the residual image amount coefficient α should be determined so that the following formula (7) is satisfied. If the residual image amount coefficient α is appropriate, the left and right sides of the following formula (7) will be a ratio (logarithmic ratio) γ, which is a quantity that is not dependent on the body thickness of the subject 20.

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[0056] Since the above-mentioned equation (7) is a nonlinear equation, it is advisable to use a solver such as the nonlinear least squares method to find the image retention coefficient α. Also, a phantom made of polyurethane with three or more thicknesses may be used, in which case the image retention coefficient α can be found with greater accuracy.

[0057] <Step S205> When the process of step S204 ends, the process proceeds to step S205. In step S205, the residual image correction unit 155 corrects the second radiographic image, that is, the high-energy radiographic image I, by using the amount of residual image estimated in step S204. H Specifically, the residual image correction unit 155 corrects the high-energy radiation image I H and Low Energy Radiography I L Using the residual image amount coefficient α estimated in step S204, the residual image-corrected high-energy radiation image I Hc Get the.

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[0058] <Step S206> When the process of step S205 ends, the process proceeds to step S206. In step S206, the bone density measurement unit 156 converts the low-energy radiation image I L and the high-energy radiation image I obtained in S205 after image lag correction. Hc Using the above, the bone image with reduced residual image d BC Get the.

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[0059] The bone image d with reduced residual image shown in Eq. (9) BC (x, y) is shown in FIG. 3(d). In the example shown in FIG. 3, the bone density measurement unit 156 obtains the bone image d shown in FIG. BC The average pixel values ​​of L1, L2, L3, and L4 from the top of the lumbar vertebra 301 at (x, y) were calculated, and the calculated average value was used as the bone density [g / cm 2 ] is measured.

[0060] <Step S207> When the process of step S206 ends, the process proceeds to step S207. In step S207, the display control unit 157, via the general control unit 110, causes the bone density measured in step S206 to be displayed on the monitor 130, which is the display unit.

[0061] When the process of step S207 ends, the process of the flowchart shown in FIG. 2 ends.

[0062] Next, experimental results showing the effects of this embodiment will be described. FIG. 5 is a diagram for explaining experimental results showing the effects of the embodiment of the present disclosure.

[0063] As shown in Fig. 5(a), in this experiment, a phantom 500 simulating the lumbar vertebrae of a human body was used as the subject 20. Specifically, the phantom 500 is formed of polyurethane 510 simulating soft tissue, and hydroxyapatites 511-1 to 511-3 simulating the lumbar vertebrae are embedded in the polyurethane 510. The bone densities of the hydroxyapatites 511-1, 511-2, and 511-3 are 0.7 [g / cm 2 ], 1.0 [g / cm 2 ], 1.3 [g / cm 2 In this experiment, the anti-scatter grid 240 was a cross grid with a focal length of 110 cm, a grid density of 52 lines / cm, and a grid ratio of 24:1. In this case, the anti-scatter grid 240 was placed between the radiation detection device 230 and the subject 20 as shown in FIG. 1. The imaging conditions were a low tube voltage of 80 kV when irradiating the low-energy (first energy) radiation 221, a high tube voltage of 140 kV when irradiating the high-energy (second energy) radiation 221, a tube current of 25 mA, and a pulse width of 32 msec. In this experiment, the thickness of the phantom 500 was changed to 15 cm, 20 cm, and 25 cm to examine the body thickness dependency.

[0064] FIG. 5(b) shows a comparative example, in which a bone image d calculated using equation (1) is BIn FIG. 5(b), the dependence of the phantom 500 on the thickness of three hydroxyapatites 511-1 to 511-3 simulating the lumbar vertebrae, which is the region of interest, is plotted. In the bone densities of the hydroxyapatites 511-1 to 511-3 shown in FIG. 5(b), the body thickness dependence occurs when the thickness of the phantom 500 is 15 cm, 20 cm, and 25 cm, and the coefficient of variation is 2.67%. Here, the approval standard for X-ray bone densitometers (Yakushoku Notification No. 0401050 dated April 1, 2005) requires that the coefficient of variation of the body thickness dependence be 2% or less, and FIG. 5(b) does not meet the approval standard.

[0065] FIG. 5(c) shows an embodiment of the present disclosure, in which a bone image d calculated using equation (9) is BC In Fig. 5(c), the body thickness dependency of the phantom 500 is plotted for three hydroxyapatites 511-1 to 511-3 simulating the lumbar vertebrae, which is the region of interest. In the bone density of the hydroxyapatites 511-1 to 511-3 shown in Fig. 5(c), when the thickness of the phantom 500 is 15 cm, 20 cm, and 25 cm, the body thickness dependency has less variation compared to the case shown in Fig. 5(b). Specifically, in the case shown in Fig. 5(c), the coefficient of variation of the body thickness dependency is 0.65%, which is found to satisfy the above-mentioned approval criteria.

[0066] The image processing unit 150 (image processing device) included in the radiation imaging system 10 described above performs the following processes. Specifically, the radiation image acquisition unit 151 acquires a low-energy radiation image I obtained by irradiating the subject 20 with low-energy (first energy) radiation 221. L Next, the radiation image acquisition unit 151 acquires a high-energy radiation image I (first radiation image) obtained by irradiating the subject 20 with radiation 221 of high energy (second energy) different from the low energy (first energy). H Next, the residual image amount estimating unit 154 acquires the low-energy radiation image I L and High Energy Radiation Imaging I HBased on the information about the soft tissue region of the subject 20 in H Low Energy Radiation Imaging in I L Then, the residual image correction unit 155 uses the amount of residual image estimated by the residual image amount estimation unit 154 to correct the high-energy radiation image I H Correct the following. According to this configuration, the high-energy radiation image I H (Second radiographic image) Low-energy radiographic image I from the first radiographic examination L The residual image of the (first radiation image) can be appropriately corrected. As a result, the high-energy radiation image I H When the bone density of the subject 20 is measured by, for example, a DXA method using this, it becomes possible to measure the bone density more appropriately with reduced dependency on the body thickness of the subject 20 .

[0067] (Other embodiments) The present disclosure can also be realized by a process in which a program for implementing one or more functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that implements one or more functions. This program and a computer-readable storage medium having the program stored thereon are included in the present disclosure.

[0068] It should be noted that the above-described embodiments of the present disclosure are merely illustrative examples of the implementation of the present disclosure, and the technical scope of the present disclosure should not be interpreted as being limited by these. In other words, the present disclosure can be implemented in various forms without departing from its technical concept or main features.

[0069] Embodiments of the present disclosure include the following configurations, methods, and programs. [Configuration 1] an image acquisition means for acquiring a first radiographic image obtained by irradiating a subject with radiation of a first energy, and a second radiographic image obtained after the first radiographic image, the second radiographic image being obtained by irradiating the subject with radiation of a second energy different from the first energy; an estimation means for estimating information regarding an afterimage of the first radiographic image in the second radiographic image based on information regarding a soft tissue region of the subject in the first radiographic image and the second radiographic image; a correcting means for correcting the second radiographic image using information about the residual image; An image processing device comprising: [Configuration 2] The estimation means acquires information indicating an amount proportional to a body thickness of the subject in the soft tissue region as information regarding the soft tissue region of the subject, and estimates information regarding the afterimage using the acquired information. 2. The image processing device according to configuration 1. [Configuration 3] The estimation means estimates the information about the afterimage using a ratio between information indicating a quantity proportional to a body thickness of the subject in the first radiographic image and information indicating a quantity proportional to a body thickness of the subject in the second radiographic image. 3. The image processing device according to configuration 2. [Configuration 4] the image processing device measures a bone density of the bone using a bone image including a bone of the subject different from the soft tissue in the soft tissue region; A first acquisition time interval between the first radiographic image and the second radiographic image used when calculating the ratio is longer than a second acquisition time interval between the first radiographic image and the second radiographic image used when measuring the bone density. 4. The image processing device according to configuration 3. [Configuration 5] During a time interval between acquisition of the first radiographic image and the second radiographic image used to calculate the ratio, an electrical signal is read out from a radiation detection device that detects the radiation that has passed through the subject as an electrical signal. 5. The image processing device according to configuration 3 or 4. [Configuration 6] The ratio is obtained using a phantom that is a model of the subject. 6. The image processing device according to any one of configurations 3 to 5. [Configuration 7] a soft tissue region detecting unit for detecting a soft tissue region of the subject; The estimation means acquires information about the soft tissue region of the subject in the first radiographic image and information about the soft tissue region of the subject in the second radiographic image according to the soft tissue region identified by the identification means, and estimates information about the afterimage using the acquired information. 7. The image processing device according to any one of configurations 1 to 6. [Configuration 8] The identifying means identifies a region designated in an image displayed on a display screen on which the first radiographic image, the second radiographic image, or a processed image obtained by performing energy subtraction processing on the first radiographic image and the second radiographic image is displayed, as the soft tissue region of the subject. 8. The image processing device according to configuration 7. [Configuration 9] The identifying means identifies the soft tissue region of the subject by performing image processing on the first radiation image, the second radiation image, or a processed image obtained by performing energy subtraction processing on the first radiation image and the second radiation image. 8. The image processing device according to configuration 7. [Configuration 10] a measuring unit for measuring a bone density of the bone using a bone image obtained by performing energy subtraction processing on the first radiographic image and the second radiographic image corrected by the correcting unit, the bone image including a bone of the subject different from the soft tissue in the soft tissue region. 10. The image processing device according to any one of configurations 1 to 9. [Configuration 11] The measuring means measures a bone density of the bone by using a pixel value of the bone in the bone image. 11. The image processing device according to configuration 10. [Configuration 12] The measuring means can measure bone densities of a plurality of the bones at a plurality of different body thicknesses of the subject. 12. The image processing device according to configuration 10 or 11. [Configuration 13] The bone density measurement device further includes a display control unit for displaying the bone density measured by the measurement unit on a display unit. 13. The image processing device according to any one of configurations 10 to 12. [Configuration 14] The radiation of the second energy is radiation having higher energy than the radiation of the first energy. 14. The image processing device according to any one of configurations 1 to 13. [Configuration 15] An image processing device according to any one of configurations 1 to 14, A radiation generating device that generates the radiation; a radiation detection device that detects the radiation that has passed through the subject as an electrical signal; Including, The image acquisition means processes the electrical signal to acquire the first radiation image and the second radiation image. Radiography system. [Method 1] an image acquiring step of acquiring a first radiographic image obtained by irradiating a subject with radiation of a first energy, and a second radiographic image obtained after the first radiographic image, the second radiographic image being obtained by irradiating the subject with radiation of a second energy different from the first energy; an estimating step of estimating information regarding an afterimage of the first radiographic image in the second radiographic image based on information regarding a soft tissue region of the subject in the first radiographic image and the second radiographic image; a correcting step of correcting the second radiographic image using information about the residual image; An image processing method comprising: [Program 1] A program for causing a computer to execute each step of the image processing method described in Method 1. [Explanation of symbols]

[0070] 10: radiography system, 20: subject, 21: body thickness of subject, 100: information processing device, 110: general control unit, 120: operation unit, 130: monitor, 140: memory unit, 150: image processing unit, 151: radiographic image acquisition unit, 152: region identification unit, 153: subject information estimation unit, 154: residual image amount estimation unit, 155: residual image correction unit, 156: bone density measurement unit, 157: display control unit, 200: radiation generation device, 210: exposure switch, 220: radiation tube, 221: radiation, 230: radiation detection device, 240: scatter reduction grid

Claims

1. an image acquisition means for acquiring a first radiographic image obtained by irradiating a subject with radiation of a first energy, and a second radiographic image obtained after the first radiographic image, the second radiographic image being obtained by irradiating the subject with radiation of a second energy different from the first energy; an estimation means for estimating information regarding an afterimage of the first radiographic image in the second radiographic image based on information regarding a soft tissue region of the subject in the first radiographic image and the second radiographic image; a correcting means for correcting the second radiographic image using information about the residual image; An image processing device comprising:

2. The estimation means acquires information indicating an amount proportional to a body thickness of the subject in the soft tissue region as information regarding the soft tissue region of the subject, and estimates information regarding the afterimage using the acquired information. The image processing device according to claim 1 .

3. The estimation means estimates the information about the afterimage using a ratio between information indicating an amount proportional to a body thickness of the subject in the first radiographic image and information indicating an amount proportional to a body thickness of the subject in the second radiographic image. The image processing device according to claim 2 .

4. the image processing device measures a bone density of the bone using a bone image including a bone of the subject different from the soft tissue in the soft tissue region; A first acquisition time interval between the first radiographic image and the second radiographic image used when calculating the ratio is longer than a second acquisition time interval between the first radiographic image and the second radiographic image used when measuring the bone density. The image processing device according to claim 3 .

5. During a time interval between acquisition of the first radiographic image and the second radiographic image used to calculate the ratio, an electrical signal is read out from a radiation detection device that detects the radiation that has passed through the subject as an electrical signal. The image processing device according to claim 3 .

6. The ratio is obtained using a phantom that is a model of the subject. The image processing device according to claim 3 .

7. a soft tissue region detecting unit for detecting a soft tissue region of the subject; The estimation means acquires information about the soft tissue region of the subject in the first radiographic image and information about the soft tissue region of the subject in the second radiographic image according to the soft tissue region identified by the identification means, and estimates information about the afterimage using the acquired information. The image processing device according to claim 1 .

8. The identifying means identifies a region designated in an image displayed on a display screen on which the first radiographic image, the second radiographic image, or a processed image obtained by performing energy subtraction processing on the first radiographic image and the second radiographic image is displayed, as the soft tissue region of the subject. The image processing device according to claim 7.

9. The identifying means identifies the soft tissue region of the subject by image processing the first radiation image, the second radiation image, or a processed image obtained by performing energy subtraction processing on the first radiation image and the second radiation image. The image processing device according to claim 7.

10. a measuring unit for measuring a bone density of the bone using a bone image obtained by performing energy subtraction processing on the first radiographic image and the second radiographic image corrected by the correcting unit, the bone image including a bone of the subject different from the soft tissue in the soft tissue region. The image processing device according to claim 1 .

11. The measuring means measures a bone density of the bone by using a pixel value of the bone in the bone image. The image processing device according to claim 10.

12. The measuring means can measure bone densities of a plurality of the bones at a plurality of different body thicknesses of the subject. The image processing device according to claim 10.

13. The bone density measurement device further includes a display control unit for displaying the bone density measured by the measurement unit on a display unit. The image processing device according to claim 10.

14. The radiation of the second energy is radiation having higher energy than the radiation of the first energy. The image processing device according to claim 1 .

15. An image processing device according to any one of claims 1 to 14, A radiation generating device that generates the radiation; a radiation detection device that detects the radiation that has passed through the subject as an electrical signal; Including, The image acquisition means processes the electrical signal to acquire the first radiation image and the second radiation image. Radiography system.

16. an image acquiring step of acquiring a first radiographic image obtained by irradiating a subject with radiation of a first energy, and a second radiographic image obtained after the first radiographic image, the second radiographic image being obtained by irradiating the subject with radiation of a second energy different from the first energy; an estimating step of estimating information regarding an afterimage of the first radiographic image in the second radiographic image based on information regarding a soft tissue region of the subject in the first radiographic image and the second radiographic image; a correcting step of correcting the second radiographic image using information about the residual image; An image processing method comprising:

17. A program for causing a computer to execute each step of the image processing method according to claim 16.

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