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

Abstract

To accurately acquire the thickness of a material.SOLUTION: An image processing device includes processing means for acquiring a thickness image of a first material and a thickness image of a second material in which scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies acquired by irradiating a subject with radiation and performing imaging, and information on a transmittance including scattered rays and a transmittance not including the scattered rays, which are preset for each thickness of a first material and a second material different from the first material.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The disclosed technology relates to an image processing device, a radiation imaging system, an image processing method, and a program, and more specifically to an image processing device, a radiation imaging system, an image processing method, and a program used for still image capture such as general imaging in medical diagnosis and for moving image capture such as fluoroscopy. [Background technology]

[0002] 2. Description of the Related Art Currently, radiation imaging devices using flat panel detectors (hereinafter abbreviated as FPD) made of semiconductor materials are in widespread use as imaging devices used in medical image diagnosis and non-destructive testing using X-rays.

[0003] Energy subtraction, an imaging method using an FPD, can obtain material-separated images, such as bone images or soft tissue images, by processing multiple images of different energies obtained by irradiating X-rays with different tube voltages (Patent Document 1).

[0004] Patent Document 2 discloses performing scattered radiation correction in order to reduce errors that occur in the thickness of bones and soft tissues when generating bone images and soft tissue images using energy subtraction. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-167948 A [Patent Document 2] JP 2019-166155 A Summary of the Invention [Problem to be solved by the invention]

[0006] To perform scattered radiation correction, it is necessary to estimate the amount of scattered radiation. However, since the amount of scattered radiation varies depending on the material of the subject, if correction is performed without taking the material into account, errors may occur in the thickness of bones and soft tissues.

[0007] The disclosed technology aims to provide an image processing technology that can obtain the thickness of a substance with high accuracy. [Means for solving the problem]

[0008] An image processing device according to one aspect of the disclosed technology includes a processing means for acquiring a thickness image of the first material and a thickness image of the second material in which the scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating a subject with radiation and taking an image, and information regarding a transmittance including scattered rays and a transmittance excluding scattered rays that are preset for each thickness of a first material and a second material different from the first material. Effect of the Invention

[0009] According to the disclosed technology, it becomes possible to obtain the thickness of a substance with high accuracy. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of the arrangement of an X-ray imaging system according to an embodiment. [Diagram 2] FIG. 2 is an equivalent circuit diagram of a pixel of the X-ray imaging device according to the embodiment. [Diagram 3] 4 is a timing chart of the X-ray imaging apparatus according to the embodiment. [Figure 4] 4 is a timing chart of the X-ray imaging apparatus according to the embodiment. [Diagram 5] 5A to 5C are views for explaining a correction process according to the embodiment. [Figure 6] FIG. 2 is a block diagram of signal processing according to the embodiment. [Figure 7] 11 is a flowchart illustrating a flow of table generation according to the embodiment. [Figure 8] FIG. 4 is a schematic diagram of a table according to the embodiment. [Figure 9] FIG. 4 is a diagram showing an example of a table for scattered radiation correction according to the embodiment. [Figure 10] FIG. 4 is a block diagram of scattered radiation correction processing according to the embodiment. [Figure 11] FIG. 13 is a diagram showing a table for scattered radiation correction taking into account two materials according to the embodiment. [Figure 12] FIG. 13 is a block diagram of a transmittance calculation method taking into account two materials according to an embodiment. [Figure 13] FIG. 4 is a block diagram of thickness calculation according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] In addition, radiation in the disclosed technology includes α-rays, β-rays, γ-rays, etc., which are beams made of particles (including photons) emitted by radioactive decay, as well as beams having the same or higher energy, such as X-rays, particle beams, and cosmic rays. In the following embodiment, an apparatus using X-rays as an example of radiation will be described. Therefore, hereinafter, a radiation imaging apparatus and a radiation imaging system will be described as an X-ray imaging apparatus and an X-ray imaging system, respectively.

[0013] (Embodiment) 1 is a block diagram showing an example of the configuration of an X-ray imaging system as an example of a radiation imaging system according to an embodiment. The X-ray imaging system of the embodiment includes an X-ray generating device 101 (radiation source), an X-ray control device 102, an imaging control device 103, and an X-ray imaging device 104 (radiography device). The radiation imaging system includes an X-ray imaging device 104 (radiography device) including a two-dimensional detector 106, and an imaging control device 103 (image processing device) that acquires an image in which scattered rays contained in radiation detected by the two-dimensional detector 106 have been corrected.

[0014] The X-ray generating device 101 generates X-rays and irradiates the subject with the X-rays. The X-ray control device 102 controls the generation of X-rays in the X-ray generating device 101. The imaging control device 103 has, for example, one or more processors (CPUs) and a memory (storage unit 136), and the processor executes a program stored in the memory (storage unit 136) to acquire an X-ray image and perform image processing. Note that each process including the image processing by the imaging control device 103 may be realized by dedicated hardware, or may be realized by a combination of hardware and software. The X-ray imaging device 104 has a phosphor 105 that converts X-rays into visible light, and a two-dimensional detector 106 that detects visible light. The two-dimensional detector is a sensor in which pixels 20 that detect X-ray quanta are arranged in an array of X columns by Y rows, and outputs image information.

[0015] The imaging control device 103 functions as an image processing device that processes radiation images using the above-mentioned processor. The acquisition unit 131, the correction unit 132, the signal processing unit 133, the image processing unit 134, the display control unit 135, and the storage unit 136 indicate an example of the functional configuration of the image processing device.

[0016] The acquisition unit 131 acquires a plurality of radiation images having different energies obtained by irradiating a subject with radiation and capturing an image of the subject. The acquisition unit 131 acquires, as the plurality of radiation images, radiation images obtained by performing sample-holding a plurality of times during exposure to one shot of radiation.

[0017] The correction unit 132 corrects the multiple radiation images acquired by the acquisition unit 131 to generate multiple images to be used in the energy subtraction process.

[0018] The signal processing unit 133 acquires a thickness image of the first material and a thickness image of the second material in which scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating the subject with radiation and taking an image, and information on the transmittance including scattered rays and the transmittance not including scattered rays, which are preset for each thickness of the first material and the second material different from the first material.

[0019] Here, the plurality of pieces of information include information corresponding to a first radiation energy among the plurality of radiation energies, and information corresponding to a second radiation energy different from the first radiation energy. The information corresponding to the first radiation energy includes a first radiation image, and the information corresponding to the second radiation energy includes a second radiation image.

[0020] The first radiographic image captured by the X-ray imaging device 104 includes data in which the signal values ​​(pixel values) of the electric signals (image signals) obtained by imaging correspond to position information indicating a two-dimensional arrangement of the signal values. The second radiographic image captured by the X-ray imaging device 104 includes data in which the signal values ​​(pixel values) of the electric signals (image signals) obtained by imaging correspond to position information indicating a two-dimensional arrangement of the signal values. The multiple images (first radiographic image, second radiographic image) include images (material-separated images) that show materials separated from one another, such as bone and soft tissue.

[0021] The signal processing unit 133 generates, for example, a first image showing the thickness of a first material and a second image showing the thickness of a second material different from the first material, based on a plurality of radiation images captured with different radiation energies. Here, the first material includes at least calcium, hydroxyapatite, or bone, and the second material includes at least water, fat, or a soft material not containing calcium. The signal processing unit 133 will be described in detail later. The image processing unit 134 generates an image for display using the first image and the second image acquired by the signal processing by the signal processing unit 133. The display control unit 135 displays the thickness image of the first material and the thickness image of the second material acquired by the signal processing unit 133 on a display device 137 configured, for example, by a liquid crystal display or an organic EL display.

[0022] The memory unit 136 stores various programs and databases (data tables), and one or more processors (CPUs) execute the programs stored in the memory unit 136 to realize the functions of the acquisition unit 131, correction unit 132, signal processing unit 133, image processing unit 134, and display control unit 135.

[0023] 2 is an equivalent circuit diagram of a pixel 20 according to an embodiment. The pixel 20 includes a photoelectric conversion element 201 and an output circuit unit 202. The photoelectric conversion element 201 can typically be a photodiode. The output circuit unit 202 includes an amplifier circuit unit 204, a clamp circuit unit 206, a sample-and-hold circuit 207, and a selection circuit unit 208.

[0024] The photoelectric conversion element 201 includes a charge accumulation section, and the charge accumulation section is connected to the gate of a MOS transistor 204a of the amplifier circuit section 204. The source of the MOS transistor 204a is connected to a current source 204c via a MOS transistor 204b. The MOS transistor 204a and the current source 204c form a source follower circuit. The MOS transistor 204b is an enable switch that turns on when an enable signal EN supplied to the gate of the MOS transistor 204b becomes active level, putting the source follower circuit into an operating state.

[0025] 2, the charge storage section of the photoelectric conversion element 201 and the gate of the MOS transistor 204a form a common node, and this node functions as a charge-voltage conversion section that converts the charge stored in the charge storage section into a voltage. That is, a voltage V (=Q / C) appears in the charge-voltage conversion section, which is determined by the charge Q stored in the charge storage section and the capacitance value C of the charge-voltage conversion section. The charge-voltage conversion section is connected to a reset potential Vres via a reset switch 203. When the reset signal PRES becomes active level, the reset switch 203 is turned on, and the potential of the charge-voltage conversion section is reset to the reset potential Vres.

[0026] The clamp circuit unit 206 clamps the noise output by the amplifier circuit unit 204 according to the potential of the reset charge-voltage converter by the clamp capacitance 206a. That is, the clamp circuit unit 206 is a circuit for canceling this noise from the signal output from the source follower circuit according to the charge generated by photoelectric conversion in the photoelectric conversion element 201. This noise includes the kTC noise at the time of reset. The clamp is performed by setting the clamp signal PCL to an active level to turn on the MOS transistor 206b, and then setting the clamp signal PCL to an inactive level to turn off the MOS transistor 206b. The output side of the clamp capacitance 206a is connected to the gate of the MOS transistor 206c. The source of the MOS transistor 206c is connected to the current source 206e via the MOS transistor 206d. The MOS transistor 206c and the current source 206e form a source follower circuit. The MOS transistor 206d is an enable switch that turns on when the enable signal EN0 supplied to its gate becomes an active level to activate the source follower circuit.

[0027] A signal output from the clamp circuit unit 206 according to the charge generated by photoelectric conversion in the photoelectric conversion element 201 is written as an optical signal to the capacitance 207Sb via the switch 207Sa when the optical signal sampling signal TS becomes active level. A signal output from the clamp circuit unit 206 when the MOS transistor 206b is turned on immediately after the potential of the charge-voltage conversion unit is reset is a clamp voltage. A noise signal is written to the capacitance 207Nb via the switch 207Na when the noise sampling signal TN becomes active level. This noise signal includes an offset component of the clamp circuit unit 206. The switch 207Sa and the capacitance 207Sb form a signal sample-and-hold circuit 207S, and the switch 207Na and the capacitance 207Nb form a noise sample-and-hold circuit 207N. The sample-and-hold circuit unit 207 includes a signal sample-and-hold circuit 207S and a noise sample-and-hold circuit 207N.

[0028] When the driving circuit unit drives the row selection signal to an active level, the signal (optical signal) held in the capacitance 207Sb is output to the signal line 21S via the MOS transistor 208Sa and the row selection switch 208Sb. At the same time, the signal (noise) held in the capacitance 207Nb is output to the signal line 21N via the MOS transistor 208Na and the row selection switch 208Nb. The MOS transistor 208Sa constitutes a source follower circuit together with a constant current source (not shown) provided on the signal line 21S. Similarly, the MOS transistor 208Na constitutes a source follower circuit together with a constant current source (not shown) provided on the signal line 21N. The MOS transistor 208Sa and the row selection switch 208Sb constitute the signal selection circuit unit 208S, and the MOS transistor 208Na and the row selection switch 208Nb constitute the noise selection circuit unit 208N. The selection circuit section 208 includes a signal selection circuit section 208S and a noise selection circuit section 208N.

[0029] The pixel 20 may have an addition switch 209S that adds optical signals of adjacent pixels 20. In the addition mode, an addition mode signal ADD becomes an active level, and the addition switch 209S is turned on. As a result, the capacitances 207Sb of adjacent pixels 20 are connected to each other by the addition switch 209S, and the optical signals are averaged. Similarly, the pixel 20 may have an addition switch 209N that adds noises of adjacent pixels 20. When the addition switch 209N is turned on, the capacitances 207Nb of adjacent pixels 20 are connected to each other by the addition switch 209N, and the noises are averaged. The adder 209 includes an addition switch 209S and an addition switch 209N.

[0030] The pixel 20 may also have a sensitivity change unit 205 for changing the sensitivity. The pixel 20 may include, for example, a first sensitivity change switch 205a and a second sensitivity change switch 205'a, as well as circuit elements associated therewith. When the first change signal WIDE becomes active level, the first sensitivity change switch 205a is turned on, and the capacitance value of the first additional capacitance 205b is added to the capacitance value of the charge-voltage conversion unit. This reduces the sensitivity of the pixel 20. When the second change signal WIDE2 becomes active level, the second sensitivity change switch 205'a is turned on, and the capacitance value of the second additional capacitance 205'b is added to the capacitance value of the charge-voltage conversion unit. This further reduces the sensitivity of the pixel 20. By adding a function for reducing the sensitivity of the pixel 20 in this way, it becomes possible to receive a larger amount of light, and the dynamic range can be expanded. When the first change signal WIDE becomes active level, the enable signal ENw may be turned to active level, and the MOS transistor 204'a may be changed to a source follower operation instead of the MOS transistor 204a.

[0031] The X-ray imaging device 104 reads out the output of the pixel circuits as described above from the two-dimensional detector 106 , converts it into a digital value by an AD converter (not shown), and then transfers the image to the imaging control device 103 .

[0032] Next, the operation of the X-ray imaging system according to the embodiment having the above-mentioned configuration will be described. Fig. 3 shows the drive timing of the X-ray imaging device 104 when energy subtraction is performed in the X-ray imaging system according to the embodiment. The waveform in Fig. 3 shows the timing of X-ray exposure, synchronization signal, reset of the photoelectric conversion element 201, sample hold circuit 207, and image readout from the signal line 21, with the horizontal axis representing time.

[0033] X-rays are emitted after the photoelectric conversion element 201 is reset by a reset signal. Ideally, the X-ray tube voltage is a square wave, but it takes a finite amount of time for the tube voltage to rise and fall. In particular, when the exposure time is short with pulsed X-rays, the tube voltage is no longer considered to be a square wave, and takes a waveform such as that shown by X-rays 301 to 303. X-rays 301 in the rising phase, X-rays 302 in the stable phase, and X-rays 303 in the falling phase each have different X-ray energies. Therefore, by obtaining X-ray images corresponding to radiation in periods separated by sample hold, multiple types of X-ray images with different energies can be obtained.

[0034] The X-ray imaging device 104 performs sampling in the noise sample hold circuit 207N after X-rays 301 in the rising phase are irradiated, and performs sampling in the signal sample hold circuit 207S after X-rays 302 in the stable phase are irradiated. After that, the X-ray imaging device 104 reads out the difference between the signal line 21N and the signal line 21S as an image. At this time, the noise sample hold circuit 207N holds the signal (R1) of the X-rays 301 in the rising phase, and the signal sample hold circuit 207S holds the sum (R1+B) of the signal of the X-rays 301 in the rising phase and the signal (B) of the X-rays 302 in the stable phase. Therefore, an image 304 corresponding to the signal of the X-rays 302 in the stable phase is read out.

[0035] Next, after completing the exposure of X-rays 303 in the falling period and the reading of the image 304, the X-ray imaging device 104 performs sampling again in the signal sample-and-hold circuit 207S. After that, the X-ray imaging device 104 resets the photoelectric conversion element 201, performs sampling again in the noise sample-and-hold circuit 207N, and reads out the difference between the signal line 21N and the signal line 21S as an image. At this time, the noise sample-and-hold circuit 207N holds a signal in a state where X-rays are not irradiated, and the signal sample-and-hold circuit 207S holds the sum (R1+B+R2) of the signal of X-rays 301 in the rising period, the signal of X-rays 302 in the stable period, and the signal of X-rays 303 in the falling period. Therefore, an image 306 corresponding to the signal of X-rays 301 in the rising period, the signal of X-rays 302 in the stable period, and the signal of X-rays 303 in the falling period is read out. Thereafter, by calculating the difference between the image 306 and the image 304, an image 305 corresponding to the sum of the X-rays 301 in the rising edge and the X-rays 303 in the falling edge is obtained. This calculation may be performed by the X-ray imaging device 104 or the imaging control device 103.

[0036] The timing for resetting the sample-and-hold circuit 207 and the photoelectric conversion element 201 is determined by using a synchronization signal 307 indicating that X-ray exposure has started from the X-ray generator 101. As a method for detecting the start of X-ray exposure, a configuration for measuring the tube current of the X-ray generator 101 and determining whether or not the current value exceeds a preset threshold can be used, but is not limited to this. For example, a configuration for detecting the start of X-ray exposure by repeatedly reading out the pixel 20 after the resetting of the photoelectric conversion element 201 is completed and determining whether or not the pixel value exceeds a preset threshold can be used.

[0037] Alternatively, for example, an X-ray imaging device 104 may have an X-ray detector built therein other than the two-dimensional detector 106, and the start of X-ray exposure may be detected by determining whether or not the measurement value of the X-ray detector exceeds a preset threshold value. In either method, sampling by the signal sample-and-hold circuit 207S, sampling by the noise sample-and-hold circuit 207N, and resetting of the photoelectric conversion element 201 are performed after a pre-specified time has elapsed since the input of a synchronization signal 307 indicating the start of X-ray exposure.

[0038] In this manner, an image 304 corresponding to the stable period of the pulsed X-rays and an image 305 corresponding to the sum of the rising period and the falling period are obtained. Since the energies of the X-rays irradiated when forming these two X-ray images are different, energy subtraction processing can be performed by performing calculations between these X-ray images.

[0039] Fig. 4 shows the drive timing of the X-ray imaging device 104 when energy subtraction is performed in the X-ray imaging system according to the embodiment. The drive timing shown in Fig. 4 differs from the drive timing in Fig. 3 in that the tube voltage of the X-ray generation device 101 is actively switched.

[0040] First, after the photoelectric conversion element 201 is reset, the X-ray generation device 101 irradiates low-energy X-rays 401. In this state, the X-ray imaging device 104 performs sampling by the noise sample-and-hold circuit 207N. After that, the X-ray generation device 101 switches the tube voltage to irradiate high-energy X-rays 402. In this state, the X-ray imaging device 104 performs sampling by the signal sample-and-hold circuit 207S. After that, the X-ray generation device 101 switches the tube voltage to irradiate low-energy X-rays 403. The X-ray imaging device 104 reads out the difference between the signal line 21N and the signal line 21S as an image. At this time, the noise sample-and-hold circuit 207N holds the signal (R1) of the low-energy X-rays 401, and the signal sample-and-hold circuit 207S holds the sum (R1+B) of the signal of the low-energy X-rays 401 and the signal (B) of the high-energy X-rays 402. Therefore, an image 404 corresponding to the signal of the high-energy X-rays 402 is read out.

[0041] Next, after completing the exposure of the low-energy X-ray 403 and the readout of the image 404, the X-ray imaging device 104 performs sampling again in the signal sample-and-hold circuit 207S. After that, the X-ray imaging device 104 resets the photoelectric conversion element 201, performs sampling again in the noise sample-and-hold circuit 207N, and reads out the difference between the signal line 21N and the signal line 21S as an image. At this time, the noise sample-and-hold circuit 207N holds a signal in a state where no X-ray is irradiated, and the signal sample-and-hold circuit 207S holds the sum (R1+B+R2) of the signal of the low-energy X-ray 401, the signal of the high-energy X-ray 402, and the signal of the low-energy X-ray 403 (R2). Therefore, an image 406 corresponding to the signal of the low-energy X-ray 401, the signal of the high-energy X-ray 402, and the signal of the low-energy X-ray 403 is read out.

[0042] Thereafter, by calculating the difference between image 406 and image 404, an image 405 corresponding to the sum of low-energy X-rays 401 and low-energy X-rays 403 is obtained. This calculation may be performed by the X-ray imaging device 104 or the imaging control device 103. The synchronization signal 407 is the same as in Fig. 3. In this way, by acquiring images while actively switching the tube voltage, it is possible to make the energy difference between low-energy and high-energy radiation images larger than in the method of Fig. 3.

[0043] Next, a description will be given of the energy subtraction processing by the imaging control device 103. The energy subtraction processing in this embodiment is divided into three stages: correction processing by the correction unit 132, signal processing by the signal processing unit 133, and image processing by the image processing unit 134. Each process will be described below.

[0044] The correction process is a process for processing a plurality of radiation images acquired from the X-ray imaging device 104 to generate a plurality of images to be used in the signal processing described below in the energy subtraction process.

[0045] 5 shows a block diagram of the correction process for the energy subtraction process according to the embodiment. First, the acquisition unit 131 causes the X-ray imaging device 104 to perform imaging without irradiating X-rays, and acquires an image by the drive shown in FIG. 3 or FIG. 4. Two images are read out by this drive. Hereinafter, the first image (image 304 or image 404) is referred to as F_ODD, and the second image (image 306 or image 406) is referred to as F_EVEN. F_ODD and F_EVEN are images corresponding to fixed pattern noise (FPN: Fixed Pattern Noise) of the X-ray imaging device 104.

[0046] Next, the acquisition unit 131 causes the X-ray imaging device 104 to irradiate X-rays in a state where there is no subject, and acquires an image for gain correction output from the X-ray imaging device 104 by driving as shown in FIG. 3 or FIG. 4. By this driving, two images are read out in the same manner as above. Hereinafter, the first image for gain correction (image 304 or image 404) is referred to as W_ODD, and the second image for gain correction (image 306 or image 406) is referred to as W_EVEN. W_ODD and W_EVEN are images corresponding to the sum of the FPN of the X-ray imaging device 104 and the signal due to X-rays. The correction unit 132 obtains images WF_ODD and WF_EVEN from which the FPN of the X-ray imaging device 104 has been removed, by subtracting F_ODD from W_ODD and F_EVEN from W_EVEN. This is called offset correction.

[0047] WF_ODD is an image corresponding to the X-ray 302 in the stable period, and WF_EVEN is an image corresponding to the sum of the X-ray 301 in the rising period, the X-ray 302 in the stable period, and the X-ray 303 in the falling period. Therefore, the correction unit 132 obtains an image corresponding to the sum of the X-ray 301 in the rising period and the X-ray 303 in the falling period by subtracting WF_ODD from WF_EVEN. The process of obtaining an image corresponding to the X-ray in a specific period separated by sample hold by subtracting multiple images in this way is called color correction. The energy of the X-ray 301 in the rising period and the X-ray 303 in the falling period is lower than the energy of the X-ray 302 in the stable period. Therefore, by subtracting WF_ODD from WF_EVEN through color correction, a low-energy image W_Low when there is no subject is obtained. Also, a high-energy image W_High when there is no subject is obtained from WF_ODD.

[0048] Next, the acquisition unit 131 causes the X-ray imaging device 104 to irradiate X-rays and perform imaging in the presence of an object, and acquires an image output from the X-ray imaging device 104 by the driving shown in FIG. 3 or FIG. 4. At this time, two images are read out. Hereinafter, the first image (image 304 or image 404) is referred to as X_ODD, and the second image (image 306 or image 406) is referred to as X_EVEN. The correction unit 132 performs offset correction and color correction similar to those performed when there is no object, thereby obtaining a low-energy image X_Low when there is an object, and a high-energy image X_High when there is an object.

[0049] Here, if the thickness of the subject is d, the linear attenuation coefficient of the subject is μ, the output of pixel 20 when there is no subject is I0, and the output of pixel 20 when there is a subject is I, the following equation [Mathematical formula 1] holds.

[0050]

number

[0051] By transforming equation (1), we obtain the following equation (2). The right-hand side of equation (2) indicates the transmittance of the subject. The transmittance of the subject is a real number between 0 and 1.

[0052]

number

[0053] Therefore, the correction unit 132 obtains an image of transmittance L at low energy (hereinafter also referred to as "low energy image L") by dividing the low energy image X_Low when a subject is present by the low energy image W_Low when no subject is present. Similarly, the correction unit 132 obtains an image of transmittance H at high energy (hereinafter also referred to as "high energy image H") by dividing the high energy image X_High when a subject is present by the high energy image W_High when no subject is present. The process of obtaining an image (L, H) of transmittance at low energy or transmittance at high energy by dividing an image obtained based on a radiation image obtained in a state with a subject by an image obtained based on a radiation image obtained in a state without a subject is called gain correction.

[0054] 6 shows a block diagram of signal processing of the energy subtraction processing according to the embodiment. The signal processing unit 133 generates a material separation image using a plurality of images obtained from the correction unit 132. The process of generating a material separation image consisting of a bone thickness image B (hereinafter also referred to as bone image B) and a soft tissue thickness image S (hereinafter also referred to as soft tissue image S) will be described below. The signal processing unit 133 obtains the bone thickness image B and the soft tissue thickness image S from the image of transmittance L at low energy and the image of transmittance H at high energy obtained by the correction shown in FIG. 5 through the following process.

[0055] First, let E be the energy of the X-ray photon, N(E) be the number of photons at energy E, B be the thickness in the bone thickness image, S be the thickness in the soft tissue thickness image, and μ be the linear attenuation coefficient of bone at energy E. B (E), the linear attenuation coefficient of soft tissue at energy E is μ S (E) If the transmittance is I / I0, the following equation (3) holds:

[0056]

number

[0057] The number of photons N(E) at energy E is the spectrum of X-rays. The spectrum of X-rays can be obtained by simulation or actual measurement. The linear attenuation coefficient μ of bone at energy E is B (E) and the linear attenuation coefficient μ of soft tissue at energy E S (E) can be obtained from databases such as NIST (National Institute of Standards and Technology). Therefore, according to [Equation 3], it is possible to calculate the thickness B in an arbitrary bone thickness image, the thickness S in a soft tissue thickness image, and the transmittance I / I0 in the X-ray spectrum N(E).

[0058] Here, the spectrum of low energy X-rays is N L (E) The spectrum of high-energy X-rays H If (E), the following equations in [Equation 4] hold for the transmittance in image L and the transmittance in image H. In the following description, the transmittance in image L shown in [Equation 4] is also simply referred to as low-energy transmittance L, and the transmittance in image H is also simply referred to as high-energy transmittance H.

[0059]

number

[0060] By solving the nonlinear simultaneous equations in [Equation 4], the thickness B in the bone thickness image and the thickness S in the soft tissue thickness image can be obtained. As a representative method for solving the nonlinear simultaneous equations, the Newton-Raphson method will be described here. First, the number of iterations of the Newton-Raphson method is set to m, and the bone thickness after the mth iteration is defined as B. m , the thickness of the soft tissue after the mth iteration is S m Then, the high energy transmission rate after the mth iteration, H m , the low-energy transmission rate L after the mth iteration m is expressed by the following equation [5].

[0061]

number

[0062] Moreover, the rate of change in transmittance when the thickness is changed slightly is expressed by the following formula [6].

[0063]

number

[0064] In this case, the bone thickness after the m+1th iteration is B m+1 and the soft tissue thickness S m+1 is expressed by the following equation (7) using the high energy transmittance H and the low energy transmittance L.

[0065]

number

[0066] The inverse matrix of a 2x2 matrix, whose determinant is det, is expressed by the following equation (8) using Cramer's rule.

[0067]

number

[0068] Therefore, by substituting equation (8) into equation (7), we obtain the following equation (9).

[0069]

number

[0070] By repeating the above calculations, the high energy transmission rate H after the mth iteration is m The difference between the measured high-energy transmittance H and the measured high-energy transmittance H approaches 0. The same is true for the low-energy transmittance L. This results in the bone thickness B after the mth iteration. m converges to the bone thickness B, and the m-th soft tissue thickness S mconverges to the thickness S of the soft tissue. In this manner, the nonlinear simultaneous equations shown in Equation 4 can be solved. Therefore, by calculating Equation 4 for all pixels, it is possible to obtain an image B of the bone thickness and an image S of the soft tissue thickness from an image with transmittance L at low energy and an image with transmittance H at high energy.

[0071] In the embodiment, the image B of the bone thickness and the image S of the soft tissue thickness are calculated, but the disclosed technology is not limited to this form. For example, the thickness W of water and the thickness I of the contrast agent may be calculated, or the thickness of acrylic resin (PMMA) and the thickness of aluminum (AL) may be calculated as the configuration of the phantom, or the thickness of a first material and the thickness of a second material different from the first material may be calculated. In other words, it may be decomposed into the thicknesses of any two types of materials.

[0072] In the embodiment, the nonlinear simultaneous equations are solved using the Newton-Raphson method. However, the disclosed technology is not limited to this. For example, an iterative solution method such as the least squares method or the bisection method may be used.

[0073] In addition, in the embodiment, the nonlinear simultaneous equations are solved by an iterative solution method, but the disclosed technology is not limited to this form. Numerical integration is required in this process. Moreover, recalculation is required every m iterative calculations. Furthermore, such calculations are performed for all pixels. Therefore, there is a problem that the signal processing of the energy subtraction shown in FIG. 6 takes a very long time. In particular, since this embodiment assumes video shooting, the time allowed for signal processing is one frame or less. For example, if the frame rate is 20 fps, the signal processing and image processing must be performed within 50 ms or less, otherwise the processing will not be able to keep up. Therefore, a table may be generated by previously determining the bone thickness B and the soft tissue thickness S for various combinations of high energy transmittance H and low energy transmittance L, and the bone thickness B and the soft tissue thickness S may be determined quickly by referring to this table.

[0074] 7 shows a flowchart for explaining the flow of table generation according to this embodiment. The number of divisions of the table is M, the coordinate of low energy in the table is l, and the coordinate of high energy in the table is h. The coordinates l and h in the table are integers. First, the imaging control device 103 initializes the coordinate of low energy as l=0 (S701), and then initializes the coordinate of high energy as h=0 (S702).

[0075] Next, the imaging control device 103 calculates the low energy transmittance L[l] at the coordinate l and the high energy transmittance H[h] at the coordinate h by the following formula

[10] .

[0076]

number

[0077] The thus obtained low energy transmittance L[l] and high energy transmittance H[h] are solved by the nonlinear simultaneous equations shown in [Equation 4] to obtain the bone thickness B and the soft tissue thickness S. The results are stored in table B[l,h] for the bone thickness B and table S[l,h] for the soft tissue thickness S (S703). After that, the imaging control device 103 sets h=h+1 (S704). If the high energy coordinate h does not exceed the division number M of the table (NO in S705), the imaging control device 103 repeats the process from S703. If the high energy coordinate h exceeds the division number M of the table (YES in S705), the imaging control device 103 sets l=l+1 (S706). If the low-energy coordinate l does not exceed the division number M of the table (NO in S707), the imaging control device 103 sets h=0 (S702) and repeats the processes of S703 to S705. If the low-energy coordinate l exceeds the division number M of the table (YES in S707), table generation ends. In this manner, the bone thickness B and soft tissue thickness S can be found for all combinations of l and h and stored in the table.

[0078] 8 shows a schematic diagram of a table in this embodiment. This table is a two-dimensional table in which the thickness of bones and soft tissues are registered according to a combination of the transmittance shown by the high-energy image and the transmittance shown by the low-energy image. Assume that the actually measured low-energy transmittance at a certain pixel is L, and the actually measured high-energy transmittance is H. In this case, the transmittance is converted into the table coordinates l' and h' by the following [Equation 11].

[0079]

number

[0080] Using these coordinates, the bone thickness B can be obtained by referring to table B[l,h] for the bone thickness B. The same applies to the thickness of the soft tissue. However, although the coordinates l' and h' in the table are decimals, the table is stored in an array, so only integers can be referenced. Therefore, a configuration is used in which the coordinates l' and h' are converted to integers, and then the bone thickness B and the soft tissue thickness S are obtained by interpolation. For example, the value obtained by truncating the decimal point of the coordinate l' and converting it to an integer is l, and the value obtained by truncating the decimal point of the coordinate h' and converting it to an integer is h, and when the bone thickness B and the soft tissue thickness S are obtained by bilinear interpolation, the following formula

[12] can be used.

[0081]

number

[0082] Therefore, if a table is generated in advance, the bone thickness B and the soft tissue thickness S can be obtained with a significantly smaller amount of computation compared to solving non-linear simultaneous equations. Such a table is valid as long as the radiation spectrum N(E) does not change. Generally, since the radiation spectrum N(E) does not change during video shooting, it is sufficient to generate the table once before shooting. Naturally, the generation and reference of the tables shown in FIGS. 7 and 8 can be similarly used for the calculation of obtaining the thicknesses of any two substances. In this way, the signal processing of energy subtraction shown in FIG. 6 can be performed with high precision and high speed.

[0083] In this embodiment, the low-energy transmittance L[l] at the coordinate l and the high-energy transmittance H[h] at the coordinate h were obtained using Equation

[10] . The table generated in this way has the high-energy transmittance H on the vertical axis coordinate and the low-energy transmittance L on the horizontal axis coordinate, and the table is one in which the transmittance between 0 and 1 is divided at equal intervals. However, the transmittance with respect to the composition and thickness of the human body often has a value near 0. For this reason, when the number of divisions M of the table is small, an error may occur between the value obtained by referring to and interpolating the table and the value obtained by solving the non-linear simultaneous equations. Therefore, when the constant for determining the coordinate range is k (0 < k), it is possible to use a configuration in which the transmittance is obtained using the following Equation

[13] .

[0084]

Equation

[0085] When the transmittance is obtained from the coordinate by Equation

[13] , the coordinate can be obtained using the following Equation

[14] .

[0086]

Equation

[0087] Regarding the reference and interpolation of the table, it is possible to use Equation

[14] . In the table generated in this way, the coordinate on the vertical axis is -ln(H), and the coordinate on the horizontal axis is -ln(L). Therefore, even when the transmittance value is near 0, the error between the value obtained by referring to and interpolating the table and the value obtained by solving the non-linear simultaneous equations can be reduced.

[0088] In this embodiment, when generating and referring to the table, a combination of a high-energy transmittance H and a low-energy transmittance L for which no solution exists may occur. For example, the high-energy transmittance H is usually larger than the low-energy transmittance L. Therefore, in the table generated using Equation

[10] or Equation

[13] , no solution can be obtained in the region where H < L.

[0089] In this embodiment, the coordinates may be selected so as to reduce the region where no solution can be obtained on the table. For example, the coordinate on the vertical axis may be ln(L) / ln(H), and the coordinate on the horizontal axis may be -ln(H), etc. Also, when referring to the table in this embodiment, there is a possibility that coordinates outside the range of the table or a region where no solution can be obtained may be specified. In such a case, a configuration that uses the values in the region where a solution exists around the specified coordinates is used.

[0090] In the signal processing of FIG. 6, the high-energy transmittance H and the low-energy transmittance L were obtained using Equation [4]. However, the transmittance obtained using Equation [4] is a theoretical value estimated from the linear attenuation coefficient and the spectrum.

[0091] However, in actual imaging, the transmittance can increase due to scattered rays generated inside the subject. Therefore, when performing the signal processing of FIG. 6 using the high-energy transmittance H' including scattered rays and the low-energy transmittance L' including scattered rays, considering the generation of scattered rays, an error may occur in the bone thickness B and the soft tissue thickness S. To solve this problem, in this embodiment, scattered ray correction is performed to reduce the influence of scattered rays in the signal processing.

[0092] In order to correct the scattered radiation, it is necessary to estimate the amount of scattered radiation, and in this embodiment, a table for scattered radiation correction is used to estimate the amount of scattered radiation. Fig. 9 is a diagram showing an example of the table for scattered radiation correction according to this embodiment.

[0093] The scattered radiation correction table shown in FIG. 9 is created by placing, for example, a flat plate between the two-dimensional detector 106 and the X-ray generator 101 (X-ray source) and measuring the transmittance when X-rays are irradiated. When the X-ray irradiation range is sufficiently expanded, scattered radiation is generated inside the flat plate. The high energy transmittance measured in this state is called the transmittance including scattered radiation H real Let us assume that.

[0094] In addition, narrowing the X-ray irradiation range reduces the scattered radiation generated inside the plate, and the transmittance measured in this state approaches the theoretical transmittance value obtained using [Equation 4]. The high-energy transmittance measured with the X-ray irradiation range narrowed is called the transmittance without scattered radiation, H ideal Let us assume that.

[0095] By preparing multiple plates of the same material with different thicknesses (n patterns: n is an integer of 2 or more), and repeating the same measurement while changing the plate thickness S[n], the transmittance H real [n] and the high energy transmittance H without scattered radiation ideal [n] and stored in the table 901 for scattered radiation correction. As a result, the table 901 for scattered radiation correction contains the transmittance H real [n] and the transmittance without scattered radiation H ideal [n] is stored for each thickness S[n] of the flat plate.

[0096] The same measurement was also performed for the transmittance of low energy. That is, measurements were performed with the X-ray irradiation range sufficiently widened and with the X-ray irradiation range narrowed, and the transmittance L real [n] and the transmittance L without scattered radiation ideal[n] is obtained and stored in the table 902 for scattered radiation correction. As a result, the table 902 for scattered radiation correction contains the transmittance L including scattered radiation with respect to the transmittance of low energy. real [n] and the transmittance L without scattered radiation ideal [n] is stored for each thickness S[n] of the flat plate.

[0097] The material of the plate may be, for example, acrylic resin (PMMA), which has a linear attenuation coefficient close to that of soft tissue (for example, fat). Similarly, Tough Water, which has a linear attenuation coefficient similar to that of water, or water itself, contained in a container, may be used. Alternatively, a material such as aluminum (AL), which has a linear attenuation coefficient close to that of bone, which is a hard material, may be used.

[0098] The shape of the object to be photographed is not limited to a flat plate, and for example, a circular cylinder or an elliptical cylinder may be photographed to create scattered radiation correction tables 901, 902. In addition, in the explanation of Fig. 9, an example is explained in which the transmittance including scattered radiation and the transmittance excluding scattered radiation are obtained by experiment, but the present invention is not limited to this example, and the transmittance including scattered radiation and the transmittance excluding scattered radiation may be obtained from a database or by simulation.

[0099] The signal processing unit 133 acquires a transmittance not including scattered rays corresponding to the thickness of a predetermined material by performing correction using the transmittance including scattered rays and the transmittance not including scattered rays stored in the storage unit 136. Fig. 10 shows a block diagram of the scattered rays correction process according to the embodiment. In the scattered rays correction process, the signal processing unit 133 acquires a transmittance H not including scattered rays at high energy from a transmittance H' including scattered rays at high energy using a table 901 for scattered rays correction shown in Fig. 9.

[0100] The signal processing unit 133 searches the table 901 for high-energy scattered radiation correction and determines whether the transmittance H′ of high energy including scattered radiation is greater than the transmittance H real [n] or more, and the transmittance including scattered radiation H realObtain information about the thickness of the material (n) that is less than [n+1] (H real [n]≦H' <H real [n+1]).

[0101] The signal processing unit 133 uses information (n) regarding the thickness of the material to correct (interpolate) the data in table 901 using the following equation [Mathematical Formula 15A] to obtain the transmittance H not including scattered rays from the high energy transmittance H' including scattered rays.

[0102] [Number 15A] H=H ideal [n]*(H real [n+1]-H') / (H real [n+1]-H real [n]) +H ideal [n+1]*(H'-H real [n]) / (H real [n+1]-H real [n])

[0103] The same is true for the low-energy transmittance L' including scattered rays and the low-energy transmittance L not including scattered rays. In this way, scattered rays can be corrected by converting the high-energy transmittance H' including scattered rays and the low-energy transmittance L' including scattered rays into the high-energy transmittance H not including scattered rays and the low-energy transmittance L not including scattered rays.

[0104] The scattered radiation correction table explained in Fig. 9 shows the transmittance (H real , L real ) and the transmittance without scattered radiation (H ideal , L ideal )

[0105] However, the table for scattered ray correction is not limited to the example in Fig. 9, and for example, the area where X-rays directly reach the two-dimensional detector 106 from the X-ray generator 101 may be regarded as a direct ray area, and values ​​expressing the components of X-rays as ratios may be stored in the table. For example, SPR (Scatter Primary Ratio), which is the ratio between the scattered ray component (Scatter) and the direct ray component (Primary) as shown in [Equation 15B], may be stored. In the following explanation of the equations, the scattered ray component (Scatter) is also simply referred to as "scattered ray component S" or "S", and the direct ray component (Primary) is also simply referred to as "direct ray component P" or "P".

[0106] [Math 15B] SPR=scattered radiation component S / direct radiation component P Here, the transmittance including scattered rays at high energy, H real is the sum of the direct radiation component P and the scattered radiation component S (direct radiation component P + scattered radiation component S), and the transmittance H ideal is the direct radiation component P (H ideal = direct line component P).

[0107] From the relationship between the scattered radiation component S and the direct radiation component P, the transmittance including the scattered radiation H real and the transmittance without scattered radiation H ideal The relationship between SPR and the transmittance including scattered radiation, H real and the transmittance without scattered radiation H ideal The relationship between these is shown in the following equation [15D].

[0108] [Number 15C] Scattered radiation component S=P+SP=H real -H ideal [Number 15D] SPR=S / P=(H real -H ideal ) / H ideal By rearranging the formula [15D], the transmittance without scattered radiation, H ideal is the transmittance including scattered radiation H real Using and SPR, it is expressed as the following equation [15E].

[0109] [Number 15E] SPR*H ideal =H real -H ideal SPR*H ideal +H ideal =H real (1+SPR)*H ideal =H real H ideal =H real / (1+SPR) Transmittance including scattered radiation at thickness [n] H real [n] and the transmittance without scattered radiation H ideal [n] is expressed by the following formula [15F].

[0110] [Number 15F] H ideal [n] = H real [n] / (1+SPR[n]) H ideal [n+1] = H real [n+1] / (1+SPR[n+1]) By substituting equation (15F) into equation (15A), we obtain equation (15G), which corrects the transmittance using SPR, which is the ratio between the scattered radiation component and the direct radiation component.

[0111] [Number 15G] H=H real [n] / (1+SPR[n])*(H real [n+1]-H') / (H real [n+1]-H real [n]) +H real [n+1] / (1+SPR[n+1])*(H'-H real [n]) / (H real [n+1]-H real [n])

[0112] In the processing of the formulas (15B) to (15G), the storage unit 136 stores the ratio of the scattered ray component contained in the radiation to the direct ray component of the radiation that directly reaches the two-dimensional detector 106 from the X-ray generating device 101 (radiation source) for each thickness of a predetermined material. The signal processing unit 133 calculates the ratio SPR and the transmittance including the scattered ray real By performing correction using the above formula, the transmittance H, excluding scattered radiation, corresponding to the thickness of a given material is obtained.

[0113] In addition, the scattered radiation correction table is not limited to the example of FIG. 9. For example, real and the transmittance without scattered radiation H ideal The difference SB between the thickness [n] and the scattered radiation transmittance H real [n] and the transmittance without scattered radiation H ideal [n] is expressed by the following formula [15H].

[0114] [Number 15H] Difference SB = H real -H ideal H ideal [n] = H real [n] - SB[n] H ideal [n+1] = H real [n+1] - SB[n+1]

[0115] Substituting Equation 15H into Equation 15A, the transmittance including scattered radiation, H real and the transmittance without scattered radiation H ideal The transmittance is corrected using the difference SB between these values, and the following equation (15J) is obtained.

[0116] [Number 15J] H=(H real [n]-SB[n])*(H real [n+1]-H') / (H real [n+1]-H real [n]) +(H real [n+1]-SB[n+1])*(H'-H real [n]) / (Hreal [n+1]-H real [n])

[0117] In the processing of the formulas [15H] to [15J], the memory unit 136 stores the transmittance including the scattered radiation. real and the transmittance without scattered radiation H ideal The difference SB between the difference SB and the transmittance including scattered radiation is stored for each thickness of a predetermined material. real By performing correction using the above formula, the transmittance H, excluding scattered radiation, corresponding to the thickness of a given material is obtained.

[0118] In the description of FIG. 10, the scattered radiation is corrected using a table for scattered radiation correction. However, the correction of the scattered radiation is not limited to such an embodiment. For example, the relationship between the transmittance including the scattered radiation and the transmittance not including the scattered radiation may be approximated by a function. For example, the transmittance not including the scattered radiation H ideal and the transmittance including scattered radiation H real A function that corresponds to the scattered radiation is calculated and the transmittance H real When you input the value, the function outputs the transmittance without scattered radiation H ideal When such a function is approximated, the relationship between pixel values ​​containing scattered radiation and pixel values ​​not containing scattered radiation is included in the function, so that the interpolation process using formulas (15A) to (15J) is not necessary.

[0119] The table for scattered radiation correction shown in Figure 9 was created by preparing n patterns (n ​​is an integer greater than or equal to 2) of flat plates of the same material but with different thicknesses, and obtaining the transmittance while changing the thickness S[n] of the flat plates.

[0120] However, an actual subject is composed of multiple materials, such as bones and soft tissues. When the type of material that composes the subject changes, the amount of scattered radiation generated may also change. When the inventors of the present application performed scattered radiation correction using the scattered radiation correction table shown in Fig. 9, they found that when the subject is composed of multiple materials, a non-negligible error may occur in the corrected transmittance H and transmittance L.

[0121] It was also found that when the subject is a material different from that used to create the table for scattered radiation correction, a non-negligible error may occur in the transmittance H and transmittance L after scattered radiation correction. As a result, a problem was discovered in that an error may occur in the bone thickness B and soft tissue thickness S determined by the signal processing shown in Fig. 6. Therefore, the present inventors propose in this embodiment scattered radiation correction that takes into account multiple materials (two materials) that make up the subject, such as bone and soft tissue.

[0122] In this embodiment, information regarding the transmittance including scattered radiation obtained for each combination of the thickness of the first material and the thickness of the second material is stored in the memory unit 136. The signal processing unit 133 obtains information regarding the transmittance H', L' including scattered radiation for the combination by correcting the information obtained from the memory unit 136 based on the combination of the thickness of the first material and the thickness of the second material.

[0123] 11 is a diagram showing a table for scattering correction taking into account two materials according to the embodiment. A table 1101 shown in FIG. 11 shows a transmittance H real [m, n]. Table 1102 is a table for correcting the transmittance L including scattered radiation at low energy. real The tables 1101 and 1102 for scattering correction considering two substances shown in FIG. 11 can be created in the same manner as the tables 901 and 902 for scattering correction shown in FIG. 9. Note that the transmittance H ideal Tables for correcting [m, n] and transmittance L without scattered radiation at low energies ideal It is possible to create a table for correcting [m, n] in the same way as tables 1101 and 1102, but the correction process based on the combination of the thicknesses of the two materials becomes complicated and the calculation load may become high. ideal and the transmittance without scattered radiation at low energies L idealis substantially the same as the transmittance H and transmittance L obtained by theoretical calculation using equation (4). Therefore, for the transmittance not including scattered rays, the information (H, L) obtained by the calculation using equation (4) can be used.

[0124] In creating tables 1101 and 1102, m patterns (m is an integer of 2 or more) of plates of different thicknesses made of a first material and n patterns (n ​​is an integer of 2 or more) of plates of different thicknesses made of a second material different from the first material are prepared as plates to be placed between two-dimensional detector 106 and X-ray generator 101 (X-ray source). Measurement is performed in a state where the plates of the first material and the plates of the second material are arranged so as to overlap in the direction in which X-rays are irradiated from X-ray generator 101 toward two-dimensional detector 106.

[0125] The same measurements are repeated while changing the combination of the thickness B[m] of the first material plate in the m pattern and the thickness S[n] of the second material plate in the n pattern. By such measurements, the transmittance H real [m,n] and the scattering-free transmittance at high energy H ideal [m, n] is measured (obtained). Similarly, for low energy, the transmittance L real [m,n] and the transmittance without scattering at low energy L ideal By repeating measurements with different combinations of the thickness B[m] of the plate of the first material in the m pattern and the thickness S[n] of the plate of the second material in the n pattern, it is possible to obtain tables 1101 and 1102 for scattered radiation correction taking into account the two materials shown in FIG.

[0126] In scattered radiation correction taking into account two substances (a first substance and a second substance), the signal processing unit 133 uses a table for scattered radiation correction taking into account two substances shown in Figure 11 to obtain the thickness B of the bone and the thickness S of the soft tissue not including scattered radiation from the transmittance H' including scattered radiation at high energy and the transmittance L' including scattered radiation at low energy.

[0127] The signal processing unit 133 executes such processing in two stages, a first processing and a second processing.

[0128] (1) First process: The signal processing unit 133 generates a first function (function H'=h(B,S)) for outputting information on the transmittance including the scattered rays in a combination of the thickness of the first material (bone thickness B) and the thickness of the second material (soft tissue thickness S) for a first radiation energy (high energy) when no scattered rays are included, from the combination of the thickness of the first material (bone thickness B) and the thickness of the second material (soft tissue thickness S) for a second radiation energy (low energy) when no scattered rays are included. In particular, the signal processing unit 133 generates a function H'=h(B,S) for calculating the transmittance H' including the scattered rays at high energy and a function L'=l(B,S) for calculating the transmittance L' including the scattered rays at low energy, from the thickness of the bone B and the thickness of the soft tissue S not including scattered rays.

[0129] The formula [4] explained above is a formula for calculating the transmittances H and L not including scattered radiation from the bone thickness B not including scattered radiation and the soft tissue thickness S. In the first process, the signal processing unit 133 generates a function H'=h(B,S) for calculating the transmittance H' including scattered radiation and a function L'=l(B,S) for calculating the transmittance L' including scattered radiation as functions equivalent to the formula [4] from the bone thickness B not including scattered radiation and the soft tissue thickness S, based on the data in tables 1101 and 1102.

[0130] In generating the function H'=h(B,S), the signal processing unit 133 refers to table 1101. For example, if the bone thickness B is 1.5, data of B=1 and B=2 is searched for in table 1101, and if the soft tissue thickness S is 13, data of S=10 and S=15 is searched for in table 1101. The signal processing unit 133 acquires the transmittance H' including scattered radiation corrected (interpolated) based on the data searched for in table 1101 based on the combination of the bone thickness B and the soft tissue thickness S, as the function H'=h(B,S).

[0131] In generating the function L'=l(B,S), the signal processing unit 133 refers to the table 1102. For example, if the bone thickness B is 1.5, data of B=1 and B=2 is searched for in the table 1102, and if the soft tissue thickness S is 13, data of S=10 and S=15 is searched for in the table 1102. The signal processing unit 133 acquires the transmittance L' including scattered radiation corrected (interpolated) based on the data searched for in the table 1102 based on the combination of the bone thickness B and the soft tissue thickness S, as the function L'=l(B,S).

[0132] (2) Second processing: Next, the signal processing unit 133 uses the function H'=h(B, S) and the function L'=l(B, S) obtained in the first processing to determine the thickness B of the bone and the thickness S of the soft tissue not including scattered rays from the transmittance H' including scattered rays at high energy and the transmittance L' including scattered rays at low energy.

[0133] Fig. 12 is a block diagram of transmittance calculation for the first process considering two materials according to the embodiment. In the process not including scattered radiation, the calculation process of [Equation 4] is included in the block diagram of Fig. 6, but in the case including scattered radiation, correction (interpolation) process obtained from the data in tables 1101 and 1102 is included instead of the calculation process of [Equation 4].

[0134] The signal processing unit 133 obtains a function H' = h(B, S) for obtaining the transmittance H' including scattered rays at high energy and a function L' = l(B, S) for obtaining the transmittance L' including scattered rays at low energy from the thickness B of bone without scattered rays and the thickness S of soft tissue, which are set as initial values. The signal processing unit 133 obtains the functions H' = h(B, S) and L' = l(B, S) by searching and interpolating the tables 1101 and 1102.

[0135] First, the signal processing unit 133 searches the two-material table 1101 using the thickness B and obtains m such that B[m] ≤ B < B[m + 1].

[0136] Similarly, the signal processing unit 133 searches the two-material table 1101 using the thickness S and obtains n such that S[n] ≤ S < S[n + 1].

[0137] Then, the signal processing unit 133 corrects (interpolates) the data obtained by searching the table 1101 using the following [Equation 16A] and obtains the transmittance H' including scattered rays as the function H' = h(B, S). The signal processing unit 133 obtains information H' regarding the transmittance including scattered rays at the first radiation energy (high energy) through correction.

[0138] [Equation 16A] H'[n] = H ideal [m, n] * (B[m + 1] - B) / (B[m + 1] - B[m]) + H ideal [m + 1, n] * (B - B[m]) / (B[m + 1] - B[m]) H'[n + 1] = H ideal [m, n + 1] * (B[m + 1] - B) / (B[m + 1] - B[m]) + H ideal [m + 1, n + 1] * (B - B[m]) / (B[m + 1] - B[m]) H' = H'[n] * (S[n + 1] - S) / (S[n + 1] - S[n]) + H'[n + 1] * (S - S[n]) / (S[n + 1] - S[n])

[0139] The same is true for the low energy transmittance L', and the signal processing unit 133 corrects (interpolates) the data acquired by searching table 1102 using the following formula [Mathematical Expression 16B] to acquire the transmittance L' including scattered radiation as a function L'=I(B,S). Through the correction, the signal processing unit 133 acquires information regarding the transmittance L' including scattered radiation at the second radiation energy (low energy).

[0140] [Number 16B] L'[n] = L ideal [m,n]*(B[m+1]-B) / (B[m+1]-B[m]) +L ideal [m+1,n]*(BB[m]) / (B[m+1]-B[m]) L'[n+1] = L ideal [m,n+1]*(B[m+1]-B) / (B[m+1]-B[m]) +L ideal [m+1,n+1]*(BB[m]) / (B[m+1]-B[m]) L'=L'[n]*(S[n+1]-S) / (S[n+1]-S[n])+L'[n+1]*(SS[n]) / (S[n+1]-S[n])

[0141] In this way, a function H'=h(B,S) for calculating the transmittance H' including scattered radiation at high energy and a function L'=l(B,S) for calculating the transmittance L' including scattered radiation at low energy are generated from the bone thickness B not including scattered radiation and the soft tissue thickness S, which are set as initial values.

[0142] FIG. 13 shows a block diagram of thickness calculation for the second processing according to the embodiment. The block diagram of FIG. 12 described above is included in the block diagram of FIG. 13, and when executing the processing of the block diagram of FIG. 13, the processing of the block diagram of FIG. 12 is executed first, and the processing of the block diagram of FIG. 13 is executed using the result. The processing of the block diagram of FIG. 13 is executed repeatedly until the bone thickness B and the soft tissue thickness S converge. In the processing of the block diagram of FIG. 13, the bone thickness B and the soft tissue thickness S obtained in the process of repeated calculation become inputs to the block diagram of FIG. 12, the processing of the block diagram of FIG. 12 is executed, and the processing of the block diagram of FIG. 13 is executed repeatedly using the result.

[0143] In the processing of the block diagram of Fig. 13, the signal processing unit 133 obtains the thickness B of the bone not including scattered rays (with reduced scattered rays) and the thickness S of the soft tissue from the transmittance H' including scattered rays at any high energy and the transmittance L' including scattered rays at any low energy. Here, the signal processing unit 133 obtains the thickness B of the bone not including scattered rays (with reduced scattered rays) and the thickness S of the soft tissue from the transmittance H' including scattered rays and the transmittance L' including scattered rays using the function H'=h(B,S) and the function L'=l(B,S) obtained in the processing of the block diagram of Fig. 12.

[0144] The signal processing unit 133 can obtain the bone thickness B and the soft tissue thickness S by solving the nonlinear simultaneous equations using the Newton-Raphson method. The calculation method of the Newton-Raphson method is almost the same as the signal processing in FIG. 6, but it differs in that processing is performed using a function for calculating the transmittance (H', L') including scattered radiation, instead of the theoretical formula for the transmittance (H, L) shown in [Equation 4]. That is, the number of iterations of the Newton-Raphson method is m, and the bone thickness after the mth iteration is B. m , the thickness of the soft tissue after the mth iteration is S m Then, the high energy transmission rate after the mth iteration is H' m , the low-energy transmission rate L' after the mth iteration m When the transmittance is H' m , and transmittance L' mis expressed by the following equation

[17] instead of equation [5]. [Number 17] H' m =h(B m ,S m ) L' m =l(B m ,S m )

[0145] In addition, the rate of change in transmittance when the thickness changes slightly is calculated from the difference based on the change in material thickness (amount of change) as shown in the following formula

[17] , instead of formula [6] which is calculated by differentiation. In other words, when the minute change in the thickness of each material in the bone thickness B and the soft tissue thickness S is δ, the transmittance is H' m , and transmittance L' m The rate of change of is expressed by the following equation

[18] .

[0146]

number

[0147] The signal processing unit 133 acquires a first rate of change (first equation in [Equation 18]) of the first function (function H'=h(B,S)) and a second rate of change (second equation in [Equation 18]) of the second function (function L'=l(B,S)) based on the change (minute change δ) of the thickness of the first material (bone thickness B). The signal processing unit 133 also acquires a third rate of change (third equation in [Equation 18]) of the first function (function H'=h(B,S)) and a fourth rate of change (fourth equation in [Equation 18]) of the second function (function L'=l(B,S)) based on the change (minute change δ) of the thickness of the second material (soft tissue thickness S).

[0148] By calculating from [Equation 7] to [Equation 9] using the rate of change of [Equation 18], the bone thickness B after the m+1th iteration is calculated as in FIG. 6. m+1 and the soft tissue thickness S m+1 can be sought.

[0149] In the calculation of [Equation 7] to [Equation 9], the signal processing unit 133 calculates a first difference (HH′) between the transmittance H not including scattered radiation at the first radiation energy (high energy) and the transmittance H′ including scattered radiation output from the first function (function H′=h(B,S)). m In addition, the signal processing unit 133 obtains a second difference (LL') between the transmittance L not including scattered radiation at the second radiation energy (low energy) and the transmittance L' including scattered radiation output from the second function (function L'=l(B,S)). m ) and get.

[0150] In the calculation of the formula [9], the signal processing unit 133 calculates the fourth rate of change (ΔL′ m / δS m ) and the first difference (HH' m ) and the third rate of change (δH' m / δS m ) and the second difference (LL' m ) and the energy subtraction process using m+1 In addition, the signal processing unit 133 obtains the second rate of change (ΔL′ m / δB m ) and the first difference (HH' m ) and the first rate of change (δH' m / δB m ) and the second difference (LL' m ) and the energy subtraction process using m+1 ) to get the

[0151] By repeating the above calculations, the high energy transmission rate H' after the mth iteration is m Rate of change of and low energy transmittance L' m That is, the rate of change of the transmittance H' of high energy including scattered rays after the mth iteration by the repeated calculation of the signal processing unit 133 converges. m converges to the scattered radiation-free transmittance H, and after the mth iteration, the scattered radiation-containing low-energy transmittance L' mconverges to the transmittance L that does not include scattered radiation. For example, in [Equation 9], the high-energy transmittance H' m The difference between the transmittance H that does not include scattered radiation and the transmittance L' that does not include scattered radiation approaches 0. m The difference between this and the transmittance L not including scattered radiation approaches zero infinitely.

[0152] The signal processing unit 133 acquires an image of a first transmittance H not including scattered rays, in which the scattered ray components are reduced from the transmittance H' including scattered rays output from the first function, and an image of a second transmittance L not including scattered rays, in which the scattered ray components are reduced from the transmittance L' including scattered rays output from the second function.Then, the signal processing unit 133 acquires a thickness image of the first material and a thickness image of the second material in which the scattered ray components are reduced, using the image of the first transmittance H and the image of the second transmittance L.

[0153] Specifically, the signal processing unit 133 acquires a transmittance H not including scattered rays (an image of the first transmittance H) as a transmittance in which the scattered ray components are reduced from the high-energy transmittance H' including scattered rays, by repeated calculations using Equation

[18] and Equation [7] to Equation [9].

[0154] Similarly, the signal processing unit 133 acquires a transmittance L not including scattered rays (an image of a second transmittance L) as a transmittance in which the scattered ray components are reduced from the low-energy transmittance L' including scattered rays, by repeated calculations using Equation

[18] and Equation [7] to Equation [9].

[0155] The signal processing unit 133 calculates the high energy transmittance H' m and low energy transmittance L' m The bone thickness B after the mth iteration is calculated by the energy subtraction process using m converges to the bone thickness B, and the m-th soft tissue thickness S mconverges to the thickness S of the soft tissue. In this manner, the nonlinear simultaneous equations shown in Equation 17 can be solved. By calculating Equation 17 for all pixels, it is possible to obtain the thickness B of the bone and the thickness S of the soft tissue that do not contain scattered rays (scattered rays are reduced) from the image with high energy transmittance H' including scattered rays and the image with low energy transmittance L' including scattered rays.

[0156] In this embodiment, an example in which the Newton-Raphson method is used as a method for analyzing a nonlinear simultaneous equation has been described, but the present invention is not limited to this analysis example, and an analysis method of an iterative solution such as the least squares method or the bisection method may be used. In addition, by generating the table shown in Fig. 7 or by referring to the table shown in Fig. 8, signal processing of energy subtraction can be performed with high precision and at high speed, and various modifications can be used for the configuration for speeding up calculations.

[0157] In addition, in this embodiment, an example has been described in which the thickness B of the bone not including scattered rays and the thickness S of the soft tissue are obtained from an image of the transmittance H' including scattered rays at high energy and an image of the transmittance L' including scattered rays at low energy. However, this embodiment is not limited to this example, and scattered ray correction may be performed, for example, by using a table for scattered ray correction considering two materials shown in Fig. 11 to obtain the transmittance H not including scattered rays at high energy and the transmittance L not including scattered rays at low energy from the transmittance H' including scattered rays at high energy and the transmittance L' including scattered rays at low energy.

[0158] In addition, in this embodiment, for the sake of simplicity, no specific limitations are set on the placement position of the flat plate when creating a table for scattered radiation correction that takes two materials into consideration. However, the disclosed technology is not limited to this example, and a table for scattered radiation correction may be created by specifically setting the placement position of the flat plate and performing measurements. For example, the distance (OID) between the subject (e.g., flat plate) and the two-dimensional detector 106 in the X-ray imaging device 104 may be measured, and a table for scattered radiation correction may be created for each distance (OID). If the distance (OID) between the two-dimensional detector 106 and the subject (e.g., flat plate) changes, the amount of scattered radiation may also change, so it is also possible to switch the table for scattered radiation correction to be used depending on the distance (OID). It is also possible to calculate the amount of scattered radiation by interpolating tables for scattered radiation correction at multiple distances (OID). This makes it possible to calculate the amount of scattered radiation with higher accuracy depending on the placement position of any flat plate and perform correction processing. For example, the memory unit 136 stores information on the transmittance acquired for each distance between the subject and the two-dimensional detector 106 that detects radiation. The signal processing unit 133 may obtain information regarding the transmittance corresponding to the thickness of a specific material, or information regarding the transmittance corresponding to a combination of the thickness of a first material and the thickness of a second material, using information (information regarding the transmittance) interpolated based on the distance at which the subject is photographed.

[0159] In this embodiment, the X-ray imaging device 104 is an indirect X-ray sensor using a phosphor. However, the disclosed technology is not limited to this. For example, a direct X-ray sensor using a direct conversion material such as CdTe may be used.

[0160] In the present embodiment, the tube voltage of the X-ray generating device 101 is changed. However, the disclosed technology is not limited to this. The energy of the X-rays irradiated to the X-ray imaging device 104 may be changed by, for example, switching the filter of the X-ray generating device 101 over time.

[0161] In this embodiment, images with different energies are obtained by changing the energy of X-rays. However, the disclosed technology is not limited to this form. By stacking a plurality of phosphors 105 and two-dimensional detectors 106, images with different energies may be obtained from a two-dimensional detector on the front side and a two-dimensional detector on the back side with respect to the incidence direction of X-rays. The two-dimensional detector 106 is not limited to a medical one, and may be an industrial two-dimensional detector.

[0162] In the present embodiment, the energy subtraction process is performed using the imaging control device 103 of the radiation imaging system. However, the disclosed technology is not limited to this form. The image acquired by the imaging control device 103 may be transferred to another computer and the energy subtraction process may be performed thereon. For example, the acquired image may be transferred to another image processing device (image viewer) via a medical PACS and displayed after the energy subtraction process is performed thereon.

[0163] The disclosure of this specification includes the following image processing device, radiation imaging system, image processing method, and program. (Item 1) An image processing device comprising a processing means for acquiring a thickness image of a first material and a thickness image of a second material in which the scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating a subject with radiation and taking an image, and information regarding a transmittance including scattered rays and a transmittance not including the scattered rays that are preset for each thickness of a first material and a second material different from the first material. (Item 2) An image processing device according to item 1, wherein information corresponding to a first radiation energy among the plurality of radiation energies includes a first radiation image, and information corresponding to a second radiation energy different from the first radiation energy includes a second radiation image. (Item 3) The image processing device according to item 2, wherein the first radiographic image includes data in which signal values ​​of electrical signals obtained by the imaging correspond to position information indicating a two-dimensional arrangement of the signal values, and the second radiographic image includes data in which signal values ​​of electrical signals obtained by the imaging correspond to position information indicating a two-dimensional arrangement of the signal values. (Item 4) The radiation source further includes a storage means for storing information on the transmittance obtained for each thickness of a predetermined material for each of a first radiation energy and a second radiation energy different from the first radiation energy, 2. The image processing device according to item 1, wherein the processing means obtains information regarding the transmittance corresponding to the thickness of the predetermined substance by correcting the information stored in the storage means. (Item 5) The storage means stores information about the transmittance including the scattered radiation obtained for each combination of the thickness of the first material and the thickness of the second material, 5. The image processing device according to item 4, wherein the processing means obtains information regarding the transmittance including the scattered radiation in the combination by correcting the information obtained from the storage means based on the combination. (Item 6) The image processing device according to item 5, wherein the processing means acquires, by the correction, information regarding the transmittance at which the scattered rays are contained in the first radiation energy and information regarding the transmittance at which the scattered rays are contained in the second radiation energy. (Item 7) The processing means generates a first function for outputting information on transmittance including the scattered radiation in a combination of a thickness of the first material and a thickness of the second material with respect to the first radiation energy in a case where the scattered radiation is not included, 7. The image processing device according to item 5 or 6, which generates a second function that outputs information about the transmittance including the scattered radiation in a combination of the thickness of the first material and the thickness of the second material for the second radiation energy when the scattered radiation is not included, from the combination of the thickness of the first material and the thickness of the second material. (Item 8) The processing means obtains a first rate of change of the first function and a second rate of change of the second function based on a change in thickness of the first substance; 8. The image processing device according to item 7, further comprising: acquiring a third rate of change of the first function and a fourth rate of change of the second function based on a change in thickness of the second substance. (Item 9) The processing means calculates a first difference between the transmittance including the scattered radiation output from the first function and the transmittance not including the scattered radiation at the first radiation energy, 9. The image processing device according to item 8, further comprising: a second difference between the transmittance including the scattered radiation output from the second function and the transmittance not including the scattered radiation at the second radiation energy. (Item 10) The processing means acquires a thickness image of the first material by repeatedly executing an energy subtraction process using the fourth change rate and the first difference, and the third change rate and the second difference; 10. The image processing device according to item 9, wherein the energy subtraction process is repeatedly performed using the second rate of change and the first difference, and the first rate of change and the second difference, to obtain a thickness image of the second material. (Item 11) The processing means includes: a first transmittance image not including the scattered radiation, the first transmittance image being obtained by reducing the scattered radiation component from the transmittance including the scattered radiation output from the first function; and acquiring a second transmittance image not including the scattered radiation, the second transmittance image being obtained by reducing the scattered radiation component from the transmittance including the scattered radiation output from the second function; 11. The image processing device according to item 10, which uses the first transmittance image and the second transmittance image to obtain a thickness image of the first material and a thickness image of the second material in which the scattered radiation components are reduced. (Item 12) The image processing device described in Item 4, wherein the processing means acquires the transmittance not including scattered radiation corresponding to the thickness of the specified material by correction using the transmittance including scattered radiation and the transmittance not including scattered radiation stored in the memory means. (Item 13) The storage means stores a ratio of a scattered ray component contained in the radiation to a direct ray component of the radiation that directly reaches the two-dimensional detector from the radiation source for each thickness of the predetermined material, Item 13. The image processing device according to item 12, wherein the processing means obtains the transmittance not including scattered radiation corresponding to the thickness of the specified material by performing correction using the ratio and the transmittance including scattered radiation. (Item 14) The storage means stores a difference between the transmittance including the scattered radiation and the transmittance not including the scattered radiation for each thickness of the predetermined material, Item 13. The image processing device according to item 12, wherein the processing means acquires the transmittance not including scattered radiation corresponding to the thickness of the specified material by performing correction using the difference and the transmittance including scattered radiation. (Item 15) The storage means stores information regarding the transmittance obtained for each distance between the subject and a detection means for detecting the radiation, 5. The image processing device according to item 4, wherein the processing means acquires information regarding the transmittance corresponding to a thickness of the specified substance using the information interpolated based on the distance in shooting the subject. (Item 16) The image processing device according to any one of items 1 to 15, wherein the first material thickness image is a bone image, and the second material thickness image is a soft tissue image. (Item 17) An image processing device described in any one of items 1 to 16, further comprising a display control means for causing a display means to display the thickness image of the first material and the thickness image of the second material acquired by the processing means. (Item 18) A radiation imaging device including a two-dimensional detector; 18. The image processing device according to any one of items 1 to 17, which acquires an image in which scattered rays contained in the radiation detected by the two-dimensional detector have been corrected; A radiation imaging system comprising: (Item 19) An image processing method comprising the steps of: acquiring a thickness image of a first material and a thickness image of a second material in which the scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating a subject with radiation and taking an image; and information regarding a transmittance including scattered rays and a transmittance not including the scattered rays, which are preset for each thickness of a first material and a second material different from the first material. (Item 20) A program for causing a computer to execute the image processing method described in Item 19.

[0164] [Other embodiments] The disclosed technology can also be realized by a process in which a program for realizing 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) for realizing one or more functions.

[0165] The disclosed technology is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to disclose the scope of the invention. [Explanation of symbols]

[0166] 101: X-ray generator, 102: X-ray control device, 103: imaging control device (image processing device), 104: X-ray imaging device, 106: two-dimensional detector

Claims

1. An image processing device comprising: a processing means for acquiring a thickness image of a first material and a thickness image of a second material in which the scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating a subject with radiation and taking an image, and information regarding a transmittance including scattered rays and a transmittance not including the scattered rays, which are preset for each thickness of a first material and a second material different from the first material.

2. 2. The image processing device according to claim 1, wherein the information corresponding to a first radiation energy among the plurality of radiation energies includes a first radiation image, and the information corresponding to a second radiation energy different from the first radiation energy includes a second radiation image.

3. 3. The image processing device according to claim 2, wherein the first radiographic image includes data in which signal values ​​of electrical signals obtained by the imaging correspond to position information indicating a two-dimensional arrangement of the signal values, and the second radiographic image includes data in which signal values ​​of electrical signals obtained by the imaging correspond to position information indicating a two-dimensional arrangement of the signal values.

4. a storage means for storing information on the transmittance obtained for each thickness of a predetermined material for each of a first radiation energy and a second radiation energy different from the first radiation energy, 2 . The image processing apparatus according to claim 1 , wherein the processing means obtains information on the transmittance corresponding to the thickness of the predetermined substance by correcting the information stored in the storage means.

5. the storage means stores information about the transmittance including the scattered radiation obtained for each combination of the thickness of the first material and the thickness of the second material, The image processing apparatus according to claim 4 , wherein the processing means obtains information on the transmittance including the scattered radiation in the combination by correcting the information obtained from the storage means based on the combination.

6. The image processing device according to claim 5 , wherein the processing means acquires, by the correction, information regarding the transmittance at which the scattered radiation is contained in the first radiation energy and information regarding the transmittance at which the scattered radiation is contained in the second radiation energy.

7. the processing means generates a first function for outputting information on transmittance including the scattered radiation in a combination of a thickness of the first material and a thickness of the second material with respect to the first radiation energy in a case where the scattered radiation is not included, and The image processing device according to claim 5, further comprising: a second function that generates, from a combination of the thickness of the first material and the thickness of the second material, information regarding the transmittance including the scattered radiation in the combination for the second radiation energy when the scattered radiation is not included, the second function outputting information regarding the transmittance including the scattered radiation in the combination.

8. the processing means obtains a first rate of change of the first function and a second rate of change of the second function based on a change in thickness of the first material; The image processing apparatus according to claim 7 , further comprising: a third rate of change of the first function and a fourth rate of change of the second function, the third rate of change being obtained based on a change in thickness of the second substance.

9. The processing means calculates a first difference between the transmittance not including the scattered radiation at the first radiation energy and the transmittance including the scattered radiation output from the first function; The image processing apparatus according to claim 8 , further comprising: acquiring a second difference between the transmittance not including the scattered radiation at the second radiation energy and the transmittance including the scattered radiation output from the second function.

10. the processing means repeatedly executes an energy subtraction process using the fourth rate of change and the first difference, and the third rate of change and the second difference, to obtain a thickness image of the first material; The image processing device according to claim 9 , wherein a thickness image of the second material is obtained by repeatedly executing the energy subtraction process using the second rate of change and the first difference, and the first rate of change and the second difference.

11. The processing means outputs a first transmittance image not including the scattered radiation, the first transmittance image being obtained by reducing the scattered radiation component from the transmittance image including the scattered radiation output from the first function; and and acquiring a second transmittance image not including the scattered radiation, the second transmittance image being obtained by reducing the scattered radiation component from the transmittance including the scattered radiation output from the second function; The image processing device according to claim 10 , further comprising: a thickness image of the first material and a thickness image of the second material in which the scattered radiation components are reduced, the thickness image of the first material and the thickness image of the second material being obtained using the first transmittance image and the second transmittance image.

12. The image processing device according to claim 4, wherein the processing means obtains the transmittance not including scattered radiation corresponding to the thickness of the specified material by performing correction using the transmittance including scattered radiation and the transmittance not including scattered radiation stored in the memory means.

13. the storage means stores a ratio of a scattered ray component contained in the radiation to a direct ray component of the radiation that directly reaches the two-dimensional detector from the radiation source for each thickness of the predetermined material; The image processing device according to claim 12 , wherein the processing means acquires the transmittance not including the scattered radiation corresponding to the thickness of the specified material by performing a correction using the ratio and the transmittance including the scattered radiation.

14. the storage means stores a difference between the transmittance including the scattered radiation and the transmittance not including the scattered radiation for each thickness of the predetermined material, The image processing device according to claim 12 , wherein the processing means acquires the transmittance not including the scattered radiation corresponding to the thickness of the predetermined material by performing correction using the difference and the transmittance including the scattered radiation.

15. the storage means stores information regarding the transmittance acquired for each distance between the subject and a two-dimensional detector that detects the radiation, 5. The image processing apparatus according to claim 4, wherein the processing means obtains information about the transmittance corresponding to the thickness of the predetermined substance by using the information interpolated based on the distance in shooting the subject.

16. 2. The image processing apparatus of claim 1, wherein the first material thickness image is a bone image and the second material thickness image is a soft tissue image.

17. The image processing apparatus according to claim 1 , further comprising a display control means for causing a display means to display the thickness image of the first material and the thickness image of the second material acquired by the processing means.

18. a radiation imaging device including a two-dimensional detector; an image processing device according to claim 1 , which acquires an image in which scattered rays contained in the radiation detected by the two-dimensional detector have been corrected; A radiation imaging system comprising:

19. An image processing method comprising a step of acquiring a thickness image of a first material and a thickness image of a second material in which the scattered rays have been corrected, using a plurality of pieces of information corresponding to a plurality of different radiation energies obtained by irradiating a subject with radiation and taking an image, and information regarding a transmittance including scattered rays and a transmittance not including the scattered rays, which are preset for each thickness of a first material and a second material different from the first material.

20. A program for causing a computer to execute the image processing method according to claim 19.

Images