JP2017127638A - X-ray ct apparatus, information processing device and information processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To discriminate materials with high accuracy.SOLUTION: An X-ray CT apparatus includes an X-ray generator, an X-ray detector, a calculation unit, and a determination unit. The X-ray generator irradiates a subject with X-rays. The X-ray detector detects X-rays transmitted through the subject. The calculation unit calculates an irradiation spectrum as a spectrum up to reaching at the subject, and an estimated spectrum estimated as a spectrum after the X-rays transmit through the subject on the basis of an estimated distance showing an estimated value of an X-ray transmission distance of a material to be discriminated, and information showing distortion of the spectrum caused in a path of the X-rays transmitted through the subject, among spectra showing distribution of photon counts for each energy of the X-rays. The determination unit determines an X-ray transmission distance of the material to be discriminated, on the basis of the estimated spectrum and a detected spectrum, which is a spectrum after transmitting through the subject detected by the X-ray detection unit.SELECTED DRAWING: Figure 1

Description

  Embodiments described herein relate generally to an X-ray CT apparatus, an information processing apparatus, and an information processing method.

  As an application of X-ray CT (Computed Tomography), there is a technique for estimating the type, atomic number, density, etc. of a substance contained in a subject. This is called substance discrimination. Substance discrimination uses the fact that the interaction between X-rays and substances differs for each X-ray energy.

  As a technique for performing material discrimination, a technique is known that uses a spectrum obtained by measuring the energy of each photon that reaches a detector via a subject. For example, a technique for calculating the abundance of a substance from an average absorption coefficient of the substance and an image obtained by irradiating a subject with radiation of a plurality of types of energy is disclosed.

JP-A-5-161633

  However, in the prior art, the spectrum and image used for substance discrimination may differ from the actual spectrum transmitted through the subject and the image obtained from the spectrum. For this reason, it has been difficult to perform material discrimination with high accuracy using the conventional technology for performing material discrimination from such spectra and images.

  The present invention has been made in view of the above, and an object of the present invention is to provide an X-ray CT apparatus, an information processing apparatus, and an information processing method capable of accurately performing material discrimination.

  The X-ray CT apparatus according to the embodiment includes an X-ray generator, an X-ray detector, a calculation unit, and a specifying unit. The X-ray generator irradiates the subject with X-rays. The X-ray detector detects X-rays that have passed through the subject. The calculation unit calculates an irradiation spectrum, which is a spectrum until reaching the subject, of the spectrum indicating the distribution of the photon count number for each energy of the X-ray, and an estimated value of the X-ray transmission distance of the substance to be discriminated. An estimated spectrum estimated as a spectrum after the X-ray passes through the subject is calculated based on the indicated estimated distance and information indicating the distortion of the spectrum generated in the X-ray path that has passed through the subject. . The specifying unit specifies an X-ray transmission distance of the substance to be discriminated based on the estimated spectrum and a detection spectrum detected by the X-ray detection unit and transmitted through the subject. To do.

The figure which shows an example of the X-ray CT apparatus which concerns on this Embodiment. Explanatory drawing of an example of a spectrum. Explanatory drawing of the outline | summary of the process which a processing circuit performs. The flowchart which shows an example of the procedure of a discrimination process. Hardware configuration diagram.

  Hereinafter, an X-ray CT (Computed Tomography) apparatus, an information processing apparatus, and an information processing method according to the present embodiment will be described in detail with reference to the drawings. In the following embodiments, parts denoted by the same reference numerals perform the same operation, and redundant description will be omitted as appropriate.

  FIG. 1 is a diagram showing an example of an X-ray CT apparatus 1 according to the present embodiment.

  The X-ray CT apparatus 1 irradiates the subject P with X-rays that are an example of radiation. The X-ray CT apparatus 1 is, for example, a spectral CT apparatus that obtains a cross-sectional image of the subject P, a photon counting CT apparatus, or the like.

  The X-ray CT apparatus 1 includes a gantry device 10, a bed device 20, and an information processing device 30. The gantry device 10 and the couch device 20 are connected to the information processing device 30 so as to be able to exchange data and signals.

  The gantry device 10 irradiates the subject P with X-rays and collects spectral information of the X-rays transmitted through the subject P. The gantry device 10 includes an irradiation control circuit 11, an X-ray generator 12, a rotating frame 15, a detector 13, and a collection circuit 14.

  The X-ray generator 12 generates X-rays and irradiates the subject P. The X-ray generator 12 includes an X-ray tube 12A, a wedge 12B, and a collimator 12C.

  The X-ray tube 12 </ b> A is a vacuum tube that generates X-rays by a voltage supplied from the irradiation control circuit 11. Then, the X-ray tube 12A irradiates the subject P with the generated X-rays. For example, the X-ray tube 12 </ b> A generates beam-shaped X-rays having a conical or pyramidal spread.

  The wedge 12B is a filter that adjusts the X-ray dose of X-rays emitted from the X-ray tube 12A. The collimator 12C is a slit that narrows the X-ray irradiation range in which the X-ray dose is adjusted by the wedge 12B.

  For this reason, the spectrum of the X-ray irradiated from the X-ray generator 12 to the subject P includes the high voltage and current supplied to the X-ray tube 12A, the type of target (for example, tungsten) used for the radiation source, the target It is determined by the angle, the type (for example, beryllium) and thickness of the filter, and the type and thickness of the wedge 12B.

  The rotating frame 15 is a ring-shaped support member. The rotating frame 15 supports the X-ray generator 12 and the detector 13 so as to face each other with the subject P interposed therebetween. Note that the subject P is located at the center of the circle of the rotating frame 15. The rotation frame 15 is provided so as to be rotatable about the subject P as a rotation center. For this reason, the X-ray generator 12 and the detector 13 are rotatably arranged with the subject P as the center of rotation while maintaining the opposed state.

  The detector 13 detects the spectrum of X-rays irradiated from the X-ray tube 12A and transmitted through the subject P. In other words, the detector 13 detects a spectrum indicating the number of photons counted for each energy of the X-ray transmitted through the subject P.

  In the present embodiment, the spectrum detected by the detector 13 will be described as a detected spectrum. Details of the detection spectrum will be described later.

The detector 13 detects a detection spectrum for each position of the X-ray generator 12 while rotating in the circumferential direction along the rotating frame 15. The position of the X-ray generator 12 indicates the relative position of the X-ray generator 12 (specifically, the X-ray tube 12A) with respect to the subject P. The position of the X-ray generator 12 may be referred to as a “view”. For example, the position of the X-ray generator 12 is represented by an angle when a predetermined reference position is set to 0 ° in one turn of 360 ° in the circumferential direction of the rotating frame 15. For example, it is assumed that the gantry device 10 detects a detection spectrum every 0.5 °. In this case, the position of the X-ray generator 12 can be expressed by an angle of 0.5 °.
The detector 13 includes a plurality of detection elements 16. The detection element 16 outputs a signal corresponding to the incident X-ray. The detection element 16 is, for example, a cadmium telluride (CdTe) based semiconductor element.

  In the present embodiment, the detector 13 has a circumferential direction of the rotating frame 15 (refer to the arrow Q direction in FIG. 1) and an orthogonal direction orthogonal to the circumferential direction on the facing surface facing the X-ray generator 12. A two-dimensional array is provided along (refer to the arrow Z direction in FIG. 1).

In the present embodiment, each time one X-ray photon is incident, the detection element 16 outputs a pulse-like signal capable of measuring the energy of the incident photon and the number of photons to the collection circuit 14.
The detector 13 may be either a direct conversion type or an indirect conversion type. The direct conversion type detector 13 is a detector 13 that directly converts a photon incident on the detection element 16 into an electrical signal. The indirect conversion type detector 13 has a configuration in which a scintillator is disposed on the X-ray incident side of the detection element 16.

  The irradiation control circuit 11 controls the operations of the X-ray generator 12 and the rotating frame 15. The irradiation control circuit 11 includes a high voltage generation function 11A, an adjustment function 11B, and a gantry drive function 11C.

  Each processing function performed by the high voltage generation function 11A, the adjustment function 11B, and the gantry driving function 11C is stored in the storage circuit in the form of a program executable by a computer. The irradiation control circuit 11 is a processor that implements a function corresponding to each program by reading the program from the storage circuit and executing the program. In other words, the irradiation control circuit 11 in a state where the respective programs are read out has each function shown in the irradiation control circuit 11 of FIG.

  In FIG. 1, it is assumed that the single irradiation control circuit 11 realizes processing functions performed by the high voltage generation function 11A, the adjustment function 11B, and the gantry drive function 11C. However, the irradiation control circuit 11 may be configured by combining a plurality of independent processors, and the functions may be realized by each processor executing a program.

  The high voltage generation function 11A generates a high voltage and supplies the generated high voltage to the X-ray tube 12A. The adjustment function 11B adjusts the opening degree and position of the collimator 12C. By adjusting the aperture and position of the collimator 12C, the X-ray irradiation range irradiated from the X-ray tube 12A to the subject P is adjusted. For example, the adjustment function 11B adjusts the X-ray irradiation range, that is, the X-ray fan angle and cone angle, by adjusting the aperture of the collimator 12C.

  The gantry drive function 11C controls the rotary frame 15 to rotate. The rotating frame 15 rotates along a circular orbit centered on the subject P by the control by the gantry driving function 11C. For this reason, the X-ray generator 12 and the detector 13 are rotated around the subject P as the rotating frame 15 rotates while the rotating frame 15 maintains the positional relationship facing the subject P. Rotate.

  The gantry device 10 is not limited to a configuration that rotates the X-ray generator 12 and the detector 13.

  For example, the gantry device 10 may be configured to rotate only the X-ray generator 12. In this case, the detector 13 may have a configuration in which the detection elements 16 are arranged over one circumference in the circumferential direction of the rotating frame 15. The rotating frame 15 may be configured to support the X-ray generator 12.

  The collection circuit 14 counts the number of X-ray photons incident on each of the detection elements 16 using signals received from each of the plurality of detection elements 16 provided in the detector 13. Further, the collection circuit 14 performs arithmetic processing based on a pulsed signal waveform indicated by a signal received from each of the plurality of detection elements 16. By this arithmetic processing, the collection circuit 14 measures the energy of the photon that caused the output of the signal for each of the plurality of detection elements 16.

  Through these processes, the collection circuit 14 collects the detection spectrum indicating the photon count number with respect to the energy of the X-rays for each of the detection elements 16 and outputs the detection spectrum to the information processing apparatus 30. Therefore, the detection spectrum detected by the detector 13 is specifically a spectrum obtained by collecting the signal output from the detection element 16 by the collection circuit 14.

  Here, as described above, the X-ray generator 12 rotates around the subject P and emits X-rays from a plurality of different positions with respect to the subject P. Therefore, in the present embodiment, the collection circuit 14 receives a signal from each of the plurality of detection elements 16 provided in the detector 13 for each position of the X-ray generator 12.

  The collection circuit 14 collects the photon count for the X-ray energy for each position of the X-ray generator 12 and each detection element 16. Then, the collection circuit 14 outputs a detection spectrum indicating the number of photon counts with respect to the X-ray energy to the information processing device 30 for each position of the X-ray generator 12 and each detection element 16. At this time, the collection circuit 14 outputs collection information including the position of the X-ray generator 12, the identification information of the detection element 16, and the corresponding detection spectrum to the information processing apparatus 30.

  In the present embodiment, the case where the position of the detection element 16 in the detector 13 is used as the identification information of the detection element 16 will be described as an example. For this reason, hereinafter, the identification information of the detection element 16 may be referred to as the position of the detection element 16 in some cases.

  The couch device 20 places the subject P thereon. The couch device 20 includes a couch driving device 21 and a top plate 22.

The top plate 22 is a bed such as a bed on which the subject P is placed. The top plate 22 is provided to be movable along the body axis direction of the subject P placed on the top plate 22 (see the arrow Z direction in FIG. 1). The couch driving device 21 moves the top 22 along the body axis direction of the subject P under the control of the information processing device 30. In the present embodiment, description will be made assuming that the body axis direction of the subject P and the rotation axis direction of the rotating frame 15 coincide. For this reason, when the top 22 moves along the body axis direction of the subject P, the subject P placed on the top 22 enters the inside of the rotating frame 15 or from the inside of the rotating frame 15. It will be conveyed to the outside.
The information processing device 30 controls the gantry device 10 and the couch device 20.
The information processing apparatus 30 includes an interface circuit 31, a display 32, an input circuit 33, a storage circuit 34, and a processing circuit 35.

  The interface circuit 31 communicates with each of the gantry device 10 and the couch device 20.

  The input circuit 33 receives various operation instructions from the user. The input circuit 33 outputs an instruction signal corresponding to the accepted operation instruction to the processing circuit 35. The input circuit 33 is, for example, a mouse, a keyboard, a button, a trackball, or a joystick.

  The display 32 displays various images. The display 32 displays, for example, a display image (details will be described later), a CT image (details will be described later), and the like. The display 32 is, for example, a CRT (Cathode Ray Tube) display, an LCD (Liquid Crystal Display), an organic EL (Organic Electro-Luminescence), or the like.

  The storage circuit 34 stores various data. The storage circuit 34 is, for example, an HDD (Hard Disk Drive), an SSD (Solid State Drive), a RAM (Random Access Memory), a semiconductor memory element such as a flash memory, an optical disk, or the like.

  The processing circuit 35 performs control of the gantry device 10 and the couch device 20 and information processing using various data acquired from the gantry device 10.

  The processing circuit 35 includes a scan control function 35A, an acquisition function 35B, a calculation function 35C, a calculation function 35D, an update function 35E, a specifying function 35F, a generation function 35G, a reconstruction function 35H, and a display control function. 35I and a reception function 35J.

  Some or all of the scan control function 35A, the acquisition function 35B, the calculation function 35C, the calculation function 35D, the update function 35E, the identification function 35F, the generation function 35G, the reconstruction function 35H, the display control function 35I, and the reception function 35J For example, a program may be executed by an electronic circuit such as a CPU (Central Processing Unit), that is, realized by software.

  For example, each process performed by the scan control function 35A, the acquisition function 35B, the calculation function 35C, the calculation function 35D, the update function 35E, the specifying function 35F, the generation function 35G, the reconstruction function 35H, the display control function 35I, and the reception function 35J. The functions are stored in the storage circuit 34 in the form of a program that can be executed by a computer.

  The information processing apparatus 30 is a processor that implements a function corresponding to each program by reading the program from the storage circuit 34 and executing the program. In other words, the processing circuit 35 in the state in which the respective programs are read has the functions shown in the processing circuit 35 of FIG.

  In FIG. 1, the scan processing function 35A, the acquisition function 35B, the calculation function 35C, the calculation function 35D, the update function 35E, the identification function 35F, the generation function 35G, the reconstruction function 35H, and the display are displayed by a single processing circuit 35. A description will be given assuming that processing functions performed by the control function 35I and the reception function 35J are realized. However, the processing circuit 35 may be configured by combining a plurality of independent processors, and the functions may be realized by each processor executing a program.

  Also, some or all of the scan control function 35A, the acquisition function 35B, the calculation function 35C, the calculation function 35D, the update function 35E, the identification function 35F, the generation function 35G, the reconstruction function 35H, the display control function 35I, and the reception function 35J. May be realized by hardware, or may be realized by using software and hardware together.

  When realized by the hardware, it may be realized by an electronic circuit such as an IC (Integrated Circuit), an ASIC (Application Specific Integrated Circuit), or an FPGA (Field Programmable Gate Array).

  The scan control function 35 </ b> A controls the irradiation control circuit 11, the collection circuit 14, and the bed driving device 21. Specifically, the scan control function 35A controls the irradiation control circuit 11 to rotate the rotating frame 15 to irradiate the subject P with the X-rays from the X-ray tube 12A, and the opening degree and position of the collimator 12C. Make adjustments. By this control, the gantry device 10 irradiates the subject P with X-rays from the X-ray generator 12 continuously or intermittently while rotating the rotating frame 15.

  For example, the scan control function 35A controls the gantry device 10 so as to execute a helical scan or a non-helical scan.

  Further, the scan control function 35 </ b> A receives the collection information from the collection circuit 14 and stores it in the storage circuit 34. As described above, the collected information includes the position of the X-ray generator 12, the position of the detection element 16, and the corresponding detection spectrum.

  Here, the spectrum will be described. 2A to 2D are explanatory diagrams of examples of spectra.

  X-rays irradiated from the X-ray generator 12 pass through the subject P, reach the detector 13, and are detected by the detector 13.

  Hereinafter, the X-ray spectrum irradiated from the X-ray tube 12 </ b> A until reaching the subject P is referred to as an irradiation spectrum 50. For example, as shown in FIG. 2A, it is assumed that the X-ray generator 12 irradiates the detection element 16 with X-rays a1. In this case, the X-ray spectrum of the region XA in the X-ray a1 corresponds to the irradiation spectrum 50 (see FIG. 2B). Note that the region XA is an air region, and the attenuation of X-rays can be ignored.

  The irradiation spectrum 50 is determined by the high voltage and current supplied to the X-ray tube 12A, the type of target (for example, tungsten) used for the radiation source, the type and thickness of the wedge 12B, and the like. For this reason, the irradiation spectrum 50 is obtained by precise actual measurement or highly accurate simulation. For this reason, in the present embodiment, the storage circuit 34 stores the irradiation spectrum 50 in advance.

  In the present embodiment, the spectrum of X-rays that passes through the subject P and reaches the detection element 16 is referred to as a transmission spectrum 52. For example, as shown in FIG. 2A, the X-ray spectrum of the region XB in the X-ray a1 irradiated from the X-ray generator 12 to the detection element 16 corresponds to the transmission spectrum 52 (FIG. 2C )reference).

  The irradiation spectrum 50 irradiated to the subject P from the X-ray tube 12A is attenuated by passing through the subject P. This attenuation depends on the substance contained in the subject P. Therefore, the transmission spectrum 52, which is the X-ray spectrum transmitted through the subject P, has a spectrum shape that is attenuated according to the substance contained in the subject P. That is, the transmission spectrum 52 is determined by the composition of the substance present in the path through which X-rays have passed through the subject P and the irradiation spectrum 50 irradiated on the subject P.

  In the present embodiment, the X-ray spectrum detected by the detector 13 is referred to as a detection spectrum 54. Specifically, the X-ray spectrum detected by the detector 13 corresponds to the detected spectrum 54 (see FIG. 2D).

  Here, ideally, the transmission spectrum 52 and the detection spectrum 54 should be equal. However, the detection spectrum 54 actually measured is detected in consideration of the detector response of the actual detector 13 and the like. In general, the transmission spectrum 52 immediately before entering the device 13 is different.

Note that the shapes of the spectra (the irradiation spectrum 50, the transmission spectrum 52, and the detection spectrum 54) illustrated in FIGS. 2B to 2D are examples, and are not limited to these shapes.
Here, each of the irradiation spectrum 50 and the detection spectrum 54 is defined by each of the following formulas (formula (1), formula (2)).

  In the present embodiment, the irradiation spectrum 50 is defined by the above formula (1). In the formula (1), c indicates the position of the detection element 16. e represents the energy (keV) of X-rays.

In the formula (1), S ₀ (c, e) represents an irradiation spectrum 50. Specifically, S ₀ (c, e) is a group of photon count numbers corresponding to each of the energy e of the X-rays irradiated to the detection element 16 at the position c. More specifically, S ₀ (c, e) is an X-ray photon count number that is considered to be irradiated with the energy e to the detection element 16 at the position c when it is assumed that there is no subject P. A group of estimates. In other words, the irradiation spectrum 50 is, for example, an X-ray spectrum indicating a distribution of photon count numbers for each energy e of X-rays, which is finally detected by the detection element 16 at the position c, and is an X-ray generator 12. And an X-ray spectrum until X-rays irradiated from the X-ray generator 12 reach the subject P in a path connecting the detection element 16 at the position c. Since the irradiation spectrum 50 is constant regardless of the position of the X-ray generator 12, the equation (1) does not include a parameter indicating the position of the X-ray generator 12. However, Expression (1) may further include a parameter indicating the position of the X-ray generator 12. The example of the irradiation spectrum 50 is not limited to the above-described example, and the irradiation spectrum 50 may not have the position c of the detection element 16 as an argument as in S ₀ (e). In addition, examples of the irradiation spectrum 50, as S _0, the irradiation spectrum 50 which is integrated over the energy e of X-ray may be used.

In the present embodiment, the detection spectrum 54 is defined by the above formula (2). In Expression (2), c represents the position of the detection element 16. e represents the energy (keV) of X-rays. In Expression (2), v indicates the position of the X-ray generator 12. S ₁ (c, v, e) represents the detection spectrum 54. Specifically, S ₁ (c, v, e) is a photon count corresponding to each energy e of X-rays detected by the detection element 16 at position c when the X-ray generator 12 is at position v. A group of numbers.

  E in Formula (1) and Formula (2) will be specifically described. In the present embodiment, each of the irradiation spectrum 50 and the detection spectrum 54 indicates a photon count number for each energy band having a predetermined energy width (hereinafter sometimes simply referred to as energy).

  The energy width of the energy band corresponding to each of the photon count numbers indicated by the detection spectrum 54 is referred to as a first energy width E1 (see FIG. 2D).

  Further, the energy width of the energy band corresponding to each of the photon count numbers indicated by the irradiation spectrum 50 is referred to as a second energy width E2 (see FIG. 2B).

  In the present embodiment, the second energy width E2 is equal to or smaller than the first energy width E1.

  That is, the detection spectrum 54 represents the photon count number of each energy band for each first energy width E1 from 0 keV toward the maximum energy. The maximum energy may be determined, for example, as the maximum value of photon energy that can be detected by the detector 13.

  The irradiation spectrum 50 represents the number of photons counted in the energy band for each second energy width E2 from 0 keV toward the maximum energy.

  It should be noted that at least a part of each energy band may or may not overlap.

  The values of the first energy width E1 and the second energy width E2 are not limited, but are preferably less than 20 keV, more preferably 1 keV or less.

  In the present embodiment, as an example, the first energy width E1 and the second energy width E2 are described as being 1 keV.

  That is, in the present embodiment, each of the irradiation spectrum 50 and the irradiation spectrum 50 indicates a photon count corresponding to the energy band of each energy width for each energy width of 1 keV.

  Further, in the present embodiment, in the expressions (1) and (2), e is described as indicating the upper limit value of energy indicated by the energy band of the corresponding energy width. For this reason, in the present embodiment, e = 1 represents an energy band of 0 keV or more and less than 1 keV. E = 2 represents an energy band of 1 keV or more and 2 keV or less. Similarly, e = 100 will be described as indicating an energy band of 99 keV to 100 keV.

  Next, spectral distortion and attenuation occurring in the X-ray path transmitted through the subject P will be described. Here, the spectrum distortion means a deviation of a detection spectrum 54 actually detected from a spectrum expected when the system is ideal. Here, the spectrum expected when the system is ideal is, for example, using a simple calculation such as the Lambert-Beer law from the irradiation spectrum 50 and information on the substance contained in the subject P, and a detector. This is the spectrum expected when 13 responses are assumed to be ideal.

  As a first cause of spectrum distortion, distortion of the irradiation spectrum 50 due to the characteristics of the X-ray generator 12 can be cited. For example, the X-ray irradiation spectrum 50 irradiated from the X-ray tube 12a of the X-ray generator 12 is distorted due to the focal movement of the X-ray irradiation and the characteristics of the collimator provided on the X-ray tube 12a side. .

  A second cause of spectrum distortion is distortion of the irradiation spectrum 50 in the path from the X-ray generator 12 to the subject P. For example, the X-ray irradiation spectrum 50 irradiated from the X-ray tube 12a of the X-ray generator 12 varies in the mechanical accuracy of the Cu filter for generating a uniform irradiation spectrum 50 of, for example, a wedge in the irradiation path. Causes distortion.

  As a third cause of spectrum distortion, distortion of the transmission spectrum 52 caused by beam hardening when X-rays pass through the subject P can be cited. Here, beam hardening means that when X-rays pass through a substance, the X-ray absorption rate of the substance is energy-dependent, so X-rays with a long wavelength and low energy are compared with X-rays with high energy. Therefore, the X-ray quality changes, and as a result, linearity is lost in the material mass attenuation coefficient. Due to beam hardening, a deviation from the spectrum calculated using a simple method appears in the transmission spectrum 52. In this manner, spectrum distortion occurs in the X-ray path that has passed through the subject P due to the attenuation of the X-rays due to the subject's transmission. In such a case, the spectrum is distorted due to the attenuation of X-rays due to the transmission of the subject. If beam hardening can be accurately measured or estimated as energy spectrum distortion, it can be said that the degree of beam hardening (or spectrum distortion caused only by beam hardening) represents the composition of the substance in the transmission path of the subject.

  A fourth cause of spectrum distortion is distortion of the transmission spectrum 52 caused by scattering when X-rays pass through the subject P. Usually, X-rays irradiated to the subject P from the X-ray tube 12A are detected by the detector 13 in the irradiated traveling direction. However, when scattering occurs, the X-ray irradiated from the X-ray tube 12A to the subject P changes its traveling direction on the subject P, and the detector 13 is located at an angle different from the traveling direction initially irradiated. Is detected. As a result, distortion occurs in the transmission spectrum.

  As a fifth cause of spectrum distortion, distortion of the transmission spectrum 52 caused by a collimator (e.g. post-patient filter) or the like on the detector 13 side can be cited.

  The sixth cause of spectral distortion is the response characteristic of the detector 13. That is, the detection spectrum 54 actually detected by the detector 13 has a shape in which the transmission spectrum 52 is further distorted. Here, the response characteristic of the detector 13 is specifically the response characteristic of each of the detection elements 16 provided in the detector 13. The response characteristic of the detection element 16 is a factor that causes distortion in the transmission spectrum 52 (or the detection spectrum 54) of X-rays incident on the detection element 16. The response characteristics of the detection element 16 include, for example, the probability of occurrence of escape, fluorescence, crosstalk, and scattering for each X-ray energy incident on the detection element 16 and variations in detected energy. In this case, due to the response characteristics of the X-ray detector 13, spectral distortion occurs in the X-ray path that has passed through the subject P.

  Returning to FIG. 1, therefore, in the X-ray CT apparatus 1 of the present embodiment, the processing circuit 35 of the information processing apparatus 30 has a specific function. Details of each function will be described below.

  Hereinafter, the outline of the overall processing flow will be described, and then the details of each processing will be described. Here, the purpose of a series of processes performed by the processing circuit 35 is to accurately calculate the distance (X-ray transmission distance) through which X-rays have passed through the substance to be discriminated contained in the subject P. It may be difficult to obtain the calculation of the line transmission distance directly from the detection spectrum 54 due to the aforementioned distortion of the spectrum.

  Therefore, in the embodiment, the processing circuit 35 uses the calculation function 35C to calculate the estimated distance indicating the provisional estimated value of the X-ray transmission distance of the substance to be discriminated in which the X-ray is included in the subject P, and the subject P. A predetermined simulation is performed based on the information indicating the distortion of the spectrum generated in the X-ray path transmitted through the X-ray, and an estimated spectrum estimated as the spectrum after the X-ray has transmitted through the subject P is calculated. Subsequently, the processing circuit 35 uses the calculation function 35D to calculate the estimated spectrum, which is the theoretical value of the detection spectrum 54, and the actual value of the detection spectrum 54, which is the spectrum after passing through the subject, detected by the detector 13. Compare. Specifically, the processing circuit 35 calculates an error between the estimated spectrum and the detected spectrum 54 by the calculation function 35D.

  Here, the processing circuit 35 typically calculates an estimated spectrum for each of two or more estimated distance candidates, and calculates an error from the detected spectrum 54 based on the calculated estimated spectrum. To do. Subsequently, the processing circuit 35 specifies an X-ray transmission distance that is considered to be closest to the true value based on the calculated error by the specifying function 35F. For example, the processing circuit 35 specifies, as the X-ray transmission distance of the substance, an estimated distance that minimizes the error from among errors calculated for two or more estimated distance candidates by the specifying function 35F. .

  Subsequently, the processing circuit 35 uses the update function 35E to update the estimated distance value based on the specified X-ray transmission distance and use it as an argument in the next iteration.

  As described above, the processing circuit 35 causes the calculation function 35D to update the estimated distance by the specifying function 35, to cause the calculation function 35C to calculate the estimated spectrum based on the updated estimated distance, and to calculate the error. The processing and a specific series of processing of the X-ray transmission distance are repeated until a predetermined condition is satisfied. After that, the processing circuit 35 specifies the estimated distance when the predetermined condition is satisfied as the X-ray transmission distance of the substance by the specifying function 35F. The X-ray transmission distance of the substance thus obtained becomes the X-ray transmission distance of the substance to be finally obtained.

  The processing circuit 35 may perform the above-described processing on two or more kinds of substances such as soft tissue and contrast medium. In such a case, the processing circuit 35 calculates an estimated spectrum for a plurality of estimated distance candidates for each of the two or more types of substances, and estimates such that the error is minimized based on the calculated estimated spectrum. The distance is specified as the X-ray transmission distance of the substance.

  Next, functions of the processing circuit 35 and processing procedures performed by the processing circuit 35 will be described in more detail.

  The reception function 35J receives various operation instructions from the user from the input circuit 33.

  The acquisition function 35B acquires the detection spectrum 54 and the initial value of the estimated distance.

  The detection spectrum 54 is a spectrum detected by the detector 13 as described above. The acquisition function 35B reads the collected information stored in the storage circuit 34 by the scan control function 35A. Accordingly, the acquisition function 35B acquires the detection spectrum 54 detected by each of the plurality of detection elements 16 at each position of the X-ray generator 12.

  The estimated distance indicates an estimated value of the distance (effective distance) through which X-rays have passed through the substance to be discriminated included in the subject P. Specifically, the estimated distance is the distance that X-rays irradiated from the X-ray generator 12 and transmitted through the subject P and incident on the detector 13 have passed through the substance to be discriminated in the optical path of the X-rays. It is the value which estimated the total value of.

  Here, the estimated distance is updated by an update function 35E described later. The initial value of the estimated distance acquired by the acquisition function 35B is not the estimated distance updated by the update function 35E but the estimated distance acquired separately.

  When there are a plurality of types of substances to be discriminated, the acquisition function 35B acquires the initial value of the estimated distance for each of the plurality of types of substances.

  For example, the acquisition function 35B acquires an initial value of the estimated distance of the substance to be discriminated from the input circuit 33. In the present embodiment, a case will be described in which there are multiple types of substances to be discriminated. For this reason, the acquisition function 35 </ b> B acquires the initial value of the estimated distance of each of the plurality of substances from the input circuit 33 from the input circuit 33.

  The storage circuit 34 may store in advance the type information indicating the type of each of the plurality of substances in association with the initial value of the estimated distance. In this case, for example, the user operates the input circuit 33 to input the type of substance to be discriminated. The receiving function 35J receives input of type information indicating the type of substance to be discriminated from the input circuit 33. Then, the acquisition function 35B reads the initial value of the estimated distance corresponding to the received type information from the storage circuit 34. Thereby, the acquisition function 35B may acquire the initial value of the estimated distance of each substance to be discriminated.

  The substance to be discriminated may be any substance that causes spectral attenuation when transmitted through the substance. The substance to be discriminated may be an atom or molecule, or a specific part or composition (bone, muscle, etc.) of the human body. Substances to be discriminated are, for example, water, iodine, calcium, gadolinium, muscle, fat and the like.

  The substance to be discriminated may be one type or a plurality of types. In the present embodiment, a case will be described in which there are multiple types of substances to be discriminated. Further, in the present embodiment, the case where the substances to be discriminated are three kinds of water, iodine, and calcium will be described as an example.

  The type of the substance to be discriminated is input by the operation of the input circuit 33 by the user, for example. Note that the storage circuit 34 may store in advance type information indicating each of a plurality of types of substances. And according to the operation instruction of the input circuit 33 by the user, the substance of the type information desired by the user may be selected as the discrimination target from the type information. In addition, the acquisition function 35B may acquire advance information such as a medical record, past examination information, examination request information, and examination site from the storage circuit 34 and the like, and automatically set a substance to be discriminated from the advance information.

  Although details will be described later, the estimated distance is updated by the update function 35E so as to become a more optimal value. For this reason, it is preferable that the initial value of the estimated distance is closer to the actual X-ray transmission distance of the substance contained in the subject P. For this reason, it is preferable that the acquisition function 35B acquires by calculating the initial value of the estimated distance of the substance to be discriminated.

  By obtaining the initial value by the acquisition function 35B, it is possible to reduce the number of repetitions of a series of processes described later, and to improve the accuracy and stability of the X-ray transmission distance of the substance specified by the specifying function 35F described later. it can.

  For example, the acquisition function 35B may acquire the initial value by calculating the initial value of the estimated distance of the substance to be discriminated using a known method. For example, the acquisition function 35B calculates the thickness of the substance using the detection spectrum 54 and the average absorption coefficient of each substance to be discriminated using the method disclosed in JP-A-5-161633. May be. Then, the acquisition function 35B may use the calculated thickness of the substance as an initial value of the estimated distance of the discrimination target substance.

  For example, the acquisition function 35B may generate an X-ray CT image from the irradiation spectrum 50 and the detection spectrum 54 and simply calculate an initial value from the generated X-ray CT image. For example, the acquisition function 35 </ b> B generates an X-ray CT image from the total photon count number or effective energy of each of the irradiation spectrum 50 and the detection spectrum 54. At this time, it is preferable to generate an X-ray CT image using the entire energy band of each of the irradiation spectrum 50 and the detection spectrum 54. Thereby, the influence of the response characteristic of the detector 13 can be canceled, or information on a high SN ratio can be used. Then, the acquisition function 35B sets the value obtained by dividing the total of CT values (linear attenuation coefficients) in the optical path of the X-rays by the average linear attenuation coefficient of water as the initial value of water that is one of the discrimination target substances. . Then, the acquisition function 35B sets the initial values of the remaining discrimination target substances to zero. This makes it possible to obtain a simple estimated initial value when it is assumed that the subject is all water.

  Further, the acquisition function 35B may extract only a high-luminance region such as a bone or a contrast agent by performing threshold processing on the CT value in the X-ray CT image. The acquisition function 35B may use an initial value that is simply estimated by assigning the total of CT values in the optical path of the X-ray in the extracted region to iodine or calcium. X-ray CT images can be classified into four regions, that is, an air region, a fat region, a muscle / blood region, and a bone / contrast agent region with relatively high accuracy even when a fixed threshold is used. Also, many methods for dynamically determining a threshold using CT value histogram information have been proposed.

  When the acquisition function 35B calculates the initial value of the estimated distance using the method disclosed in JP-A-5-161633, a plurality of positions of the X-ray generator 12 and a plurality of detection elements It is preferable to classify the 16 positions into a plurality of groups and perform calculation for each group. By performing calculation for each group, it is possible to improve stability by improving the SN ratio and shorten calculation time by reducing the number of calculations.

  The calculation function 35C calculates the estimated spectrum. The estimated spectrum is an estimate of the spectrum of X-rays transmitted through the subject P and detected by the detector 13.

  The calculation function 35C calculates the estimated spectrum based on the irradiation spectrum 50, the estimated distance, and the response information.

  The response information is information indicating spectral distortion caused by the response characteristics of the detector 13 and the like. Since the response characteristic has been described above, the description thereof is omitted here. The response information may be stored in the storage circuit 34 in advance. For example, the storage circuit 34 stores the identification information of the detection element 16 and the response information in advance in association with each other.

  The irradiation spectrum 50 may be stored in the storage circuit 34 in advance. For example, the storage circuit 34 previously stores an irradiation spectrum 50 corresponding to the position of the X-ray generator 12 and the position of the detection element 16. The irradiation spectrum 50 is constant regardless of the position of the X-ray generator 12. For this reason, the storage circuit 34 may store only the irradiation spectrum 50 of a type corresponding to the position of the detection element 16 in advance.

  The calculation function 35C calculates an estimated spectrum for each of the positions of the X-ray generator 12 using the estimated distance, the response information, and the irradiation spectrum 50 for each of the plurality of detection elements 16.

  FIG. 3 is an explanatory diagram of an outline of processing performed by the processing circuit 35.

  For example, it is assumed that the calculation function 35C uses the irradiation spectrum 50 shown in FIG. Further, it is assumed that the transmission spectrum 52 in which the X-rays indicated by the irradiation spectrum 50 are transmitted through the subject P is the spectrum shown in FIG. Then, it is assumed that the detection spectrum 54 actually detected by the detection element 16 is the spectrum shown in FIG.

  In this case, the calculation function 35C calculates the estimated spectrum 56 based on the irradiation spectrum 50, the estimated distance, and the response information (see FIG. 3D).

  Specifically, the calculation function 35C attenuates the irradiation spectrum 50 according to the linear attenuation coefficient and the estimated distance of the substance to be discriminated, and distorts the irradiation spectrum 50 according to the distortion indicated by the response information. Is calculated.

  The material linear attenuation coefficient indicates the linear attenuation coefficient of the material to be discriminated with respect to the X-ray energy. What is necessary is just to memorize | store the linear attenuation coefficient of the substance for every X-ray energy in the memory | storage circuit 34 previously. For example, the storage circuit 34 stores the type information indicating the type of the substance, the X-ray energy, and the linear attenuation coefficient in advance in association with each other. The calculation function 35C may calculate the estimated spectrum 56 by reading the linear attenuation coefficient corresponding to the type information of the substance to be discriminated from the storage circuit 34 when calculating the estimated spectrum 56.

  First, the case where the cause of the spectrum distortion is the above-described sixth cause, that is, the case where the estimated spectrum 56 considering the spectrum distortion based on the response characteristic of the detector 13 is calculated will be briefly described. In such a case, the processing circuit 35 calculates the estimated spectrum 56 based on the response characteristic of the detector 13 which is information representing the distortion of the spectrum and the estimated distance by the calculation function 35C.

Specifically, the calculation function 35C calculates the estimated spectrum 56 by the following equation (3).

  In Formula (3), c is the same as Formula (1) and Formula (2). Moreover, in Formula (3), e is the same as Formula (1). In the formula (3), v is the same as that in the formula (2).

Moreover, in Formula (3), m shows this serial number when assign | subjecting a serial number for every kind to the substance of discrimination object. N _M indicates the number of discrimination substance (number of types). As described above, in the present embodiment, the case where the substances to be discriminated are three kinds of water, iodine, and calcium will be described as an example. Therefore, it is assumed that m = 1 represents water, m = 2 represents iodine, m = 3 represents calcium, and N _M = 3.

In the formula (3), the mu _m, a linear attenuation coefficient of the material m the discrimination target. l _m represents the estimated distance of the substance m to be discriminated. F _{c, e} is the response information of the detector 13. Specifically, F _{c, e} represents response information corresponding to the X-ray energy e of the detection element 16 at the position c.

S ₂ (c, v, e) indicates the estimated spectrum 56. Specifically, S ₂ (c, v, e) is the photon count number corresponding to the energy e of the X-ray detected by the detection element 16 at the position c when the X-ray generator 12 is at the position v. A group of estimates.

That is, the part other than F _{c, e on} the right side of Equation (3) represents the irradiation spectrum 50 represented by S _o (c, e), the estimated distance l _m and the linear attenuation coefficient of each substance to be discriminated. Shows the attenuated spectrum. Then, an estimated spectrum 56 indicated by S ₂ (c, v, e) is calculated by distorting this spectrum according to the distortion indicated by the response information F _{c, e} .

  Here, as described above, in Expression (3), e is the same as Expression (1). That is, the energy width of the energy e used when calculating the estimated spectrum 56 coincides with the second energy width E2 of the irradiation spectrum 50 (see FIG. 2B).

  For this reason, as described above, in the present embodiment, e = 1 in Expression (3) indicates an energy band of 0 keV or more and less than 1 keV. E = 2 represents an energy band of 1 keV or more and 2 keV or less. Similarly, e = 100 will be described as indicating an energy band of 99 keV to 100 keV.

  The calculation function 35C may read the line attenuation coefficient from the storage circuit 34 and use it for the above calculation. For this reason, the storage circuit 34 may store in advance the linear attenuation coefficient of each energy band corresponding to each photon count number in the estimated spectrum 56. For example, the storage circuit 34 may store in advance, for each substance to be discriminated, a linear attenuation coefficient corresponding to each of the median values of the upper limit value and the lower limit value of each energy band. The median value between the upper limit value and the lower limit value of the energy band is, for example, 99.5 keV when the energy band is in the range of 99 keV to 100 keV.

  As described above, in the present embodiment, the first energy width E1 and the second energy width E2 are described as having the same energy width. For this reason, in the present embodiment, the calculation function 35C will be described with respect to the case where the estimated spectrum 56 is calculated using the above equation (3). However, when the first energy width E1 is larger than the second energy width E2, the energy band is normalized (average operation) so as to correspond to the first energy width E1 with respect to the right side of the equation (3). Just do it. A known method may be used for this normalization.

  The response information may change depending on the number of photons incident on the detector 13 per unit time. For example, when photons continuously enter the detector 13, the reaction of the detector 13 becomes insufficient, and it may be counted as the number of photons having lower energy (or higher energy) than the actual appearance. For this reason, the calculation function 35C may calculate the estimated spectrum 56 using response information corresponding to the photon count number indicated by the detection spectrum 54.

  In this case, the storage circuit 34 may store response information corresponding to each of the position of the detection element 16 and the photon count number in advance.

Next, the case where the estimated spectrum 56 in consideration of the spectrum distortion is calculated when the cause of the spectrum distortion is the above-described first to fifth causes will be briefly described.
When the cause of the distortion of the spectrum is the cause on the tube side such as the above-mentioned first cause (focus movement of X-ray irradiation, characteristics of the collimator on the tube side, etc.) or the second cause (wedge etc.) The processing circuit 35 evaluates the right side of the expression (3) using an expression in which s ₀ (c, e) is replaced with F ₁ (s _O (c, e)) in the expression (3). Thus, the estimated spectrum 56 is calculated by the calculation function 35C. Here, F ₁ is a function representing the distortion of the irradiation spectrum 50 caused by the first cause or the second cause. A specific expression of such a function is held in the storage circuit 34 in the form of a predetermined table, for example. Note that F _{c, e} = 1 when the correction due to the sixth cause is not performed.

When the cause of the spectrum distortion is the above-mentioned fifth cause (detector-side collimator or the like), that is, the detector-side cause, the processing circuit 35 _calculates F _c, By taking these effects into the expression of _e and evaluating the right side of the expression (3), the estimated spectrum 56 is calculated by the calculation function 35C. A specific expression of such a function is held in the storage circuit 34 in the form of a predetermined table, for example.

  When the cause of the spectrum distortion is the third cause (beam hardening) described above, the processing circuit 35 uses a Monte Carlo simulation, for example, to create a table indicating the spectrum change caused by the beam hardening. Can be generated. With this table, it is possible to know the energy dependence of the X-ray attenuation caused by the substance composition of the subject, and the substance composition can be estimated based on this. The processing circuit 35 stores the generated table in the storage circuit 34. The processing circuit 35 uses the calculation function 35C to calculate the estimated spectrum 56 based on the table stored in the storage circuit 34 and the estimated distance during the processing related to imaging of the subject P. In such a case, the above-described table (information indicating attenuation of X-rays due to subject transmission) is an example of information indicating spectral distortion. In such a case, the embodiment does not require a Monte Carlo simulation and may create such a table by a simpler method.

  When the cause of the distortion of the spectrum is the fourth cause (scattering) described above, the X-ray tube 12 </ b> A uses the X-ray of the known irradiation spectrum 50 for a known substance such as a phantom, for example. Irradiate. The detector 13 detects the detection spectrum 54. Since the detection spectrum 54 includes a scattering effect, the processing circuit 35 performs scattering indicating the scattering effect based on the information on the substance constituting the phantom, the irradiation spectrum 50, and the detection spectrum 54. An information table can be generated. The processing circuit 35 stores the generated scattering information table in the storage circuit 34. The processing circuit 35 calculates the estimated spectrum 56 by the calculation function 35C based on the scattering information table stored in the storage circuit 34 and the estimated distance in the process related to imaging of the subject P. In this case, the above-described scattering information table (X-ray scattering characteristics) is an example of information representing spectral distortion.

  Next, the calculation function 35D will be described. The calculation function 35D calculates an error between the estimated spectrum 56 and the detected spectrum 54.

  Specifically, the calculation function 35D calculates an error between the estimated spectrum 56 and the detection spectrum 54 for each of the detection elements 16 for each position of the X-ray tube 12A. The calculation function 35D calculates an error for each combination of the estimated spectrum 56 and the detection spectrum 54 in which the position of the X-ray generator 12 and the position of the detection element 16 are the same.

  Note that a series of processes of updating the estimated distance by the update function 35E, which will be described later, calculating the estimated spectrum 56 by the calculation function 35C, and calculating the error by the calculation function 35D, are repeatedly executed under the control of the specific function 35F described later. (Details will be described later).

  For this reason, the calculation function 35D uses the estimated spectrum 56 calculated immediately before by the calculation function 35C as an error calculation target. That is, the calculation function 35D calculates an error from the detected spectrum 54 using the newly calculated estimated spectrum 56 every time the estimated spectrum 56 is newly calculated by the calculation function 35C.

  For example, the calculation function 35D calculates an error based on the difference in the number of photon counts for each of the energy between the detected spectrum 54 and the estimated spectrum 56. More specifically, the calculation function 35D calculates, as an error, an integrated value (for example, a mean square) of the difference in the number of photon counts for each of the energy of the detected spectrum 54 and the estimated spectrum 56.

  Specifically, the calculation function 35D calculates, as an error, the integrated value of the difference in the number of photon counts corresponding to each of the energy bands of the second energy width E2 between the detected spectrum 54 and the estimated spectrum 56.

Specifically, for example, the calculation function 35D calculates an error using the following equation (4).

In equation (4), d indicates an error. In the formula (4), S ₁ (c, v, e) is the same as the above formula (2). In the formula (4), S ₂ (c, v, e) is the same as the above formula (3). Moreover, in Formula (4), each of c, e, and v is the same as Formula (3).

In formula (4), _NE is the upper limit value of energy e. In the present embodiment, as an example, it is assumed that N _E = 120.

Note that the calculation function 35D may calculate an error using the following equation (5).

In formula (5), each of d, S ₁ (c, v, e), S ₂ (c, v, e), c, e, and v is the same as in formula (4). In equation (5), E _min represents a value larger than the minimum value of photon energy detectable by the detector 13. E _max indicates a value smaller than the maximum value of photon energy detectable by the detector 13.

That is, as shown in Expression (5), the calculation function 35D may limit the energy range used when calculating the error to an arbitrary energy range from E _{min to} E _max .
Further, the calculation function 35D may calculate the error using any one of the following formulas (6), (7), and (8).

In formulas (6) to (8), each of d, S ₁ (c, v, e), S ₂ (c, v, e), c, e, v, E _min , and E _max is This is the same as the above formula (5). The function g is a predetermined function used as a filter function, and the operator “*” represents a convolution operation.

  In equation (6), a (e) represents a weighting coefficient corresponding to the energy e. An arbitrary value may be determined in advance as the weighting coefficient for each energy.

  In other words, the calculation function 35D performs estimation based on the difference in the photon count number for each of the energy of the detection spectrum 54 and the estimation spectrum 56 and the weighting coefficient corresponding to the energy based on the characteristics of the substance to be discriminated. An error between the spectrum and the detected spectrum is calculated. As an example, as shown in Expression (6), the calculation function 35D calculates the energy based on the square of the difference between the photon count numbers of the detected spectrum 54 and the estimated spectrum 56 and the characteristics of the substance to be discriminated. An error between the estimated spectrum and the detected spectrum is calculated based on the linear sum with the corresponding weight coefficient.

  For example, an energy band assumed to be important for error calculation is set in advance. Then, the calculation function 35D may set in advance a higher weighting coefficient for the energy in the energy band than in the energy other than the energy band.

  As an example, the calculation function 35D calculates an error based on a weighting coefficient such that a weighting coefficient in an energy whose energy difference from a predetermined energy is equal to or less than a first threshold becomes a weighting coefficient smaller than the weighting coefficients in other energy. calculate. Here, for example, the predetermined energy is energy corresponding to the K absorption edge. The reason for adopting such a weighting factor is, for example, as follows. That is, in the energy region close to the K absorption edge, an error is likely to occur in the detection spectrum 54, so it is desirable to reduce the weighting coefficient of these energy regions. Accordingly, the calculation function 35D decreases the weighting coefficient of the energy region near the K absorption edge.

  As another example, in the calculation function 35D, the weighting coefficient in the energy whose energy difference from the predetermined energy is greater than the first threshold and equal to or less than the second threshold becomes a weighting coefficient larger than the weighting coefficients in the other energy. An error is calculated based on such a weighting coefficient. Here, for example, the predetermined energy is energy corresponding to the K absorption edge. The reason for adopting such a weighting factor is as follows, for example. That is, if the energy region close to the K absorption edge is excluded, the X-ray transmission distance can be estimated more accurately as the energy is closer to the K absorption edge. Accordingly, the calculation function 35D increases the weighting coefficient of the energy region such that the energy difference from the K absorption edge is greater than the first threshold and less than or equal to the second threshold greater than the first threshold. In other words, it is desirable to use a large weighting coefficient because the spectrum information in the vicinity of the K edge is effective for substance discrimination. However, in the vicinity of the K edge (the energy difference from the K edge is equal to or less than the first threshold value), the error is Use a small weighting factor because it can be large.

  Further, the first threshold value may be “0”. In such a case, the calculation function 35D calculates an error based on a weighting coefficient such that the weighting coefficient in the energy whose energy difference from the predetermined energy is equal to or smaller than the second threshold becomes a larger weighting coefficient than the weighting coefficient in the other energy. calculate.

  The weighting coefficient may be determined according to energy based on the characteristics of the substance to be discriminated. The characteristic of the substance to be discriminated includes, for example, at least one of an attenuation characteristic with respect to X-rays of the substance and energy of the K absorption edge.

  For example, the calculation function 35D presets an energy band region in which an attenuation rate is greater than or less than a threshold value and is indicated by an attenuation characteristic with respect to X-rays of at least one material among a plurality of types of substances to be discriminated. Any value may be set as the threshold. Further, this threshold value may be appropriately updated by the operation of the input circuit 33 by the user. Then, the calculation function 35D may set in advance a higher weighting coefficient for the energy in the energy band than in the energy other than the energy band.

  Further, for example, a higher weighting coefficient may be set in the energy band including the K absorption edge (for example, around 33 keV in the case of iodine) of the substance to be discriminated compared to the energy other than the energy band.

That is, the calculation function 35D calculates the integrated value after adding a weighting coefficient corresponding to the energy based on the characteristics of the substance to each of the difference in the photon count number for each of the energy of the detected spectrum 54 and the estimated spectrum 56. The error may be calculated.
Further, as shown in Expression (7), the calculation function 35D may calculate an error by performing a convolution operation further using the function g in order to consider the influence of noise included in the detection spectrum 54 and the like. Good.

  The function g is, for example, a linear filter such as an average value filter or a Gaussian filter, or a nonlinear filter such as an ε filter. Filter parameters such as filter size (range) and ε of each filter are more preferably switched according to energy, for example, so as not to blur energy in the vicinity of 33 keV, which is the k absorption edge of iodine.

  That is, the calculation function 35D may adjust the filter range by using the function g in Expression (7) so that the spectrum near 33 keV in the detection spectrum 54 and the estimated spectrum 56 that are the error calculation target is not blurred.

In Formula (8), b is a serial number of each group when the continuous energy e is classified into a plurality of groups for each predetermined number (2 or more). As the serial number of each group, for example, a numerical value assigned to each group in ascending order from the smaller energy e to the larger one may be used. In the formula (8), _{N B} denotes the number of said group.

In the formula (8), E ₀ and E ₁ are each, showing the respective lower and upper limits of energy e belonging to the group b. For example, E ₀ (1) = 1, E ₁ (1) = 20, E ₀ (2) = 21, E ₁ (2) = 40,... And N _B = 6.

That is, as shown in Expression (8), the calculation function 35D calculates the photon count in an arbitrary energy range (a range from E ₀ (b) to E ₁ (b)) in each of the estimated spectrum 56 and the detected spectrum 54. A difference in the sum of the numbers may be calculated as an error.

  The update function 35E updates the estimated distance. Specifically, the update function 35E updates the estimated distance corresponding to the estimated spectrum 56 used by the calculation function 35D when calculating the previous error. The estimated distance corresponding to the estimated spectrum 56 used in the previous error calculation is the estimated distance used in the calculation function 35 </ b> C when calculating the estimated spectrum 56. In the following description, the estimated distance corresponding to the estimated spectrum 56 used when calculating the previous error may be referred to as the previously used estimated distance.

  A method of updating the estimated distance used last time by the update function 35E will be described later.

  The update by the update function 35E means that the estimated distance after changing the value of the estimated distance is further stored in the storage circuit 34 while the estimated distance already stored in the storage circuit 34 remains. . For this reason, every time the update function 35E updates the estimated distance, the updated new estimated distance is sequentially stored in the storage circuit 34.

  The specifying function 35F specifies the X-ray transmission distance of the substance based on the error calculated by the calculation function 35D.

  For example, the specifying function 35F specifies an estimated distance at which the error calculated by the calculation function 35D is equal to or less than a threshold as the X-ray transmission distance of the substance.

  In the present embodiment, the specifying function 35F includes the update function 35E, the calculation function so as to repeat a series of processes of updating the estimated distance, calculating the estimated spectrum 56 based on the updated estimated distance, and calculating the error. 35C and the calculation function 35D are controlled.

  As shown in FIG. 3, the estimated distance is updated for each of the series of processes under the control of the specifying function 35F (see FIG. 3E). In FIG. 3E, L1 represents an example of an estimated distance of water that is a substance to be discriminated, L2 represents an example of an estimated distance of iodine, and L3 represents an example of an estimated distance of calcium.

  That is, the specifying function 35F updates the estimated distance (see FIG. 3E), the estimated spectrum 56 based on the irradiation spectrum 50 (see FIG. 3A) and the updated estimated distance (FIG. 3D). A series of processes of calculation of (see) and calculation of an error between the estimated spectrum 56 and the detected spectrum 54 (see FIG. 3C) are repeatedly executed.

  Returning to FIG. 1, the specifying function 35 </ b> F specifies the estimated distance used in the series of processes in which the error below the threshold is calculated as the X-ray transmission distance of the substance.

  That is, the specific function 35F repeats a series of processes of updating the estimated distance, calculating the estimated spectrum 56 based on the updated estimated distance, and calculating the error until an error equal to or less than the threshold is calculated. The update function 35E, the calculation function 35C, and the calculation function 35D are controlled.

  The threshold value used by the specific function 35F may be a value that can be used to determine whether the error or the estimated distance has been optimized. For example, the threshold value used by the specific function 35F may be a minimum value of errors calculated for each series of processes. In this case, the specifying function 35F specifies the minimum value among the errors calculated for each series of processes as an error equal to or less than the threshold value. Then, the specifying function 35F may specify the estimated distance used in a series of processes in which an error equal to or less than the threshold is calculated as the X-ray transmission distance of the substance. That is, the specifying function 35F may specify the estimated distance that minimizes the error as the X-ray transmission distance of the substance.

  In addition, it is preferable that the update function 35E updates the estimated distance used last time so that an error smaller than the error calculated last time is calculated for each of the series of processes. As a method for updating the estimated distance by the update function 35E, a known optimization method such as a greedy method, a steepest descent method, and a conjugate gradient method may be used.

  In performing the optimization, the processing circuit 35 typically calculates an error for two or more estimated distance candidates by the calculation function 35D.

  In such a case, when using the greedy method by the specific function 35F, for example, the processing circuit 35 has an estimated distance such that the error becomes a minimum value among the errors calculated for the two or more estimated distance candidates. Candidates are estimated distances in the next iteration step.

  In addition, when it is necessary to calculate the differential coefficient for updating the iteration step, such as the steepest descent method, the processing circuit 35 uses the specifying function 35F to calculate the error calculated for the above two or more estimated distance candidates. For example, a differential coefficient is calculated using a difference method, and an estimated distance in the next iteration step is calculated using the calculated differential coefficient.

  Thus, the estimated distance is updated for each of the series of processes under the control of the specifying function 35F. For this reason, the calculation function 35D uses the weighting coefficient (see a (e) in the above equations (6) and (7)) and energy ranges Emin, Emax, E0 (b), E1 (b ) The filter function g may be changed for each of the series of processes according to the updated estimated distance.

  For example, it is assumed that the estimated distance of iodine is the largest among the estimated distances after each of the substances to be discriminated. In this case, it is preferable that the difference in the number of photon counts near 33 keV that is the k absorption edge of iodine in the detected spectrum 54 and the estimated spectrum 56 calculated in the next series of processing is smaller than that in the current series of processing. .

  Therefore, in this case, the calculation function 35D calculates the weighting coefficient a (e) corresponding to the energy band around 33 keV for the weighting coefficient a (e) shown in each of the above formulas (6) to (7). It is preferable to make it larger than energy other than the energy band.

The calculation function 35D may limit an arbitrary energy range indicated by E _min and E _max in the above formula (5) to around 33 keV.

  As described above, the calculation function 35D can increase the influence of the difference in the number of photon counts near 33 keV on the error, for example, by updating the weighting coefficient according to the updated estimated distance.

Further, the calculation function 35D is configured so that 33 keV is included in the energy band defined by the lower limit value and the upper limit value of the energy e belonging to the group b represented by E ₀ and E ₁ in the equation (8). It is preferable to design.

  On the other hand, it is assumed that the estimated distance of iodine is the shortest among the updated estimated distances of each substance to be discriminated. In this case, the calculation function 35D calculates the weighting coefficient a (e) corresponding to the energy band near 33 keV for the weighting coefficient a (e) shown in each of the above formulas (6) to (7). It is preferable to make it smaller than other energy.

  In this case, the calculation function 35D can reduce the influence of the difference in the number of photon counts near 33 keV on the error, for example, by changing the weighting coefficient according to the updated estimated distance. Therefore, in this case, the effect of the difference in the number of photon counts near 33 keV that is susceptible to noise due to the small number of transmitted photons can be reduced on the error calculated next time. That is, in this case, it is possible to suppress a decrease in error calculation accuracy due to the influence of noise.

  Next, the reconstruction function 35H will be described. The reconstruction function 35H reconstructs the CT image by a known reconstruction method using the detection spectrum 54 detected by each detection element 16 for each position of the X-ray generator 12 acquired by the acquisition function 35B. Configure. The reconstruction method is, for example, back projection processing. The back projection process is, for example, an FBP (Filtered Back Projection) method.

  The generation function 35G generates a display image corresponding to the X-ray transmission distance specified by the specification function 35F.

  The display image includes the X-ray transmission distance of each substance to be discriminated specified by the specifying function 35F, the density of the substance calculated from the X-ray transmission distance, the type of the substance to be discriminated, and the substance to be discriminated. The image further includes at least one of the atomic numbers.

  For example, the generation function 35G is configured to identify substances to be discriminated (for example, water, iodine, calcium) identified for each detection element 16 provided in the detector 13 corresponding to each position of the X-ray generator 12. Using each X-ray transmission distance, the density distribution in the subject P of each substance to be discriminated included in the subject P is calculated. A known method may be used to calculate the density distribution.

  The generation function 35G generates, for example, a color image in which the water density distribution is assigned to the G channel, the iodine density distribution to the R channel, and the calcium density distribution to the B channel as a display image.

  The generation function 35G may generate, as a display image, a color image indicating the density of each substance P to be discriminated in the subject P with a color and density corresponding to the density.

  Further, the generation function 35G may generate a monochromatic X-ray CT image or an X-ray CT image of only a high energy band from an image indicating the density of the substance in the subject P, and use it as a display image.

  The generation function 35G may generate a composite image obtained by combining the color image and the CT image generated by the reconstruction function 35H as a display image. In this case, the generation function 35G may generate a composite image (display image) by α blending the CT image and the color image.

  The display control function 35I performs control to display various images on the display 32. In the present embodiment, the display control function 35I performs control to display the display image generated by the generation function 35G and the CT image generated by the reconstruction function 35H on the display 32.

  Next, the procedure of the discrimination process executed by the processing circuit 35 will be described. FIG. 4 is a flowchart showing an example of the procedure of the discrimination process executed by the processing circuit 35.

  First, the acquisition function 35B acquires the detection spectrum 54 and the initial value of the estimated distance (step S100).

  Next, the calculation function 35C calculates the estimated spectrum 56 based on the irradiation spectrum 50, the initial value of the estimated distance acquired in step S100, and the response information (step S102).

  Next, the calculation function 35D calculates an error between the estimated spectrum 56 calculated in step S102 and the detected spectrum 54 acquired in step S100 (step S104). Then, the calculation function 35D stores the error calculated in step S104 in the storage circuit 34 in association with the estimated distance acquired in step S100 (step S106).

  Next, the update function 35E updates the estimated distance (step S108). The update function 35E updates the latest estimated distance stored in the storage circuit 34. Then, the update function 35E newly stores the updated estimated distance in the storage circuit 34 (step S110).

  Next, the calculation function 35C calculates the estimated spectrum 56 based on the irradiation spectrum 50, the updated estimated distance updated in step S108, and the response information (step S112).

  Next, the calculation function 35D calculates an error between the estimated spectrum 56 calculated in step S112 and the detected spectrum 54 acquired in step S100 (step S114). Then, the calculation function 35D stores the error calculated in step S114 in the storage circuit 34 in association with the estimated distance updated in step S108 (step S116).

  For this reason, in the storage circuit 34, the error between the detected spectrum 54 and the estimated spectrum 56 and the estimated distance of each substance to be discriminated are sequentially stored in correspondence with each other in a series of processing from step S108 to step S116. It will be done.

  Next, the specific function 35F determines whether or not to end the process (step S118). For example, the specific function 35F performs the determination in step S118 by determining whether or not the series of processing in steps S108 to S116 has been repeated a predetermined number of times.

  Note that the specific function 35F may make the determination in step S118 by determining whether or not an instruction signal indicating the end of processing has been received from the input circuit 33 by the reception function 35J. Further, the specific function 35F may perform the determination in step S118 by determining whether or not the error calculated in step S114 is equal to or less than the above-described threshold value used by the specific function 35F.

  Further, the specifying function 35F may perform the determination in step S118 by determining whether or not the error calculated in step S114 is less than or equal to a predetermined fluctuation value from the previously calculated error. .

  If a negative determination is made in step S118 (step S118: No), the process returns to step S108. That is, the specific function 35F controls to repeat the processes of Step S108 to Step S118: No. That is, the specifying function 35F repeats the update function so as to repeat a series of processes of updating the estimated distance, calculating the estimated spectrum 56 based on the updated estimated distance, and calculating an error using the estimated spectrum 56. 35E, the calculation function 35C, and the calculation function 35D are controlled.

  On the other hand, if an affirmative determination is made in step S118 (step S118: Yes), the process proceeds to step S120.

  In step S120, the specifying function 35F specifies the X-ray transmission distance of the substance (step S120).

  For example, the specifying function 35 </ b> F specifies the minimum error from the storage circuit 34 among the errors calculated for each of the series of processes in steps S <b> 108 to S <b> 116 stored in the storage circuit 34. Then, the specifying function 35F reads the estimated distance corresponding to the specified minimum error from the storage circuit 34. Thereby, the specifying function 35F specifies the estimated distance corresponding to the minimum error as the X-ray transmission distance of the substance. That is, the specifying function 35F specifies the X-ray transmission distance of each substance to be discriminated.

  The specific function 35F repeats the series of processes a predetermined number of times in the determination of step S118 out of the errors calculated for each series of processes of steps S108 to S116 stored in the storage circuit 34. The error calculated when it is determined that the error has occurred may be specified from the storage circuit 34, and the specifying function 35F reads the estimated distance corresponding to the specified minimum error from the storage circuit 34. Accordingly, the specifying function 35F may specify the estimated distance when the above series of processes is repeated a predetermined number of times as the X-ray transmission distance of the substance.

  In the determination in step S118, it is assumed that the specifying function 35F has determined in step S118 by determining whether or not the variation from the previously calculated error is equal to or less than a predetermined variation value. In this case, the specifying function 35F may specify the estimated distance corresponding to the error calculated when an affirmative determination is made in step S118 as the X-ray transmission distance of the substance. As described above, the specifying function 35F may specify an estimated distance at which the calculated error fluctuation is equal to or less than a predetermined fluctuation value as the X-ray transmission distance of the substance.

  Then, the specifying function 35F stores the specified X-ray transmission distance in the storage circuit 34 in association with the type information indicating the type of the corresponding substance.

  Next, the generation function 35G generates a display image corresponding to the X-ray transmission distance of each substance stored in the storage circuit 34 in step S120 (step S122). Next, the display control function 35I displays the display image generated in step S122 on the display 32 (step S124). Then, this routine ends.

  Note that the processing of step S122 and step S124 may be executed when display of the display image is instructed by an operation instruction of the input circuit 33 by the user.

  As described above, the X-ray CT apparatus 1 according to the present embodiment includes the calculation function 35C, the calculation function 35D, and the specifying function 35F. The calculation function 35C includes an X-ray irradiation spectrum 50 applied to the subject P from the X-ray generator 12, an estimated distance indicating an estimated value of the X-ray transmission distance of the substance to be discriminated, and a detector that detects the X-rays. The estimated spectrum 56 detected by the X-ray detector 13 that has passed through the subject P is calculated based on the response information indicating the distortion of the spectrum caused by the 13 response characteristics. The calculation function 35D calculates an error between the estimated spectrum 56 and the X-ray detection spectrum 54 detected by the detector 13 and transmitted through the subject P. The specifying function 35F specifies the X-ray transmission distance of the substance based on the error.

  As described above, in the X-ray CT apparatus 1 of the present embodiment, an error between the estimated spectrum 56 calculated from the irradiation spectrum 50 based on the estimated distance and the response information and the detected spectrum 54 that is the actually detected spectrum. Is calculated. Then, the X-ray CT apparatus 1 specifies the X-ray transmission distance of the substance based on the error.

  For this reason, in the X-ray CT apparatus 1 of this Embodiment, compared with the case where the estimated spectrum 56 is estimated from the detection spectrum 54, the influence of an estimation error can be suppressed.

Therefore, the X-ray CT apparatus 1 of the present embodiment can perform highly accurate material discrimination.
Further, the specifying function 35F can specify an estimated distance at which the error is equal to or less than a threshold value as the X-ray transmission distance of the substance. In addition, the specifying function 35F may specify the estimated distance that minimizes the error as the X-ray transmission distance of the substance.

  Further, the specifying function 35F may specify an estimated distance at which the calculated error fluctuation is equal to or less than a predetermined fluctuation value as the X-ray transmission distance of the substance. Further, the specifying function 35F calculates the estimated distance when a series of processes of updating the estimated distance, calculating the estimated spectrum 56 based on the updated estimated distance, and calculating the error is repeated a predetermined number of times. It may be specified as an X-ray transmission distance.

  Here, the initial value of the estimated distance initially used when the calculation function 35C calculates the estimated spectrum 56 includes effects such as the beam hardening effect and the response characteristics of the detection element 16. However, in the X-ray CT apparatus 1 of the present embodiment, the estimated distance in which the error is less than or equal to the threshold, the estimated distance in which the error is the minimum, the estimated distance in which the error variation is less than or equal to a predetermined change value, or the series of The estimated distance when the process is repeated a predetermined number of times is specified as the X-ray transmission distance of the substance. For this reason, the X-ray CT apparatus 1 can perform material discrimination with high accuracy.

  In the present embodiment, the energy width of the energy band corresponding to each of the photon count numbers indicated by the detection spectrum 54 used by the information processing device 30 is the first energy width E1 (FIG. 2D). reference). The energy width of the energy band corresponding to each of the photon count numbers indicated by the irradiation spectrum 50 is the second energy width E2 (see FIG. 2B). In the present embodiment, the second energy width E2 is equal to or smaller than the first energy width E1.

  The first energy width E1 and the second energy width E2 are narrow widths such as less than 20 keV and 1 keV or less.

  For this reason, in the X-ray CT apparatus 1 of the present embodiment, the X of the substance to be discriminated using the spectrum in which the influence of the beam hardening effect in which the ratio of photons in the energy band changes during the substance transmission is suppressed. The line transmission distance can be specified.

  For this reason, in the X-ray CT apparatus 1 of this Embodiment, in addition to the said effect, the highly accurate substance discrimination which reduced the influence of the beam hardening effect can be performed.

  Next, an example of the hardware configuration of the information processing apparatus 30 according to the present embodiment will be described. FIG. 5 is an example of a hardware configuration diagram of the information processing apparatus 30 according to the present embodiment. The information processing device 30 is an I / F 76 that is an interface between a control device such as a CPU (Central Processing Unit) 70, a storage device such as a ROM (Read Only Memory) 72 and a RAM (Random Access Memory) 74, and various devices. And a bus 78 for connecting each part, and has a hardware configuration using a normal computer. Note that the CPU 70, ROM 72, RAM 74, and I / F 76 are connected to each other via a bus 78.

  In the information processing apparatus 30 according to the above-described embodiment, the CPU 70 reads out a program from the ROM 72 onto the RAM 74 and executes it, whereby the above functions are realized on the computer.

  Note that a program for executing each of the processes executed by the information processing apparatus 30 of the embodiment may be stored in a hard disk connected via the I / F 76. In addition, a program for executing each of the processes executed by the information processing apparatus 30 according to the embodiment may be provided by being incorporated in the ROM 72 in advance.

  A program for executing the above-described processing executed by the information processing apparatus 30 of the above-described embodiment is a file in an installable format or an executable format, and is a CD-ROM, CD-R, memory card, DVD ( It may be stored in a computer-readable storage medium such as a digital versatile disk (FD) or a flexible disk (FD) and provided as a computer program product. Further, the program for executing the processing executed by the information processing apparatus 30 according to the embodiment is stored on a computer connected to a network such as the Internet and provided by being downloaded via the network. May be. Further, a program for executing the above-described processing executed by the information processing apparatus 30 of the above-described embodiment may be provided or distributed via a network such as the Internet.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 X-ray CT apparatus 12 X-ray generator 30 Information processing apparatus 35B Acquisition function 35C Arithmetic function 35D Calculation function 35E Update function 35F Specific function 35G Generation function 35I Display control function 35J Reception function

Claims (19)

An X-ray generator that irradiates the subject with X-rays;
An X-ray detector for detecting X-rays transmitted through the subject;
Of the spectrum showing the distribution of the photon count number for each energy of the irradiated X-ray, the irradiation spectrum which is the spectrum until reaching the subject and the estimation showing the estimated value of the X-ray transmission distance of the substance to be discriminated An arithmetic unit that calculates an estimated spectrum that is estimated as a spectrum after the X-ray has transmitted through the subject based on the distance and information indicating the distortion of the spectrum that occurs in the X-ray path that has passed through the subject. When,
A specifying unit that specifies an X-ray transmission distance of the substance to be discriminated based on the estimated spectrum and a detection spectrum that is detected by the X-ray detection unit and is a spectrum after passing through the subject; ,
An X-ray CT apparatus comprising:   2. The X-ray CT apparatus according to claim 1, wherein spectral distortion generated in a path of X-rays transmitted through the subject is caused by at least one of response characteristics of the X-ray detector and attenuation of X-rays due to subject transmission. . A calculation unit that calculates an error between the estimated spectrum and the detection spectrum;
The X-ray CT apparatus according to claim 1, wherein the specifying unit specifies the estimated distance at which the error is equal to or less than a threshold as an X-ray transmission distance of the substance. A calculation unit that calculates an error between the estimated spectrum and the detection spectrum;
The X-ray CT apparatus according to claim 1, wherein the specifying unit specifies the estimated distance that minimizes the error as an X-ray transmission distance of the substance. A calculation unit that calculates an error between the estimated spectrum and the detection spectrum;
The specifying unit specifies the X-ray transmission distance based on the calculated error, updates the estimated distance based on the specified X-ray transmission distance,
The specifying unit updates the estimated distance, performs processing of the estimated spectrum based on the updated estimated distance, performs processing of causing the calculating unit to perform calculation of the error, and processing of the X A specific series of processing of the line transmission distance is repeated until a predetermined condition is satisfied,
The X-ray CT apparatus according to claim 1, wherein the specifying unit specifies the estimated distance when the predetermined condition is satisfied as an X-ray transmission distance of the substance.   The X-ray CT apparatus according to claim 5, wherein the predetermined condition is that a variation of the calculated error from the previously calculated error is equal to or less than a predetermined variation value.   The X-ray CT apparatus according to claim 5, wherein the predetermined condition is that the series of processes is repeated a predetermined number of times.   The calculation unit calculates the estimated spectrum obtained by attenuating the irradiation spectrum according to a linear attenuation coefficient of the substance and the estimated distance and distorting the irradiation spectrum according to the distortion indicated by the information. The X-ray CT apparatus according to 3.   The X-ray CT apparatus according to claim 3, wherein the calculation unit calculates the error based on a difference in a photon count number for each energy between the detection spectrum and the estimated spectrum. An error between the estimated spectrum and the detected spectrum based on a difference in the number of photon counts for each energy between the detected spectrum and the estimated spectrum, and a weighting factor according to energy based on characteristics of the substance. A calculation unit for calculating
The X-ray CT apparatus according to claim 1, wherein the specifying unit specifies an X-ray transmission distance of the substance based on the calculated error.   The calculation unit calculates the error based on the weighting coefficient such that a weighting coefficient in an energy whose energy difference from a predetermined energy is equal to or less than a first threshold value is smaller than a weighting coefficient in other energy. The X-ray CT apparatus according to claim 10.   The calculation unit is based on the weighting coefficient such that the weighting coefficient in the energy that is larger than the first threshold and less than or equal to the second threshold is larger than the weighting coefficient in the other energy. The X-ray CT apparatus according to claim 10, wherein the error is calculated.   The X-ray CT apparatus according to claim 11, wherein the predetermined energy is energy corresponding to a K absorption edge. The detection spectrum indicates a photon count for each energy band of the first energy width,
The X-ray CT apparatus according to claim 1, wherein the irradiation spectrum indicates a photon count number for each energy band having a second energy width equal to or less than the first energy width.   The X-ray CT apparatus according to claim 1, wherein the detection part spectrum and the irradiation spectrum indicate a photon count number for each energy width of 1 keV.   The X-ray CT apparatus according to claim 1, further comprising a generation unit that generates a display image corresponding to the X-ray transmission distance. A reception unit that receives input of type information indicating the type of the substance to be discriminated;
The calculation unit calculates the estimated spectrum based on the irradiation spectrum, the estimated distance of the substance indicated by the type information, and the information. The X-ray CT apparatus described. Of the spectrum showing the distribution of the photon count number for each X-ray energy irradiated to the subject from the X-ray generator, the irradiation spectrum that is the spectrum until reaching the subject, and the substance to be discriminated Based on an estimated distance indicating an estimated value of an X-ray transmission distance and information indicating a distortion of the spectrum generated in the X-ray path that has passed through the subject, a spectrum after the X-ray has passed through the subject is obtained. An arithmetic unit for calculating an estimated spectrum;
A specifying unit that specifies an X-ray transmission distance of the substance to be discriminated based on the estimated spectrum and an X-ray detection spectrum detected by an X-ray detector that detects X-rays transmitted through the subject; An information processing apparatus comprising: An information processing method executed by a processing circuit,
Of the spectrum showing the distribution of the photon count number for each X-ray energy irradiated to the subject from the X-ray generator, the irradiation spectrum that is the spectrum until reaching the subject, and the substance to be discriminated Based on an estimated distance indicating an estimated value of an X-ray transmission distance and information indicating a distortion of the spectrum generated in the X-ray path that has passed through the subject, a spectrum after the X-ray has passed through the subject is obtained. Calculate the estimated spectrum,
Based on the estimated spectrum and an X-ray detection spectrum detected by an X-ray detector that detects X-rays transmitted through the subject, an X-ray transmission distance of the substance to be discriminated is specified.
Information processing method.

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