JP2016104125A - X-ray ct apparatus, image processing apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which is improved in accuracy in correction of distortion of the spectrum of X-rays detected by a detector, and an image processing apparatus and a program.SOLUTION: In an X-ray CT apparatus, radiation energy transmitted through a subject is detected by a detector and image processing is performed on data based on the energy. The X-ray CT apparatus includes a generation unit and a regeneration unit. The generation unit generates second projection data by correcting first projection data based on a first spectrum showing the dosage of X-rays for each radiation energy detected by the detector with response information corresponding to the detector. The regeneration unit regenerates second projection data.SELECTED DRAWING: Figure 7

Description

  Embodiments described herein relate generally to an X-ray CT apparatus, an image processing apparatus, and a program.

  In recent years, development of silicon-based photomultipliers has become active, and radiation detection apparatuses such as X-ray CT (Computed Tomography) apparatuses using photomultipliers have been developed. As an example of such an X-ray CT apparatus, there is a spectral CT apparatus or a photon counting CT apparatus that detects a spectrum indicated by the number of photons for each X-ray energy transmitted through a subject. In a photon counting CT apparatus or the like, a reconstructed image indicating the line attenuation coefficient of the subject is reconstructed from the degree of attenuation of the spectrum of the X-ray transmitted through the subject.

  The X-ray spectrum observed by the detector is a fluctuation generated in the process of converting the X-ray energy into an observed value, a shift in the X-ray energy due to noise, or the X-ray is photoelectrically converted from the detector element. Alternatively, the detection is distorted from the spectrum of the X-rays incident on the detector by causing an interaction such as scattering. The linear attenuation coefficient of the subject calculated using the distorted spectrum as it is becomes a value different from the true value.

  As one of the X-ray spectrum detection methods for correcting such a distorted spectrum, the measured value of the number of photons of X-rays obtained using a calibration object and the theoretical value obtained by simulation. Therefore, a technique for obtaining a correction formula for approximating a measured value to a theoretical value for each energy has been proposed. Then, a correction formula is applied to the detected X-ray spectrum.

  However, in the technique that applies the above-described correction formula, there is a problem that the number of photons cannot be corrected accurately when the composition of the subject is different from the conditions for creating the correction formula. In this case, the pixel value of the sinogram obtained from the X-ray spectrum transmitted through the subject is different from the true value.

“Image-based spectral distortion correction for photo-counting x-ray detectors”, Medical physics, 39 (4), pp. 199 1864-1876, 2012.

  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 image processing apparatus, and a program that improve the accuracy of correction of distortion of the X-ray spectrum detected by the detector. And

  An X-ray CT apparatus according to an embodiment is an X-ray CT apparatus that detects energy of radiation that has passed through a subject by a detector and performs image processing on data based on the energy. A section. The generation unit corrects the first projection data based on the first spectrum indicating the amount of X-rays for each energy of the radiation detected by the detector with the response information corresponding to the detector, and generates the second projection data. To do. The reconstruction unit reconstructs the second projection data.

1 is an overall configuration diagram of an X-ray inspection apparatus according to a first embodiment. It is a figure explaining a sinogram. It is a figure which shows an example of the spectrum of the energy detected by the specific channel. It is a figure which shows the example of a subject sinogram. It is a figure explaining the physical phenomenon of the X-ray which injects into a detector. It is a figure explaining a detection spectrum. It is a figure which shows an example of the block configuration of the image processing part of 1st Embodiment. 3 is a flowchart illustrating an example of an operation of an image processing unit according to the first embodiment. It is a figure which shows an example of the block configuration of the image process part of the modification 1 of 1st Embodiment. It is a figure which shows the example of the decompression | restoration spectrum by which the detection spectrum was correct | amended. It is a figure which shows an example of the block configuration of the image processing part of 2nd Embodiment. It is a figure which shows an example of an emission spectrum and a subject spectrum. It is a flowchart which shows an example of operation | movement of the image process part of 2nd Embodiment. It is a figure which shows an example of the block configuration of the image process part of the modification of 2nd Embodiment. It is a figure which shows an example of the block configuration of the production | generation part of the image processing part of 3rd Embodiment. It is a flowchart which shows an example of operation | movement of the image process part of 3rd Embodiment.

  Hereinafter, an X-ray CT apparatus, an image processing apparatus, and a program according to an embodiment of the present invention will be described in detail with reference to the drawings. Moreover, in the following drawings, the same code | symbol is attached | subjected to the same part. However, since the drawings are schematic, a specific configuration should be determined in consideration of the following description.

(First embodiment)
FIG. 1 is an overall configuration diagram of the X-ray inspection apparatus according to the first embodiment. An overview of the overall configuration of the X-ray inspection apparatus 1 will be described with reference to FIG.

  As shown in FIG. 1, an X-ray inspection apparatus 1 that is an example of an X-ray CT apparatus transmits X-rays that are an example of radiation to a subject 40 and detects it as a spectrum indicated by the number of photons for each energy. Thus, a spectral CT apparatus, a photon counting CT apparatus, or the like that obtains a cross-sectional image of the projected cross section 41 of the subject 40. As shown in FIG. 1, the X-ray inspection apparatus 1 includes a gantry device 10, a bed device 20, and a console device 30 (image processing device).

  The gantry device 10 is a device that irradiates and transmits the subject 40 with X-rays and detects the above-described spectrum. The gantry device 10 includes an X-ray tube 11, a rotating frame 12, a detector 13, an irradiation control unit 14, a gantry driving unit 15, and a data collecting unit 16 (collecting unit).

  The X-ray tube 11 is a vacuum tube that generates X-rays with a high voltage supplied from the irradiation control unit 14, and irradiates the subject 40 with the X-ray beam 11 a. The spectrum indicated by the number of photons for each X-ray energy irradiated from the X-ray tube 11 is determined by the tube voltage of the X-ray tube 11, the tube current, and the type of target (for example, tungsten) used for the radiation source. . When the X-rays irradiated from the X-ray tube 11 pass through the subject 40, the energy of the X-rays is attenuated according to the state of the substance constituting the subject 40, and the number of photons of each energy is reduced. As a result, the spectrum changes.

  The rotating frame 12 is a ring-shaped support member that supports the X-ray tube 11 and the detector 13 so as to face each other with the subject 40 interposed therebetween.

  The detector 13 is a detector that detects, for each channel, the number of photons for each energy of the X-ray beam 11 b that is an X-ray irradiated from the X-ray tube 11 and transmitted through the subject 40. That is, the detector 13 detects, for each channel, a spectrum indicated by the number of photons for each X-ray energy as shown in FIG. 4 described later. Here, it is assumed that the spectrum detected by the detector 13 may be hereinafter referred to as “detected spectrum”. As shown in FIG. 1, the detector 13 detects a spectrum for each view while rotating in the circumferential direction of the rotating frame 12. Here, the view means an angle when a spectrum is detected by the detector 13 at every predetermined angle out of 360 degrees in the circumferential direction of the rotating frame 12. That is, when the detector 13 detects a spectrum every 0.5 °, it is assumed that 1 view = 0.5 °. The detector 13 has a detection element array in which a plurality of detection elements are arranged in the channel direction (circumferential direction of the rotating frame 12) in the body axis direction (slice direction) of the subject 40 (Z-axis direction shown in FIG. 1). It is a two-dimensional array type detector arranged in a plurality of rows along. Note that the detection element array of the detector 13 may be configured by a combination of a photocounting type detection element and an integration type detection element. A plurality of sets of the X-ray tube 11 and the detector 13 may be installed.

  The irradiation control unit 14 is a device that generates a high voltage and supplies the generated high voltage to the X-ray tube 11.

  The gantry driving unit 15 is a device that rotationally drives the rotary frame 12 to rotationally drive the X-ray tube 11 and the detector 13 on a circular orbit around the subject 40. The gantry driving unit 15 is not limited to a configuration that rotationally drives both the X-ray tube 11 and the detector 13. For example, the detector 13 may be configured such that detection elements are arranged for one turn in the circumferential direction of the rotating frame 12, and the gantry driving unit 15 may be configured to rotate only the X-ray tube 11. .

  The data collection unit 16 is a device that collects data of a spectrum (first spectrum) indicated by the number of photons for each energy detected for each channel by the detector 13. Then, the data collection unit 16 performs amplification processing or A / D conversion processing on each collected spectrum data, for each energy band of a predetermined width (hereinafter, sometimes simply referred to as “every energy”). ) Is generated and output to the console device 30.

  The couch device 20 is a device on which the subject 40 is placed, and includes a couch driving device 21 and a top plate 22 as shown in FIG.

  The top plate 22 is a bed such as a bed on which the subject 40 is placed. The couch driving device 21 is a device that moves the subject 40 into the rotary frame 12 by moving the subject 40 placed on the top plate 22 in the body axis direction (Z-axis direction).

  The console device 30 is a device that receives an operation on the X-ray inspection apparatus 1 by an operator and reconstructs a cross-sectional image (restored image) from data collected by the gantry device 10. As shown in FIG. 1, the console device 30 includes an input device 31 (input unit), a display device 32, a scan control unit 33, an image processing unit 34, an image storage unit 35, a system control unit 36, It has.

  The input device 31 is a device for an operator who operates the X-ray inspection apparatus 1 to input various instructions, and is a device that transmits various commands input by the operation to the system control unit 36. The input device 31 is, for example, a mouse, a keyboard, a button, a trackball, or a joystick.

  The display device 32 displays a GUI (Graphical User Interface) for accepting an operation instruction from the operator via the input device 31, or displays a restored image (cross-sectional image) stored in the image storage unit 35 described later. It is. The display device 32 is, for example, a CRT (Cathode Ray Tube) display, an LCD (Liquid Crystal Display), an organic EL (Organic Electro-Luminescence) display, or the like.

  The scan control unit 33 is a processing unit that controls operations of the irradiation control unit 14, the gantry driving unit 15, the data collection unit 16, and the bed driving device 21. Specifically, the scan control unit 33 causes the X-ray scan to be executed by continuously or intermittently emitting X-rays from the X-ray tube 11 while rotating the rotary frame 12. For example, the scan control unit 33 performs helical scanning in which imaging is performed by continuously rotating the rotating frame 12 while moving the top plate 22, or imaging is performed by rotating the rotating frame 12 once around the subject 40, and then Then, the top plate 22 on which the subject 40 is placed is slightly shifted, and the rotating frame 12 is rotated once again to perform non-helical scanning for imaging.

  The image processing unit 34 is a processing unit that reconstructs a cross-sectional image of the subject from the sinogram received from the data collection unit 16. Details of the block configuration and operation of the image processing unit 34 will be described later.

  The image storage unit 35 is a functional unit that stores a cross-sectional image (restored image) generated by the reconstruction processing by the image processing unit 34. The image storage unit 35 is, for example, a storage device such as an HDD (Hard Disk Drive), an SSD (Solid State Drive), or an optical disk.

  The system control unit 36 is a processing unit that controls the entire X-ray inspection apparatus 1 by controlling operations of the gantry device 10, the couch device 20, and the console device 30. Specifically, the system control unit 36 controls the scan control unit 33 to control the spectrum data collection operation of the subject 40 by the gantry device 10 and the couch device 20. In addition, the system control unit 36 controls the image processing unit 34 to control the cross-sectional image reconstruction process. Further, the system control unit 36 reads the cross-sectional image from the image storage unit 35 and causes the display device 32 to display the cross-sectional image.

  The data collection unit 16 generates a sinogram for each predetermined energy band from the collected spectrum data, but the present invention is not limited to this. That is, the data collection unit 16 may transmit collected spectrum data to the image processing unit 34, and the image processing unit 34 may generate a sinogram for each energy band having a predetermined width from the spectrum data.

  FIG. 2 is a diagram illustrating a sinogram. FIG. 3 is a diagram showing an example of a spectrum of energy detected in a specific channel of the detector. FIG. 4 is a diagram illustrating an example of a subject sinogram. FIG. 5 is a diagram for explaining the physical phenomenon of X-rays incident on the detector. FIG. 6 is a diagram for explaining the detection spectrum. A sinogram, an X-ray energy spectrum, a detection spectrum detected by the detector 13, and the like will be described with reference to FIGS.

  The data collection unit 16 of the gantry device 10 generates a sinogram from a spectrum (detection spectrum) indicated by the number of photons for each energy detected by the detector 13 as shown in FIG. Here, the sinogram is data in which measured values for each view of the X-ray tube 11 and for each channel of the detector 13 are arranged like a sinogram 1001 shown in FIG. A measured value for each channel is regarded as a pixel value, and a sinogram is handled as an image. Among these, a sinogram generated from a spectrum (see FIG. 3) in which X-rays irradiated from the X-ray tube 11 pass through the subject 40 and are detected by the detector 13 is referred to as a subject sinogram. A sinogram generated from a spectrum detected by the detector 13 with X-rays passing through only air without placing the subject 40 is referred to as an air sinogram. The pixel values of the subject sinogram and the air sinogram are, for example, the number of photons detected as a measurement value by the detector 13.

  In addition, since the detector 13 detects the spectrum indicated by the number of photons for each energy for each view and for each channel, the data acquisition unit 16 performs the X-ray scan for one round of the X-ray tube 11 to display the spectrum. As shown in FIG. 4, a subject sinogram 1011 for each energy can be obtained. In the example shown in FIG. 4, the spectrum is divided into four energy bands, and four object sinograms 1011a to 1011d are obtained for each energy band. In addition, although the example divided | segmented into four energy bands was shown in FIG. 4, it is not limited to this division | segmentation number. In addition, from the viewpoint of improving the S / N ratio of a restored image (an image showing an attenuation coefficient) and a density image described later, the number of photons in the energy band used for reconstruction and estimation of the material density is uniform. May be desirable. In order to realize this, for example, there are the following two methods.

(Method 1) At the stage of creating first projection data, which will be described later, the image data is divided into energy bands so that the number of photons is uniform.
(Method 2) First, it is finely divided (for example, divided every 1 [keV]), and the number of photons is added at the stage of reconstruction or estimation of material density.

  Since the subject sinogram has a different spectrum shape depending on the pixel, it may be divided based on the air sinogram.

  Next, a physical phenomenon that occurs with respect to X-rays incident on the detector 13 will be described with reference to FIG. As shown in FIG. 5, the detector 13 is a so-called indirect conversion type detector having a scintillator 50, an adhesive layer 51, and a SiPM (Si Photomultiplier) 52.

  When the X-ray beam 11b (see FIG. 1) is incident, the scintillator 50 converts it into an electromagnetic wave (hereinafter referred to as scintillation light) containing at least one of ultraviolet rays, visible rays, and infrared rays having a wavelength longer than that of the X-rays. It is a member to do. The scintillator 50 is a plate member that is defined in a mat-like shape in the circumferential direction of the rotating frame 12 and in the slice direction by the reflecting plate 50a. The reflector 50a transmits X-rays but reflects scintillation light.

  The adhesive layer 51 is a layer that bonds the emission surface side of the scintillator 50 and the incident surface side of the SiPM 52.

  The SiPM 52 is a photoelectric conversion element configured by connecting a plurality of series circuits of an avalanche photodiode (APD) and a quench resistor in parallel. The SiPM 52 photoelectrically converts the scintillation light into current when the scintillator 50 receives the scintillation light from the X-rays through the adhesive layer 51. The portion of the SiPM 52 where the scintillator 50 faces each cell defined by the reflector 50 a forms each channel of the detector 13.

  When X-rays are incident on a specific channel in the detector 13, photoelectric conversion that reacts with the elements constituting the scintillator 50 occurs, a part of the X-ray energy is re-radiated as fluorescent X-rays, and the remaining energy is converted into the scintillator. 50 reacts with scintillation light (visible light). Then, as shown in FIG. 5, the fluorescence may pass through the reflecting plate 50a and move to another channel (such as an adjacent channel). This phenomenon is called “escape” in order to distinguish it from fluorescence described later. When the escape occurs, X-rays that have moved to another channel do not contribute to the detection of photons in the specific channel.

  Further, when X-rays are incident on a channel around a specific channel in the detector 13, similarly to the above, it reacts with an element constituting the scintillator 50 to generate a photoelectric effect and emit fluorescence. Then, as shown in FIG. 5, the fluorescence may pass through the reflection plate 50a from the surrounding channel and move to the specific channel. This phenomenon is simply referred to as “fluorescence” in distinction from the above-mentioned escape. When fluorescence is generated, a part of the energy of the X-rays incident on the other channel is detected by the specific channel.

  Further, the X-rays incident on the scintillator 50 generate not only the photoelectric conversion described above but also Compton scattering and Rayleigh scattering (hereinafter simply referred to as “scattering”) due to collision with the elements constituting the scintillator 50, As shown in FIG. 5, the X-ray whose traveling direction is bent may pass through the reflection plate 50a from the surrounding channel and move to the specific channel. When this scattering occurs, some or all of the energy of the X-rays incident on the other channel is detected in the specific channel.

  Further, part or all of the energy of the X-rays incident on the scintillator 50 reacts with the scintillator 50 and is converted into scintillation light. As described above, the scintillation light reflects the reflecting plate 50a. However, as shown in FIG. 5, when the reflecting plate 50a does not extend to the adhesive layer 51, a channel around the specific channel is used. In some cases, the scintillation light generated in step 1 passes through the adhesive layer 51 and is detected by the SiPM 52 of the specific channel. This phenomenon is hereinafter referred to as “crosstalk”. When crosstalk occurs, X-rays originally incident on another channel and a part of the energy of X-rays from which photons should be detected are detected by the specific channel. Become.

  Due to the influence of the occurrence of escape, fluorescence, scattering, and crosstalk as described above, as shown in FIG. 6, an incident spectrum that is an X-ray spectrum transmitted through the subject 40 and incident on a specific channel of the detector 13. 2001 is detected by the detector 13 in a spectrum having a waveform such as a detection spectrum 2011. As shown in FIG. 6, the detection spectrum 2011 includes a spectrum corresponding to the amount of X-rays incident on the specific channel detected by the SiPM 52 corresponding to the specific channel, and the spectra 2012 to 2014. Expressed in sum. A spectrum 2012 is a spectrum in which part of the energy is transferred to another channel due to escape and the remaining energy is detected in the specific channel. A spectrum 2013 is a spectrum in which a part of the energy of X-rays incident on another channel is detected by the specific channel due to fluorescence. A spectrum 2014 is a spectrum in which part or all of the energy of X-rays incident on another channel is detected in the specific channel due to crosstalk and scattering. That is, the detection spectrum 2011 is a spectrum having a waveform distorted from the incident spectrum 2001 which is a correct waveform to be detected by the detector 13. The image processing unit 34 of the X-ray inspection apparatus 1 according to the present embodiment performs an operation of bringing the detection spectrum 2011 that is a distorted waveform closer to the waveform of the incident spectrum 2001 by a correction operation described later. The configuration and operation of the image processing unit 34 will be described in detail below.

  Although the detector 13 has been described as a so-called indirect conversion type detector as shown in FIG. 5, the detector 13 is not limited to this, and does not have a scintillator and directly detects X-rays. A conversion type detector may be used. Even in the case of a direct conversion type detector, when X-rays pass through the material constituting the detector, a physical phenomenon having an action equivalent to the above-described escape, fluorescence, crosstalk, and scattering can occur.

  FIG. 7 is a diagram illustrating an example of a block configuration of the image processing unit according to the first embodiment. A block configuration of the image processing unit 34 of the present embodiment will be described with reference to FIG.

  As illustrated in FIG. 7, the image processing unit 34 includes a storage unit 341, a generation unit 342, and a reconstruction unit 343.

  The storage unit 341 is a functional unit that stores detector response data (response information) described later. The storage unit 341 is not limited to the configuration provided in the image processing unit 34. For example, the image storage unit 35 illustrated in FIG. 1 may serve as the storage unit 341.

  The generation unit 342 receives a subject sinogram that is a sinogram of the subject 40 from the data collection unit 16 as first projection data, reads out detector response data from the storage unit 341, and outputs first projection data and detector response data. Is a functional unit that generates second projection data in the form of a sinogram based on. Here, the first projection data is energy of a predetermined width generated by the data collection unit 16 based on the spectrum indicated by the number of photons for each energy detected by the detector 13 for each view and for each channel. This is data composed of a collection of subject sinograms (first sinograms) for each band. The second projection data is also in the same data format as the first projection data, and is data composed of a collection of subject sinograms (second sinograms) for each energy band having a predetermined width. Hereinafter, data having the same data format as the first projection data and the second projection data may be simply referred to as “projection data”. In the following description, the first projection data and the second projection data are the number of photons per 1 [keV], that is, 0 to 1 [keV], with respect to the photon number spectrum at energies from 0 to 140 [keV]. A collection of 140 subject sinograms whose pixel values are the number of photons in each energy band (first energy band, second energy band) of 1 to 2 [keV], ..., 139 to 140 [keV]. It is assumed that

Specifically, the generation unit 342 calculates and generates second projection data in which distortion included in the first projection data is corrected from the first projection data and the detector response data according to the following equation (1). .

In Expression (1), p represents a pixel constituting the subject sinogram as the first projection data and the second projection data. y _p is a data indicating the first projection data to be the arithmetic processing by the generation unit 342, specifically, the pixel value of the pixel p of each object sinogram constituting the first projection data (number of photons ) Is a 140-dimensional vector. _xp is data indicating the second projection data generated by the generation unit 342, and specifically includes the pixel value (number of photons) of the pixel p of each subject sinogram constituting the second projection data. 140-dimensional vector. M represents an energy response in which X-rays incident on a specific channel (channel corresponding to the pixel p) of the detector 13 are detected in the specific channel, and the amount of energy detected in the specific channel This is a 140 × 140 matrix consisting of a spectrum of a portion detected by the specific channel due to escape, a spectrum detected by the specific channel due to scattering, and the like. The j-th column component of M represents a response spectrum to X-rays in the energy band of (j−1) to j [keV] among the energy of X-rays incident on a specific channel of the detector 13.

Further, x _i in equation (1) is a 140-dimensional vector composed of pixel values (number of photons) of the pixel i around the pixel p of each subject sinogram constituting the second projection data. C _i is detected in the specific channel of the X-ray input to the channel corresponding to the pixel i around the specific channel (channel corresponding to the pixel p) of the detector 13 by fluorescence, crosstalk, and scattering. This is data indicating a response to the spectrum of the amount of the data, and is a 140 × 140 matrix. The component in the j column of C _i is the energy band of (j−1) to j [keV] among the energy of the X-rays incident on the channel corresponding to the pixel i of the detector 13, and the specific channel ( The response spectrum when X-rays are detected in the channel corresponding to the pixel p) is shown. N represents a set of pixels in a predetermined range (range in the channel direction and slice direction) around the pixel p in the same view as the pixel p, and the pixel i described above is included in the set N.

In the following description, it is assumed that the matrix M and the matrix C _i are invariant at all pixels of the subject sinogram as the first projection data and the second projection data. However, the matrix M and the matrix C _i may be set for each pixel of the subject sinogram in accordance with a specific variation caused by a manufacturing error of the detector 13. For example, a particular variation in the detector 13 in advance by measuring the response characteristics for each channel may be assumed to set the matrix M and the matrix C _i.

Here, the detector response data is data indicating the magnitude of a physical phenomenon that contributes to an error included in a spectrum output as a response characteristic of the detector 13 to which X-rays have entered. For example, the detector response data is obtained from the surrounding channel by the probability of occurrence of escape occurring in the specific channel, fluorescence, crosstalk, scattering, etc. for each energy of X-rays incident on the specific channel of the detector 13. This is data based on at least one of information on the spectrum of X-ray energy detected in a specific channel, variation in detected energy, and the like. Specifically, the detector response data indicates the data of the matrix M and the matrix C _{i in} the above equation (1). That is, the detector response data indicates data for correcting the first projection data and generating the second projection data as described above.

  The generation unit 342 generates, as the second projection data, projection data in which the result of considering the change in the spectrum due to the detector response data is close to the first projection data, as shown in the above equation (1). become.

  In the above equation (1), the result of considering the change in spectrum due to the detector response data and the proximity of the first projection data are defined by the L2 norm of the vector, but the method of calculating the proximity is limited to this. Instead, any distance defined between the vectors can be used. For example, an Lp norm such as an L1 norm, an L2 norm, or an L∞ norm can be used. Further, when calculating the norm, a weight may be given to each component. A distance considering the correlation between vector components may be used. For example, the Mahalanobis distance can be used.

  The width of the energy band in the first projection data and the second projection data is not limited to 1 [keV] described above, the unit of energy is not limited to [keV], and the spectrum The energy range is not limited to 0 to 140 [keV]. In other words, energy bands having different widths may be used, and units of energy may be different units such as [J] or [cal]. The unit in the first projection data and the second projection data is not limited to the unit of energy. For example, the unit in the first projection data may be a digital signal value read from the detector 13, and the unit in the second projection data may be [keV].

The first projection data and the second projection data are 140 object sinograms each having a pixel value corresponding to the number of photons in the 140 energy bands with respect to the spectrum of the number of photons in the energy range of 0 to 140 [keV]. However, the first projection data and the second projection data are not limited to the same number of energy bands. For example, for the first projection data, a set of 28 subject sinograms having the pixel number as the number of photons in the energy band (first energy band) every 5 [keV] for energy from 0 to 140 [keV]. As described above, the second projection data may be a collection of 140 subject sinograms whose pixel values are the number of photons in the energy band (second energy band) for each [keV]. In this case, the difference between the number of energy bands for the first projection data and the number of energy bands for the second projection data may be such that the dimensions of the matrix M and the matrix C _i are 28 × 140. At this time, the components of the jth row of the matrix M and the matrix C _i are the components of the (5 × j + 1) th row to the (5 × j + 5) th row when each matrix is created with a dimension of 140 × 140, What is necessary is just to set it as the value added for every column.

In addition, when the generation unit 342 actually calculates the second projection data by the above-described formula (1), the generation unit 342 calculates by the iterative calculation as shown in the following formula (2).

First, the generation unit 342 arbitrarily sets an initial value of the second projection data in Expression (2). For example, the generation unit 342 may use the initial value of the second projection data as the first projection data. The generation unit _342, ^{x p (k + 1)} and a (k + 1) th second projection data after the iterative calculation, calculation _{x i} equations for the _x ^{p (k + 1)} of the next update (2) ^{Use as (k)} . Note that the number of iterations may be performed, for example, a predetermined number of times, and the difference between the second projection data calculated by Expression (2) and the second projection data calculated by Expression (2) immediately before is predetermined. It is also possible to stop the iterative calculation when the threshold value is below the threshold value.

The above-mentioned detector response data may change depending on the number of X-ray photons incident on the detector 13 per unit time. For example, when X-ray photons continuously enter the detector 13, the reaction of the detector 13 becomes insufficient, and it is detected as the number of X-ray photons having an energy lower than the actual appearance. Therefore, the generation unit 342 may generate the second projection data by switching the detector response data in accordance with the number of photons of the first projection data. In this case, for example, in the storage unit 341, the number of photons incident on the detector 13 per unit time is divided into 10 stages, and detector response data for each is stored. Then, the generation unit 342 calculates the number of photons for each pixel of the subject sinogram of the first projection data, reads the detector response data corresponding to the number of photons from the storage unit 341, and uses it. The detector response data can also be read by synthesizing detector response data close to the number of photons of the pixels of the subject sinogram of the first projection data. For example, it is assumed that M ₅₀₀ and C _{i, 500} that are detector response data for the photon number “500” and M ₁₀₀₀ and C _{i, 1000 that} are detector response data for the photon number “1000” are stored. At this time, if the number of photons in the subject sinogram of the first projection data is “700”, detection is performed according to the proximity of the number of photons “700” to the number of photons “500” and “1000”. The instrument response data is weighted averaged. That is, (3/5) M ₅₀₀ + (2/5) M ₁₀₀₀ and (3/5) C _{i, 500} + (2/5) C _{i, 1000} are calculated and read as detector response data. The storage unit 341 stores a coefficient for correcting the fluctuation of the detector response data due to the change in the number of photons, and adjusts the coefficient according to the number of photons in the subject sinogram of the first projection data. In addition, the detector response data can be read after correction. Note that the X-ray incident spectrum that passes through the subject 40 and enters the specific channel of the detector 13 is sufficiently close to the X-ray incident spectrum that is incident on the spatially and temporally adjacent channel of the specific channel. In this case, the values of the first projection data can be obtained by combining neighboring data. As an example of the synthesis, a weighted average can be used. Here, with respect to the pixel in the sinogram of the first projection data, the spatially neighboring channel is a channel that is neighboring in the channel direction and the slice direction, and the temporally neighboring channel is data of neighboring views, Alternatively, in the projection data acquired by rotating the detector two or more times, it is the data of the view in the vicinity of the view that is different in view by exactly one rotation. Thereby, even when the dose is low and the first projection data cannot be observed with high accuracy due to the influence of photon fluctuation, the second projection data can be generated with high accuracy.

The reconstruction unit 343 is a functional unit that reconstructs a subject sinogram in an energy band to be restored from the subject sinograms included in the second projection data generated by the generation unit 342 and generates a restored image. is there. The restored image is, for example, an image having a line attenuation coefficient as a pixel value. As a reconstruction method, for example, a filtered back projection method (FBP (Filtered Back Projection) method) which is an example of a back projection method can be used. Here, a case where the FBP method is adopted as a method for reconstructing the subject sinogram of the second projection data will be described. In this case, the reconstruction unit 343 does not arrange the subject 40, and from the portion corresponding to the energy band to be restored in the spectrum detected by the detector 13 where X-rays pass through only air. Assume that the reference data I ₀ is a generated air sinogram. Further, among the object sinograms included in the second projection data, the object sinogram in the energy band to be restored is set as restoration target data I. The restoration target data I is calculated by the sum of the number of photons of the second projection data regarding the energy belonging to the energy band to be restored.

First, the reconstruction unit 343 first calculates the integral value M of the linear attenuation coefficient from the restoration target data I and the reference data I _{0 according} to the following equation (3).

  In equation (3), (m, n) indicates the data of the mth channel and the nth view. The integral value M (m, n) shown in Expression (3) is a path through which the X-rays emitted from the X-ray tube 11 pass until reaching the m-th channel of the detector 13 in the n-th view. A value obtained by integrating the linear attenuation coefficient of the subject 40 along the line.

  Next, the reconstruction unit 343 performs a one-dimensional Fourier transform in the channel direction on the calculated integral value M (m, n). Next, the reconstruction unit 343 performs a one-dimensional inverse Fourier transform by filtering the value obtained by the one-dimensional Fourier transform in the frequency direction with a high-pass filter such as a ramp filter or a Shepp-Logan filter. . Then, the reconstruction unit 343 generates a restored image, which is a reconstructed reconstructed image, by back projecting and adding the data subjected to the one-dimensional inverse Fourier transform for every view.

  Note that the reconstruction unit 343 has shown a case where reconstruction is performed using the FBP method which is an example of the back projection method, but the reconstruction method is not limited to this, and various methods such as a successive approximation method may be used. A reconstruction method may be used. For example, the successive approximation method is a method in which a pseudo provisional image is prepared in advance, and the attenuation rate is calculated by irradiating the subject with X-rays in each view. When the attenuation rate calculated for the temporary image is smaller than the measured value (attenuation rate) actually detected by the detector 13, the pixel value of the temporary image is increased. Conversely, when the attenuation rate calculated for the temporary image is larger than the measured value actually detected by the detector 13, the pixel value of the temporary image is decreased. By repeating this operation, the attenuation rate calculated in the temporary image is changed to be equal to the measured value (attenuation rate) actually detected by the detector 13 to obtain a reconstructed image. The successive approximation method includes an ART (Algebric Reconstruction Technique) method, an OS-EM (Ordered Subset Expansion Maximization) method, and an ML-EM (Maximum Likelihood Exodation Method) method.

  FIG. 8 is a flowchart illustrating an example of the operation of the image processing unit according to the first embodiment. The image processing operation by the image processing unit 34 of this embodiment will be described with reference to FIG.

<Step S11>
The generation unit 342 of the image processing unit 34 receives and acquires a subject sinogram that is a sinogram of the subject 40 from the data collection unit 16 as first projection data. Then, the process proceeds to step S12.

<Step S12>
The generation unit 342 reads out and acquires the detector response data from the storage unit 341 of the image processing unit 34. Then, the process proceeds to step S13.

<Step S13>
The generation unit 342 generates second projection data in which distortion included in the first projection data is corrected from the acquired first projection data and detector response data using the above-described equation (1). In this case, the detector response data indicates the data of the matrix M and the matrix C _{i in} the above equation (1). As shown in the above formula (1), the generation unit 342 generates, as the second projection data, projection data in which the projection data on which the detector response data is applied is close to the first projection data. The generation unit 342 sends the generated second projection data to the reconstruction unit 343 of the image processing unit 34. Then, the process proceeds to step S14.

<Step S14>
The reconstruction unit 343 reconstructs a subject sinogram of an energy band to be restored among the subject sinograms included in the second projection data generated by the generation unit 342, and generates a restored image. Specifically, the reconstructing unit 343 firstly, from the subject sinogram included in the second projection data, the restoration target data I that is the subject sinogram in the energy band to be restored, and the reference data I _0. Then, the integral value M (m, n) of the linear attenuation coefficient is calculated by the above equation (3). Next, the reconstruction unit 343 performs a one-dimensional Fourier transform in the channel direction on the calculated integral value M (m, n). Next, the reconstruction unit 343 performs a one-dimensional inverse Fourier transform by filtering the value obtained by the one-dimensional Fourier transform in the frequency direction with a high-pass filter such as a ramp filter or a Shepp-Logan filter. . Then, the reconstruction unit 343 generates a restored image, which is a reconstructed reconstructed image, by back projecting and adding the data subjected to the one-dimensional inverse Fourier transform for every view.

  Image processing by the image processing unit 34 is executed by the operations in steps S11 to S14 described above.

  As described above, the probability of escape occurring in the specific channel for each X-ray energy incident on the specific channel of the detector 13, and the specific channel from the surrounding channel due to fluorescence, crosstalk, scattering, and the like. The generation unit 342 calculates second projection data in which distortion included in the first projection data is corrected, using the detector response data that is information about the spectrum of the X-ray energy detected in step S2. That is, the generation unit 342 generates, as the second projection data, projection data in which the projection data on which the detector response data is applied is close to the first projection data, as shown in the above formula (1). . Then, the reconstruction unit 343 reconstructs the subject sinogram of the energy band to be restored among the subject sinograms included in the second projection data generated by the generation unit 342, and generates a restored image. Thus, the pixel value of the subject sinogram for each predetermined energy band can be corrected so as to approach the theoretical value (first projection data) regardless of the composition of the subject, and the X-ray spectrum detected by the detector 13 can be obtained. Thus, it is possible to improve the accuracy of correcting the distortion of the image and improve the accuracy of the finally reconstructed restored image. For example, a restored spectrum when the spectrum is restored from each subject sinogram of the second projection data (hereinafter simply referred to as “reconstructed spectrum of the second projection data”) passes through the subject 40 and is detected by the detector. 13 can be made close to the shape of the spectrum of X-rays incident on (incident spectrum).

  The detector 13 detects the spectrum indicated by the number of photons for each energy for each channel (detection element) arranged in the circumferential direction of the rotating frame 12, but as described above, the detector 13 Detection elements are also arranged in the body axis direction of the subject 40. Therefore, a sinogram may be generated for each ring-shaped array of detection elements in the body axis direction (slice direction), and the above-described image processing may be executed. Alternatively, when the rotating frame 12 is continuously rotated while moving the top plate 22 as in the helical scan, not only the data detected by the same circumferential channel (detection element) is used, but the body axis direction. A sinogram may be generated by interpolation using data detected by a channel shifted in the (slice direction). In addition, as in a dual energy X-ray CT apparatus, the energy of X-rays emitted from the X-ray tube 11 is divided into two types and switched for every round (for example, the first round is 140 [keV], 2 The circumference may be 80 [keV]), and a spectrum of different energies may be synthesized to generate a sinogram.

  In addition, although the pixel value of the restored image generated by the reconstruction unit 343 is a line attenuation coefficient, the pixel value is not limited to this, and may be a value representing the amount of X-ray attenuation such as a CT value. Any of them is effective. Similarly, the pixel value of the sinogram may be a value indicating the amount of X-rays. For example, it may be a value representing the amount of X-rays themselves or the number of photons, or a value representing a change rate of the amount of X-rays or the number of photons.

<Modification 1>
The image processing unit 34a of the present modification will be described focusing on differences from the image processing unit 34 of the first embodiment. The image processing unit 34a of the present modification example is different from the storage unit 341 and the generation unit 342 included in the image processing unit 34 of the first embodiment illustrated in FIG. A portion 342a is provided.

  FIG. 9 is a diagram illustrating an example of a block configuration of an image processing unit according to Modification 1 of the first embodiment. FIG. 10 is a diagram illustrating an example of a restored spectrum in which the detected spectrum is corrected. The block configuration and operation of the image processing unit 34a of this modification will be described with reference to FIG.

  As illustrated in FIG. 9, the image processing unit 34a includes a storage unit 341a, a generation unit 342a, and a reconstruction unit 343.

  The storage unit 341a is a functional unit that stores filter coefficients for converting the first projection data into the second projection data, as will be described later.

The generation unit 342a receives a subject sinogram that is a sinogram of the subject 40 from the data collection unit 16 as first projection data, and a filter for converting the first projection data into second projection data from the storage unit 341a. It is a functional unit that reads out the coefficient and generates second projection data in the form of a sinogram based on the first projection data and the filter coefficient. Here, for example, the X-ray incident spectrum that passes through the subject 40 and enters the specific channel of the detector 13 is sufficiently close to the X-ray incident spectrum that is incident on a channel in the spatial vicinity of the specific channel. Suppose you are. In this case, the above formula (1) can be replaced by the following formula (4).

However, H is a 140 × 140 matrix expressed by the following equation (5).

The generation unit 342a corrects the distortion included in the first projection data from the first projection data and the data of the matrix H (corresponding to the detector response data described above) by the above equation (4). It is also possible to calculate data. However, here, in order to provide tolerance to noise or detection error of the detector 13, the regularization term λf (information indicating continuity) is added to the value shown in Expression (4) as shown in Expression (6) below. ) Shall be added.

In Expression (6), λ is a weight representing the strength of regularization, and f (x _p ) is a function representing the smoothness (continuity) of the spectrum in the energy direction of x _p . The function f (x) is represented by the following formula (7), for example.

Alternatively, the function f (x) may be expressed as the following formula (8).

  X (j) in Equations (7) and (8) is the j-th component (number of photons) of x. Both f (x) shown in Equations (7) and (8) are functions that increase in value as the difference between adjacent components (number of photons) increases.

  The above equation (6) can be solved by an iterative algorithm such as a gradient method or a subgradient method. Further, when the function f (x) is represented by the above-described equation (7), it can be solved analytically, and this case will be described below.

When the function f (x) is expressed by the above equation (7), the above equation (6) is equivalent to the following equation (9).

S in Equation (9) is a (140 + 139) × 140 matrix expressed by Equation (10) below.

In addition, y ′ _p in Expression (9) is a (140 + 139) -dimensional vector represented by Expression (11) below.

X _p which is a solution of the above-described equation (9) can be calculated by the following equation (12) using the pseudo inverse matrix S ⁺ of the matrix S. Further, the pseudo inverse matrix S ⁺ can be calculated using singular value decomposition of the matrix S.

Here, the filter coefficient stored in the storage unit 341a described above is a value of a component of the pseudo inverse matrix S ⁺ . The generation unit 342a generates second projection data in which the distortion included in the first projection data is corrected by convolving the filter coefficient read from the storage unit 341a with the first projection data by using the equation (12). To do. That is, the filter coefficient stored in the storage unit 341a is data for correcting the first projection data and generating the second projection data, and can be regarded as the above-described detector response data.

  The reconstruction unit 343 is a functional unit that reconstructs a subject sinogram in an energy band to be restored from the subject sinograms included in the second projection data generated by the generation unit 342a to generate a restored image. is there. The reconstruction method by the reconstruction unit 343 is the same as that in the first embodiment.

  Here, it is assumed that an X-ray indicated by an incident spectrum 2101 shown in FIG. 10 is incident on a specific channel of the detector 13 through the subject 40. In this case, the spectrum detected by the detector 13 becomes a detection spectrum 2111 distorted as compared with the incident spectrum 2101 due to escape, fluorescence, crosstalk, scattering, and the like, as described above. Then, the data collection unit 16 performs an amplification process or an A / D conversion process on each of the spectrum data collected from the detector 13 (the detected spectrum 2111 is an example thereof) to obtain a subject sinogram for each energy band. Generated and sent to the generating unit 342a as the first projection data. Further, the spectrum when the spectrum is restored from each object sinogram of the second projection data calculated by using the equation (12) in which the regularization term of the above equation (6) is added by the generation unit 342a is obtained. This is a restored spectrum 2121 shown in FIG. As described above, the second projection data is calculated by the generation unit 342a using the expression (12) in which the regularization term is added, so that the restored spectrum 2121 has an incident spectrum 2101 compared with the detected spectrum 2111. Can be approximated to the shape approximated to. Moreover, since the production | generation part 342a can produce | generate 2nd projection data using the simplified Formula (12), it can reduce a calculation load.

The storage unit 341a stores the values of the components of the pseudo matrix S ⁺ shown in the above equation (12) as coefficients, and the generation unit 342a uses the equation (12) to calculate the first projection data. The second projection data is generated by convolving the filter coefficient read from the storage unit 341a. However, the present invention is not limited to this. For example, the storage unit 341a may store the matrix H and the weight λ as detector response data, instead of storing the coefficients that are the values of the components of the pseudo matrix S ⁺ . In this case, the generation unit 342a may calculate and generate the second projection data according to the equations (6) to (12) using the detector response data stored in the storage unit 341a.

<Modification 2>
The operation of the reconstruction unit 343 according to the present modification will be described focusing on differences from the reconstruction unit 343 according to the first embodiment. The reconstruction unit 343 of the present modification estimates the density of the specific material based on the linear attenuation coefficient that is the pixel of the reconstructed restored image and the mass attenuation coefficient of the specific material. The details of the substance density estimation operation will be described below.

  The reconstruction unit 343 of the image processing unit 34 according to the present modification does not reconstruct the subject sinogram of one target energy band from the second projection data as in the first embodiment described above. The object sinogram of a plurality of energy bands is reconstructed to generate a plurality of restored images whose pixel values are linear attenuation coefficients. Here, for example, assuming that water and iodine are assumed as the substances contained in the subject 40, the density of each substance is estimated.

First, the reconstruction unit 343 includes a specific energy band (for example, 35 to 50 [keV]) and an energy band different from a specific energy band (for example, 55 to 55 [keV]) in the subject sinogram included in the second projection data. 70 [keV]) (second energy band) of the subject sinogram is reconstructed to generate two restored images whose pixel values are linear attenuation coefficients. And the reconstruction part 343 calculates a material density for every pixel of a decompression | restoration image from the simultaneous equation shown to the following formula | equation (13).

(S, t) in Expression (13) indicates the coordinates of the restored image. [rho] _w and [rho] _I are the material densities of water and iodine, respectively, and are values to be calculated by the equation (13). v _{w, 1} and v _{I, 1} are respectively a mass attenuation coefficient of water and a mass attenuation coefficient of iodine in a specific energy band, and are known values. v _{w, 2} and v _{I, 2} are the mass attenuation coefficient of water and the mass attenuation coefficient of iodine, which are different from the specific energy band, respectively, and are known values. μ ₁ is a pixel value that is a linear attenuation coefficient of the restored image reconstructed by the reconstruction unit 343 from the subject sinogram of a specific energy band. Μ ₂ is a pixel value that is a linear attenuation coefficient of the restored image reconstructed by the reconstruction unit 343 from the subject sinogram of an energy band different from the specific energy band. As described above, since v _{w, 1} , v _{I, 1} , v _{w, 2} and v _{I, 2} are known values, and μ ₁ and μ ₂ are obtained values, the equation (13) is a simultaneous equations for ρ _w and ρ _I. Reconstruction unit 343, by solving equation (13) is a simultaneous equation for [rho _w and [rho _I, pixel (coordinates (s, t)) of the restored images for each, to calculate the [rho _w and [rho _I Can do. The reconstruction unit 343 can generate a density image of water or iodine by replacing the pixel value of each pixel of the restored image with the substance density ρ _w or ρ _I corresponding to the pixel.

  As described above, since the reconstruction unit 343 generates a restored image composed of linear attenuation coefficients with improved accuracy, a density image indicating an accurate density for a specific substance of the subject 40 is generated from the restored image. Can be generated.

  In addition, although the example which calculates | requires the density of water and iodine was shown above, the substance which calculates | requires a density is not limited to these, For example, the density | concentration image of arbitrary substances, such as bone or gadolinium, can be produced | generated. it can. Moreover, although the example which calculates | requires the density of two types of substances of water and iodine was shown in the above-mentioned example, it is not limited to this, It is also possible to obtain | require the density of three or more types of substances. In this case, a restored image whose pixel value is a linear attenuation coefficient may be generated from subject sinograms of energy bands of the same number or more as the number of types of substances whose density is to be calculated.

  In addition to estimating the material density after generating the reconstructed image as described above, the material density can be set as the pixel value by performing the reconstruction process after estimating the material density for each pixel of the sinogram. It is also possible to generate a cross-sectional image having the same.

As an example, a method for calculating a material density image of water and iodine will be described. First, among subject sinograms included in the second projection data, a specific energy band (for example, 35 to 50 [keV]) (hereinafter referred to as energy band A) and an energy band different from the specific energy band ( For example, for 55 to 70 [keV]) (hereinafter referred to as energy band B), the sum of the values of the subject sinogram of the energy belonging to each energy band is calculated (respectively, I ₁ and I ₂ ). Similarly, the sum of the values of the air sinogram corresponding to energy band A and energy band B is also calculated (respectively, I _0,1 and I _0,2 ).

Next, the integral values M1 and M2 of the linear attenuation coefficient are calculated for each energy band by the following equation (14).

Then, the transmission distance of the substance is calculated for each pixel of the sinogram from the simultaneous equations shown in the following formula (15).

Here, l _w and l _I are the distances of water and iodine on the X-ray transmission path related to the pixel (m, n) of the sinogram, respectively, and are values to be calculated. Further, z _{w, 1} and z _{I, 1} are the linear attenuation coefficients of water and iodine in the energy band A, respectively, and z _{w, 2} and z _{I, 2} are the linear attenuation of water and iodine in the energy band B, respectively. It is a coefficient. Next, after reconstructing l _w and l _I by the FBP method or the like, a cross-sectional image showing the density of water and a cross-sectional image showing the density of iodine are obtained by multiplying the density of water and iodine, respectively. be able to. In the example described above, water and iodine are examples of substances, but the substance is not limited to these. It is also possible to calculate three or more material densities. In this case, it is only necessary to create more energy bands than the number of substances, and make the number of simultaneous equations more than the number of substances.

(Second Embodiment)
The image processing unit 34b of the present embodiment will be described focusing on differences from the image processing unit 34a of the first modification of the first embodiment. The image processing unit 34b of the present embodiment uses the pixel value (number of photons) in the adjacent energy band for the function f (x) represented by the equation (7) or the like in the regularization term λf in the above equation (6). Calculate by adding weight to the difference. Hereinafter, the details of the calculation operation in consideration of the weight will be described.

  FIG. 11 is a diagram illustrating an example of a block configuration of an image processing unit according to the second embodiment. FIG. 12 is a diagram showing an example of the emission spectrum and the subject spectrum emitted from the X-ray tube. The block configuration of the image processing unit 34b of the present embodiment will be described with reference to FIG.

  As illustrated in FIG. 11, the image processing unit 34b includes a storage unit 341b, a generation unit 342b, a reconstruction unit 343, and a calculation unit 344.

  The storage unit 341b is a functional unit that stores the matrix H and the weight λ shown in the above equation (6) as detector response data.

  The generation unit 342b receives from the data collection unit 16 a subject sinogram that is a sinogram of the subject 40 as first projection data, reads out detector response data from the storage unit 341b, and outputs first projection data and detector response data. Is a functional unit that generates second projection data in the form of a sinogram based on. Similarly to the first modification of the first embodiment, the X-ray incident spectrum that passes through the subject 40 and enters a specific channel of the detector 13 is incident on a channel in the vicinity of the specific channel. Suppose that it is sufficiently close to the incident spectrum of the line.

The above formula (6) is obtained by adding the regularization term λf to the above formula (4). In the present embodiment, the function f (x) of the regularization term λf is expressed by the following formula ( As shown in 16), the calculation is performed by multiplying the first weight w _j for each difference in pixel values (number of photons) in adjacent energy bands.

Alternatively, the function f (x) may be expressed as the following formula (17).

Further, when the function f (x) is expressed by the above equation (16), the above equation (6) is equivalent to the following equation (18).

However, W in Expression (18) is a matrix of (140 + 139) × (140 + 139) represented by Expression (19) below.

X _p which is the solution of the above equation (18) can be calculated by the following equation (20) using a pseudo inverse matrix (WS) ⁺ of the matrix WS which is a matrix of (140 + 139) × 140. The pseudo inverse matrix (WS) ⁺ can be calculated using singular value decomposition of the matrix S.

The generation unit 342b uses the first weight w _j calculated by the calculation unit 344, which will be described later, and the detector response data (matrix H, weight λ) read from the storage unit 341b, to formulas (6), (16), And (18) to (20), the second projection data in which the distortion included in the first projection data is corrected is calculated and generated.

  The reconstruction unit 343 is a functional unit that reconstructs a subject sinogram in an energy band to be restored from the subject sinograms included in the second projection data generated by the generation unit 342b to generate a restored image. is there. The reconstruction method by the reconstruction unit 343 is the same as that in the first embodiment.

The calculation unit 344 is a functional unit that calculates the first weight w _j of the above equation (16) in order for the generation unit 342b to calculate the second projection data. The calculation unit 344 includes, for example, information on an emission spectrum that is an X-ray spectrum emitted from the X-ray tube 11, a target member of the X-ray tube 11, an angle of the target of the X-ray tube 11, and the X-ray tube 11. Based on the information of at least one of the filter, the tube voltage of the X-ray tube 11 at the time of imaging, the contrast agent used, and the composition of the subject 40 assumed, the restored spectrum of the second projection data is in the energy direction. The first weight w _j is calculated by obtaining the degree assumed to be sharply converted to. The calculation unit 344 decreases the first weight w _j in the energy that is assumed to change sharply in the energy direction in the restored spectrum of the second projection data, and increases the energy in the energy that is assumed to be smooth in the energy direction. Calculate as follows. Note that the calculation unit 344 outputs information on the emission spectrum that is the spectrum of the X-ray emitted from the X-ray tube 11 described above, the target member of the X-ray tube 11, the angle of the target of the X-ray tube 11, and the X-ray tube 11. For example, information such as a filter, a tube voltage of the X-ray tube 11 at the time of imaging, a used contrast agent, and an assumed composition of the subject 40 may be acquired from the system control unit 36 or the like. .

Hereinafter, a specific example of a method of calculating the first weight w _j by the calculation unit 344 will be described with reference to FIG. For example, as illustrated in FIG. 12A, the calculation unit 344 acquires information on an emission spectrum 2200 that is an X-ray spectrum emitted from the X-ray tube 11 from the system control unit 36 or the like. The calculation unit 344 detects from the shape of the emission spectrum 2200 that the energy E1 and E2 changes sharply in the energy direction, and the first weight w _j is reduced by the energy around the energy E1 and E2. To calculate. For example, the calculation unit 344 sets the first weight w _j to “1” in the range of energy E1 ± 3 [keV] and E2 ± 3 [keV], and sets the first weight w _j to “10” for other energies. And This is because the steep change in the energy direction included in the emission spectrum 2200 is based on the fact that the spectrum changes steeply in the energy direction at the same energy even after transmission through the subject 40.

  The X-ray emission spectrum emitted from the X-ray tube 11 does not directly acquire the shape information, but the target member of the X-ray tube 11, the target angle of the X-ray tube 11, the X-ray tube The shape of the emission spectrum can be calculated from the filter included in 11 and the tube voltage information of the X-ray tube 11 at the time of imaging, and the calculation result can also be used. Even without calculating the shape of the emission spectrum, the target member of the X-ray tube 11, the angle of the target of the X-ray tube 11, the filter provided in the X-ray tube 11, and the tube voltage of the X-ray tube 11 at the time of imaging It is also possible to obtain information on energy at which the emission spectrum of X-rays emitted from the X-ray tube 11 changes sharply in the energy direction from at least one of the above information.

If the subject 40 contains a contrast agent such as iodine or cadolinium, the subject spectrum (X-ray spectrum attenuated by the passage of the subject 40 with the energy of the K absorption edge of each contrast agent) ( In the sense of being incident on the detector 13, it can be said to be an incident spectrum) sharply changes in the energy direction. FIG. 12B shows a subject spectrum 2201 as an example of a spectrum when the subject 40 containing iodine as a contrast agent is attenuated by X-rays. The subject spectrum 2201 shown in FIG. 12B has a sharp change in energy E3. Therefore, the calculation unit 344 detects from the shape of the subject spectrum 2201 that the energy E3 is abruptly changing in the energy direction, and the first weight w _j is reduced by the energy around the energy E3. calculate. For example, the calculation unit 344 sets the first weight w _j at the energy E3 ± 3 [keV] to “1” in addition to the ranges of the energy E1 ± 3 [keV] and E2 ± 3 [keV], and other energy. Then, the first weight w _{j is set} to “10”. Here, the energy at the K absorption edge is energy specific to each contrast agent, and is 33 [keV] for iodine and 50 [keV] for gadolinium.

In addition, the calculation of the first weight w _j by the calculation unit 344 based on the information on the contrast agent is preferably performed only when the subject 40 includes the contrast agent, but when the presence or absence of the contrast agent is unclear The first weight w _j may be calculated in correspondence with a contrast agent that may be used. The calculation unit 344 can also receive information about the contrast agent to be used from the system control unit 36 or the like. Further, the user may input information about the contrast medium to be used via the input device 31. Furthermore, the user may be allowed to directly input an energy value that is assumed that the second projection data changes sharply in the energy direction via the input device 31.

As described above, the generation unit 342b generates the second projection data by using the first weight w _j calculated by the calculation unit 344. That is, the generating unit 342b is to act to increase the smoothness of the energy direction in the first weight w _j is greater energy, so as to mitigate the effect of improving the smoothness of the energy direction in the energy first weight w _j is small The projection data on which the detector response data is applied generates projection data that is close to the first projection data as the second projection data.

Here, it is assumed that the X-ray indicated by the incident spectrum 2101 shown in FIG. 10 is incident on a specific channel of the detector 13 through the subject 40. In this case, the spectrum detected by the detector 13 becomes a detection spectrum 2111 distorted as compared with the incident spectrum 2101 due to escape, fluorescence, crosstalk, scattering, and the like, as described above. Then, the data collection unit 16 performs an amplification process or an A / D conversion process on each of the spectrum data collected from the detector 13 (the detected spectrum 2111 is an example thereof) to obtain a subject sinogram for each energy band. Generated and sent to the generating unit 342b as the first projection data. Further, the generation unit 342b calculates a spectrum from each subject sinogram of the calculated second projection data using the regularization term of the above-described formula (6) and the formula (20) in which the first weight w _j is added. The spectrum when restored is the restored spectrum 2122 shown in FIG. In this way, the second projection data is calculated by the generation unit 342a using the expression (20) in which the regularization term and the first weight w _j are added, so that the restored spectrum 2122 is compared with the detected spectrum 2111. Thus, a portion that changes sharply in the energy direction in the incident spectrum 2101 can also be approximated to an approximate shape.

The generation unit 342b may further generate the second projection data by weighting the spectrum error for each energy band. If the weight weighted for each energy band with respect to this spectrum error is the second weight u _j , the following equation (21) is expressed instead of the matrix W shown in the above equation (17). .

That is, the generation unit 342b uses the above-described second weight u _j , the first weight w _j calculated by the calculation unit 344, and the detector response data (matrix H, weight λ) read from the storage unit 341b. The second projection data may be calculated and generated by (6), (16), (17), (19) and (21). It is desirable that the second weight u _j be small for the first projection data corresponding to an energy band with low reliability. For example, the low energy spectrum is affected by the noise of the detector 13 and can be said to have low reliability. Therefore, for example, in the smaller energy band than a predetermined energy, the second weight u _j is "1", the large energy band, a second weight u _j may be assumed to be "10". In addition, it can be determined that the energy band having a small number of photons in the first projection data has low reliability.

The second weight u _j may be calculated individually for each pixel of the subject sinogram. For example, in a subject sinogram pixel in which attenuation of X-rays by the subject 40 is small, the influence on the noise of the detector 13 is small, so the reliability of the pixel value (number of photons) is high even at a small energy. The second weight u _j is made uniform for all energies. On the other hand, in a subject sinogram pixel in which attenuation of X-rays by the subject 40 is large, the subject is such that the second weight u _j is set to be small with small energy in consideration of the influence of noise of the detector 13. The second weight u _j can be adjusted for each pixel of the sinogram.

  FIG. 13 is a flowchart illustrating an example of the operation of the image processing unit according to the second embodiment. The image processing operation by the image processing unit 34b of the present embodiment will be described with reference to FIG.

<Step S21>
The generation unit 342b of the image processing unit 34b receives and acquires a subject sinogram, which is a sinogram of the subject 40, from the data collection unit 16 as first projection data. Then, the process proceeds to step S22.

<Step S22>
The generation unit 342b reads and acquires the detector response data from the storage unit 341b of the image processing unit 34b. Then, the process proceeds to step S23.

<Step S23>
The calculating unit 344 calculates the first weight w _j of the above equation (16) that is used by the generating unit 342b to calculate the second projection data. First, the calculation unit 344, for example, information on the emission spectrum, which is the spectrum of the X-ray emitted from the X-ray tube 11, the target member of the X-ray tube 11, the angle of the target of the X-ray tube 11, the X-ray tube 11 The information of at least one of the filter included in the X-ray tube 11, the tube voltage of the X-ray tube 11 at the time of imaging, the contrast agent used, and the assumed composition of the subject 40 is acquired from the system control unit 36 or the like. Then, the calculation unit 344 calculates the first weight w _j by obtaining the degree that the restoration spectrum of the second projection data is assumed to be sharply converted in the energy direction based on the acquired information. That is, the calculation unit 344 reduces the first weight w _j with the energy assumed to change sharply in the energy direction in the restored spectrum of the second projection data, and with the energy assumed to be smooth in the energy direction. Calculate to increase. The calculation unit 344 sends the calculated first weight w _j to the generation unit 342b. Then, the process proceeds to step S24.

<Step S24>
The generation unit 342b converts the first weight w _j received from the calculation unit 344 and the detector response data into the first projection data using Expressions (6), (16), and (18) to (20). Second projection data corrected for the included distortion is calculated and generated. In this case, the detector response data indicates the data of the matrix H and the weight λ in the above equation (6). The generation unit 342b generates, as second projection data, projection data in which the projection data on which the detector response data is applied becomes close to the first projection data. The generation unit 342b sends the generated second projection data to the reconstruction unit 343 of the image processing unit 34. Then, the process proceeds to step S25.

<Step S25>
The reconstruction unit 343 reconstructs a subject sinogram in an energy band to be restored among the subject sinograms included in the second projection data generated by the generation unit 342b, and generates a restored image.

  Image processing by the image processing unit 34b is executed by the operations in steps S21 to S25 described above.

As described above, the second projection data is calculated by the generation unit 342a using the expression (20) in which the regularization term and the first weight w _j are added, so that the restored spectrum of the second projection data is Compared with the detection spectrum detected by the detector 13, the portion of the incident spectrum (subject spectrum) that is steeply changed in the energy direction can also be approximated to an approximate shape.

<Modification>
The image processing unit 34c of the present modification will be described focusing on differences from the image processing unit 34b of the second embodiment. The image processing unit 34c of the present modification will be described focusing on the operation in which the calculation unit calculates the first weight w _j using the material density calculated by the reconstruction unit.

  FIG. 14 is a diagram illustrating an example of a block configuration of an image processing unit according to a modification of the second embodiment. The block configuration and operation of the image processing unit 34c of the present modification will be described with reference to FIG.

  As illustrated in FIG. 14, the image processing unit 34c includes a storage unit 341c, a generation unit 342c, a reconstruction unit 343c, and a calculation unit 344c.

  The storage unit 341c is a functional unit that stores the matrix H and the weight λ shown in the above equation (6) as detector response data.

The generation unit 342c receives a subject sinogram, which is a sinogram of the subject 40, from the data collection unit 16 as first projection data, reads out detector response data from the storage unit 341c, and outputs first projection data and detector response data. Is a functional unit that generates second projection data in the form of a sinogram based on. The generating unit 342c uses the first weight w _j calculated by the calculating unit 344c, which will be described later, and the detector response data (matrix H, weight λ) read from the storage unit 341c. ) And (18) to (20), the second projection data in which the distortion included in the first projection data is corrected is calculated and generated.

The calculation unit 344c is a functional unit that calculates the first weight w _j of the above equation (16) so that the generation unit 342c calculates the second projection data. The calculation unit 344c includes, for example, information on an emission spectrum that is an X-ray spectrum emitted from the X-ray tube 11, a target member of the X-ray tube 11, an angle of the target of the X-ray tube 11, and the X-ray tube 11. Based on the information of at least one of the filter, the tube voltage of the X-ray tube 11 at the time of imaging, the contrast agent used, and the composition of the subject 40 assumed, the restored spectrum of the second projection data is in the energy direction. The first weight w _j is calculated by obtaining the degree assumed to be sharply converted to. Furthermore, the calculation unit 344c is assumed to rapidly convert the restoration spectrum of the second projection data in the energy direction based on the substance density of the substance included in the subject 40 calculated by the reconstruction unit 343c described later. The first weight w _j is calculated by obtaining the degree.

  The reconstruction unit 343c is a functional unit that reconstructs a subject sinogram in an energy band to be restored from the subject sinograms included in the second projection data generated by the generation unit 342c and generates a restored image. is there. The reconfiguration method by the reconfiguration unit 343c is the same as in the first embodiment.

  The reconstruction unit 343c first identifies a substance that is expected to be included in the subject 40, and extracts from the second projection data subject sinograms of energy bands of the same number or more as the number of types of the substance. Then, the extracted object sinogram is reconstructed to generate a plurality of restored images whose pixel values are linear attenuation coefficients. Next, the reconstruction unit 343c calculates the specified substance density for each pixel of the generated plurality of restored images by the same method as that of the second modification of the first embodiment. Then, the reconstruction unit 343c sends the calculated material density information to the calculation unit 344c.

For example, when the calculation unit 344c determines that iodine is included in the subject 40 based on the material density received from the reconstruction unit 343c, the first weight w _j is reduced with energy around 33 [keV]. Calculate as follows. Then, as described above, the generation unit 342c generates the second projection data using the first weight w _j calculated by the calculation unit 344c, and the reconstruction unit 343c is newly generated by the generation unit 342c. Of the subject sinograms included in the second projection data, a subject sinogram in the energy band to be restored is reconstructed to generate a restored image.

Note that the calculation unit 344c may calculate the first weight w _j according to the level of the material density received from the reconstruction unit 343c. For example, since the change in the energy direction of the restoration spectrum of the second projection data increases as the material density increases, the first weight w _j may be reduced.

The first weight w _j may be calculated individually for each pixel of the subject sinogram. For example, the calculation unit 344c receives the material density from the reconstruction unit 343c in the form of a density image, specifies a pixel including iodine in the density image, and includes the first projection data of the first projection data including the pixel in the X-ray transmission path. For only the pixel, the first weight w _j may be calculated to be small around the energy at the K absorption edge of iodine.

As described above, the calculation unit 344c uses the first weight based on the material density calculated by the reconstruction unit 343c as well as information on the emission spectrum that is the spectrum of the X-rays emitted from the X-ray tube 11. Calculate w _j . Therefore, compared with the detection spectrum detected by the detector 13, the restored spectrum of the second projection data has a shape that more closely approximates the portion of the incident spectrum (subject spectrum) that changes sharply in the energy direction. You can get closer.

(Third embodiment)
The image processing unit of the present embodiment will be described focusing on differences from the image processing unit 34b of the second embodiment. The generation unit 342d of the image processing unit according to the present embodiment generates projection data for two spectra having different smoothness in the energy direction of the spectrum, and synthesizes them. Hereinafter, details of the operation of generating and synthesizing projection data for these two spectra will be described.

  FIG. 15 is a diagram illustrating an example of a block configuration of a generation unit of the image processing unit according to the third embodiment. A block configuration of the generation unit 342d of the image processing unit of the present embodiment will be described with reference to FIG. Note that the block configuration of the image processing unit of the present embodiment is the same as the block configuration of the image processing unit 34b of the second embodiment.

  As illustrated in FIG. 15, the generation unit 342d includes a first auxiliary generation unit 3421, a second auxiliary generation unit 3422, and a synthesis unit 3423.

The first auxiliary generation unit 3421 uses the first weight w _j calculated by the calculation unit 344 and the detector response data (matrix H, weight λ) read out from the storage unit 341b, to formulas (6) and (16). And (18) to (20) are functional units that calculate and generate third projection data in which distortion included in the first projection data is corrected.

The second auxiliary generation unit 3422 uses the first weight w _j calculated by the calculation unit 344 and the detector response data (matrix H, weight λ) read out from the storage unit 341b, to formulas (6) and (16). And (18) to (20) are functional units that calculate and generate fourth projection data in which distortion included in the first projection data is corrected. At this time, the second auxiliary generation unit 3422 has a smoother energy direction in the restored spectrum (fourth spectrum) for the fourth projection data than in the restored spectrum (third spectrum) for the third projection data. Generate to increase. That is, the second auxiliary generation unit 3422 uses the first weight used in Expression (16) (indicated by w _{j in} Expression (16)), and the first auxiliary generation unit 3421 similarly uses the first weight used in Expression (16). Is also increased to generate fourth projection data. As an example, there is a method in which a value obtained by multiplying the first weight used in the first auxiliary generation unit 3421 by a predetermined value of 1 or more is used as the first weight used in the second auxiliary generation unit 3422. As another example, there is a method of setting a predetermined fixed value so that the first weight used in the second auxiliary generation unit 3422 is larger than the first weight used in the first auxiliary generation unit 3421.

The combining unit 3423 is a functional unit that combines the third projection data generated by the first auxiliary generation unit 3421 and the fourth projection data generated by the second auxiliary generation unit 3422 to generate second projection data. is there. The synthesizer 3423 gives higher weight to the third projection data as the first weight w _j received from the calculator 344 is smaller. Specifically, the synthesis unit 3423 generates the second projection data using the following formula (22).

E in the formula (22) represents an index indicating the value of the energy band. x _p is a data indicating the second projection data generated by the synthesizing unit 3423, specifically, constituted by pixel values of the pixel p of each object sinogram constituting the second projection data (number of photons) Is a vector. _{xp, 3} is data indicating the third projection data generated by the first auxiliary generation unit 3421. Specifically, the pixel value (x) of the pixel p of each subject sinogram constituting the third projection data ( (A number of photons). _{xp, 4} is data indicating the fourth projection data generated by the second auxiliary generation unit 3422. Specifically, the pixel value (x) of the pixel p of each subject sinogram constituting the fourth projection data ( (A number of photons). g is a weight having a value in the range of 0 to 1, and is set to a smaller value as the first weight w _j is smaller. For example, a value obtained by dividing the first weight w _j by a predetermined constant may be a weight g.

  FIG. 16 is a flowchart illustrating an example of the operation of the image processing unit according to the third embodiment. The image processing operation of the image processing unit of this embodiment will be described with reference to FIG.

<Step S31>
The generation unit 342d of the image processing unit receives and acquires a subject sinogram, which is a sinogram of the subject 40, from the data collection unit 16 as first projection data. Then, the process proceeds to step S32.

<Step S32>
The generation unit 342d reads and obtains detector response data from the storage unit 341b of the image processing unit. Then, the process proceeds to step S33.

<Step S33>
The calculating unit 344 calculates the first weight w _j of the above equation (16) that is used by the generating unit 342b to calculate the second projection data. First, the calculation unit 344, for example, information on the emission spectrum, which is the spectrum of the X-ray emitted from the X-ray tube 11, the target member of the X-ray tube 11, the angle of the target of the X-ray tube 11, the X-ray tube 11 The information of at least one of the filter included in the X-ray tube 11, the tube voltage of the X-ray tube 11 at the time of imaging, the contrast agent used, and the assumed composition of the subject 40 is acquired from the system control unit 36 or the like. Then, the calculation unit 344 calculates the first weight w _j by obtaining the degree that the restoration spectrum of the second projection data is assumed to be sharply converted in the energy direction based on the acquired information. The calculation unit 344 sends the calculated first weight w _j to the generation unit 342d.

<Step S34>
The first auxiliary generation unit 3421 uses the first weight w _j calculated by the calculation unit 344 and the detector response data (matrix H, weight λ) read out from the storage unit 341b, to formulas (6) and (16). And (18) to (20), the third projection data in which the distortion included in the first projection data is corrected is calculated and generated. The first auxiliary generation unit 3421 sends the generated third projection data to the synthesis unit 3423. Then, the process proceeds to step S35.

<Step S35>
The second auxiliary generation unit 3422 uses the first weight w _j calculated by the calculation unit 344 and the detector response data (matrix H, weight λ) read out from the storage unit 341b, to formulas (6) and (16). And (18) to (20) are functional units that calculate and generate fourth projection data in which distortion included in the first projection data is corrected. At this time, the second auxiliary generation unit 3422 generates the restoration spectrum for the fourth projection data so that the energy direction is smoother than the restoration spectrum for the third projection data. That is, the second auxiliary generation unit 3422 uses the first weight used in Expression (16) (indicated by w _{j in} Expression (16)), and the first auxiliary generation unit 3421 similarly uses the first weight used in Expression (16). Is also increased to generate fourth projection data. The second auxiliary generation unit 3422 sends the generated fourth projection data to the synthesis unit 3423. Then, the process proceeds to step S36.

<Step S36>
The combining unit 3423 is a functional unit that combines the third projection data generated by the first auxiliary generation unit 3421 and the fourth projection data generated by the second auxiliary generation unit 3422 to generate second projection data. is there. The synthesizer 3423 generates the second projection data by using the above-described equation (22) so that the smaller the first weight w _j received from the calculator 344 is, the greater the weight is given to the third projection data. .

  Image processing by the image processing unit of the present embodiment is executed by the operations in steps S31 to S36 described above.

  As described above, the generation unit 342d combines the third projection data in which the smoothness in the energy direction of the restored spectrum is suppressed and the fourth projection data in which the smoothness in the energy direction of the restored spectrum is enhanced, Projection data is generated. As a result, the restored spectrum of the second projection data has a shape that approximates the portion that changes sharply in the energy direction in the incident spectrum (subject spectrum) as compared with the detection spectrum detected by the detector 13. You can get closer.

  An image processing apparatus (console apparatus 30) according to the above-described embodiment and its modification includes a microprocessor such as a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an HDD ( It has an external storage device such as Hard Disk Drive), a display device such as a display, and an input device such as a keyboard or a mouse, and has a hardware configuration using a normal computer.

  A program executed by the image processing apparatus (console apparatus 30) according to the above-described embodiment and its modifications is a file in an installable format or an executable format, and is a CD-ROM (Compact Disk Read Only Memory), a flexible disk. (FD), CD-R (Compact Disk Recordable), DVD (Digital Versatile Disk) and the like may be recorded on a computer-readable recording medium and provided as a computer program product.

  In addition, the program executed by the image processing apparatus (console apparatus 30) according to the above-described embodiment and its modification is stored on a computer connected to a network such as the Internet and downloaded via the network. It may be configured to provide. The program executed by the image processing apparatus (console apparatus 30) according to the above-described embodiment and its modification may be provided or distributed via a network such as the Internet.

  The program executed by the image processing apparatus (console apparatus 30) according to the above-described embodiment and its modification may be provided by being incorporated in advance in a ROM or the like.

  The program executed by the image processing apparatus (console device 30) according to the above-described embodiment and the modification thereof is the computer of each unit (generation units 342, 342a to 342d, reconfiguration units 343, 343c, The calculation units 344 and 344c, the first auxiliary generation unit 3421, the second auxiliary generation unit 3422, and the synthesis unit 3423) may function. In this computer, the CPU can read a program from a computer-readable storage medium onto a main storage device and execute the program. Note that some or all of the units of the above-described image processing apparatus may be realized by a hardware circuit instead of a program that is software.

  Although several embodiments of the present invention and modifications thereof have been described, these embodiments and modifications thereof are presented as examples, and are not intended to limit the scope of the invention. These novel embodiments and modifications thereof 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 modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

DESCRIPTION OF SYMBOLS 1 X-ray inspection apparatus 10 Base apparatus 11 X-ray tube 11a, 11b X-ray beam 12 Rotating frame 13 Detector 14 Irradiation control part 15 Base drive part 16 Data collection part 20 Bed apparatus 21 Bed drive apparatus 22 Top plate 30 Console apparatus 31 Input device 32 Display device 33 Scan control unit 34, 34a to 34c Image processing unit 35 Image storage unit 36 System control unit 40 Subject 41 Projected cross section 50 Scintillator 50a Reflector plate 51 Adhesive layer 52 SiPM
341, 341a to 341c storage unit 342, 342a to 342d generation unit 343, 343c reconstruction unit 344, 344c calculation unit 1001 sinogram 1011, 1011a to 1011d subject sinogram 2001 incident spectrum 2011 detection spectrum 2012-2014 spectrum 2101 incident spectrum 2111 detection Spectrum 2121, 2122 Restoration spectrum 2200 Output spectrum 2201 Subject spectrum 3421 First auxiliary generation unit 3422 Second auxiliary generation unit 3423 Synthesis unit

Claims (21)

An X-ray CT apparatus that detects energy of radiation transmitted through a subject by a detector and performs image processing on data based on the energy,
First projection data based on a first spectrum indicating an amount of X-rays for each energy of the radiation detected by the detector is corrected by response information based on response characteristics of the detector, and second projection data is obtained. A generating unit to generate;
A reconstruction unit for reconstructing the second projection data;
X-ray CT apparatus provided with An X-ray tube that irradiates radiation around the subject;
The detector for detecting the first spectrum;
A collection unit for collecting the first spectrum from the detector to generate the first projection data;
The X-ray CT apparatus according to claim 1, further comprising: The first projection data is a set of first sinograms based on the number of photons for each first energy band in the first spectrum;
2. The X projection according to claim 1, wherein the second projection data is a set of second sinograms based on the number of photons for each second energy band, in which the first sinogram of the first projection data is corrected by the generation unit. Line CT device.   2. The X-ray CT apparatus according to claim 1, wherein the generation unit replaces data obtained by synthesizing neighboring data in at least one direction of a view direction, a channel direction, and a slice direction of the first projection data as the first projection data.   2. The X-ray according to claim 1, wherein the response information is information indicating a magnitude of a degree of a physical phenomenon contributing to an error included in the first spectrum output as the response characteristic of the detector to which radiation is incident. CT device.   The response information includes the probability of occurrence of escape occurring at the detector, fluorescence from the channel around the specific channel of the detector to the specific channel, crosstalk and scattering, and energy detected at the detector The X-ray CT apparatus according to claim 5, wherein the information is information based on at least one of the fluctuations.   The X-ray CT apparatus according to claim 1, wherein the generation unit switches the response information for generating the second projection data according to the number of photons per unit time of the radiation detected by the detector. .   The reconstruction unit includes a line attenuation coefficient that is a pixel value of an image generated by reconstructing the second sinogram corresponding to the second energy band having one or more energy widths, and the one or more types of energy. The X-ray CT apparatus according to claim 3, wherein the material density of the specific material is calculated based on a mass attenuation coefficient of the specific material corresponding to the second energy band of width.   The generation unit further generates the second projection data based on information indicating continuity in the energy direction of the second spectrum indicating the amount of X-rays for each energy based on the second projection data. X-ray CT apparatus described in 1. A calculation unit for calculating a first weight for weighting the continuity of the portion corresponding to the second energy band adjacent in the second spectrum;
The X-ray CT apparatus according to claim 9, wherein the calculation unit decreases the first weight of the second energy band corresponding to the change rate as the change rate in the energy direction in the second spectrum increases. The generator is
Based on the response information and information indicating the continuity of the energy direction of the third spectrum indicating the amount of X-rays for each energy based on the third projection data, the third projection data is obtained from the first projection data. A first auxiliary generator for generating;
Based on the response information and information indicating the continuity of the energy direction of the fourth spectrum indicating the amount of X-rays for each energy based on the fourth projection data, the fourth projection data is obtained from the first projection data. A second auxiliary generator for generating;
A combining unit that obtains the second projection data by weighting and combining the third projection data and the fourth projection data;
Have
In the third spectrum and the fourth spectrum, further includes a calculation unit that calculates a first weight for weighting the continuity of the portions corresponding to predetermined adjacent energy bands,
The second auxiliary generation unit generates the fourth projection data using the information indicating the continuity including the first weight of the fourth spectrum that is larger than the first weight of the third spectrum,
10. The X-ray CT apparatus according to claim 9, wherein the synthesizing unit synthesizes by increasing the weight for the third projection data and decreasing the weight for the fourth projection data as the first weight is small. A calculation unit for calculating a first weight for weighting the continuity of the portion corresponding to the second energy band adjacent in the second spectrum;
The calculation unit includes information on a spectrum of radiation emitted from the X-ray tube, a target member of the X-ray tube, an angle of the target of the X-ray tube, a filter included in the X-ray tube, and the X-ray at the time of imaging 12. The X according to claim 10, wherein the first weight is calculated based on at least one of a tube voltage of a tube, a contrast agent used for the subject, and an assumed composition of the subject. Line CT device. A calculation unit for calculating a first weight for weighting the continuity of the portion corresponding to the second energy band adjacent in the second spectrum;
An input unit for receiving an operation input of information on a contrast agent used for the subject or energy information on which the second spectrum is expected to change sharply in an energy direction;
Further comprising
The X-ray CT apparatus according to claim 10, wherein the calculation unit calculates the first weight based on information on the operation input received by the input unit. A calculation unit for calculating a first weight for weighting the continuity of the portion corresponding to the second energy band adjacent in the second spectrum;
The reconfiguration unit calculates the substance density of the specific substance based on the mass attenuation coefficient of the specific substance,
The X-ray CT apparatus according to claim 10, wherein the calculation unit calculates the first weight based on the material density.   The X-ray CT apparatus according to claim 1, wherein the generation unit generates the second projection data so that a distance between the first projection data and the second projection data corrected by the response information is small. A calculating unit for calculating a second weight for weighting the distance for each of the first energy bands;
The X-ray CT apparatus according to claim 15, wherein the generation unit generates the second projection data using the second weight.   The X-ray CT apparatus according to claim 16, wherein the calculation unit calculates the second weight smaller as the second energy band is smaller. A storage unit for storing the response information;
The X-ray CT apparatus according to claim 1, wherein the generation unit reads and acquires the response information from the storage unit. The storage unit stores, as the response information, a filter coefficient for converting the first projection data into the second projection data.
The X-ray CT apparatus according to claim 18, wherein the generation unit reads the filter coefficient from the storage unit and generates the second projection data by convolving the filter coefficient with the first projection data. An image processing apparatus that performs image processing on data based on energy of radiation transmitted through a subject,
A first projection data based on a first spectrum indicating the amount of X-rays for each energy of the radiation is corrected by response information corresponding to the detector to generate second projection data;
A reconstruction unit for reconstructing the second projection data;
An image processing apparatus. A program for performing image processing on data based on the energy of radiation transmitted through a subject,
Generating the second projection data by correcting the first projection data based on the first spectrum indicating the amount of X-rays for each energy of the radiation with the response information corresponding to the detector;
A reconstruction step of reconstructing the second projection data;
A program that causes a computer to execute.

Images