JP5725930B2 - Image generating apparatus, X-ray CT apparatus, and program - Google Patents

Description

The present invention relates to an image generation apparatus, an X-ray CT (Computed Tomography) apparatus, and a program therefor, and more particularly, to dual energy (dual).
energy) relates to a technique for improving the quality of an image obtained by shooting.

  Conventionally, in X-ray CT imaging, it corresponds to an image obtained when X-ray CT imaging is performed with an X-ray having a desired effective X-ray energy based on data (data) obtained by dual energy imaging. Methods for reconstructing a monochromatic image are known. This method is a model that represents an object to be imaged by combining two different base materials, that is, the X-ray absorption coefficient of each material constituting the image object, and the X-ray absorption coefficients of the two types of base materials having different X-ray absorption coefficients. This is based on a model expressed by weighted addition of linear absorption coefficients. For example, the following method can be cited.

  First, dual energy imaging is performed on an imaging target using an X-ray CT apparatus, and projection data of a plurality of views when the X-ray tube voltage is 80 kVp and projection of a plurality of views when the X-ray tube voltage is 140 kVp. Collect data and.

  Next, a water density image representing a virtual water density distribution in the imaging target is obtained by performing predetermined processing based on projection data of 80 kVp and projection data of 140 kVp, with the base material constituting the imaging target being water and iodine. An iodine density image representing a virtual iodine density distribution in the object is reconstructed.

  Then, the water density image and the iodine density image are subjected to weighted addition processing using a weighting coefficient determined according to the effective X-ray energy of the monochrome image to be obtained to obtain a monochrome match image (for example, Patent Document 1, FIG. 5, (See Patent Document 2 etc.).

JP 2010-172590 A US7724865 (US2009 / 0052612)

  In the above-described monochrome image reconstruction method, the pixel value of each pixel in the monochrome image is obtained by weighted addition processing of the pixel value of the corresponding pixel in the water density image and the pixel value of the corresponding pixel in the iodine density image. ing.

  As shown in FIG. 6, this weighted addition process can be easily understood by using a vector.

  The graph of FIG. 6 shows, for each substance of water and iodine, the pixel value of the substance in the CT image when the X-ray tube voltage is 80 kVp, and the substance in the CT image when the X-ray tube voltage is 140 kVp. The relationship with the pixel value is shown.

  Each straight line in the graph of FIG. 6 corresponds to each substance, and the slope of the straight line has a correlation with the magnitude of the X-ray absorption coefficient of the substance. The vector along the straight line of the substance corresponds to the pixel value in the substance density image of each substance, that is, the density of the substance. Specifically, the vector Vw along the water straight line Lw corresponds to the pixel value in the water density image, that is, the water density, and the vector Vio along the iodine straight line Lio is the pixel value in the iodine density image. That is, it corresponds to the density of iodine. Therefore, the weighted addition process of the pixel value of the corresponding pixel in the water density image and the pixel value of the corresponding pixel in the iodine density image to obtain the pixel value of each pixel in the monochrome image is shown in FIG. It can be expressed as a linear sum of Vw and iodine vector Vio.

  Here, the pixel value of the substance Q1 having an X-ray absorption coefficient larger than that of water and smaller than that of the iodine is represented by a pixel value representing water density (water vector Vw) and a pixel value representing iodine density (iodine vector Vio). Consider the weighted addition (linear sum). In this case, as shown in FIG. 6, the pixel value (+ w11 · Vw) in the water density image used for the weighted addition and the pixel value (+ w12 · Vio) in the iodine density image are both positive (plus). The process corresponds to an “interpolation process” using a pixel value representing the density of water and a pixel value representing the density of iodine.

  In the case of the “interpolation process”, the X-ray absorption coefficient of the substance represented by the pixel whose pixel value is to be obtained is relatively close to the X-ray absorption coefficients of water and iodine, so that the accuracy of the obtained pixel value is good.

  The substance Q1 is, for example, a soft tissue such as a heart or a liver or a bone tissue.

  Next, the pixel value of the substance Q2 whose X-ray absorption coefficient is smaller than water, or the substance Q3 whose X-ray absorption coefficient is larger than iodine, the pixel value (water vector Vw) representing the density of water, and the iodine density It is assumed that the pixel value is expressed by weighted addition (linear addition sum) with the pixel value (iodine vector Vio). In this case, as shown in FIG. 6, the pixel value (+ w21 · Vw, −w32 · Vw) in the water density image used for the weighted addition and the pixel value (−w22 · Vio, + w31 · Vio) in the iodine density image are Either of them becomes negative (minus), and this weighted addition processing corresponds to “extrapolation processing” using a pixel value representing the density of water and a pixel value representing the density of iodine.

  In the case of the above “extrapolation process”, the X-ray absorption coefficient of the substance represented by the pixel whose pixel value is to be obtained deviates relatively greatly from the X-ray absorption coefficient of either the water or iodine substance. Compared with the above “interpolation process”, the accuracy of the obtained pixel value is deteriorated and the error is increased.

  The substance Q2 is, for example, fat, and the substance Q3 is, for example, metal, ceramics, an organic compound having a relatively large X-ray absorption coefficient, or the like.

  Of course, if two types of substances having a large X-ray absorption coefficient are selected as the basis material, the X-ray absorption coefficient of the substance represented by the pixel to be obtained will be the basis even if it corresponds to the “interpolation process”. Since at least one of the substances is relatively far from the X-ray absorption coefficient, the accuracy is still lowered.

  Therefore, according to the conventional method of reconstructing a monochromatic image using only the material density images of two types of base materials, the accuracy of pixel values of corresponding pixels in the monochromatic image is reduced depending on the material. There is room for improvement in image quality.

  Under such circumstances, an improvement in image quality in a monochrome image reconstructed based on X-ray CT dual energy imaging is desired.

  The invention according to the first aspect is based on the data obtained by X-ray CT dual energy imaging of the imaging target, and the base material of the imaging target in a model that represents the imaging target by combining two types of base materials. A first generation means for generating a material density image representing a virtual density distribution for each base material for two or more sets of base materials in which at least one base material constituting the set is different; and the first generation Second generation for generating a monochromatic image corresponding to an image obtained when X-ray CT imaging of the imaging object is performed with X-rays having a desired effective X-ray energy based on the material density image generated by the means The pixel value of each pixel of the monochrome image is determined based on information relating to the magnitude of the X-ray absorption coefficient of the substance represented by the pixel among the two or more sets of base substances. To provide an image generating apparatus and a second generating means obtained by weighted addition process between pixel values of corresponding pixels of the pixel in each material density image by a set of basis material was.

  According to a second aspect of the invention, when the first generation means uses the first substance and the second substance having an X-ray absorption coefficient larger than that of the first substance as the first set of base substances. Generate each material density image and each material density image when the first or second material and a third material having a larger X-ray absorption coefficient than the second material are used as the second set of base materials. And when the second generation means sets the pixel value of each pixel of the monochromatic image and the pixel values of the corresponding pixels in each material density image of the first set of base materials are both positive, When the pixel values of the corresponding pixels in the substance density image of the first substance based on the first set of base substances are negative, the substance density images based on the second set of base substances are obtained by weighted addition processing. Obtained by weighted addition of pixel values of corresponding pixels in To provide an image generating apparatus of the serial first aspect.

  According to a third aspect of the invention, when the first generation means uses the first substance and the second substance having an X-ray absorption coefficient larger than that of the first substance as the first set of base substances. Generate each material density image and each material density image when the first or second material and a fourth material having an X-ray absorption coefficient smaller than that of the first material are used as a third set of base materials. And when the second generation means sets the pixel value of each pixel of the monochromatic image and the pixel values of the corresponding pixels in each material density image of the first set of base materials are both positive, When the pixel value of the corresponding pixel in the material density image of the second material by the first set of base materials is negative, the respective material density images by the third set of base materials are obtained by weighted addition processing. Obtained by weighted addition of pixel values of corresponding pixels in To provide an image generating apparatus of the serial first aspect.

  According to a fourth aspect of the present invention, based on the data, an image obtained when X-ray CT imaging of the imaging object with X-rays having a predetermined effective X-ray energy or an image corresponding to the image is generated. 3 generating means, wherein the first generating means uses the first substance and the second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base substances. Each material density image, and each material density image when the first or second material and a third material having a larger X-ray absorption coefficient than the second material are used as the second set of base materials, The second generation means sets the pixel value of each pixel of the monochrome image, the pixel value of the corresponding pixel in the image generated by the third generation means, and the X-ray absorption coefficient of the first value. A picture that is larger than the substance and smaller than the second substance When the value is a value, the pixel values of the corresponding pixels in each material density image by the first set of base materials are obtained by weighted addition processing, and the pixel values of the corresponding pixels in the image generated by the third generation unit are obtained. When the X-ray absorption coefficient is a pixel value corresponding to a substance larger than the second substance, the pixel values of the corresponding pixels in each substance density image by the second set of base substances are obtained by weighted addition processing. An image generation apparatus according to one aspect is provided.

  According to a fifth aspect of the invention, based on the data, an image obtained when X-ray CT imaging of the imaging object with X-rays having a predetermined effective X-ray energy or an image corresponding to the image is generated. 3 generating means, wherein the first generating means uses the first substance and the second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base substances. Each material density image, and each material density image when the first or second material and a fourth material having an X-ray absorption coefficient smaller than that of the first material are a third set of base materials, The second generation means sets the pixel value of each pixel of the monochrome image, the pixel value of the corresponding pixel in the image generated by the third generation means, and the X-ray absorption coefficient of the first value. A picture that is larger than the substance and smaller than the second substance When the value is a value, the pixel values of the corresponding pixels in each material density image by the first set of base materials are obtained by weighted addition processing, and the pixel values of the corresponding pixels in the image generated by the third generation unit are obtained. When the X-ray absorption coefficient is a pixel value corresponding to a substance larger than the second substance, the pixel value of the corresponding pixel in each substance density image by the third set of base substances is obtained by weighted addition processing. An image generation apparatus according to one aspect is provided.

  According to a sixth aspect of the invention, the first substance is water, the second substance is iodine, and the third substance is a metal having a larger X-ray absorption coefficient than iodine, ceramics ( The image generating apparatus according to the second aspect or the fourth aspect is ceramics) or an organic compound.

  According to a seventh aspect of the present invention, the first substance is water, the second substance is iodine, and the fourth substance is an organic compound having a smaller X-ray absorption coefficient than fat or water. The image generating apparatus according to the third or fifth aspect is provided.

  According to an eighth aspect of the present invention, the first generating means has a projection data by X-rays having a first effective X-ray energy and a second effective X-ray energy different from the first effective X-ray energy. Any one of the first to seventh aspects of generating a material density image by performing image reconstruction processing on data obtained by linear weighted subtraction or nonlinear weighted subtraction of projection data by X-rays having An image generation apparatus according to one aspect is provided.

  A ninth aspect of the invention provides an X-ray CT apparatus comprising the image generating apparatus according to any one of the first to eighth aspects.

  The invention according to a tenth aspect provides a program for causing a computer to function as the image generating apparatus according to any one of the first to eighth aspects.

  The “density” mentioned here is not an actual density but an imaginary density. Two predetermined types of substances are used as a base substance, and an X-ray absorption coefficient of a certain substance is set to X of the two types of base substances. When expressed by the weighted addition value of the linear absorption coefficient, it corresponds to a weighting coefficient to be multiplied by the X-ray absorption coefficient of the two types of base materials. Therefore, for example, a water density image when water and iodine are a set of base materials is the X-ray absorption coefficient of the substance represented by each pixel in the image to be imaged, the X-ray absorption coefficient of water and the X-ray of iodine. When expressed by a weighted addition value with an absorption coefficient, a distribution diagram of values corresponding to the weighting coefficient multiplied by the water X-ray absorption coefficient is obtained.

  According to the invention of the above aspect, the base material of the material density image used for calculation of the pixel value of the monochrome image is not fixed to two predetermined types of materials, but the X-ray absorption coefficient of the material represented by the pixel is used. Based on such information, it is possible to switch to a combination in which the accuracy of the calculated pixel value is relatively improved, so that the accuracy of the pixel value in the monochrome image can be improved. As a result, the image quality of the monochrome image is improved. Can be improved.

It is a figure which shows the structure of the X-ray CT apparatus by 1st embodiment. It is a figure which shows the flow of a process in the X-ray CT apparatus which concerns on 1st embodiment. It is a figure which shows the 1st example of the method of reconstructing an iodine density image and a water density image. It is a figure which shows the 2nd example of the method of reconstructing an iodine density image and a water density image. It is a figure which shows an example of the method of reconstructing a 1st monochrome image. It is a figure for demonstrating the weighted addition process of a water density image and an iodine density image. It is a figure which shows an example of the method of reconstructing an iodine density image and a polyethylene (polyethylene) density image. It is a figure for demonstrating the weighted addition process of a polyethylene density image and an iodine density image. It is a figure which shows an example of the method of reconstructing a water density image and a ceramic density image. It is a figure for demonstrating the weighted addition process of a water density image and a ceramics density image. It is a figure which shows the flow of a process in the X-ray CT apparatus which concerns on 2nd embodiment.

  Embodiments of the invention will be described below.

(First embodiment)
FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus according to the first embodiment.

  The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry 20.

  The operation console 1 includes an input device 2 that receives input from an operator, a central processing unit 3 that performs control of various units and various calculations for scanning the subject 40, and data acquired by the scanning gantry 20. A data collection buffer (buffer) 5, a monitor 6 for displaying images, and a storage device 7 for storing programs and data.

  The imaging table 10 includes a cradle 12 on which the subject 40 is placed and carried into and out of the opening of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10.

The scanning gantry 20 includes a rotating unit 15 and a support unit 16 that rotatably supports the rotating unit 15. The rotating unit 15 includes an X-ray tube 21, an X-ray controller (controller) 22 that controls the X-ray tube 21, and a collimator that collimates and shapes the X-ray 81 generated from the X-ray tube 21. ) 23, an X-ray detector 24 that detects X-rays 81 irradiated from the X-ray tube 21 and transmitted through the subject 40, and a data collection device that converts and collects the output of the X-ray detector 24 into projection data (DAS; Data
Acquisition System) 25 and a rotating unit controller 26 for controlling the X-ray controller 22, the collimator 23, and the DAS 25 are mounted. The support unit 16 includes a control controller 29 that communicates control signals and the like with the operation console 1 and the imaging table 10. The rotating portion 15 and the support portion 16 are provided with a slip ring (slip
ring) 30 and is electrically connected.

  Hereafter, the flow of processing in the X-ray CT apparatus according to the first embodiment will be described. In the first embodiment, for each pixel of a monochromatic image to be reconstructed, the threshold value determination of the pixel value of the corresponding pixel in the substance density image of a predetermined basis substance is performed using a combination of the substance density image used to calculate the pixel value. Switch with.

  FIG. 2 is a flowchart showing a flow of processing in the X-ray CT apparatus according to the first embodiment.

  In step S1, dual energy imaging of an imaging target is performed, and the first tube voltage projection data Pv1, which is projection data of a plurality of views by the first X-ray tube voltage, and the projection data of a plurality of views by the second X-ray tube voltage. Certain second tube voltage projection data Pv2 is collected.

  For example, the X-ray tube voltage is switched between the first X-ray tube voltage and the second X-ray tube voltage in units of one or several views, the scan of the object to be imaged is scanned, and the fan of 180 ° + X-ray beam (beam) (Fan) Collect projection data with angle α ° or more. In this case, the projection data of the missing view is obtained by performing weighted addition processing on the projection data of the view adjacent to the missing view.

  Also, for example, the X-ray tube voltage is first set to the first X-ray tube voltage and scanning is performed to collect projection data of 180 ° + fan angle α ° or more, and then the X-ray tube voltage is set to the second X-ray tube voltage. To scan and collect projection data of 180 ° + fan angle α ° or more.

  The first and second X-ray tube voltages are 80 kVp and 140 kVp, for example.

  In step S2, water and iodine are used as base materials, and based on the first and second tube voltage projection data Pv1 and Pv2, a water density image Gw representing a virtual water density distribution in the imaging target and a virtual density in the imaging target are displayed. An iodine density image Gio representing a typical iodine density distribution is reconstructed.

  FIGS. 3 and 4 are diagrams conceptually illustrating an example of a method for reconstructing an iodine density image and a water density image based on the first and second tube voltage projection data using water and iodine as a base material. .

  For example, as shown in FIG. 3, the first intermediate projection representing the water component is obtained by performing weighted subtraction processing on the first tube voltage projection data Pv1 and the second tube voltage projection data Pv2 so that the iodine component is eliminated. Data Pw is obtained. In addition, the first tube voltage projection data Pv1 and the second tube voltage projection data Pv2 are subjected to weighted subtraction processing so that the water component is eliminated, thereby obtaining second intermediate projection data Pio representing the iodine component. Next, the first intermediate projection data Pw is subjected to image reconstruction processing to reconstruct the water density image Gw. Further, the second intermediate projection data Pio is subjected to image reconstruction processing by back projection processing or the like to reconstruct the iodine density image Gio.

  Further, for example, as shown in FIG. 4, the first tube voltage projection data Pv1 is subjected to image reconstruction processing to reconstruct the first tube voltage image Gv1. Further, the second tube voltage projection data Pv2 is subjected to image reconstruction processing to reconstruct the second tube voltage image Gv2. Next, a weighted subtraction process is performed on the first tube voltage image Gv1 and the second tube voltage image Gv2 so as to eliminate the iodine component, thereby reconstructing the water density image Gw. Further, the iodine density image Gio is reconstructed by performing weighted subtraction processing on the first tube voltage image Gv1 and the second tube voltage image Gv2 so that the water component is eliminated.

  In step S3, a first monochrome image corresponding to an image obtained when X-ray CT imaging of an imaging target is performed using X-rays having a desired effective X-ray energy E1 based on the water density image Gw and the iodine density image Gio. The image Gm1 is reconstructed. The effective X-ray energy E1 is, for example, 120 keV.

  FIG. 5 is a diagram conceptually illustrating an example of a method for reconstructing the first monochrome image based on the water density image and the iodine density image.

  For example, as shown in FIG. 5, the water density image Gw and the iodine density image Gio are weighted and added with a predetermined weighting factor corresponding to the effective X-ray energy E1, and the first monochrome image Gm1 is reconstructed.

  FIG. 6 is a diagram for explaining a weighted addition process of a water density image and an iodine density image.

  As shown in FIG. 6, in the first monochrome image Gm1, the pixel value of the substance Q1 having an X-ray absorption coefficient larger than water and smaller than iodine represents the pixel value (Vw) representing the density of water and the iodine density. Since it is obtained by “interpolation processing” with the pixel value (Vio), that is, weighted addition processing in which the weighting coefficients are all positive, the accuracy is high.

  However, the pixel value of the substance Q2 having a smaller X-ray absorption coefficient than water and the substance Q3 having a larger X-ray absorption coefficient than iodine have a pixel value (Vw) representing the density of water and a pixel value (Vio) representing the density of iodine. Therefore, the accuracy is not good.

  Therefore, the pixel values of the substance Q2 having an X-ray absorption coefficient smaller than that of water and the substance Q3 having an X-ray absorption coefficient larger than that of iodine are obtained using a substance density image based on another combination of base substances. Replace with.

  In step S4, the target pixel (x, y) is selected in the first monochrome image Gm1.

  In step S5, it is determined whether or not the pixel value Gio (x, y) of the corresponding pixel of the target pixel in the iodine density image Gio is negative.

  Here, when it is determined that the pixel value Gio (x, y) of the corresponding pixel in the iodine density image Gio is negative, the X-ray absorption coefficient of the substance represented by the target pixel is smaller than the water X-ray absorption coefficient, The pixel value Gm1 (x, y) of the target pixel is determined to be obtained by extrapolation processing of the pixel value representing the water density and the pixel value representing the iodine density, and the process proceeds to step S6.

  On the other hand, when it is determined that the pixel value Gio (x, y) of the corresponding pixel in the iodine density image Gio is not negative, that is, 0 or more, the X-ray absorption coefficient of the substance represented by the target pixel is the X-ray absorption of water. It is determined that the coefficient is equal to or larger than the coefficient, and the process proceeds to step S9.

  In step S6, a substance density image of these base substances is reproduced based on the first and second tube voltage projection data Pv1 and Pv2 using the substance A having a smaller X-ray absorption coefficient than water and iodine as the base substance. Configure.

  As the substance A, for example, an organic compound having a smaller X-ray absorption coefficient than water, such as polypropylene and polyethylene, and fat can be considered.

  Here, the substance A is made of polyethylene, and a polyethylene density image Ga representing a virtual polyethylene density distribution in the photographing object and an iodine density image Gio representing the virtual iodine density distribution in the photographing object are reconstructed.

  FIG. 7 is a diagram conceptually illustrating an example of a method for reconstructing a polyethylene density image and an iodine density image based on the first and second tube voltage projection data using polyethylene and iodine as a base material.

  For example, as shown in FIG. 7, the first intermediate voltage projection data Pv1 and the second intermediate voltage projection data Pv2 are weighted and subtracted so that the iodine component is subtracted, and the third intermediate projection representing the polyethylene component is obtained. Data Pa is obtained. Further, the first tube voltage projection data Pv1 and the second tube voltage projection data Pv2 are subjected to weighted subtraction processing so that the polyethylene component is subtracted to obtain fourth intermediate projection data Pio representing the iodine component. Next, the third intermediate projection data Pa is subjected to image reconstruction processing to reconstruct a polyethylene density image Ga. Further, the iodine density image Gio is reconstructed by performing image reconstruction processing on the fourth intermediate projection data Pio by backprojection processing or the like.

  In step S7, the polyethylene density image Ga and the iodine density image Gio are weighted and added with a predetermined weighting factor corresponding to the effective X-ray energy E1, thereby reconstructing the second monochrome image Gm2.

  FIG. 8 is a diagram for explaining weighted addition processing of a polyethylene density image and an iodine density image. The substance represented by the pixel of interest here is a substance Q2 having an X-ray absorption coefficient smaller than that of water. Moreover, the polyethylene which is the substance A also has an X-ray absorption coefficient smaller than that of water. Therefore, as shown in FIG. 8, the pixel value Gm2 (x, y) of the corresponding pixel of the pixel of interest in the second monochrome image Gm2, that is, the pixel value of the substance Q2 is a pixel value (Va) representing the density of polyethylene. And a pixel value (Va) representing the density of polyethylene which is a substance having an X-ray absorption coefficient closer to the substance of the pixel of interest compared to water. Thus, it is obtained by extrapolation with a pixel value (Vio) representing the density of iodine. As a result, the pixel value Gm2 (x, y) of the corresponding pixel in the second monochrome image Gm2 is more accurate than the pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1.

  In step S8, the pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1 is replaced with the pixel value Gm2 (x, y) of the corresponding pixel of the target pixel in the second monochrome image Gm2. Thereafter, the process proceeds to step S13.

  Note that the steps S6 and S7 for reconstructing the second monochrome image Gm2 are the first time when the pixel value Gio (x, y) of the corresponding pixel in the iodine density image Gio is first determined to be negative in step S5. Only execute, and skip the second and subsequent times.

  In step S9, it is determined whether or not the pixel value Gw (x, y) of the corresponding pixel of the target pixel in the water density image Gw is negative.

  Here, when it is determined that the pixel value Gw (x, y) of the corresponding pixel in the water density image Gw is negative, the X-ray absorption coefficient of the substance represented by the target pixel is larger than the X-ray absorption coefficient of iodine, It is determined that the pixel value Gm1 (x, y) of the target pixel is obtained by extrapolation processing of the pixel value representing the iodine density and the pixel value representing the water density, and the process proceeds to step S10.

  On the other hand, when the pixel value Gw (x, y) of the corresponding pixel in the water density image Gw is not negative, that is, 0 or more, is the X-ray absorption coefficient of the substance represented by the pixel of interest the same as the X-ray absorption coefficient of iodine? It can be considered smaller than that. That is, when considered together with the determination result in step S5, the X-ray absorption coefficient of the substance represented by the pixel of interest can be regarded as not less than the X-ray absorption coefficient of water and not more than the X-ray absorption coefficient of iodine. The pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1 is considered to be obtained by interpolation processing between the pixel value representing the iodine density and the pixel value representing the water density. it can. Therefore, in this case, it is determined that the pixel value Gm1 (x, y) of the target pixel is obtained with high accuracy, and the pixel value Gm1 (x, y) of the target pixel is left as it is, and the process proceeds to step S13.

  In step S10, water and a substance B having an X-ray absorption coefficient larger than that of iodine are used as base substances, and based on the first and second tube voltage projection data Pv1 and Pv2, substance density images of these base substances are reproduced. Configure.

  As the substance B, for example, metals, ceramics, organic compounds, etc. having a larger X-ray absorption coefficient than iodine, such as zirconium ceramics, can be considered.

  Here, the substance B is assumed to be ceramics, and a water density image Gw representing the virtual water density distribution in the photographing target and a ceramic density image Gb representing the virtual ceramic density distribution in the photographing target are reconstructed.

  FIG. 9 is a diagram conceptually illustrating an example of a method for reconstructing a water density image and a ceramic density image based on the first and second tube voltage projection data using water and ceramics as a base material.

  For example, as shown in FIG. 9, a first intermediate voltage projection data Pv1 and a second intermediate voltage projection data Pv2 are subjected to a weighted subtraction process so that a ceramic component is subtracted, and a fifth intermediate projection representing a water component is obtained. Data Pw is obtained. Further, the first tube voltage projection data Pv1 and the second tube voltage projection data Pv2 are weighted and subtracted so that the water component is subtracted, thereby obtaining sixth intermediate projection data Pb representing the ceramic component. Next, the fifth intermediate projection data Pw is subjected to image reconstruction processing by back projection processing or the like to reconstruct the water density image Gw. Further, the sixth intermediate projection data Pb is subjected to image reconstruction processing to reconstruct the ceramic density image Gb.

  In step S11, the water density image Gw and the ceramics density image Gb are subjected to a weighted addition process with a predetermined weighting factor corresponding to the effective X-ray energy E1, thereby reconstructing the third monochrome image Gm3.

  FIG. 10 is a diagram for explaining the weighted addition processing of the water density image and the ceramic density image. The substance represented by the pixel of interest here is a substance Q3 having a larger X-ray absorption coefficient than iodine. Moreover, the ceramics which is the substance B also has a larger X-ray absorption coefficient than iodine. Therefore, as shown in FIG. 10, the pixel value Gm3 (x, y) of the corresponding pixel of the pixel of interest in the third monochrome image Gm3, that is, the pixel value of the substance Q3 is a pixel value (Vw) representing the density of water. Or a pixel value (Vb) representing the density of ceramics, or a pixel value (Vw) representing the density of water and a substance having an X-ray absorption coefficient closer to that of the pixel of interest compared to iodine. It is obtained by extrapolation with a pixel value (Vb) representing the density of a certain ceramic. As a result, the pixel value Gm3 (x, y) of the corresponding pixel in the third monochrome image Gm3 is more accurate than the pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1.

  In step S12, the pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1 is replaced with the pixel value Gm3 (x, y) of the corresponding pixel of the target pixel in the third monochrome image Gm3. Thereafter, the process proceeds to step S13.

  In step S13, it is determined whether or not there is still another pixel to be selected next as the target pixel in the first monochrome image Gm1. When it is determined that there is still a pixel to be selected as the target pixel, the process returns to step S4 to select a new pixel as the target pixel and continue the processing. If it is determined that there are no more pixels to be selected as the pixel of interest, the process ends.

(Second embodiment)
A processing flow in the X-ray CT apparatus according to the second embodiment will be described. In the second embodiment, for each pixel of the reconstructed monochrome image, the base material of the material density image used for calculating the pixel value is determined by threshold determination of the pixel value of the corresponding pixel in the temporarily reconstructed monochrome image. Switch.

  FIG. 11 is a flowchart showing a flow of processing in the X-ray CT apparatus according to the second embodiment.

  Since steps T1 to T4 are the same as steps S1 to S4, description thereof is omitted.

  In step T5, it is determined whether or not the pixel value (CT value) Gm1 (x, y) of the pixel of interest is smaller than a predetermined threshold, for example, 0HU.

  Here, when it is determined that the pixel value Gm1 (x, y) of the target pixel is smaller than 0HU, the X-ray absorption coefficient of the substance represented by the target pixel is smaller than the water X-ray absorption coefficient, and the pixel value of the target pixel is It is determined that Gm1 (x, y) is obtained by extrapolation of the pixel value representing iodine and the pixel value representing water, and the process proceeds to step T6.

  On the other hand, when the pixel value Gm1 (x, y) of the target pixel is not smaller than 0HU, that is, 0HU or more, the X-ray absorption coefficient of the substance represented by the target pixel is equal to or higher than the water X-ray absorption coefficient. It judges that it is large, and proceeds to step T9.

  Since steps T6 to T8 are the same as steps S6 to S8, description thereof is omitted.

  In step T9, it is determined whether or not the pixel value (CT value) Gm1 (x, y) of the target pixel is greater than a predetermined threshold, for example, 1500HU.

  When it is determined that the pixel value Gm1 (x, y) of the target pixel is larger than 1500 HU, the X-ray absorption coefficient of the substance represented by the target pixel is larger than the X-ray absorption coefficient of iodine, and the pixel value Gm1 (x , y) is determined to be obtained by extrapolation processing of a pixel value representing the density of water and a pixel value representing the density of iodine, and the process proceeds to step T10.

  On the other hand, when it is determined that the pixel value Gm1 (x, y) of the target pixel is 1500 HU or less, the X-ray absorption coefficient of the substance represented by the target pixel is equal to or smaller than the X-ray absorption coefficient of iodine. I reckon. That is, when considered together with the determination result of step T5, the X-ray absorption coefficient of the substance represented by the pixel of interest can be regarded as not less than the X-ray absorption coefficient of water and not more than the X-ray absorption coefficient of iodine. The pixel value Gm1 (x, y) of the target pixel in the first monochrome image Gm1 can be regarded as being obtained by interpolation between a pixel value representing the water density and a pixel value representing the iodine density. it can. Therefore, in this case, it is determined that the pixel value Gm1 (x, y) of the target pixel is accurately obtained, and the pixel value Gm1 (x, y) of the target pixel is left as it is, and the process proceeds to Step T13.

  Since steps T10 to T13 are the same as steps S10 to S13, the description thereof is omitted.

  As described above, according to each of the embodiments described above, the base material of the material density image used for calculating the pixel value of the monochrome image is not fixed to two predetermined types of materials, but the X-rays of the material represented by the pixels Based on the information related to the absorption coefficient, it is possible to switch to a combination in which the accuracy of the calculated pixel value is relatively improved, so that the accuracy of the pixel value in the monochrome image can be improved, and as a result, the monochrome image Image quality can be improved.

  As mentioned above, although embodiment of invention was described, embodiment of invention is not limited to said thing, In the range which does not deviate from the meaning of invention, various forms can be taken.

  For example, in each of the above embodiments, water and iodine are described as examples of the main base material, but other materials may be used as the main base material.

  Further, for example, in each of the above embodiments, for a pixel representing a substance having an X-ray absorption coefficient smaller than that of water, two kinds of substance density images having a substance A having an X-ray absorption coefficient smaller than that of water and iodine as a base substance are obtained. However, the pixel value may be obtained using two kinds of substance density images having substance A and water as base substances. Similarly, in each of the above-described embodiments, two types of substance density images using water and a substance B having a larger X-ray absorption coefficient than iodine as a base substance are obtained for pixels representing a substance having a larger X-ray absorption coefficient than iodine. Although the pixel value is obtained using the pixel value, the pixel value may be obtained using two kinds of substance density images having iodine and the substance B as base substances.

  Further, for example, in the second embodiment, for each pixel in the reconstructed monochrome image, the substance density image used for calculating the pixel value by determining the threshold value of the pixel value of the pixel of interest in the temporarily reconstructed monochrome image The basis material is determined. However, the first tube voltage projection data obtained by image reconstruction processing of the first tube voltage projection data and the second tube voltage image obtained by image reconstruction processing of the second tube voltage projection data represent the imaging target. The base material of the material density image used for calculating the pixel value may be determined by determining the threshold value of the pixel value of the corresponding pixel in another CT image.

  Further, for example, each of the above embodiments is an X-ray CT apparatus, but an image generation apparatus having an image generation (reconstruction) function in the X-ray CT apparatus is also an embodiment of the invention.

Further, for example, a program for causing a computer to function as the above-described image generation apparatus is also an embodiment of the invention.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Rotation part 16 Support part 20 Scanning gantry 21 X-ray tube 22 X-ray tube controller 23 Collimator 24 X-ray detector 25 Data collection device (DAS)
26 Rotating part controller 29 Control controller 30 Slip ring 40 Subject 81 X-ray

Claims (10)

Based on data obtained by X-ray CT dual energy imaging of an imaging target, a virtual density distribution of the base material of the imaging target in a model representing the imaging target by combining two types of base materials is represented. First generation means for generating a material density image for each base material for two or more sets of base materials in which at least one base material constituting the set is different;
Based on the material density image generated by the first generation unit, a monochrome image corresponding to an image obtained when X-ray CT imaging of the imaging target with X-rays having a desired effective X-ray energy is generated Second generation means for determining a pixel value of each pixel of the monochrome image based on information relating to the magnitude of the X-ray absorption coefficient of the substance represented by the pixel among the two or more sets of base substances. An image generation apparatus comprising: a second generation unit that obtains the pixel values of the corresponding pixels of each substance density image by a set of base materials by weighted addition processing. Each of the first generation means includes a first substance and a second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base materials, Generating each material density image when the first material or the second material and the third material having a larger X-ray absorption coefficient than the second material are used as the second set of base materials;
The second generation means calculates the pixel value of each pixel of the monochrome image when the pixel values of the corresponding pixels in the material density images of the first set of base materials are both positive. When the pixel value of the corresponding pixel in the substance density image of the first substance by the first set of base substances is negative, the correspondence in each substance density image by the second set of base substances is obtained by weighted addition processing The image generating apparatus according to claim 1, wherein pixel values of pixels are obtained by weighted addition processing. Each of the first generation means includes a first substance and a second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base materials, Generating each material density image when the first or second material and a fourth material having an X-ray absorption coefficient smaller than that of the first material are used as a third set of base materials;
The second generation means calculates the pixel value of each pixel of the monochrome image when the pixel values of the corresponding pixels in the material density images of the first set of base materials are both positive. When the pixel value of the corresponding pixel in the substance density image of the second substance by the first set of base substances is negative, the correspondence in each substance density image by the third set of base substances is obtained by weighted addition processing The image generating apparatus according to claim 1, wherein pixel values of pixels are obtained by weighted addition processing. The image processing apparatus further includes third generation means for generating an image obtained when X-ray CT imaging of the imaging object is performed with X-rays having predetermined effective X-ray energy based on the data or an image corresponding to the image. And
Each of the first generation means includes a first substance and a second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base materials, Generating each material density image when the first material or the second material and the third material having a larger X-ray absorption coefficient than the second material are used as the second set of base materials;
The second generation means sets the pixel value of each pixel of the monochrome image, the pixel value of the corresponding pixel in the image generated by the third generation means has an X-ray absorption coefficient larger than that of the first substance. When the pixel value corresponds to a material smaller than the second material, the pixel values of the corresponding pixels in each material density image by the first set of base materials are obtained by weighted addition processing and generated by the third generation unit When the pixel value of the corresponding pixel in the obtained image is a pixel value corresponding to a substance having an X-ray absorption coefficient larger than that of the second substance, the pixel value of the corresponding pixel in each substance density image by the second set of base substances The image generation apparatus according to claim 1, wherein the image generation apparatuses are obtained by performing weighted addition processing on each other. The image processing apparatus further includes third generation means for generating an image obtained when X-ray CT imaging of the imaging object is performed with X-rays having predetermined effective X-ray energy based on the data or an image corresponding to the image. And
Each of the first generation means includes a first substance and a second substance having an X-ray absorption coefficient larger than that of the first substance as a first set of base materials, Generating each material density image when the first or second material and a fourth material having an X-ray absorption coefficient smaller than that of the first material are used as a third set of base materials;
The second generation means sets the pixel value of each pixel of the monochrome image, the pixel value of the corresponding pixel in the image generated by the third generation means has an X-ray absorption coefficient larger than that of the first substance. When the pixel value corresponds to a material smaller than the second material, the pixel values of the corresponding pixels in each material density image by the first set of base materials are obtained by weighted addition processing and generated by the third generation unit When the pixel value of the corresponding pixel in the obtained image is a pixel value corresponding to a substance having an X-ray absorption coefficient smaller than the first substance, the pixel value of the corresponding pixel in each substance density image by the third set of base substances The image generation apparatus according to claim 1, wherein the image generation apparatuses are obtained by performing weighted addition processing on each other.   The first material is water, the second material is iodine, and the third material is a metal, ceramics, or organic compound having a larger X-ray absorption coefficient than iodine. The image generation apparatus according to claim 4.   The first material is water, the second material is iodine, and the fourth material is an organic compound having an X-ray absorption coefficient smaller than that of fat or water. 5. The image generating apparatus according to 5.   The first generation means includes projection data based on X-rays having a first effective X-ray energy, projection data based on X-rays having a second effective X-ray energy different from the first effective X-ray energy, and The image generation apparatus according to any one of claims 1 to 7, wherein a material density image is generated by performing image reconstruction processing on data obtained by performing linear weighted subtraction or nonlinear weighted subtraction.   An X-ray CT apparatus provided with the image generation apparatus according to claim 1.   The program for functioning a computer as an image generation apparatus as described in any one of Claims 1-8.

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