JP5588697B2 - X-ray CT system - Google Patents

Description

  The present invention relates to an X-ray CT (Computed Tomography) apparatus, and sequentially measures the X-ray absorptance distribution image so that the measured measurement projection data and the calculated projection data obtained by forward projection processing of the X-ray absorptivity distribution image are equal. It is a technology to correct.

  An X-ray CT apparatus is an image diagnostic apparatus that calculates an X-ray absorption rate at each point from X-ray projection data obtained by imaging a subject from multiple directions and obtains a tomographic image (hereinafter referred to as a CT image) of the subject. One. CT images acquired from this device can diagnose a patient's medical condition accurately and immediately in a medical field, and are clinically useful. However, in order to obtain high-quality images necessary for doctors' diagnosis, a certain amount of exposure is involved. On the other hand, the lower the dose to achieve low exposure, the greater the ratio of noise to the signal and the more line-shaped artifacts and graininess noise that cause misdiagnosis. Therefore, if artifacts and noise can be reduced during low-dose imaging, high-quality diagnosis and low exposure can be realized. In order to solve this problem, the technique disclosed in Patent Document 1 proposes a successive approximation reconstruction method that sequentially corrects CT images so that measured projection data and calculated projection data are equal.

JP 2006-25868 A

Keesing, Daniel B. , Et al. "Missing data estimation for full 3D spiral CT image reconstruction", Proceedings of the SPIE, Volume 6510, pp. 65105V (2007).

  As described in Non-Patent Document 1, in the image reconstruction of the X-ray CT apparatus, an incomplete measurement region in which only a part of the angle X-rays in the rotation measurement is measured cannot satisfy the conditions necessary for the reconstruction. Therefore, there is a problem that the quantitativeness of the image is lowered. If the image is used as it is as the initial value of the successive approximation reconstruction method (hereinafter referred to as the initial image), the projection data is calculated and the image is corrected, including the pixels with reduced quantification. There is.

  In order to solve this problem, before or after normal imaging (hereinafter referred to as main imaging), an area that protrudes from an incomplete measurement area at the time of actual imaging is moved to a complete measurement area and imaging is performed. This shooting will be referred to as overshoot correction shooting. At this time, a subject, a table, a phantom fixture, a catheter, or the like can be considered as a target that protrudes from an incomplete measurement region. Next, using the CT image acquired in the overshoot correction imaging, the incomplete measurement area or a part of the complete measurement area in the CT image at the time of the actual imaging is replaced, thereby improving the quantitativeness of the initial image. . By using this image for the successive approximation reconstruction method, calculation projection data can be calculated and the image can be corrected without degrading the quantitativeness. Therefore, a high-quality CT image can be acquired.

  Specifically, the following X-ray CT apparatus is provided.

  The X-ray CT apparatus according to the first aspect of the present invention includes an X-ray generation unit that generates X-rays, an X-ray detection unit that detects the X-rays after passing through a subject, the X-ray generation unit, and the X-rays A projection data measurement unit that generates projection data from a detection signal of the X-ray detection unit measured by rotating the detection unit, and a complete measurement region in which X-rays of a rotation angle for one rotation are measured in the rotation measurement, or An image calculation unit that calculates a first CT image that is a distribution of X-ray absorption rate from the projection data with respect to an incomplete measurement region where X-rays at some angles are measured.

  Here, the image calculation unit corrects the first CT image located in the incomplete measurement region or a part of the complete measurement region by using the second CT image measured before or after photographing the subject. A function, a function of calculating projection data as an integral value of the corrected first CT image on a path connecting the X-ray generation unit and the X-ray detection unit, and a calculation value of the projection data equal to a measurement value And a function of sequentially correcting the corrected first CT image. In this way, the first CT image located in the incomplete measurement region is corrected using the second CT image located in the complete measurement region, so that the first CT image is not deteriorated. Can be corrected sequentially.

  In the X-ray CT apparatus according to the second aspect of the present invention, the image calculation unit includes a function of excluding a part of the corrected first CT image from the calculation, the X-ray generation unit, and the X-ray detection unit. A function of calculating first projection data as an integral value of the excluded first CT image on a path connecting the two, a function of creating a projection data difference value by subtracting the first projection data calculation value from a measurement value, A function of calculating second projection data as an integral value of the first CT image of the partial region on a path connecting the X-ray generation unit and the X-ray detection unit; and a calculation value of the second projection data is the projection data A function of sequentially correcting the corrected first CT image of the partial region so as to be equal to the difference value; Thus, the X-ray CT apparatus according to the second aspect of the present invention corrects the first CT image located in the incomplete measurement region by using the second CT image located in the complete measurement region. Thus, the CT image of a partial region can be sequentially corrected without degrading the quantitativeness of the image.

  In the X-ray CT apparatus of the third aspect of the present invention, an X-ray generation unit that generates X-rays, an X-ray detection unit that detects the X-rays after transmission through an object, the X-ray generation unit, and the X-rays A projection data measurement unit that generates projection data from a detection signal of the X-ray detection unit measured by rotating the detection unit, and a complete measurement region in which X-rays of a rotation angle for one rotation are measured in the rotation measurement, or An image calculation unit that calculates a first CT image that is a distribution of X-ray absorption rate from the projection data with respect to an incomplete measurement region where X-rays at some angles are measured.

  Here, the image calculation unit has a function of aligning the partial region of the second CT image measured before or after imaging of the subject with the partial region of the first CT image, the X-ray generation unit, and the X-ray detection. A function of calculating first projection data as an integral value of the second CT image subjected to the alignment processing on the path connecting the parts, and creating a first projection data difference value by subtracting the first projection data calculation value from the measurement value A function of calculating a third CT image that is a distribution of X-ray absorption rate from the first projection data difference value, and a third CT image on a path connecting the X-ray generation unit and the X-ray detection unit. A function of calculating second projection data as an integral value; and a function of sequentially correcting the third CT image so that the calculated value of the second projection data is equal to the first projection data difference value. As described above, the X-ray CT apparatus according to the third aspect of the present invention obtains the quantitativeness of the image by subtracting the projection data calculation value of the second CT image located in the incomplete measurement area from the projection data measurement value. The CT image can be sequentially corrected without lowering.

  In the X-ray CT apparatus according to the fourth aspect of the present invention, the image calculation unit connects a function of excluding a partial region in the third CT image from the calculation, and the X-ray generation unit and the X-ray detection unit. A function of calculating third projection data as an integral value of the excluded third CT image on the path, and calculating a second projection data difference value by subtracting the third projection data calculation value from the first projection data difference value. A function of creating, a function of calculating fourth projection data as an integral value of the third CT image of the partial region on a path connecting the X-ray generation unit and the X-ray detection unit, and calculation of the fourth projection data A function of sequentially correcting the third CT image so that the value becomes equal to the second projection data difference value; As described above, the X-ray CT apparatus according to the fourth aspect of the present invention obtains the quantitativeness of the image by subtracting the projection data calculation value of the second CT image located in the incomplete measurement region from the projection data measurement value. The CT image of a partial region can be sequentially corrected without lowering the image quality.

  The X-ray CT apparatus according to the fifth aspect of the present invention includes a table for fixing a subject and a position measuring unit for measuring a table position at the time of photographing the subject.

  Here, the image calculation unit uses the position information measured by the position measurement unit to correct the first CT image from the second CT image so as to match the table position of the first CT image, or A function of aligning the position of the second CT image; As described above, the X-ray CT apparatus according to the fifth aspect of the present invention can acquire a CT image with a small correction error or alignment error by using the measured table position information.

  In the X-ray CT apparatus of the sixth aspect of the present invention, the image calculation unit uses the position information measured by the position measurement unit to calculate the amount of table deformation at the time of subject photographing, and the first CT image. A function of correcting the first CT image from the second CT image using a deformation amount of the table or a function of aligning the position of the second CT image so as to match the table position. As described above, the X-ray CT apparatus according to the sixth aspect of the present invention reduces the correction error or alignment error by reflecting the deformation amount of the table similar to that at the time of subject photographing on the second CT image. CT images can be acquired.

  The X-ray CT apparatus according to the seventh aspect of the present invention includes a deformation amount storage unit that stores the deformation amount of the table according to the subject.

  Here, the image calculation unit reads the deformation amount from the deformation amount storage unit according to the subject to be photographed, and uses the deformation amount of the table so as to match the table position of the first CT image. A function of correcting the first CT image from the image or a function of aligning the position of the second CT image; As described above, the X-ray CT apparatus according to the seventh aspect of the present invention can obtain a CT image with a small correction error or alignment error by using a deformation amount stored in advance.

  The X-ray CT apparatus according to the eighth aspect of the present invention includes a similarity calculation unit that calculates a similarity that is an evaluation index for alignment with respect to the table position in the first CT image and the table position in the second CT image.

  Here, the image calculation unit calculates a table position of the second CT image showing a high similarity from the values calculated by the similarity calculation unit, and uses the calculated table position to calculate the second CT image from the second CT image. A function of correcting the 1CT image or a function of aligning the position of the second CT image; As described above, the X-ray CT apparatus according to the eighth aspect of the present invention uses the table position of the second CT image having a high similarity to the first CT image to reduce the correction error or the alignment error. The CT image can be corrected.

  In the X-ray CT apparatus according to the ninth aspect of the present invention, in the image calculation unit, the function of correcting the first CT image from the second CT image or the function of aligning the position of the second CT image, and the table position of the first CT image So that the second CT image is deformed using rigid transformation or non-rigid transformation. As described above, the X-ray CT apparatus according to the ninth aspect of the present invention deforms the table of the second CT image so as to approximate the table shape of the first CT image, thereby correcting the correction error or alignment error. CT image with a small size can be acquired.

  In the X-ray CT apparatus according to the tenth aspect of the present invention, when X-rays having a plurality of different types of energy spectra are generated from the X-ray generation unit, a plurality of types of different energy spectra acquired from the image calculation unit are obtained. An X-ray absorption rate storage unit that stores the X-ray absorption rate distribution is provided.

  Here, the image calculation unit reads out the X-ray absorption rate corresponding to the X-ray energy spectrum at the time of photographing the subject from the X-ray absorption rate storage unit, and corrects the X-ray absorption rate of the first CT image or the second CT image. And a function of correcting the first CT image from the second CT image or a function of aligning the position of the second CT image. As described above, the X-ray CT apparatus according to the tenth aspect of the present invention uses the X-ray absorption rate corresponding to the X-ray energy spectrum stored in advance, so that the CT image with a small correction error or alignment error can be obtained. Can be obtained.

  In the X-ray CT apparatus according to the eleventh aspect of the present invention, the image calculation unit uses the X-ray absorption rate located in the complete measurement region of the first CT image, thereby using the incomplete measurement region of the second CT image. It has a function of correcting the X-ray absorption rate located in the area. As described above, the X-ray CT apparatus according to the eleventh aspect of the present invention uses the X-ray absorption rate of the first CT image for the second CT image, so that the CT image with a small correction error or alignment error is obtained. Can be obtained.

  In the X-ray CT apparatus of the twelfth aspect of the present invention, the image calculation unit displays that the correction or the alignment process cannot be applied when the similarity calculated by the similarity calculation unit does not reach a threshold value. Have As described above, the X-ray CT apparatus according to the twelfth aspect of the present invention can notify whether correction or alignment processing is possible based on the calculated similarity.

The present invention relates to an X-ray CT apparatus, and corrects an incomplete measurement area or a part of a complete measurement area of a normal CT image using a correction CT image taken before or after normal imaging. As a result, by applying the corrected CT image to the initial image of the successive approximation reconstruction method, the CT image can be sequentially corrected without degrading the quantitativeness of the image.

The figure for demonstrating the structure of the hardware of each part of the apparatus in this invention. Example 1 The figure for demonstrating the flow of imaging | photography in this invention. Example 1 The figure for demonstrating the example of a screen of the imaging condition input part in this invention. Example 1 The figure for demonstrating each function of the reconstruction process part in this invention. Example 1 The figure for demonstrating the calculation procedure of the successive approximation reconstruction method in this invention. Example 1 The figure for demonstrating the imaging | photography range of CT image at the time of rotation imaging | photography in this invention. Example 1 The figure for demonstrating the protrusion correction | amendment in this invention. Example 1 The figure for demonstrating the effect of the phantom used for the simulation in this invention, and this invention method. Example 1 The figure for demonstrating each function of the reconstruction process part in this invention. (Example 2) The figure for demonstrating the calculation procedure of the local projection data creation function in this invention. (Example 2) The figure for demonstrating the effect of the phantom used for the simulation in this invention, and this invention method. (Example 2) The figure for demonstrating each function of the reconstruction process part in this invention. (Example 3) The figure for demonstrating the calculation procedure of the protrusion correction function and projection data difference function in this invention. (Example 3) The figure for demonstrating each function of the reconstruction process part in this invention. Example 4 The figure for demonstrating the measuring method of the table variation | change_quantity (xy direction) in this invention. (Example 5) The figure for demonstrating the measuring method of the table variation | change_quantity (z direction) in this invention. (Example 5) The figure for demonstrating the calculation procedure of the protrusion correction function in this invention. (Example 6) The figure for demonstrating the structure for calculating the similarity in this invention. (Example 6) The figure for demonstrating the alignment process in this invention. (Example 7) The figure for demonstrating the alert function in this invention. (Example 8)

  In the present embodiment, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 describes a hardware configuration for realizing an X-ray CT apparatus equipped with the successive approximation reconstruction software of the first embodiment.

  The apparatus shown in FIG. 1 includes an input unit 101 for inputting imaging conditions such as X-ray irradiation conditions and image reconstruction conditions, an imaging unit 102 for controlling imaging and X-ray irradiation and detection, and for detected signals. The image generating means 103 is configured to output an image by performing correction and image reconstruction. Note that the input unit 101 and the image generation unit 103 do not have to be configured integrally with the present apparatus, and may be configured as an apparatus installed at a remote location via a network, for example.

  In the input unit 101, the photographing conditions can be input by using, for example, a keyboard 111, a mouse 112, a pen tablet, a touch panel, and the like. The data input by the input means 101 is transmitted as a signal to the photographing means 102 by developing and starting a predetermined program in the central processing unit 114, the memory 113, the HDD (Hard Disk Drive) device 115, and the like. The respective components are connected by a data bus 101a.

  In the imaging unit 102, imaging control can be realized by the X-ray controller 117, the gantry controller 116, and the table controller 118 when the X-ray tube 1, the gantry 3, and the table 5 are operated. Next, X-ray irradiation and detection can be realized by the X-ray tube 1 and the X-ray detector 2. A typical example of the distance between the X-ray generation point of the X-ray tube 1 and the X-ray input surface of the X-ray detector 2 is 1000 [mm]. In the center of the gantry 3, a circular opening 7 for arranging the subject 6 and the table 5 is provided. A typical example of the diameter of the opening 7 is 700 [mm]. A typical example of the time required for rotation of the rotating plate 4 is 1.0 [s]. As the X-ray detector 2, a known X-ray detector 2 composed of a scintillator and a photodiode is used. The X-ray detector 2 has a large number of detection elements (not shown) in an arc shape equidistant from the X-ray tube 1, and a representative example of the number of elements (hereinafter referred to as the number of channels) is 950. A typical example of the size of each detection element in the channel direction is 1 [mm]. The number of times of photographing in one rotation of the photographing means 102 is 900, and one photographing is performed every time the rotating plate 4 rotates 0.4 degrees. The specifications are not limited to these values, and can be variously changed according to the configuration of the X-ray CT apparatus.

  In the image generation means 103, the signal detected by the X-ray detector 2 of the imaging means 102 is converted into a digital signal by a data acquisition system (DAS; Data Acquisition System) 119. Next, correction and image reconstruction of the converted digital signal can be realized by developing and starting a predetermined program in the central processing unit 121 and the memory 120, and the HDD unit 122 or the like can store data. I / O can be realized. The display of the reconstructed CT image can be realized by a monitor 123 such as a liquid crystal display or a CRT. Each component is connected by a data bus 103a.

  FIG. 2 is a diagram for explaining an imaging flow of the X-ray CT apparatus equipped with the successive approximation reconstruction software of the first embodiment. In the present apparatus shown in FIG. 2, the input unit 101 includes a photographing condition input unit 131 for inputting photographing conditions. The imaging unit 102 includes an imaging control unit 132 that controls imaging based on the imaging conditions input by the imaging condition input unit 131 and an imaging unit 133 that performs X-ray irradiation and detection. The image generating unit 103 includes a signal collecting unit 134 that converts a detected signal into a digital signal, a correction processing unit 135 that corrects the digital signal, and a reconstruction processing unit 136 that reconstructs an image of the corrected projection data. The image display unit 137 outputs a reconstructed CT image.

  Next, the flow of shooting will be described with reference to FIG. FIG. 3 is a diagram illustrating an example of the monitor screen 141 of the imaging condition input unit 131 illustrated in FIG. The operator uses the mouse 112, the keyboard 111, and the like to set an imaging region, an X-ray condition, and other imaging conditions. This screen includes an imaging region selection list 142 for selecting an imaging region, an X-ray condition 143 for setting a tube voltage and a tube current amount corresponding to the energy and output amount of X-rays to be irradiated in the main imaging, In the same manner, the X-ray condition 144 for setting the tube voltage and the tube current amount is used in the projection correction. In addition, this screen includes an item 145 for determining an X-ray irradiation area and a correction target list 146 for selecting a correction target in photographing for correction of protrusion.

  The operator selects the imaging region or the field of view [mm] (hereinafter referred to as FOV) of the reconstructed image in the manual setting mode from the imaging area selection list 142. For example, the region is selected from the chest, abdomen, head, neck, spine, hip joint, extremities, etc., and is not limited to the region, and may be a tissue such as the heart or coronary artery.

  In the X-ray condition 143 for actual imaging in this embodiment, a typical example of the tube voltage value specified by the operator is 120 [kV] and the tube current amount is 200 [mAs]. Similarly, the tube voltage value is 120 [kV] and the tube current amount is 300 [mAs] under the X-ray condition 144 for photographing for correction of protrusion. At this time, for example, when a correction target represented by a table or the like does not have the influence of exposure, it is desirable to set the tube current amount high and perform under a condition with good S / N. In the present embodiment, X-rays having one type of energy spectrum are assumed, but multi-energy CT using two or more types of X-rays can be performed in the same manner by adding items of tube voltage and tube current amount. . The operator selects one of the high-quality mode, the high-speed shooting mode, and the manual setting mode with respect to the projection area 145 for shooting for correction. For example, in the high image quality mode, cone beam artifacts and the like caused by a known analytical reconstruction method such as the Feldkamp method can be suppressed by narrowing the irradiation region with respect to the imaging target selected from the correction target list 146. On the other hand, in the high-speed imaging mode, the imaging time required for the overshoot correction imaging can be shortened by widening the irradiation area. The numeric notation of the number of slices in the manual setting mode adds a numeric entry field to allow numeric entry. For example, FIG. 3 shows that data for 16 slices of the X-ray detector is taken in the body axis direction. Further, the operator selects from the correction target list 146 for the overshoot correction photographing. For example, a subject, a table, a catheter or the like is selected. At this time, not only the subject but also a region such as the chest and a tissue such as the heart may be selected.

  FIG. 3 is an example of an imaging region, X-ray conditions, and other imaging conditions, and is not limited to this screen configuration. Further, when settings of the imaging region, X-ray conditions, and other imaging conditions are stored in the HDD device 115 in advance, the operator does not need to input each time.

  Next, X-ray imaging is performed according to the imaging conditions input by the imaging condition input unit 131. Usually, X-ray imaging is performed in the order of main imaging and projection correction imaging.

  First, regarding the actual shooting, the operator uses the mouse 112, the keyboard 111, and the like to specify the shooting position of the subject 6 and then instructs the start of shooting. When the start of photographing is instructed, the table controller 118 of the photographing controller 132 moves the table 6 in the direction substantially perpendicular to the rotating plate 4 by the table 5. When the photographing position of the subject 6 coincides with the specified value, the movement is stopped and the placement of the subject 6 is ended. On the other hand, the gantry controller 116 of the imaging control unit 132 starts the rotation of the rotating plate 4 via the drive motor at the same time when the imaging start is instructed. When the rotation of the rotating plate 4 enters a constant speed state and the arrangement of the subject 6 is completed, the gantry controller 116 detects the X-ray irradiation timing of the X-ray tube 1 of the imaging unit and the X-ray detector 2 of the imaging unit. Instruct the shooting timing and start shooting. In this embodiment, for example, the energy spectrum and output amount of X-rays to be irradiated are determined by the tube voltage and tube current amount of the X-ray tube 1 set by the operator.

  In the present embodiment, X-rays having one type of energy spectrum are used. However, a multi-beam which obtains imaging data by irradiating X-rays having two or more types of energy spectra by switching the tube voltage at high speed every rotation. It can also be applied to energy CT.

  Next, the X-ray detector 2 of the imaging unit 133 detects X-ray photons transmitted through the subject 6 and converts them into digital signals by the DAS 119 of the signal acquisition unit 134. The acquired X-ray detection data is stored in the memory 120. The correction processing unit 135 performs a known offset correction or a known air calibration process for correcting the sensitivity between the detectors on this data, and acquires measurement projection data of the subject 6. The offset correction is a method of calibrating to zero the signal value when no X-ray is detected by subtracting the X-ray signal value measured without generating the X-ray.

Next, the reconfiguration processing unit 136 configures the functions shown in FIG. 4 and will be described using the processing flow of FIG. For the measurement projection data R (i) corrected by the correction processing unit 135, the analytical reconstruction function 151 uses an analytical reconstruction method such as a well-known Feldkamp method in step 161 to perform X-ray analysis of the subject. A CT image λ ^{k = 0} (j) representing the absorption rate is calculated. i and j represent a detector number and a pixel number, respectively. At this time, the CT image λ ^{k = 0} (j) may be repeatedly corrected using a known successive approximation reconstruction method.

  Next, after the main shooting, regarding the overshoot correction shooting, the operator instructs the start of shooting using the mouse 112, the keyboard 111, or the like.

  First, in step 162, the protrusion correction function 152 determines whether or not the correction target located in the imperfect measurement region is protruded based on the CT image acquired in the main imaging. FIG. 6A is a diagram for distinguishing X-ray measurement regions in the tomographic direction. The pair of the X-ray tube 1 and the X-ray detector 2 in FIG. 6A rotates and photographs the subject about the body axis direction z181. At this time, on the path connecting the X-ray tube 1 and a pixel j of an arbitrary CT image, an area where X-rays having a rotation angle of one round is detected is a complete measurement area 182 (indicated by hatching in FIG. 6), An area where the X-ray of the rotation angle is not detected is defined as an incomplete measurement area 183 (dot display in FIG. 6). In the present embodiment, this means that the table 5 indicated by the arrow 184 protrudes into an incomplete measurement area. FIG. 6B shows the body axis direction z185, and the measurement region is defined in the same manner as in FIG. At this time, the detector 2 has a thickness in the direction of the body axis as compared with the X-ray source 1, and therefore detects a beam (hereinafter referred to as a cone beam) spreading three-dimensionally from the X-ray source 1. Due to this influence, the imperfect measurement region 183 increases as the distance from the central slice 186 where the X-ray source 1 is located.

In step 162, the protrusion is determined by using an image processing technique such as visual judgment of the operator by the monitor 123, measuring means using a known laser or position sensor, or region extraction.
For example, in a measuring means using a laser, a laser is installed at the position of an X-ray tube assuming an arbitrary rotation angle, and a laser detector is installed at the positions of opposite ends of the CT detector. Next, the laser detectors at both ends are irradiated with laser, and the case where the laser can be detected is not projected, and the case where the laser cannot be detected is projected. At this time, the rotation angle may be assumed to be, for example, an X-ray tube at the top of the gantry and a detector at the bottom, but information on other angles or a plurality of angles may be acquired.

  For example, a means using an image processing technique uses a CT image acquired by actual imaging. A part or all of the pixels of the CT image are scanned, and the presence / absence of an imaging target is determined at the end of the image. At this time, for example, a pixel value of −900 [HU] or less may be determined as air using threshold processing. As another method, a region such as a subject or a table may be extracted from a CT image using an existing image processing technique. As a result, the extracted regions are compared with each other using a past database or the like, and the presence or absence of protrusion is determined.

  In the case of determination of the presence of protrusion, the table controller 118 controls the position of the correction target manually or automatically so that the complete measurement area 182 satisfies the correction target, and performs imaging. For example, in this embodiment, the correction target is the table 5. At this time, when the table 5 cannot be filled in the complete measurement region 182, the result of the projection correction not possible is displayed on the monitor 123. In the same way as the main shooting, the shooting for overshoot correction is performed in the order of the configuration of the shooting condition input unit 131, the shooting control unit 132, the shooting unit 133, the signal collection unit 134, the correction processing unit 135, and the reconstruction processing unit 136 in FIG. CT image is acquired.

  Next, in step 163, the incomplete measurement region 183 or a part of the complete measurement region 182 of the CT image acquired by the main imaging is corrected using the CT image acquired by the overshoot correction imaging. The CT image shown in FIG. 7A is the result of actual imaging with the subject 6 placed on the table 5, and the end of the table 5 is not filled with the complete measurement region 182 and is partially missing. Next, FIG. 7B shows a result obtained by moving upward so that the complete measurement area 182 fills the table 5 after the main imaging, and performing an overshoot correction imaging. FIG. 7C is an image obtained by matching the table position shown in FIG. 7B with the table position shown in FIG. 7A and replacing the table 5 in the incomplete measurement region 183. In the present embodiment, only the incomplete measurement area 183 is replaced, but the table 5 of the complete measurement area 182 may be replaced in addition to the incomplete measurement area 183.

  In this embodiment, the correction of the protrusion of the table 5 is assumed, but the correction may be applied to the protrusion of the subject 6, the phantom fixture, the catheter, and the like. The table 5 may be considered including a pillow, a headrest and the like.

  In the present embodiment, the overshoot correction shooting is performed after the main shooting, but the same is true even if the shooting is performed before the main shooting. In that case, by using the above-described laser measuring means and image processing technology, first, the protrusion of the correction target located in the incomplete measurement region 183 is determined. In accordance with the determination result, the table controller 118 controls the position manually or automatically so that the complete measurement area 182 satisfies the correction target, and takes an image.

  Next, the corrected CT image is used as an initial image of the successive approximation reconstruction method, and is sequentially corrected. In step 164, if the update count k being calculated is smaller than the set update count K, the image is corrected using the measured projection data R (i) obtained by the rotational imaging in steps 165 to 168.

As an algorithm for correcting an image, for example, ASIRT (Additional Simulated Reconstruction Technique), which is one of successive approximation reconstruction methods, is expressed by Equation 1.

λ ^k (j) represents the pixel value of the pixel j of the CT image at the update count k being calculated, and is assumed to be composed of J pixels. The CT image is not only a general two-dimensional (x, y direction) tomographic image, but also one-dimensional data (x direction) and three-dimensional data (x, y, z direction) obtained by superimposing the images in the body axis direction z. ), Or four-dimensional data (x, y, z, t) considering the time direction t in three dimensions. R (i) represents measured projection data, and RC ^k (i) represents calculated projection data obtained by forward projection processing on the CT image at the update count k being calculated. Further, it is assumed that there are I detectors without distinguishing the projection direction, and p (i, j) represents the probability that the X-ray passing through the pixel j is detected by the i-th detector. Further, the relaxation coefficient α represents a ratio of correction with respect to the pixel value λ ^k (j) of the update count k.
Next, in step 165, the forward projection function 153 forward-projects λ ^k (j) shown in Expression 2 to obtain calculated projection data RC ^k (i).

Next, in step 166, the data comparison function 154 compares and calculates the calculated projection data shown in Expression 3 and the measured projection data, and obtains updated projection data ΔR ^k (i).

Next, in step 167, the backprojection processing function 155 performs backprojection processing on the updated projection data shown in Expression 4 as shown in Expression 5, and obtains an updated image Δλ ^k (j).

Next, in step 168, the image update function 156 acquires the CT image λ ^{k + 1} (j) modified as shown in Equation 5 using the updated image shown in Equation 4. As an example, α = 1.0 is set, and a relaxation coefficient α of 1.0 or more is used for early convergence and less than 1.0 for late convergence.

  As described above, after completing steps 165 to 168, the update count k is incremented to k + 1 in step 169, and loop processing is performed by returning to step 164. At this time, if the updated number of updates k is greater than the set number of updates K, the update ends, and in step 170, the image display unit 137 outputs a CT image.

  As described above, steps 164 to 170 show the calculation procedure of the successive approximation reconstruction method.

  The successive approximation reconstruction method represented by Equation 1 of the first embodiment is an example, and is known SPS, OS-SPS, PWLS, OS-PWLS, MSIRT, GRADY, CONGR, ART, SART, ML-EM, OS-. You may apply to other methods, such as EM, FIRA, RAMLA, DRAMA.

  Finally, the image display unit 137 displays the calculated CT image on the monitor 123 to provide information to the operator. A network adapter can be used to connect to an external terminal via a network such as a local area network, a telephone line, or the Internet, and CT images can be transmitted to and received from the terminal.

  In this embodiment, the CT image is reconstructed using the measurement projection data acquired from the rotation for one round, but the present invention is not limited to one round and can be applied to a known half reconstruction. At this time, the complete measurement region is a region where the rotation angle satisfying the complete collection condition in the half reconstruction is acquired.

  In this embodiment, a normal scan method in which the table 5 is not moved is assumed. However, a step-and-shoot method in which normal scan is performed by repeating the operation and stop order of the table 5 at regular intervals, and a spiral scan method in which imaging is performed while moving the table. However, the definition of the complete measurement region 182 and the incomplete imaging region 183 is the same, and it goes without saying that the present invention may be applied.

  In the present embodiment, a living body X-ray CT apparatus is shown as an example, but it goes without saying that the present invention may be applied to an X-ray CT apparatus for non-destructive inspection such as explosives inspection and product inspection. Yes. This embodiment shows a known third-generation multi-slice X-ray CT apparatus as an example, but can also be applied to known first-, second-, and fourth-generation X-ray CT apparatuses. It can also be applied to a line CT apparatus and an electron beam CT.

  In order to verify the effectiveness of the present invention, a simulation experiment was performed in consideration of system noise such as quantum noise and circuits. The phantom to be photographed is assumed to be an elliptical human abdomen and a table, and the table is set downward so as to be positioned in a partially incomplete measurement region 183. The phantom of the human abdomen has a structure having an X-ray absorption rate close to that of living tissue.

As a result of the simulation, FIG. 8A shows a CT image obtained by sequentially correcting the conventional initial image, and FIG. 8B shows a CT image obtained by sequentially correcting the initial image after the overhang correction according to the present invention. In this embodiment, image reconstruction by ASIRT using a known subset method is performed, and an image having the number of updates = 10, the number of subsets = 40, and the rotational imaging relaxation coefficient α = 1.0 is displayed.
As a result of the evaluation, in the conventional method of FIG. 8A, an increase or decrease in the CT value is observed at the end of the table, but the present invention in FIG. 8B can suppress the influence of the increase or decrease in the CT value. As a result, the present invention can improve the quantitativeness of the CT image as compared with the conventional successive approximation reconstruction method.

  In the present embodiment, in addition to the first embodiment, a hardware configuration when an overhang correction is applied to sequential reconstruction only for a local region (hereinafter referred to as ROI successive approximation reconstruction technique) will be described. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the following, FIG. 9 shows each function constituting the reconfiguration processing unit 136, which will be described using the processing flow of FIG. In the analytical reconstruction function 191 and the protrusion correction function 192 in FIG. 9, the same processing as in the first embodiment is performed. Next, the local projection data creation function 193 excludes, from the CT image, the imaging region selected from the imaging area selection list 142 and the area included in the image FOV in step 201 of FIG. Next, in step 202, the CT image is forward-projected while excluding a part of the region. In this case, exclusion means that forward projection processing is not included in the calculation target. In this embodiment, the exclusion process is used. However, the present invention is not limited to this method, and the same meaning is obtained even if the forward projection process is performed without replacing the pixel value in the area with 0.

Next, in step 203, the calculated projection data is subtracted from the measured projection data, and local projection data shown in Expression 6 is created.

  After step 203, the measured projection data is replaced with local projection data, and the normal successive approximation reconstruction method is calculated in the order of the forward projection function 194, the data comparison function 195, the back projection function 196, and the image update function 197. , CT images are output.

  In order to verify the effectiveness of the present invention, verification was performed using the same simulation conditions as in Example 1. The FOV of the image was a square based on the image center, the normal reconstruction method was 512 mm, and the ROI successive approximation reconstruction method was 128 mm.

  As a result of the simulation, FIG. 11A shows a CT image obtained by sequentially correcting the conventional initial image, and FIG. 11B shows a CT image obtained by sequentially correcting the initial image after the overhang correction according to the present invention.

  In this embodiment, image reconstruction by ASIRT using a known subset method is performed, and an image having the number of updates = 10, the number of subsets = 40, and the rotational imaging relaxation coefficient α = 1.0 is displayed.

  In order to evaluate the quantitativeness, the region of interest 211 shown in FIG. 11B was installed, and the error of the CT value was measured. For the error evaluation, | average value within region of interest−true value | was used. As a result, the error was reduced to 0.7 [HU] from the present invention compared to 4.7 [HU] of the conventional method. Further, in the conventional method shown in FIG. 11 (a), the CT value increase / decrease indicated by the arrow 212 is observed, whereas the present invention shown in FIG. 11 (b) can suppress the influence of the CT value increase / decrease. As described above, the present invention can improve the quantitativeness of CT images as compared with the conventional ROI successive approximation reconstruction method.

  In this embodiment, a method of sequentially correcting CT images without degrading the quantitativeness of the image by subtracting the calculation projection data to be corrected located in the imperfect measurement area from the measurement projection data will be described. The present embodiment is a hardware configuration that realizes an X-ray CT apparatus equipped with successive approximation reconstruction software as in the first embodiment. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the following, FIG. 12 shows each function constituting the reconfiguration processing unit 136. In the analytical reconstruction function 221 shown in FIG. 12, processing similar to that in the first embodiment is performed. Next, the protrusion correction function 222 and the projection data difference function 223 will be described using the processing flow of FIG.

  First, in the protrusion correction function 222, in step 231, the presence / absence of protrusion is determined in the same manner as in the first embodiment.

  Next, in step 232, the correction target of the CT image acquired by the overshooting correction imaging is aligned with the correction target of the CT image acquired by the main imaging. For example, in this embodiment, the correction target is the table 5. At this time, the alignment means that the table position of the CT image acquired by the overshooting correction imaging is moved so as to coincide with the table position of the CT image acquired by the main imaging. Do not replace.

  Next, in step 233, the projection data difference function 223 performs forward projection processing on the CT image aligned in step 232, and acquires measurement projection data limited to the table 5. Next, in step 234, the calculated projection data limited to the table 5 is subtracted from the measured projection data, and the projection data excluding the table 5 is obtained from the measured projection data. In step 235 and thereafter, the measured projection data is replaced with the projection data after the table is excluded, and in step 235, the projection data after the table is excluded is reconstructed to obtain a CT image.

  Next, in the forward projection function 224 and thereafter, the calculation of the normal successive approximation reconstruction method is performed in the order of the data comparison function 225, the back projection function 226, and the image update function 227, as in the first embodiment, and the CT image is obtained. Output.

  In this way, by subtracting the calculated projection data to be corrected from the measured projection data, the influence of errors existing in the imperfect measurement region 183 can be excluded, and the CT images can be sequentially obtained without degrading the quantitativeness of the images. Can be corrected.

  In this embodiment, in addition to the third embodiment, a hardware configuration for realizing an X-ray CT apparatus equipped with ROI successive approximation reconstruction software for only a local region will be described. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 14 shows each function constituting the reconfiguration processing unit 136. FIG. The analytical reconstruction function 241, the protrusion correction function 242, and the projection data difference function 243 in FIG. 14 are the same processes as in the third embodiment. Next, the local data creation function 244 is the same processing as in the second embodiment. After the forward projection function 245, the calculation of the normal successive approximation reconstruction method is performed in the order of the data comparison function 246, the back projection function 247, and the image update function 248, and a CT image is output.

  In this way, by subtracting the calculated projection data to be corrected from the measured projection data, the influence of errors existing in the imperfect measurement region 183 can be excluded, and the CT images can be sequentially obtained without degrading the quantitativeness of the images. Can be corrected.

  In this embodiment, in addition to Embodiment 1 or Embodiment 3, by measuring the positions of the subject 6, the table 5, and the like at the time of actual photographing, a CT image with a small error in overshoot correction or alignment error is acquired. List the methods. In the present embodiment, in addition to the hardware configuration for realizing the X-ray CT apparatus equipped with the successive approximation reconstruction software as in the first embodiment or the third embodiment, the positions of the subject 6, the table 5 and the like at the time of actual imaging are determined. Comes with a sensor to measure. At this time, the position of the attached sensor does not need to be integrated with the apparatus.

  For example, the sensor uses a known light, laser, or camera to measure the distance from the sensor to the table 5 or the table position.

In addition, the amount of deformation of the table 5 or the like due to the load of the subject 6 can be measured using a sensor. For example, FIG. 15 shows the cross-sectional direction, and the amount of deformation of the table 5 is estimated using the fact that the distance measured using the laser 251 differs before and after the load on the subject 6. For example, in FIG. 15, the difference between the distance L _{before the} load and the distance L _after the load is used. Next, a CT image with a small correction or alignment error can be acquired by converting the shape of the table 5 of the CT image into linear or non-linear using the estimated deformation amount. FIG. 16 similarly shows the body axis direction z 252, and the thickness d of the table section in the section direction varies depending on the deformation amount of the table 5. Similar to FIG. 15, the length of the front load L _before, when the distance after the load and _{after the} L, calculates a bending angle θ255 table 5 due to the load from the distance l between the fulcrum 253 of the table 5 from equation 7 measurement points 254 it can.

For example, if the thickness d of the table cross section and the deflection angle θ of the table 5 due to the load are assumed to be 5 degrees, the thickness d ′ of the table cross section after the deformation is increased by about 0.4% from the expression 8 compared to before the deformation. To do.

  In this embodiment, the amount of deformation is calculated from the distance and position of the laser 251 and the like. However, the present invention is not limited to this method, and the amount of deformation is calculated by measuring the displacement of the table 5 using a strain gauge, a pressure sensor, or the like. Also good.

  In the present embodiment, real imaging or overshoot correction imaging can be performed in real time during imaging without being limited to before and after X-ray irradiation.

  In this embodiment, it is possible to store the results measured in advance with phantoms, clinical data, etc. as a calculation table and use it for correction or alignment processing. For example, the calculation table stores table deformation amounts [mm] in the cross-sectional direction and body axis direction corresponding to the size and load of the phantom. First, the table deformation amount is read out from the stored calculation table corresponding to the size and load of the subject measured before or after the main photographing. Next, by using the read table deformation amount for correction or alignment processing, a CT image with a small correction or alignment error can be acquired. By using this method, it is possible to omit the work of measuring the amount of table deformation at the time of each main photographing, leading to a reduction in calculation time and a reduction in calculation amount.

  In the present embodiment, in addition to the first or third embodiment, a similarity that serves as an index for aligning the position of the protrusion correction CT image is calculated with respect to the CT image correction target in the main imaging, and a high similarity is obtained. The CT image for overhang correction is moved to the position to be corrected, and the CT image for actual imaging is corrected or aligned.

Hereinafter, for example, FIG. 17 shows a processing flow of the protrusion correction function 152 of FIG. 4 when the table 5 is to be corrected. First, in order to extract only the table information from the CT image for overhang correction, in step 261, all except for the area around the CT value in the table 5 is set to air by threshold processing. For example, in this embodiment, since the CT value in Table 5 is assumed to be 0 [HU], all pixels below -200 [HU] including noise components are set to air -1000 [HU]. Next, in step 262, an area other than air is assumed to be the table 5, and the coordinates where the table is located and the range A in the x direction and the range B in the y direction are calculated. Next, in step 263, the degree of similarity between the CT image for actual imaging and the CT image for overhang correction is calculated. For example, in this embodiment, the similarity is calculated using the difference between two images shown in Expression 9.

  At this time, the start point of the CT image λorg for the actual imaging is set to (x, y), and the coordinates of the start point of the CT image λest for the overhang correction are set to (x ′, y ′). The start point refers to the coordinates located at the upper left in the table extracted from the CT image for the overshoot correction imaging. For the pixels in the range A in the x direction and the range B in the y direction, the difference value for each pixel is integrated.

Next, in step 264, a difference value Δλ shown in Equation 9 is calculated for each coordinate (x, y) in the range X, Y of the CT image acquired in the main imaging, and the minimum difference value Δλ ( Let x ″, y ″) be the table position with the maximum similarity.

  For example, for the image ranges X and Y, the similarity may be calculated only with the complete measurement region 182, or in addition, the incomplete measurement region 183 may be calculated.

  Next, in step 265, the CT image λest (x ′ + a, y ′ + b) is translated with respect to the CT image λorg (x ″ + a, y ″ + b) using a known affine transformation or the like. Replace.

  In this embodiment, the integration of the difference value is used for the evaluation of the similarity. However, the method of integrating the absolute value of the difference value, the method of calculating the square error, the method of least squares, the method of calculating the correlation coefficient, etc. You may use.

  Further, the calculation time is required as the image ranges X and Y to be calculated are increased. Since the image ranges X and Y can be limited by specifying the table position of the actual photographing in advance, the calculation time is shortened.

  In the present embodiment, the entire area of the extracted table 5 is treated as the ranges A and B in step 262. However, the similarity may be calculated by limiting to only a partial area such as the surface of the table. For example, as shown in FIG. 18A, a structure 271 having a characteristic shape is embedded in a partial region of the table 5. By using this method, it is possible to apply correction or alignment by photographing only the structure 271 in a partial region, which is effective even when the protruding region is large.

  Further, in the cross-sectional direction of FIG. 18B, the similarity can be calculated by limiting to the structure of the triangle 272 and the circle 273. In addition, by embedding a structure 271 having different widths w of the circle 272 and the square 273 with respect to the body axis direction z, the z position of the CT image for the overshooting correction is determined from the width w of the CT image acquired in the main imaging. And correction or alignment errors can be reduced.

  In the present embodiment, in addition to the sixth embodiment, a method of calculating the deformation amount of the table from the result of aligning the CT image for the overshoot correction and correcting the CT image for the overshoot correction in accordance with the calculation is given.

  Hereinafter, this embodiment will be described with reference to FIG. FIG. 19A is a CT image of the main imaging in the present example. First, as in the sixth embodiment, parallel movement is performed by a known affine transformation 281 and the like, and the table position of the CT image in the overshoot correction imaging is aligned with the CT image of the actual imaging. Next, a table in the CT image for the overshoot correction shown in FIG. 19B is used so as to match the CT image for the main imaging shown in FIG. 19A by using the known rigid body transformation and non-rigid body transformation 282. 5 is deformed. At this time, the CT image shown in FIG. 19A can correct the incomplete measurement region 183 by replacing the deformed table 5. By using this method, correction errors or alignment errors due to deformation of the table 5 can be reduced, and image quality can be improved.

  In the present embodiment, in addition to the sixth embodiment, when the degree of similarity with respect to the correction target is small, an overhang correction impossible alert is displayed. FIG. 20 is a diagram illustrating an example of the monitor screen 141 of the imaging condition input unit 131. First, the similarity is calculated for the correction target selected from the correction target list 291. If the similarity calculated at this time does not reach the specified threshold value, it is determined that the protrusion correction is not applicable, and an alert is displayed on the image display screen 292 together with the image output. The threshold can be arbitrarily set and may be changed according to the correction target.

  In the present embodiment, in addition to the fifth embodiment, a method of correcting the CT value to be corrected in the protrusion correction CT image according to the irradiated or transmitted X-ray energy spectrum will be described.

  First, after aligning the table positions in the CT image for the overshooting correction, the CT values of the table area in the CT image for the main imaging are averaged to obtain a representative CT value of the table with sufficiently reduced noise. . Next, by substituting the acquired CT value for the CT value of the table in the overshoot correction imaging, a CT image of the main imaging with a high S / N can be acquired.

  In general, the CT value of the table 5 changes according to the irradiated and transmitted X-ray energy spectrum. For this reason, the table 5 is imaged in advance with X-rays having a plurality of types of energy spectra, and a calculation table storing the CT values of the table 5 is created. Thereby, the imaging | photography of the table 5 can be abbreviate | omitted for every X-ray energy spectrum, and imaging | photography time can be shortened. At this time, in addition to the table 5, by using a phantom simulating an imaging region or the like, clinical data, or the like, the X-ray energy spectrum transmitted through the subject 6 can be taken into consideration, and the CT value error can be further reduced.

  Further, by multiplying a table CT value of one type of photographed X-ray energy spectrum by a proportional coefficient, it can be converted into a CT value by another X-ray energy spectrum. By using this method, the time for creating the calculation table can be shortened by calculating in advance the calculation table based on one type of X-ray energy spectrum and the proportionality coefficient in another X-ray energy spectrum.

DESCRIPTION OF SYMBOLS 1 ... X-ray tube, 2 ... X-ray detector, 3 ... Gantry, 4 ... Rotating plate, 5 ... Table, 6 ... Subject, 7 ... Circular opening part, 101 ... Input means, 102 ... Imaging means, 103 ... Image Generating means, 111... Keyboard, 112 .. mouse, 113 .. memory, 114... Central processing unit, 115... HDD device, 116 .. gantry controller, 117 .. X-ray controller, 118. ... Memory, 121 ... Central processing unit, 122 ... HDD device, 123 ... Monitor, 131 ... Shooting condition input unit, 132 ... Shooting control unit, 133 ... Shooting unit, 134 ... Signal collection unit, 135 ... Correction processing unit, 136 ... Reconstruction processing unit, 137 ... Image display unit, 141 ... Monitor screen, 142 ... Imaging region selection list, 143 ... X-ray condition for main imaging, 144 ... X-ray condition for imaging for overshoot correction, 45 ... X-ray irradiation area for projection for imaging, 146 ... List of correction target for imaging for projection, 151 ... Analytical reconstruction function, 152 ... Projection correction function, 153 ... Forward projection function, 154 ... Data comparison function, 155 ... Backprojection function, 156 ... Image update function, 161 to 170 ... Calculation step of successive approximation reconstruction method according to the present invention, 181 ... Body axis direction z (cross-sectional direction), 182 ... Complete measurement region, 183 ... Incomplete Measurement area, 184 ... Table in incomplete measurement area, 185 ... Body axis direction z (body axis direction), 186 ... Central slice, 191 ... Analytical reconstruction function, 192 ... Projection correction function, 193 ... Local projection data Creation function, 194 ... Forward projection function, 195 ... Data comparison function, 196 ... Back projection function, 197 ... Image update function, 201 to 203 ... Local projection according to the present invention Calculation step of data creation function, 211 ... region of interest, 212 ... display of CT value increase / decrease, 221 ... analytical reconstruction function, 222 ... protrusion correction function, 223 ... projection data difference function, 224 ... forward projection function, 225 ... Data comparison function, 226... Back projection function, 227... Image update function, 231 to 235. ... projection data difference function, 244 ... local projection data creation function, 245 ... forward projection function, 246 ... data comparison function, 247 ... back projection function, 248 ... image update function, 251 ... laser, 252 ... body axis direction z, 253 ... table fulcrum, 254 ... table measurement point, 255 ... deflection angle θ of table 5, 261 to 265 ... according to the present invention Calculation step of the correction function 271 ... Structure, 272 ... Structure (triangle), 273 ... Structure (circle), 281 ... Parallel movement by affine transformation, 282 ... Movement by rigid body transformation or non-rigid body transformation, 291 ... Projection Correction target list for correction shooting, 292... Image display screen

Claims (9)

An X-ray generator that generates X-rays;
An X-ray detection unit for detecting the X-ray after passing through the subject;
A projection data measurement unit that generates measurement projection data from a detection signal of the X-ray detection unit measured by rotating the X-ray generation unit and the X-ray detection unit;
In the rotation measurement, the X-ray absorption rate is calculated from the measurement projection data with respect to a complete measurement region in which X-rays of rotation angles for one round are measured and an incomplete measurement region in which X-rays at some angles are measured. An image calculator that calculates a first CT image that is a distribution of
The image calculation unit uses a second CT image measured before or after the first CT image is captured, corrects an incomplete measurement region of the first CT image, and creates a first CT correction image;
A function of excluding a partial region in the first CT correction image from the calculation;
A function of acquiring first calculation projection data as an integral value of the first CT correction image subjected to the exclusion process on a path connecting the X-ray generation unit and the X-ray detection unit;
A function of creating a projection data difference value by subtracting the value of the first calculation projection data from the measurement projection data;
A function of acquiring second calculation projection data as an integral value of the first CT correction image of the partial region on the path connecting the X-ray generation unit and the X-ray detection unit;
An X-ray CT apparatus comprising a function of sequentially correcting the first CT correction image of the partial region so that the value of the second calculated projection data is equal to the projection data difference value. The X-ray CT apparatus according to claim 1, further comprising a table for fixing the subject and a position measuring unit for measuring a table position at the time of photographing the subject.
  The image calculation unit uses the position information measured by the position measurement unit to correct the first CT image from the second CT image so as to match the table position of the first CT image, or the second CT image An X-ray CT apparatus having a function of aligning the position of the X-ray CT. The X-ray CT apparatus according to claim 2, wherein the image calculation unit uses a position information measured by the position measurement unit to calculate a table deformation amount at the time of photographing an object, and a table position of the first CT image. An X-ray CT apparatus comprising a function of correcting the first CT image from the second CT image using a deformation amount of the table so as to match the position of the second CT image or a function of aligning the position of the second CT image . The X-ray CT apparatus according to claim 3, further comprising a deformation amount storage unit that stores a deformation amount of the table according to the subject.
  The image calculation unit reads the deformation amount from the deformation amount storage unit according to the subject to be photographed, and uses the deformation amount of the table to match the table position of the first CT image. An X-ray CT apparatus comprising a function of correcting the first CT image or a function of aligning the position of the second CT image. 5. The X-ray CT apparatus according to claim 2, wherein the similarity calculation unit calculates a similarity that is an evaluation index for alignment with respect to a table position in the first CT image and a table position in the second CT image. Have
  The image calculation unit calculates a table position of the second CT image showing a high similarity from the values calculated by the similarity calculation unit, and uses the calculated table position to calculate the first CT image from the second CT image. An X-ray CT apparatus comprising a function of correcting or a function of aligning the position of the second CT image. 6. The X-ray CT apparatus according to claim 2, wherein the image calculation unit includes a function of correcting the first CT image from the second CT image or a function of aligning the position of the second CT image, and the first CT image. A function of deforming the corrected second CT corrected image or the aligned second CT aligned image using rigid transformation or non-rigid transformation so as to match the table position of the 1CT image is provided. X-ray CT system. 7. The X-ray CT apparatus according to claim 2, wherein when the X-ray generation unit generates X-rays having a plurality of different energy spectra, the plurality of types acquired from the image calculation unit are different. Having an X-ray absorptivity storage unit for storing the X-ray absorptivity distribution of the energy spectrum;
  The image calculation unit reads an X-ray absorption rate corresponding to an X-ray energy spectrum at the time of photographing an object from the X-ray absorption rate storage unit, and corrects the X-ray absorption rate of the first CT image or the 2CT image; An X-ray CT apparatus comprising a function of correcting the first CT corrected image from the second CT corrected image or a function of aligning the position of the second CT corrected image. 8. The X-ray CT apparatus according to claim 2, wherein the image calculation unit uses the X-ray absorption rate located in a complete measurement region of the first CT image to correct the corrected second CT correction. An X-ray CT apparatus having a function of correcting an X-ray absorption rate located in an incomplete measurement region of an image. 6. The X-ray CT apparatus according to claim 5, wherein the image calculation unit has a function of displaying that the correction or the alignment process cannot be applied when the similarity calculated by the similarity calculation unit does not reach a threshold value. An X-ray CT apparatus characterized by that.