JP6771879B2 - X-ray computed tomography equipment - Google Patents

Description

An embodiment of the present invention relates to an X-ray computed tomography apparatus.

As an automatic setting technique of a tube current in an X-ray computed tomography apparatus, there is AEC (Auto Exposure Control) that automatically determines a tube current according to an X-ray absorption index value such as water equivalent thickness. When the step & shoot method of collecting data while intermittently moving the top plate is executed in AEC, the tube current value is set in the imaging region related to the stop position of each top plate. The average value of the X-ray absorption index values of the plurality of pixels included in each imaging region is set as the X-ray absorption index value for the imaging region. When the step-and-shoot method is executed, if the shape of the crown or the like suddenly changes, a step in the tube current occurs in the adjacent imaging region, and a difference in image SD (Standard Deviation) occurs. This is because the number of X-ray detectors in X-ray computed tomography equipment has increased, and it has become possible to collect data in a wider range at one time, so that the imaging region includes many structures. It is caused by the fact.

JP-A-2010-193940

An object of the embodiment is to reduce an image SD difference between imaging volumes in an X-ray computed tomography apparatus that automatically determines a tube current from an X-ray absorption index value.

The X-ray computer tomography apparatus according to the present embodiment emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and X-rays emitted from the X-ray tube and transmitted through the subject. A first X-ray regarding the amount of X-rays absorbed by the subject in each of the plurality of imaging regions based on the gantry for detecting the X-rays with the X-ray detector and the positioning image data of the subject in a predetermined imaging direction. Of the plurality of imaging regions, a calculation unit that calculates the line absorption index value, a tube current determination unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions, and The tube of the other imaging region according to the relative relationship between the first X-ray absorption index value of the reference imaging region and the first X-ray absorption index value of the other imaging region other than the reference imaging region. It is provided with a tube current correction unit that corrects the current value.

FIG. 1 is a diagram showing a configuration of an X-ray computed tomography apparatus according to the present embodiment. FIG. 2 is a diagram showing a typical operation flow of an X-ray computed tomography apparatus performed under the control of the system control circuit of FIG. FIG. 3 is a diagram schematically showing an initial determination process of a tube current value performed in step S4 of FIG. FIG. 4 is a diagram schematically showing a reference volume determination process performed in step S5 of FIG. FIG. 5 is a diagram schematically showing the correction process of the tube current value in step S6 of FIG. FIG. 6 is a diagram for explaining a first method of adjusting the water equivalent thickness performed by the tube current setting circuit in step S6 of FIG. FIG. 7 is a diagram for explaining a second method of adjusting the water equivalent thickness performed by the tube current setting circuit in step S6 of FIG. FIG. 8 is a diagram for explaining the directional modulation of the tube current according to the application example 1 of the present embodiment, and is a diagram showing the allocation of the tube current value before the tube current is corrected by the tube current setting circuit. FIG. 9 is a diagram for explaining the directional modulation of the tube current according to the application example 1, and is the allocation of the tube current value when the tube current value is uniformly corrected by the tube current setting circuit regardless of the rotation angle. It is a figure which shows. FIG. 10 is a diagram for explaining the directional modulation of the tube current according to Application Example 1, and shows the tube current value when the tube current value is specifically corrected according to the rotation angle by the tube current setting circuit. It is a figure which shows the allocation. FIG. 11 is a diagram showing weighting of the X-ray absorption amount according to Application Example 2. FIG. 12 is a diagram for explaining the setting of the image pickup volume to be corrected according to the application example 3.

Hereinafter, the X-ray computed tomography apparatus according to the present embodiment will be described with reference to the drawings.

FIG. 1 is a diagram showing a configuration of an X-ray computed tomography apparatus according to the present embodiment. As shown in FIG. 1, the X-ray computed tomography apparatus according to the present embodiment includes a gantry 10 and a console 30. The gantry 10 rotatably supports a rotating frame 11 having a cylindrical shape around a rotation axis Z. An X-ray generating unit 13 and an X-ray detecting unit 15 are attached to the rotating frame 11 so as to face each other with the rotating shaft Z interposed therebetween. The opening of the rotating frame 11 is set to FOV (field of view). The rotating frame 11 receives power from the rotation driving device 19 and rotates around the rotation axis Z at a constant angular velocity. The rotation drive device 19 generates power for rotating the rotation frame 11 according to the control from the image pickup control circuit 37 of the console 30. The sleeper 17 has a top plate 171 on which the subject S is placed and a support base 173 that movably supports the top plate 171. For example, the support base 173 supports the top plate 171 in the Z direction of the rotation axis, the vertical direction, and the horizontal direction. The support base 173 receives power from the sleeper drive device 23 and moves the top plate 171 in an arbitrary direction. The sleeper drive device 23 moves the top plate 171 in an arbitrary direction according to the control from the image pickup control circuit 37.

The X-ray generation unit 13 generates X-rays according to a control signal from the image pickup control circuit 37. Specifically, the X-ray generation unit 13 includes an X-ray tube 131 and an X-ray high voltage device 133. The X-ray tube 131 generates X-rays in response to the application of a high voltage from the X-ray high voltage device 133 and the supply of a filament current. The X-ray high voltage device 133 applies a high voltage to the X-ray tube 131 according to the control by the image pickup control circuit 37, and supplies a filament current. The X-ray high voltage device 133 adjusts the high voltage applied to the X-ray tube 131 and the filament current supplied to the X-ray tube 131 so as to maintain the preset tube voltage value and tube current value.

The X-ray detection unit 15 detects the X-rays generated from the X-ray generation unit 13 and transmitted through the subject S, and generates digital data according to the intensity of the detected X-rays. Specifically, the X-ray detection unit 15 has an X-ray detector 151 and a data acquisition circuit 153.

The X-ray detector 151 detects the X-rays generated from the X-ray tube 133. The X-ray detector 151 is equipped with a plurality of X-ray detection elements arranged on a two-dimensional curved surface. Each X-ray detection element detects X-rays from the X-ray tube 131 and converts them into an electric signal having a peak value corresponding to the intensity of the detected X-rays. Each X-ray detection element may be a scintillator detector composed of a scintillator and a photoelectric converter and indirectly converting X-rays into an electric signal, or a semiconductor detection device that directly converts X-rays into an electric signal. It may be a vessel.

The data acquisition circuit 153 collects an electric signal from each X-ray detection element for each view, and converts the collected electric signal into digital data. The converted digital data is called raw data. Raw data is a set of digital values of X-ray intensity associated by the channel number, column number, and view number that identifies the collected view of the X-ray detector from which it originated. The raw data is supplied to the console 30.

The console 30 includes a data storage circuit 31, a reconstruction circuit 33, an image processing circuit 35, an image pickup control circuit 37, a tube current setting circuit 41, a display circuit 43, an input circuit 45, a main storage circuit 47, and a system control circuit 49. ..

The data storage circuit 31 is a storage device such as an HDD (hard disk drive) or SSD (solid state drive) that stores raw data transmitted from the gantry 10.

The reconstruction circuit 33 has a processing device (processor) such as a CPU, MPU, and GPU (Graphics Processing Unit) and a storage device (memory) such as a ROM or RAM as hardware resources. The reconstruction circuit 33 realizes the reconstruction function 331 and the positioning image generation function 333 by reading and executing the program stored in the storage device.

By executing the reconstruction function 331, the reconstruction circuit 33 performs preprocessing such as logarithmic conversion on the raw data. The raw data after preprocessing is called projection data. The pre-processing includes various correction processing such as logarithmic conversion, X-ray intensity correction, and offset correction. Then, the reconstruction circuit 33 generates a CT image expressing the spatial distribution of the CT value with respect to the subject S based on the projection data. Image reconstruction algorithms include analytical image reconstruction methods such as the FBP (filtered back projection) method and CBP (convolution back projection) method, the ML-EM (maximum likelihood expectation maximization) method, and the OS-EM (ordered subset). An existing image reconstruction algorithm such as a statistical image reconstruction method such as the expectation maximization method may be used.

By executing the positioning image generation function 333, the reconstruction circuit 33 has a fixed imaging direction (rotation axis Z) based on the raw data collected by the X-ray detection unit 15 in the positioning scan described later or the projection data based on the raw data. A positioning image showing an X-ray projection image of the subject S with respect to the rotation angle) is generated. Specifically, the reconstruction circuit 33 performs a filtering process on the projection data related to the positioning scan to generate a positioning image. Hereinafter, when the projection data related to the positioning scan and the positioning image are not particularly distinguished, they are collectively referred to as positioning image data. The positioning image data is used for automatic setting of the tube current by the tube current setting circuit 41 described later.

The reconstruction circuit 33 may have a processing circuit for the reconstruction function 331 and a processing circuit for the positioning image generation function 333.

The image processing circuit 35 has a processing device (processor) such as a CPU, MPU, or GPU, and a storage device such as a ROM or RAM as hardware resources. The image processing circuit 35 performs various image processing on the CT image. For example, the image processing circuit 35 performs three-dimensional image processing on a CT image to generate a CT image for display. Examples of the three-dimensional image processing include volume rendering, surface rendering, pixel value projection processing, MPR (Multi Planar Reconstruction), and CPR (Curved MPR).

The image pickup control circuit 37 has a processing device (processor) of a CPU or MPU and a storage device such as a ROM or RAM as hardware resources. The image pickup control circuit 37 controls the control of various devices mounted on the gantry 10. For example, the imaging control circuit 37 synchronously controls the X-ray generation unit 13, the X-ray detection unit 15, the rotation drive device 19, and the sleeper drive device 23 in order to execute data collection for the subject S. .. The rotation drive device 19 rotates at a constant angular velocity according to the control by the image pickup control circuit 37. The X-ray high voltage device 133 of the X-ray generation unit 13 applies a tube voltage having a preset tube voltage value to the X-ray tube 13 according to the control by the image pickup control circuit 37. Further, the X-ray high voltage device 133 adjusts the tube voltage and the filament current according to the tube current having a preset tube current value according to the control by the imaging control circuit 37. The data acquisition circuit 153 of the X-ray detection unit 15 collects raw data for each view in synchronization with the X-ray exposure timing according to the control by the image pickup control circuit 37. Further, the image pickup control circuit 37 controls the bed driving device 23 so as to move the top plate 171 according to the input from the user via the input circuit 45 described later. For example, the image pickup control circuit 37 intermittently moves the top plate 171 along the rotation axis Z by synchronous control of the X-ray generation unit 13, the X-ray detection unit 15, the rotation drive device 19, and the sleeper drive device 23. However, a step-and-shoot method for collecting data at each stop position of the top plate 171 and a helical scan method for collecting data while continuously moving the top plate 171 along the rotation axis Z are executed. Further, the image pickup control circuit 37 controls the rotation angle (shooting direction) of the X-ray tube 131 around the Z axis by synchronous control of the X-ray generation unit 13, the X-ray detection unit 15, the rotation drive device 19, and the sleeper drive device 23. It is also possible to perform a positioning scan (scano imaging) in which data is collected while the top plate 171 is moved along the Z axis while keeping the top plate 171 constant.

The tube current setting circuit 41 has a CPU, an MPU, a GPU processing device (processor), and a storage device such as a ROM or RAM as hardware resources. The tube current setting circuit 41 automatically sets the tube current value for each of the plurality of imaging regions based on the positioning image data. Here, the imaging region corresponds to a region for collecting projection data while the X-ray tube 131 goes around the rotation axis Z. In the case of the step-and-shoot method in which the top plate 171 is stopped when the X-ray tube 131 is rotated, the imaging region corresponds to the stop position of the top plate 171. Hereinafter, the imaging region will be referred to as an imaging volume. As the positioning image data, the projection data collected by the positioning scan may be used, or the positioning image may be used.

The tube current setting circuit 41 executes an image pickup volume setting function, an X-ray absorption index value calculation function, a tube current determination function, a reference volume determination function, and a tube current correction function by executing a tube current setting program stored in the storage device. To realize.

In the image pickup volume setting function, the tube current setting circuit 41 sets a plurality of image pickup volumes by using the positioning image data. In the X-ray absorption index value calculation function, the tube current setting circuit 41 calculates the X-ray absorption index value by the subject S based on the positioning image data for each of the plurality of imaging volumes. The X-ray absorption index value is an index value that reflects the amount of X-ray absorption by the subject S. Specifically, the tube current setting circuit 41 calculates the X-ray absorption index value for each of the plurality of imaging volumes based on the data values of the plurality of data points relating to the imaging volume in the positioning image data. Specific examples of the X-ray absorption index value include an X-ray absorption amount, a water equivalent thickness, a subject thickness, and the like. In the tube current determination function, the tube current setting circuit 41 uses the X-ray absorption index value-tube current value table stored in the main memory circuit 47 or the like to initially set the tube current value corresponding to the X-ray absorption index value. To decide. The X-ray absorption index value-tube current value table is a LUT (Look Up Table) or a database in which an optimum tube current value is associated with each of a plurality of X-ray absorption index values. The combination of the X-ray absorption index value and the tube current value is determined in advance based on experiments, predictive calculations, clinical knowledge, and the like. In the reference volume determination function, the tube current setting circuit 41 automatically determines the reference volume from the plurality of imaging volumes or according to an instruction by the user via the input circuit 45. The reference volume is determined to be the imaging volume that requires the highest image quality among the plurality of imaging volumes. In the tube current correction function, the tube current setting circuit 41 uses the relative relationship between the X-ray absorption index value of the reference volume in the plurality of imaging regions and the X-ray absorption index value of the other imaging volume other than the reference volume. Correct the tube current value of the imaging volume of. The corrected tube current value is stored in the main storage circuit 47 as a set tube current value.

The display circuit 43 displays various information on the display device. For example, the display circuit 43 displays a CT image reconstructed by the reconstruction circuit 33 and a CT image processed by the image processing circuit 35. Further, the display circuit 43 may display a positioning image or a scan plan screen. As the display device, for example, a CRT display, a liquid crystal display, an organic EL display, a plasma display, or the like can be appropriately used.

The input circuit 45 receives various commands and information inputs from the user via the input device. For example, the input circuit 45 receives an imaging start instruction from the user via the input device. As an input device, a keyboard, a mouse, a switch and the like can be used.

The main storage circuit 47 is a large-capacity storage device such as an HDD that stores various information. For example, the main memory circuit 47 stores CT image data, positioning image data, a control program of an X-ray computed tomography apparatus, and the like. Further, the main storage circuit 47 stores the X-ray absorption index value-tube current value table used by the tube current setting circuit 41.

The system control circuit 49 has a processing device (processor) of a CPU or MPU and a storage device such as a ROM or RAM as hardware resources. The system control circuit 49 functions as the center of the X-ray computed tomography apparatus. Specifically, the system control circuit 49 reads out the control program stored in the main storage circuit 47, expands it on the memory, and controls each part of the X-ray computed tomography apparatus according to the expanded control program.

Next, an operation example of the X-ray computed tomography apparatus performed under the control of the system control circuit 49 will be described. It should be noted that the X-ray absorption index value is the water equivalent thickness, the imaging method of the tube current setting target is the step & shoot method, and the positioning image data is the positioning image in order to concretely explain the following.

FIG. 2 is a diagram showing a typical operation flow of an X-ray computed tomography apparatus performed under the control of the system control circuit 49.

As shown in FIG. 2, the system control circuit 49 causes the image pickup control circuit 37 to perform a positioning scan in the pre-stage of the main imaging (step S7) by the step & shoot method (step S1). In step S1, the imaging control circuit 37, for example, follows an instruction via the input circuit 45 by the user, and performs an X-ray generation unit 13, an X-ray detection unit 15, a rotation drive device 19, and a sleeper drive device 23 in order to perform a positioning scan. To control. Specifically, the imaging control circuit 37 controls the rotation drive device 19 to rotate the rotation frame 11 so that the X-ray tube 131 is arranged in a desired imaging direction (rotation angle around the rotation axis Z). Rotate around. The imaging direction of the X-ray tube 131 can be arbitrarily set by the user via the input circuit 45. Further, the image pickup control circuit 37 controls the sleeper drive device 23, the X-ray high voltage device 133, and the data acquisition circuit 153, and generates X-rays from the X-ray tube 131 arranged in the photographing direction while generating the top plate. The 171 is moved along the rotation axis Z, and the data acquisition circuit 153 is made to collect projection data related to the photographing direction. As a result, the imaging range related to the positioning scan of the subject is positioned and scanned by X-rays. The imaging range related to the positioning scan is set to include the imaging range of the main imaging in step S7. The projection data related to the positioning imaging is transmitted to the console 30. A positioning image is generated by the reconstruction circuit 33 based on the projection data.

When step S1 is performed, the system control circuit 49 causes the tube current setting circuit 41 to execute the following steps S2, S3, S4, S5, and S6.

In step S2, the tube current setting circuit 41 executes the imaging volume setting function. By executing the imaging volume setting function, the tube current setting circuit 41 sets a plurality of imaging volumes along the rotation axis Z in the imaging range of the main imaging by using the positioning image. For example, the tube current setting circuit 41 sets a plurality of imaging volumes according to instructions on the positioning image by the user via the input circuit 45.

When step S2 is performed, the tube current setting circuit 41 executes the X-ray absorption index value calculation function. By executing the X-ray absorption index value calculation function, the tube current setting circuit 41 calculates the X-ray absorption index value (water equivalent thickness) of each imaging volume (step S3). Specifically, the tube current setting circuit 41 calculates the water equivalent thickness for each of the plurality of imaging volumes based on the pixel values of the plurality of pixels in the image region corresponding to the imaging volume included in the positioning image. More specifically, the X-ray absorption amount is calculated for each of a plurality of pixels in the image area based on the pixel value of the pixel, and the calculated X-ray absorption amount is set to the water equivalent thickness according to a predetermined conversion formula. Convert. This water equivalent thickness is the water equivalent thickness with respect to the shooting direction of the positioning image. The statistical value of the water equivalent thickness for a plurality of pixels included in each imaging volume is determined to be the water equivalent thickness representing the imaging volume. The statistical value is set to, for example, an average value or an intermediate value of water equivalent thickness for a plurality of pixels. In order to make the following description concrete, the statistical value is assumed to be an average value, and the water equivalent thickness representing the imaging volume is referred to as the average water equivalent thickness.

When step S3 is performed, the tube current setting circuit 41 executes the tube current determination function. By executing the tube current determination function, the tube current setting circuit 41 initially determines the tube current value corresponding to the average water equivalent thickness by using the X-ray absorption index value-tube current value table (step S4).

FIG. 3 is a diagram schematically showing an initial determination process of a tube current value performed in step S4. As shown in FIG. 3, a plurality of imaging volumes are set in the positioning image along the rotation axis Z. The thick black line in FIG. 3 indicates the tube current value [mA] for each Z coordinate, and the bar graph hatched by points indicates the tube current value [mA] for each imaging volume. The tube current value for each Z coordinate is not actually used. The tube current value for each Z coordinate is a tube current value determined by applying the X-ray absorption index value-tube current value table to the average value of the water equivalent thickness of the pixels with respect to the Z coordinate. The tube current value of each imaging volume indicates the average value of the tube current values of a plurality of rotation angles while the X-ray tube 131 goes around the imaging volume. In this embodiment, the directional modulation of the tube current is not considered. As shown in FIG. 3, the thickness of the head of the head changes greatly along the rotation axis Z. On the other hand, the tube current value of each imaging volume is determined based on the average value of the water equivalent thicknesses of a plurality of pixels included in the imaging volume. Therefore, for example, as shown in FIG. 4, at the boundary between the first imaging volume and the second imaging volume, the subject thicknesses of the imaging volumes are close to each other, but the subject thickness at the other end of the first imaging volume is the second imaging. It is far from the volume subject thickness. Therefore, when the tube current value is determined from the average water equivalent thickness, the tube current values of the first imaging volume and the second imaging volume are significantly different, and the image SD is significantly different accordingly.

The tube current setting circuit 41 according to the present embodiment executes the following steps S5 and S6 in order to eliminate the step difference in the image SD, and corrects the tube current values of the plurality of imaging volumes.

When step S4 is performed, the tube current setting circuit 41 executes the reference volume determination function. By executing the reference volume determination function, the tube current setting circuit 41 determines the reference volume from the plurality of imaging volumes (step S5).

FIG. 4 is a diagram schematically showing a reference volume determination process performed in step S5. As described above, the reference volume is determined to be the imaging volume that requires the highest image quality among the plurality of imaging volumes. Typically, the reference volume is determined to be the imaging volume with the thickest water equivalent thickness. In this case, the tube current setting circuit 41 automatically determines the imaging volume having the thickest water equivalent thickness as the reference volume based on the plurality of water equivalent thicknesses for the plurality of imaging volumes. The reference volume can be determined to be an arbitrary imaging volume among a plurality of imaging volumes according to instructions by the user via the input circuit 45. Hereinafter, for the sake of specific description, it is assumed that the reference volume is determined to be the imaging volume having the thickest water equivalent thickness.

When step S5 is performed, the tube current setting circuit 41 executes the tube current correction function. By executing the tube current correction function, the tube current setting circuit 41 sets the tube current value of the other imaging volume according to the relative relationship between the water equivalent thickness of the reference volume and the water equivalent thickness of the other imaging volume other than the reference volume. Correct (step S6). The corrected tube current value is stored in the main storage circuit 47 for each imaging volume as a set tube current value.

FIG. 5 is a diagram schematically showing the correction process of the tube current value in step S6. As shown in FIG. 5, the tube current setting circuit 41 sets the tube current value of each imaging volume as the reference volume tube so that a step does not occur between the image SD of the reference volume and the image SD of another imaging volume. Bring it closer to the current value. In the present embodiment, since the reference volume is the imaging volume having the thickest water equivalent thickness among the plurality of imaging volumes, the tube current values of the other imaging volumes are increased. Specifically, the tube current setting circuit 41 adjusts the water equivalent thickness based on the weight value corresponding to the relative relationship of each imaging volume with respect to the reference volume, and determines the tube current value corresponding to the adjusted water equivalent thickness. To do. Hereinafter, a specific method for adjusting the water equivalent thickness will be described.

FIG. 6 is a diagram for explaining a first method for adjusting the water equivalent thickness. The upper part of FIG. 6 is a graph showing a change curve of the water equivalent thickness with respect to the image pickup volume to be processed with respect to the Z axis, and the middle part of FIG. It is a graph which shows the change curve about the Z axis, and the lower part of FIG. As shown in FIG. 6, the tube current setting circuit 41 compares the water equivalent thickness of each of the plurality of pixels with respect to the imaging volume to be processed with respect to the average water equivalent thickness with respect to the reference volume, and for each of the plurality of pixels. Calculate the difference between the water equivalent thickness and the average water equivalent thickness for the reference volume. The tube current setting circuit 41 determines a weight value corresponding to the difference value with respect to the Z coordinate for each of the plurality of pixels, and applies the determined weight value to the water equivalent thickness with respect to the pixel. The weight value is set to a smaller value as the difference value is larger. For example, as shown in the middle part of FIG. 6, the tube current setting circuit 41 sets a threshold value for the difference value, and as shown in the lower part of FIG. 6, the weight value of the pixel having a difference value higher than the threshold value is the threshold value. It is preferable that the value is set lower than the weight value of the pixel having a lower difference value. For example, it is preferable that the weight value of the pixel having a difference value higher than the threshold value is set to zero, and the weight value of the pixel having a difference value lower than the threshold value is set to 1. By applying the weight value determined in this way to the water equivalent thickness for the pixel, the water equivalent thickness for the pixel is adjusted. The adjusted average water equivalent thickness for the imaging volume is calculated based on the multiple adjusted water equivalent thicknesses for the plurality of pixels. The tube current setting circuit 41 determines the tube current value corresponding to the adjusted average water equivalent thickness by using the X-ray absorption index value-tube current value table. Thereby, the tube current value can be corrected according to the difference value of the average water equivalent thickness for each imaging volume with respect to the average water equivalent thickness for the reference volume.

As described above, in the first method for adjusting the water equivalent thickness, the tube current setting circuit 41 compares the water equivalent thickness of each of the plurality of pixels for each imaging volume with respect to the average water equivalent thickness for the reference volume, and the plurality of the water equivalent thicknesses. The average water equivalent thickness is adjusted by a weight value corresponding to the difference value between the water equivalent thickness and the average water equivalent thickness with respect to the reference volume for each of the pixels of. The weight value depends on the relative relationship of the water equivalent thickness for each imaging volume to the water equivalent thickness for the reference volume. In the present embodiment, since the weight value inversely proportional to the difference value is applied to the water equivalent thickness for each pixel, the average water equivalent thickness after adjustment has a value higher than the average water equivalent thickness before adjustment. It becomes. That is, the average water equivalent thickness for each imaging volume can be brought close to the average water equivalent thickness for the reference volume while reflecting the unadjusted (original) average water equivalent thickness for the imaging volume. Therefore, it is possible to appropriately reduce the difference in the tube current value between the imaging volumes while maintaining the magnitude relationship of the water equivalent thickness between the imaging volumes.

FIG. 7 is a diagram for explaining a second method for adjusting the water equivalent thickness. The upper part of FIG. 7 is a graph showing a change curve of the water equivalent thickness with respect to the Z-axis of the imaging volume to be processed, and the middle part of FIG. 7 is a graph showing a change curve of the distance from the reference volume with respect to the Z-axis. The lower part of FIG. 7 is a graph showing a change curve of the weight value with respect to the Z axis according to the distance. As shown in FIG. 7, the tube current setting circuit 41 calculates the distance between each of the Z coordinates of the plurality of pixels with respect to the image pickup volume to be processed and the Z coordinate of the reference point of the reference volume. The reference point of the reference volume may be, for example, an end point of the reference volume with respect to the Z axis, an intermediate point, or an arbitrary point. The tube current setting circuit 41 determines a weight value according to the distance of the Z coordinate between the pixel and the reference point for each of the Z coordinates of the plurality of pixels, and applies the determined weight value to the water equivalent thickness of the pixel. To do. The weight value is set to a smaller value as the distance is longer. The weight value may be set linearly so as to be inversely proportional to the distance, or may be set non-linearly by a distance-dependent multidimensional function. By applying the weight value determined in this way to the water equivalent thickness for the pixel, the water equivalent thickness for the pixel is adjusted. The adjusted average water equivalent thickness (eg, average value) for the imaging volume is calculated based on the plurality of adjusted water equivalent thicknesses for the plurality of pixels. The tube current setting circuit 41 determines the tube current value corresponding to the adjusted average water equivalent thickness by using the X-ray absorption index value-tube current value table. Thereby, the tube current value can be corrected according to the distance of each imaging volume from the reference volume.

As described above, in the second water equivalent thickness adjusting method, the tube current setting circuit 41 calculates the distance between each of the plurality of pixels for each imaging volume and the reference volume, and corresponds to the distance for each of the plurality of pixels. The average water equivalent thickness is adjusted by the weight value. In general, the farther away from the reference volume, the lower the relationship between the water equivalent thickness for each imaging volume and the water equivalent thickness for the reference volume. Therefore, the weight value depends on the relative relationship of the water equivalent thickness for each imaging volume to the water equivalent thickness for the reference volume. In the present embodiment, since the weight value inversely proportional to the distance is applied to the water equivalent thickness for each pixel, the average water equivalent thickness after adjustment has a value higher than the average water equivalent thickness before adjustment. Become. That is, the average water equivalent thickness for each imaging volume can be brought close to the average water equivalent thickness for the reference volume while reflecting the unadjusted (original) average water equivalent thickness for the imaging volume. As described above, the tube current value is determined according to the average water equivalent thickness. Therefore, it is possible to appropriately reduce the step of the tube current value between the imaging volumes while maintaining the positional relationship between the imaging volumes.

This completes the tube current setting process by the tube current setting circuit 41.

When step S6 is performed, the system control circuit 49 causes the image pickup control circuit 37 to perform the main imaging (step S7). In step S7, the image pickup control circuit 37 synchronously controls the X-ray generation unit 13, the X-ray detection unit 15, the rotation drive device 19, and the sleeper drive device 23 in order to execute the step & shoot method. At this time, the imaging control circuit 37 performs CT imaging of each imaging volume according to the tube current value of each imaging volume set in step S6. Raw data for each imaging volume is transmitted to the console 30. The reconstruction circuit 33 reconstructs a CT image related to the imaging volume based on the raw data for each imaging volume. The reconstructed CT image is displayed by the display circuit 43.

This completes the description of the operation example of the X-ray computed tomography apparatus by the system control circuit 49.

In the above description, the correction amount of the tube current value, that is, the upper limit of the difference between the tube current value before the correction and the tube current value after the correction is not limited. However, this embodiment is not limited to this. That is, an upper limit may be set for the correction amount of the tube current value. In this case, the tube current setting circuit 41 determines whether or not the correction amount of the tube current value is larger than the upper limit when determining the corrected tube current value. When the correction amount of the tube current value is smaller than the upper limit value, the tube current setting circuit 41 sets the corrected tube current value to the final tube current value. When the correction amount of the tube current value is larger than the upper limit value, the tube current setting circuit 41 sets a value obtained by adding or subtracting the upper limit value to the tube current value before correction as the final tube current value. As a result, the tube current setting circuit 41 can limit the amount of correction from the tube current value before correction to the tube current value after correction to a predetermined upper limit value. The upper limit value may be set in advance according to the imaging site. By setting the upper limit value according to the imaging site, the correction amount can be limited to an appropriate value in consideration of the sensitivity of each imaging site to X-rays.

Further, the tube current setting process of the tube current setting circuit 41 according to the above embodiment is premised on the step & shoot method. However, this embodiment is also applicable to the helical scan method. Even when the tube current is set in the helical scan method, the imaging region is preferably defined as the data acquisition region while the X-ray tube 131 goes around the rotation axis Z. As a result, even in the helical scan method, the tube current setting process can be performed in the same manner as in the step & shoot method.

Further, in the above embodiment, the X-ray computed tomography apparatus is a so-called third generation. That is, the X-ray computed tomography apparatus is a rotation / rotation type (ROTATE / ROTATE-TYPE) in which the X-ray tube 131 and the X-ray detector 151 rotate around the rotation axis Z as one body. .. However, the X-ray computed tomography apparatus according to the present embodiment is not limited to this. For example, an X-ray computed tomography apparatus is a fixed / rotating type (STATIONARY / ROTATE-TYPE) in which a large number of X-ray detection elements arranged in a ring shape are fixed and only the X-ray tube 131 rotates around the rotation axis Z. ) Is fine.

As described above, the X-ray computed tomography apparatus according to the present embodiment is equipped with a gantry 10 and a tube current setting circuit 41. The gantry 10 emits X-rays from the X-ray tube 131 for a plurality of imaging volumes arranged along the body axis (rotation axis Z) direction, and emits X-rays emitted from the X-ray tube 131 and transmitted through the subject S. It is detected by the X-ray detector 151. The tube current setting circuit 41 calculates an X-ray absorption index value regarding the amount of X-rays absorbed by the subject S for each of the plurality of imaging volumes based on the positioning image data of the subject S in a predetermined imaging direction. Next, the tube current setting circuit 41 determines the tube current value corresponding to the X-ray absorption index value for each of the plurality of imaging volumes. Then, the tube current setting circuit 41 of the other imaging volume according to the relative relationship between the X-ray absorption index value of the reference volume among the plurality of imaging volumes and the X-ray absorption index value of the other imaging volume other than the reference volume. Correct the tube current value.

With the above configuration, even if the imaging volume covers a wide range, the relative relationship between the water equivalent thickness of the reference volume and the water equivalent thickness of other imaging volumes is considered, so that the tube current of the adjacent imaging volume tube is taken into consideration. The difference in value can be reduced. Therefore, according to the present embodiment, it is possible to reduce the step in the image SD between the imaging volumes due to the step in the tube current value.

(Application example 1)
In the above embodiment, the directional modulation of the tube current is not considered. In Application Example 1 of this embodiment, it is assumed that the directional modulation of the tube current is performed.

FIG. 8 is a diagram for explaining the directional modulation of the tube current, and shows the allocation of the tube current value before the tube current is corrected by the tube current setting circuit 41. In FIG. 8, the set tube current value is assumed to be 200 mA. As shown in FIG. 8, in the directional modulation, the degree of modulation of the tube current value based on the set tube current value of 200 mA is assigned to the rotation angle around the rotation axis Z of the X-ray tube 131. The degree of modulation of the tube current value is typically defined as a percentage with the set tube current value of 200 mA as 100%. This percentage will be called the tube current modulation factor. The tube current modulation factor is assigned to each rotation angle by the tube current setting circuit 41 so that the average value of the plurality of tube current values assigned to each of the plurality of rotation angles matches the set tube current value. Typically, the thicker the water equivalent thickness, the larger the tube current modulation rate, and the thinner the water equivalent thickness, the lower the tube current modulation rate. Typically, the same tube current value is assigned at both rotation angles facing each other across the Z axis. For example, as shown in FIG. 8, the tube current modulation factors of the rotation angle of 90 ° and the rotation angle of 270 ° are the same at 150%. As for the tube current values at the rotation angle of 0 ° and the rotation angle of 180 °, the same tube current modulation factor is assigned when only the water equivalent thickness is taken into consideration as described above. However, it is assumed that there is an anatomical part that is greatly affected by X-rays in the part facing the rotation angle of 0 °. In this case, when X-rays are irradiated from a rotation angle of 0 ° based on the tube current value of the reference tube current modulation factor corresponding to the water equivalent thickness, a high dose of X-rays is irradiated to the anatomical site. It ends up. Therefore, when the tube current setting circuit 41 has a specific rotation angle facing an anatomical site that is greatly affected by X-rays, the tube current modulation rate is lower than the reference tube current modulation factor at the specific rotation angle. A rate is assigned, and a tube current modulation factor higher than the reference tube current modulation factor is assigned to the rotation angle opposite to the specific rotation angle in order to compensate for the decrease in the tube current at the specific rotation angle.

FIG. 9 is a diagram for explaining the directional modulation of the tube current, and is a tube current value when the tube current value is uniformly corrected by the tube current setting circuit 41 according to the above embodiment regardless of the rotation angle. Indicates the allocation of. The set tube voltage value of FIG. 9 is higher than the set tube voltage value of FIG. 8 by 10 mA. When the tube current value is uniformly corrected regardless of the rotation angle, the tube current setting circuit 41 raises the tube current values of all the rotation angles by the same value, or 10 mA in the example of FIG.

FIG. 10 is a diagram for explaining the directional modulation of the tube current, and is a tube current when the tube current value is specifically corrected according to the rotation angle by the tube current setting circuit 41 according to the above embodiment. Indicates the value assignment. The set tube voltage value in FIG. 10 is higher than the set tube current value in FIG. 8 by 10 mA, similar to the tube current value in FIG. In this case, the tube current setting circuit 41 corrects the tube current value only at other rotation angles excluding the specific rotation angle facing the anatomical part that is greatly affected by X-rays. At this time, the correction amount of the other rotation angles other than the specific rotation angle, that is, the change in the tube current modulation rate, is set so that the average value of the tube current values of all the rotation angles matches the set tube voltage value. Calculated by the tube current setting circuit 41. The rotation angle to be corrected of the tube current value may be all other rotation angles except the specific rotation angle, or may be a part of all the other rotation angles. For example, as shown in FIG. 10, when only the rotation angle 180 ° on the opposite side of the specific rotation angle 0 ° is increased, the average value of the tube current values of all the rotation angles matches the set tube voltage value 210 mA. , A tube current modulation factor of 120% is assigned to a rotation angle of 180 °.

As described above, the tube current setting circuit 41 according to Application Example 1 is limited to other rotation angles excluding the specific rotation angle facing the anatomical part that is greatly affected by X-rays in the directional modulation of the tube current. And correct the tube current value. Therefore, it is possible to increase and correct the tube current value of each imaging volume while reducing the X-ray dose to the anatomical site that is greatly affected by X-rays.

In the above description, there is no limitation on the upper limit of the difference value of the tube current values of both rotation angles facing each other with the rotation shaft Z in between. However, this embodiment is not limited to this. That is, an upper limit may be set for the difference value of the tube current values of both rotation angles. In this case, the tube current setting circuit 41 determines whether or not the difference value of the tube current values of both rotation angles is larger than the upper limit when determining the corrected tube current value. When the difference value is smaller than the upper limit value, the tube current setting circuit 41 sets the corrected tube current value as the final tube current value. When the difference value is larger than the upper limit value, the tube current setting circuit 41 maintains the tube current value at a specific rotation angle facing the anatomical part that is greatly affected by X-rays, and faces the specific rotation angle. For the rotation angle, the value obtained by adding the upper limit value to the tube current value before correction is set as the final tube current value. If the average value of the tube current values of all rotation angles does not reach the set tube current value, it is advisable to add the shortage to the tube current values of other rotation angles. As a result, the tube current setting circuit 41 can limit the difference value of the tube current values of both rotation angles to a predetermined upper limit value. The upper limit value may be set in advance in consideration of the image quality of the CT image and the like.

(Application example 2)
In step S3 of FIG. 2 above, the tube current setting circuit 41 does not particularly consider the hardening of the radiation quality due to the heel effect when calculating the water equivalent thickness of each imaging volume. In Application Example 2, the tube current setting circuit 41 calculates the water equivalent thickness of each imaging volume in consideration of the hardening of the radiation quality due to the heel effect. More specifically, the tube current setting circuit 41 weights the X-ray absorption amount of each pixel of each imaging volume with a weight value corresponding to the hardening of the radiation quality due to the heel effect.

FIG. 11 is a diagram showing weighting of each pixel of each imaging volume according to Application Example 2. FIG. 11 is a diagram in which a change curve, a heel effect, and a weight value with respect to the Z axis of the water equivalent thickness are superimposed on a positioning image regarding the side surface of the subject. In FIG. 11, it is assumed that the processing target is the imaging volume 2.

As shown in FIG. 11, the X-ray tube 131 is provided with an anode and a cathode (not shown). The anode and cathode are arranged along the Z axis. X-rays are generated by the thermions flying from the cathode interacting with the substances that make up the anode and are emitted outside the anode. It is known that the longer the transmission path in the anode of X-rays, the harder the quality of the X-rays. Further, the anode is designed so that the diameter increases from the cathode side to the anode side along the Z axis. Therefore, the quality of the X-rays emitted from the anode hardens as it goes from the cathode side to the anode side along the Z axis. This phenomenon is called the heel effect.

The tube current setting circuit 41 calculates a plurality of X-ray absorption amounts for each of the plurality of pixels included in the image pickup volume for each of the plurality of image pickup volumes, and X-rays of the pixels at positions where the hardening of the radiation quality due to the heel effect is strong. A large weight value is given to the X-ray absorption amount of the pixel at the weak position as compared with the absorption amount. The weight value is set to a smaller value as the hardening of the radiation quality due to the heel effect increases in order to offset the fluctuation of the pixel value or the amount of X-ray absorption due to the hardening of the radiation quality due to the heel effect. The smaller the quality cure, the higher the value. In other words, the weight value is set to a value larger than that of the pixel at the position closer to the anode in the Z-axis direction with respect to the pixel at the position farther from the anode. The weight value may be calculated by the tube current setting circuit 41 based on the actually measured X-ray dose, or may be set by the user via the input circuit 45 or the like.

When the weighting is performed, the tube current setting circuit 41 calculates the water equivalent thickness of the imaging volume to be processed by the same method as in step S3 based on the plurality of weighted X-ray absorption amounts for each imaging volume. Specifically, the tube current setting circuit 41 converts each weighted X-ray absorption amount into water equivalent thickness according to a predetermined conversion formula, and is said to be based on a plurality of water equivalent thicknesses related to a plurality of pixels included in each imaging volume. Determine the water equivalent thickness of the imaging volume.

As described above, according to Application Example 2, it is possible to set the tube current value of the imaging volume more accurately by giving a larger weight to a pixel that is less affected by the heel effect than a large pixel.

In the above description, a weight value is given to the X-ray absorption amount of each pixel, but the present embodiment is not limited to this. For example, the tube current setting circuit 41 may give the above weight value to the pixel value of each pixel included in each imaging volume. In this case, the X-ray absorption amount is calculated for each pixel based on the weighted pixel value.

(Application example 3)
In the above embodiment, it is assumed that the tube current value is corrected for all of the plurality of imaging volumes other than the reference volume. However, this embodiment is not limited to this. In Application Example 3, the tube current setting circuit 41 corrects the tube current value only for the image pickup volume to be corrected among the plurality of image pickup volumes.

FIG. 12 is a diagram for explaining the setting of the imaging volume to be corrected. As shown in FIG. 12, the tube current setting circuit 41 sets the imaging volume of clinical interest among a plurality of other imaging volumes other than the reference volume as the correction target. For example, as shown in FIG. 12, when there is clinical interest in the crown as compared with the base of the skull, the imaging volume 1 corresponding to the crown is set as the correction target.

The tube current setting circuit 41 may set the image pickup volume to be corrected according to the user's instruction via the input circuit 45. Further, the tube current setting circuit 41 may be set according to the subject insertion direction selected in advance. The subject insertion direction indicates the direction of the subject S to be inserted into the opening of the gantry 10. For example, when the subject S is inserted into the opening from the head side, the subject insertion direction is defined as the head direction, and when the subject S is inserted into the opening from the leg side, the subject insertion direction is the leg direction. Is regulated. The subject insertion direction is selected by the user via the input circuit 45 when selecting the imaging plan.

Hereinafter, the automatic setting of the image pickup volume to be corrected according to the subject insertion direction will be described in detail. The tube current setting circuit 41 automatically sets the imaging volume on the subject insertion direction side from the reference volume as the correction target. When the head is the imaging site and the subject insertion direction is the head direction, the crown is often more interested than the bottom of the skull. Therefore, the tube current setting circuit 41 sets the image pickup volume on the crown side from the reference volume as the correction target. For example, when the image pickup volume 3 is the reference volume in FIG. 12, the image pickup volumes 1 and 2 are set as correction targets.

When the image pickup volume to be corrected is set, the tube current setting circuit 41 corrects the tube current value according to step S6 only for the image pickup volume to be corrected. Step S6 is not executed for the image pickup volume that is not subject to correction. In this case, the tube current value of the imaging volume that is not subject to correction may be set to the initial tube current value determined by the tube current setting circuit 41 in step S4.

As described above, according to Application Example 3, the tube current value is corrected only for the image pickup volume to be corrected among the plurality of image pickup volumes. As a result, the amount of increase in the tube current value due to the correction of the tube current value can be reduced as compared with the case where the tube current values of all the imaging volumes are corrected.

Thus, according to the present embodiment, it is possible to reduce the image SD difference between the imaging regions in the X-ray computed tomography apparatus that automatically determines the tube current from the X-ray absorption index value.

Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other embodiments, and various omissions, replacements, and changes can be made without departing from the gist of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the scope of the invention described in the claims and the equivalent scope thereof.

10 ... gantry, 11 ... rotating frame, 13 ... X-ray generator, 15 ... X-ray detector, 17 ... sleeper, 19 ... rotation drive, 23 ... sleeper drive, 30 ... console, 31 ... data storage circuit, 33 ... reconstruction circuit, 35 ... image processing circuit, 37 ... imaging control circuit, 41 ... tube current setting circuit, 43 ... display circuit, 45 ... input circuit, 47 ... main storage circuit, 49 ... system control circuit, 331 ... reconstruction Function, 333 ... Positioning image generation function.

Claims (19)

A gantry that emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and detects the X-rays emitted from the X-ray tube and transmitted through a subject with an X-ray detector.
A calculation unit that calculates a first X-ray absorption index value regarding the amount of X-rays absorbed by the subject for each of the plurality of imaging regions based on the positioning image data of the subject in a predetermined imaging direction.
A tube current determining unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions,
The relative relationship between the first X-ray absorption index value of the reference imaging region among the plurality of imaging regions and the first X-ray absorption index value of the other imaging region other than the reference imaging region, and the reference. A tube current correction unit that corrects the tube current value of the other imaging region according to the distance between the imaging region and the other imaging region.
An X-ray computed tomography apparatus comprising. A gantry that emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and detects the X-rays emitted from the X-ray tube and transmitted through a subject with an X-ray detector.
A calculation unit that calculates a first X-ray absorption index value regarding the amount of X-rays absorbed by the subject for each of the plurality of imaging regions based on the positioning image data of the subject in a predetermined imaging direction.
A tube current determining unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions,
According to the relative relationship between the first X-ray absorption index value of the reference imaging region among the plurality of imaging regions and the first X-ray absorption index value of the other imaging region other than the reference imaging region. A tube current correction unit that corrects the tube current value in another imaging region,
Equipped with
The tube current correction unit limits the fluctuation range from the tube current value before being corrected by the tube current correction unit to the tube current value after the tube current value to a value set according to the imaging site.
X-ray computed tomography equipment. A gantry that emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and detects the X-rays emitted from the X-ray tube and transmitted through a subject with an X-ray detector.
A calculation unit that calculates a first X-ray absorption index value regarding the amount of X-rays absorbed by the subject for each of the plurality of imaging regions based on the positioning image data of the subject in a predetermined imaging direction.
A tube current determining unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions,
According to the relative relationship between the first X-ray absorption index value of the reference imaging region among the plurality of imaging regions and the first X-ray absorption index value of the other imaging region other than the reference imaging region. A tube current correction unit that corrects the tube current value in another imaging region,
Equipped with
For each of the plurality of imaging regions, the tube current determining unit allocates an initial degree of modulation from the first reference tube current value based on the tube current value for each of the plurality of imaging directions.
The tube current correction unit maintains the initial degree of modulation in the imaging direction in which the influence of X-rays from the plurality of imaging directions on the subject is small, and the X-rays in the plurality of imaging directions are used. With respect to the imaging direction having a large influence on the subject, the initial modulation degree is changed to the modulation degree from the second reference tube current value based on the corrected tube current value.
X-ray computed tomography equipment. A gantry that emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and detects the X-rays emitted from the X-ray tube and transmitted through a subject with an X-ray detector.
A calculation unit that calculates a first X-ray absorption index value regarding the amount of X-rays absorbed by the subject for each of the plurality of imaging regions based on the positioning image data of the subject in a predetermined imaging direction.
A tube current determining unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions,
According to the relative relationship between the first X-ray absorption index value of the reference imaging region among the plurality of imaging regions and the first X-ray absorption index value of the other imaging region other than the reference imaging region. A tube current correction unit that corrects the tube current value in another imaging region,
Equipped with
The calculation unit calculates a plurality of second X-ray absorption index values for a plurality of pixels included in the imaging region for each of the plurality of imaging regions, and at a position where the hardening of the radiation quality due to the heel effect is strong. A large weight value is given to the second X-ray absorption index value of the pixel at a weak position as compared with the second X-ray absorption index value of the pixel, and a plurality of second X-ray absorptions to which the weight value is given are given. The first X-ray absorption index value is calculated based on the index value.
X-ray computed tomography equipment. A gantry that emits X-rays from an X-ray tube for a plurality of imaging regions arranged along the body axis direction, and detects the X-rays emitted from the X-ray tube and transmitted through a subject with an X-ray detector.
A calculation unit that calculates a first X-ray absorption index value regarding the amount of X-rays absorbed by the subject for each of the plurality of imaging regions based on the positioning image data of the subject in a predetermined imaging direction.
A tube current determining unit that determines a tube current value corresponding to the first X-ray absorption index value for each of the plurality of imaging regions,
According to the relative relationship between the first X-ray absorption index value of the reference imaging region among the plurality of imaging regions and the first X-ray absorption index value of the other imaging region other than the reference imaging region. A tube current correction unit that corrects the tube current value in another imaging region,
Equipped with
The tube current correction unit sets an imaging region to be corrected among the plurality of imaging regions according to a predetermined subject insertion direction, and limits the tube current value to the imaging region to be corrected. to correct,
X-ray computed tomography equipment. The invention according to any one of claims 1 to 5, further comprising a reference imaging region determining unit that determines the imaging region having the highest first X-ray absorption index value among the plurality of imaging regions as the reference imaging region. X-ray computed tomography equipment. The tube current correction unit determines a weight value according to the difference between the first X-ray absorption index value of the other imaging region and the first X-ray absorption index value of the reference imaging region. The X-ray computed tomography apparatus according to any one of claims 2 to 5, wherein the tube current value in the other imaging region is corrected according to the determined weight value. The calculation unit calculates a second X-ray absorption index value for a plurality of pixels included in the other imaging region, and based on the plurality of second X-ray absorption index values, said the other imaging region. Calculate the first X-ray absorption index value and
The tube current correction unit gives a plurality of weighted second X-ray absorption index values to the plurality of second X-ray absorption index values according to the difference from the first X-ray absorption index value of the reference imaging region. The X-ray absorption index value of 2 is calculated, and the tube current value of the other imaging region is corrected based on the plurality of weighted second X-ray absorption index values.
The X-ray computed tomography apparatus according to claim 7. The tube current correction unit determines a weight value according to the distance between the other imaging region and the reference imaging region, and corrects the tube current value according to the determined weight value. The X-ray computed tomography apparatus according to any one of 5. The calculation unit calculates a plurality of second X-ray absorption index values for the plurality of pixels included in the other imaging region, and calculates the average value of the plurality of second X-ray absorption index values for the other imaging. Calculated as the first X-ray absorption index value of the region,
The tube current correction unit gives a plurality of weighted second X-ray absorption index values to the plurality of second X-ray absorption index values according to the distance between the other imaging region and the reference imaging region, and a plurality of weighted second X-rays. The absorption index value is calculated, and the tube current value in the other imaging region is corrected based on the plurality of weighted second X-ray absorption index values.
The X-ray computed tomography apparatus according to claim 1 or 9. The tube current correction unit limits the fluctuation range from the tube current value before being corrected by the tube current correction unit to the tube current value after the tube current value to a value set according to the imaging site, claim 1 and The X-ray computed tomography apparatus according to any one of 3 to 5. For each of the plurality of imaging regions, the tube current determining unit allocates an initial degree of modulation from the first reference tube current value based on the tube current value for each of the plurality of imaging directions.
The tube current correction unit maintains the initial degree of modulation in the imaging direction in which the influence of X-rays from the plurality of imaging directions on the subject is small, and the X-rays in the plurality of imaging directions are used. With respect to the imaging direction having a large influence on the subject, the initial modulation degree is changed to the modulation degree from the second reference tube current value based on the corrected tube current value.
The X-ray computed tomography apparatus according to any one of claims 1 to 2 and 4 to 5. Any of claims 1 to 5, wherein the tube current determining unit determines a water equivalent thickness based on the first X-ray absorption index value, and determines a tube current value corresponding to the determined water equivalent thickness. The X-ray computed tomography apparatus according to item 1. The gantry according to any one of claims 1 to 5, wherein CT imaging is performed on the subject in the plurality of imaging regions according to the tube current value corrected by the tube current correction unit. X-ray computed tomography equipment. The calculation unit calculates a plurality of second X-ray absorption index values for a plurality of pixels included in the imaging region for each of the plurality of imaging regions, and at a position where the hardening of the radiation quality due to the heel effect is strong. A large weight value is given to the second X-ray absorption index value of the pixel at a weak position as compared with the second X-ray absorption index value of the pixel, and a plurality of second X-ray absorptions to which the weight value is given are given. The X-ray computer tomography apparatus according to any one of claims 1 to 3 and 5, which calculates the first X-ray absorption index value based on the index value. The X-ray computed tomography according to claim 4 or 15, wherein the weight value has a larger value with respect to a pixel at a position far from the anode of the X-ray tube in the body axis direction than a pixel at a position near the anode of the X-ray tube. apparatus. The X-ray computed tomography according to any one of claims 1 to 4, wherein the tube current correction unit corrects the tube current value only in the image pickup area to be corrected among the plurality of image pickup areas. apparatus. The X-ray computed tomography apparatus according to claim 17, wherein the tube current correction unit sets an imaging region of clinical interest among the plurality of imaging regions as the imaging region to be corrected. The X-ray computed tomography apparatus according to claim 17, wherein the tube current correction unit sets an imaging region to be corrected according to a predetermined subject insertion direction.