JP5601675B2 - X-ray CT apparatus and program - Google Patents

Description

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

  Conventionally, as a technique for improving image quality in a tomographic image of a subject obtained by X-ray CT imaging, various techniques for improving image quality degradation due to an X-ray high absorber existing in the subject have been proposed.

  For example, in projection data (data) before image reconstruction or projection data obtained by forward projection of a tomographic image that has been image-reconstructed mathematically in multiple views (views), it passes through an X-ray superabsorber. There is known a method of reconstructing an image after removing data corresponding to a path (X-ray superabsorber transmission region). Further, for example, a technique is known in which image reconstruction is performed after replacing data corresponding to an X-ray superabsorber transmission region in projection data with data estimated from the surrounding data (for example, Patent Document 1, Paragraph [0052] to [0058] etc.).

According to such a method, a tomographic image in which the X-ray high-absorber is represented as an easily observable image with a low contrast, and artifacts due to the influence of the X-ray high-absorber is obtained. Can do.
JP 2005-006832 A

  However, in the above method, the data corresponding to the X-ray superabsorber transmission region is simply removed or estimated and replaced. Therefore, the projection data can be obtained without the X-ray superabsorber. Information on tissues other than the produced X-ray superabsorber remains missing. Therefore, when a tomographic image is reconstructed from such projection data, the image of the tissue around the X-ray high absorber is not sufficiently restored, and a tomographic image suitable for observation cannot be obtained.

  In view of the above circumstances, the present invention provides an X-ray CT apparatus capable of obtaining a tomographic image in which a tissue around an X-ray superabsorbent body of a subject is restored with higher accuracy, and a program therefor. Objective.

  In a first aspect, the present invention provides an X-ray irradiation of a subject including an X-ray superabsorber in a plurality of views, detects transmitted X-rays of the subject with an X-ray detector having a plurality of channels, and Imaging means for obtaining measured projection data of a subject, high-absorber region specifying means for specifying an X-ray high-absorber transmission region based on the measured projection data, and the X-ray high-absorber transmission region in the measured projection data And a data replacement means for obtaining replaced projection data by replacing data estimated from data corresponding to a region near the region, and tomographic image reconstruction based on the replaced projection data Based on the forward projection data, forward projection means for calculating forward projection data by forward projecting the tomographic images reconstructed by the first reconstruction means with the plurality of views. Tomographic image reconstruction To provide an X-ray CT apparatus and a second reconstruction means for.

  In a second aspect, the present invention is directed to X-ray irradiation of a subject including an X-ray superabsorber in a plurality of views, and the transmitted X-ray of the subject is detected by an X-ray detector having a plurality of channels. Imaging means for obtaining measured projection data of the subject, high-absorber region specifying means for specifying an X-ray high-absorber transmission region based on the measured projection data, and the X-ray high absorption in the measured projection data Data replacement means for replacing the data corresponding to the body transmissive region with data estimated from the data corresponding to the vicinity region of the region to obtain replaced projection data, and tomographic images based on the replaced projection data First reconstruction means for image reconstruction, forward projection means for forwardly projecting a predetermined tomographic image with the plurality of views to calculate forward projection data, and tomographic image reconstruction based on the forward projection data The second reconstruction means and the previous Control means for repeatedly performing the processing by the sequential projection means and the second reconstructing means a plurality of times, and obtaining a tomographic image reconstructed at the end, wherein the predetermined tomographic image is the first process, There is provided an X-ray CT apparatus which is a tomographic image obtained by the first reconstruction means and is a tomogram obtained by the second reconstruction means in the previous process in the second and subsequent processes.

  In a third aspect, the present invention relates to the data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data, the data value, the replaced projection data, or the predetermined slice. Data adjustment means for obtaining adjusted projection data that is adjusted to be changed based on the difference between the channel data corresponding to the X-ray superabsorber transmission region in the projection data used for image reconstruction of an image The second reconstruction unit reconstructs a tomogram based on the adjusted projection data, and the control unit includes the forward projection unit, the data adjustment unit, and the second reconstruction unit. An X-ray CT apparatus according to the second aspect that allows the processing by the constituent means to be repeated a plurality of times is provided.

  In a fourth aspect, the present invention provides the data adjustment means, wherein the data value is replaced with the data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data. A value obtained by multiplying the difference between the completed projection data or the value of the channel data corresponding to the X-ray superabsorber transmission region in the projection data used for image reconstruction of the predetermined tomographic image by a predetermined weight. The X-ray CT apparatus according to the third aspect, which obtains the adjusted projection data by adding, is provided.

  In a fifth aspect, the present invention provides the data adjustment unit, wherein the data value is replaced with the data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data. A value derived by a predetermined function depending on the difference between the data of the channel corresponding to the transmission region of the X-ray superabsorber in the already-projected projection data or the projection data used for image reconstruction of the predetermined tomographic image The X-ray CT apparatus according to the third aspect of the invention for obtaining the adjusted projection data by adding.

  In a sixth aspect, the present invention provides the data replacement means, wherein the data corresponding to the X-ray superabsorber transmission region is replaced with data having a level value substantially the same as the data in the neighboring region. An X-ray CT apparatus according to any one of the first to fifth aspects is provided.

  In a seventh aspect, the present invention provides the data corresponding to the X-ray superabsorber transmission region, wherein the forward projection means projects the predetermined tomographic image only for a path passing through the X-ray superabsorber. From the second viewpoint, the forward projection data is calculated by overwriting the projection data used for image reconstruction of the predetermined tomographic image with data corresponding to the X-ray superabsorber transmission region. An X-ray CT apparatus according to any one of the aspects is provided.

  In an eighth aspect, the present invention provides the first aspect, wherein the X-ray superabsorber specifying unit specifies the X-ray superabsorber transmission region based on a threshold determination of a data value in the measured projection data. To X-ray CT apparatus according to any one of the seventh to seventh aspects.

  In a ninth aspect, the present invention relates to threshold value determination of a pixel value in a tomogram obtained by reconstructing an image from the measured projection data, or an operation on the tomogram. The X-ray superabsorber region of the tomogram is extracted based on the position designation by the person, and the X-ray superabsorber transmission region is specified. An X-ray CT apparatus is provided.

  In a tenth aspect, the present invention relates to an X-ray detector having a computer and X-ray irradiation of a subject including an X-ray superabsorber in a plurality of views, and the transmitted X-ray of the subject having a plurality of channels. Corresponding to the X-ray high-absorber transmission region in the actual measurement projection data, and a high-absorber region specifying means for specifying the X-ray high-absorber transmission region based on the actual measurement projection data of the subject obtained by A data replacement means for replacing the data to be estimated with data estimated from data corresponding to a neighboring region of the region to obtain replaced projection data; and a second method for reconstructing a tomographic image based on the replaced projection data. 1 reconstruction means, forward projection means for forwardly projecting the tomographic image reconstructed by the first reconstruction means by the plurality of views to calculate forward projection data, and tomography based on the forward projection data Image reconstruction It provides a program for functioning as a second reconstruction means for.

  In an eleventh aspect, the present invention relates to a computer, wherein an X-ray detector irradiates a subject including an X-ray superabsorber with a plurality of views and detects transmitted X-rays of the subject with an X-ray detector having a plurality of channels. A high-absorber region specifying means for specifying an X-ray high-absorber transmission region based on the measured projection data of the subject obtained in this manner, and data corresponding to the X-ray high-absorber transmission region in the actual projection data. A data replacement means for replacing the data estimated from data corresponding to the vicinity of the region to obtain replaced projection data, and a first reconstruction for reconstructing a tomographic image based on the replaced projection data. Constructing means; forward projection means for forwardly projecting a predetermined tomographic image in the plurality of views to calculate forward projection data; second reconstruction means for reconstructing a tomographic image based on the forward projection data; Said forward projection means and A program for causing the processing by the second reconstruction means to be repeated a plurality of times and functioning as a control means for obtaining a finally reconstructed tomographic image, wherein the predetermined tomographic image is the first process, A program is provided that is a tomographic image obtained by the first reconstruction means, and is a tomogram obtained by the second reconstruction means in the previous process in the second and subsequent processes.

  Here, the “X-ray superabsorber transmission region” means a region where X-rays actually or virtually transmit through the X-ray high absorber.

  The “channel” means an X-ray detection element constituting the X-ray detector and a unit of data obtained by the X-ray detection element.

  According to the present invention, the data replacement unit replaces the data corresponding to the X-ray superabsorber transmission region in the measured projection data with the data estimated from the data corresponding to the neighboring region, and the first reconstruction unit includes The tomographic image is reconstructed with the replaced projection data, and the forward projection means and the second reconstruction means perform forward projection and image reconstruction one time or repeatedly several times based on the tomographic image. Therefore, the information of the tissue other than the X-ray superabsorber included in the data corresponding to the region other than the superabsorber transmission region is converted into a tomographic image while suppressing the influence of the data obtained through the X-ray superabsorber. The information of the tissue other than the X-ray superabsorber carried by the tomographic image is reflected in the data corresponding to the transmission region of the X-ray superabsorber, and the contradiction between the image to be originally obtained and the projection data The group around the X-ray superabsorber of the subject can be reduced There it is possible to obtain a tomographic image reconstructed with higher accuracy.

  Embodiments of the present invention will now be described with reference to the drawings.

  FIG. 1 is a block diagram of the X-ray CT apparatus 100 according to the present embodiment. The X-ray CT apparatus 100 includes an operation console 1, an imaging table 10, and a scanning gantry (imaging means) 20.

  Here, the vertical direction is the y-axis direction, and the conveyance direction of the imaging table 10 (usually coincides with the thickness direction of the X-ray beam or the body axis direction of the subject) is the z-axis direction, the y-axis direction, and the z-axis. A direction perpendicular to the axial direction is defined as the x-axis direction.

  The operation console 1 includes an input device 2 such as a keyboard or a mouse that receives input from the operator, a central processing unit 3 that executes a scan control process, and the X collected by the scanning gantry 20. A data collection buffer (buffer) 5 for collecting line detector data is provided. The operation console 1 further includes a monitor 6 that displays various types of information and a storage device 7 that stores programs, projection data, and the like. The shooting conditions are input from the input device 2 and stored in the storage device 7.

  The imaging table 10 includes a cradle 12 on which a subject 71 is placed and taken in and out of the opening of the scanning gantry 20. The cradle 12 is moved up and down and horizontally moved by a motor built in the imaging table 10.

  The scanning gantry 20 includes an X-ray tube 21, an X-ray control unit 22 that controls X-ray tube voltage and X-ray irradiation timing (timing), and a fan-shaped X-ray beam 81 that converts X-rays emitted from the X-ray tube 21. A collimator 23 for shaping into an X-ray, an X-ray detector 24 for detecting an X-ray beam 81 transmitted through the subject 71, and a data acquisition device (DAS) for collecting projection data based on the output of the X-ray detector 24 : Data Acquisition System) 25.

  Furthermore, the scanning gantry 20 holds the X-ray tube 21, the collimator 23, and the X-ray detector 24, and rotates to control the gantry rotating unit 15 and the gantry rotating unit 15 that rotates around the body axis of the subject 71. A control unit 26 and a gantry control unit 29 that exchanges control signals between the operation console 1 and the X-ray control unit 22, the rotation control unit 26, the imaging table 10, and the like are provided.

  The X-ray tube 21, the collimator 23, and the X-ray detection unit 24 are supported by a predetermined base of the gantry rotating unit 15 and maintain a predetermined positional relationship. That is, the X-ray tube 21 and the X-ray detector 24 are disposed to face each other, and the collimator 23 is disposed between the X-ray tube 21 and the X-ray detector 24. Then, the X-rays radiated from the X-ray tube 21 pass through a slit formed by the collimator 23, whereby a predetermined thickness and spread, that is, a fan shape having a cone angle and a fan angle. X-ray beam is formed.

  The X-ray detection unit 24 includes a plurality of X-ray detection elements, for example, 1,000 X-ray detection elements 24a corresponding to 1000 channels arranged in the X-ray beam spreading direction, that is, the channel direction. This is a so-called multi-row X-ray detector in which a plurality of, for example, 64 detector element rows are arranged in the thickness direction (z-axis direction) of the X-ray beam.

  The central processing unit 3 includes a scan control unit 31, an X-ray high-absorber specifying unit (X-ray high-absorber specifying unit) 32, a data replacement unit (data replacement unit) 33, a first reconstruction unit (first reconstruction unit). (Composing means) 34, forward projecting section (forward projecting means) 35, data adjusting section (data adjusting means) 36, second reconfiguring section (second reconfiguring means) 37, and repetition control section (control means) 38. Have. The central processing unit 3 is configured by, for example, a computer, and functions as these units by reading and executing a program stored in the storage device 7.

  The scan control unit 31 performs X-ray projection of the subject 71 including the X-ray superabsorber in a plurality of views, and collects the measured projection data P0 (view, ch) necessary for image reconstruction. 22, the rotation control unit 26, and the imaging table 10 are controlled via the gantry control unit 29.

  The X-ray superabsorber specifying unit 32 specifies the X-ray superabsorber transmission region in the tomographic image space based on the actual measurement projection data P0 (view, ch). Here, the X-ray superabsorber specifying unit 32 specifies the X-ray superabsorber transmission region for each view based on the threshold determination of the data value in the measured projection data P0 (view, ch). For example, the output signal intensity of the X-ray detector 24 is represented by a data value, the maximum data value is 100%, and the portion where the data value is 20% or less is the portion corresponding to the X-ray high absorber transmission region. Identify.

  The data replacement unit 33 replaces the data corresponding to the X-ray superabsorber transmission region in the measured projection data P0 (view, ch) with the data estimated from the data corresponding to the neighboring region of the region, and the replaced projection Data P1 (view, ch) is obtained. Here, the data replacement unit 33 replaces the data corresponding to the X-ray superabsorber transmission region with data having a value substantially the same level as the neighboring region. For example, in the measured projection data P0 (view, ch) after fan-parallel conversion, each view corresponds to an X-ray superabsorbent transmission region on the profile representing the projection data of one view. The data value to be replaced is approximated by a straight line connecting the data value points corresponding to both ends of this area.

  The first reconstruction unit 34 reconstructs the tomographic image G1 based on the replaced projection data P1 (view, ch). For image reconstruction, for example, a conventionally known three-dimensional image reconstruction method using the Feldkamp method, another three-dimensional image reconstruction method, or a two-dimensional image reconstruction method can be used. The procedure is as follows. First, the projection data is subjected to a fast Fourier transform (FFT) for transforming into the frequency domain, and the reconstruction function Kernel (j) is superimposed on the projection data to perform an inverse Fourier transform. Then, back projection processing is performed on the projection data obtained by superimposing the reconstruction function Kernel (j), and this corresponds to a predetermined slice when the subject HB is sliced in the body axis direction (z-axis direction). A tomographic image (xy plane) is obtained.

  The forward projection unit 35 forward-projects a predetermined tomographic image Gi (i = 1, 2, 3,...) With a plurality of views to calculate projection data (forward projection data) Pi + 1. That is, forward projection projection data Pi + 1 (view, ch) is calculated for each view of each pixel constituting the tomographic image Gi using a predetermined algorithm (algorithm). The forward projection unit 35 performs the above-described forward projection process once or a plurality of times under the control of an after-mentioned iterative control unit 38. In the forward projection process, for example, a parallel beam is virtually irradiated and projected for each view. The tomographic image Gi is a tomographic image G1 obtained by the first reconstruction unit 34 in the first process (i = 1), and in the second and subsequent processes (i ≧ 2), This is a tomographic image Gi obtained by the second reconstruction unit 37. Here, the forward projection unit 35 calculates the projection data Pi + 1 (view, ch) by forwardly projecting a predetermined tomographic image Gi with respect to all paths passing through the X-ray superabsorber and other tissues. .

  The data adjustment unit 36 replaces the data value of the channel corresponding to the X-ray superabsorber transmission region in the projection data Pi + 1 (view, ch) obtained by the forward projection unit 35 with the value of the data. Changes according to the difference between the projection data P1 (view, ch) or the data value of the channel corresponding to the X-ray superabsorbent transmission region in the projection data Pi (view, ch) used for image reconstruction of the tomographic image Gi Projection data (adjusted projection data) P′i + 1 (view, ch) adjusted to be obtained is obtained.

  Here, for each channel data corresponding to the X-ray superabsorber transmission region in the projection data Pi + 1 (view, ch), the data adjustment unit 36 performs the channel adjustment for the channel data value. Obtained by multiplying the difference ΔPi + 1 between the data value and the data value of the channel corresponding to the channel in the projection data Pi (view, ch) used for image reconstruction of the tomographic image Gi by a predetermined weighting factor α. Projected data P′i + 1 (view, ch) is obtained by adding the data values α · ΔPi + 1.

  That is, projection data P′i + 1 (view, ch) is obtained according to the following formula 1.

P′i + 1 (view, ch) = Pi + 1 (view, ch) + α · ΔPi + 1 (view, chz)
However, ΔPi + 1 (view, chz) = Pi + 1 (view, chz) −Pi (view, chz),
α = weight coefficient, chz = channel corresponding to X-ray superabsorber transmission region
... (Formula 1)

  The second reconstruction unit 37 reconstructs the tomographic image Gi + 1 based on the projection data P′i + 1 (view, ch) obtained by the data adjustment unit 36.

  The iterative control unit 38 causes the forward projection unit 35, the data adjustment unit 36, and the second reconstruction unit 37 to repeat the process one or more times, and finally reconstructs the tomogram as a final tomogram. Get as.

  Hereafter, the flow of operation of the X-ray CT apparatus 100 according to the present embodiment will be described.

  FIG. 2 is a flowchart showing the operation flow of the X-ray CT apparatus 100 according to the present embodiment.

  In step S101, the subject 71 is subjected to X-ray CT imaging to collect actual projection data P0. Specifically, the scan control unit 31 sends control signals to the X-ray control unit 22 and the rotation control unit 26 via the gantry control unit 29 to rotate the gantry rotation unit 15 and to transmit the X-ray beam from the X-ray tube 21. The subject 71 is irradiated with 81. In the meantime, the data collecting device 25 converts the detection signal output by the X-ray detector 24 detecting and transmitting the transmitted X-rays into the actual projection data P0 and sends it to the data collection buffer 5, and the collected actual projection data P0 is stored. It is stored in the device 7.

  FIG. 3 is a diagram showing a predetermined cross section (xy plane) in the sample model of the subject 71 and projection data thereof. In FIG. 3, the view angle is 0 degree downward in the y-axis direction and 90 degrees right in the x-axis direction. The projection data shown in the figure shows data after fan-parallel conversion. As shown in FIG. 3, the cross-section SL has an elliptical shape as a whole, and includes a soft tissue 74, a small circular object 73 positioned substantially at the center of the soft tissue 74, and the object 73 in the x-axis direction. It includes two large circular X-ray superabsorbers 72a and 72b that are in a sandwiched positional relationship. The measured projection data P0 (0, ch) in the 0-degree direction of the cross section SL and the measured projection data P0 (90, ch) in the 90-degree view are respectively represented by the channel number ch of the X-ray detector 24 on the horizontal axis, The detection signal intensity p is represented as a vertical axis.

  In each projection data, the detection signal intensity corresponding to the path that transmits only the soft tissue 74 has a level corresponding to the path length of the tissue, and the detection signal intensity corresponding to the path that transmits the X-ray superabsorber 72a or 72b. Becomes substantially zero level regardless of the path length of the organization. Further, the detection signal intensity corresponding to the path that passes through the soft tissue 74 and the object 73 is lower than the level when transmitting only the soft tissue 74 and higher than the level when transmitting through the X-ray high absorbers 72a and 72b. It is an intermediate level and is a level corresponding to the path length of the organization. That is, in the measured projection data P0 (0, ch), the region between the channels ch1 and ch2, which is the region from one end to the other end of the X-ray high absorber 72a in the x-axis direction, and the X-ray high absorber 72b. The detection signal intensity in the region between channels ch5 and ch6, which is a region from one end to the other end in the x-axis direction, is approximately 0 level. Further, the detection signal intensity in the region between the channels ch3 and ch4, which is the region from one end to the other end of the object 73 in the x-axis direction, is an intermediate level and has a level corresponding to the path length of the tissue. The detection signal intensity in the region is higher and corresponds to the path length of the tissue.

  On the other hand, in the measured projection data P0 (90, ch), the detection signal intensity in the region between the channels ch7 and ch8, which is the region from one end to the other end of the X-ray high absorbers 72a and 72b, is substantially 0 level. The detection signal intensity in other regions is a higher level and a level corresponding to the path length of the tissue.

  In step S102, the X-ray high-absorber transmission region in the actually measured projection data P0 is specified by determining the threshold value of the data. Specifically, the X-ray superabsorber specifying unit 32 determines the maximum data value of the measured projection data P0 (view, ch) in all views or the measured projection data P0 (view, ch) in each view. The maximum data value is set to 100%, and for each view, the portion of the measured projection data P0 whose data value is 20% or less is specified as the X-ray high absorber transmission region. In the example of FIG. 3, in the actually measured projection data P0 (0, ch), the region between ch1 and ch2 and the region between ch5 and ch6 are specified as the X-ray high absorber transmission region, and the projection data P0 (90 , Ch), the region between ch7 and ch8 is specified as the X-ray superabsorber transmission region.

  FIG. 4 is a diagram showing a part of the replacement projection data P1 (view, ch) and a tomographic image G1 reconstructed from the replacement projection data P1 (view, ch). The projection data shown in the figure shows the data after fan-para conversion.

  In step S103, the data corresponding to the X-ray superabsorber transmission region in the actually measured projection data P0 (view, ch) is replaced with the data estimated from the neighboring region, and the replaced projection data P1 (view, ch) is replaced. obtain. Specifically, for each view, the data replacement unit 33 sets the data value corresponding to the X-ray superabsorbent transmission region to the data value corresponding to both ends of this region on the profile representing the projection data of one view. Approximate and replace with a straight line connecting the points. For example, in the measured projection data P0 (0, ch) of the view in the 0 degree direction as shown in FIG. 3, approximation is performed with a straight line connecting the point P0 (0, ch1) and the point P0 (0, ch2). , P0 (0, ch5) and P0 (0, ch6) are approximated and replaced by a straight line to generate replaced projection data P1 (0, ch) as shown in FIG. Similarly, in the actually measured projection data P0 (90, ch) of the 90-degree view as shown in FIG. 3, it is approximated by a straight line connecting the point P0 (90, ch7) and the point P0 (90, ch8). And the replacement projection data P1 (90, ch) as shown in FIG. 4 is generated. Such replacement processing is performed for each view to obtain replaced projection data P1 (view, ch) of a plurality of views.

  In step S104, the tomographic image G1 is reconstructed based on the replaced projection data P1 (view, ch). Specifically, the first reconstruction unit 34 performs a three-dimensional image reconstruction process by the Feldkamp method on the replaced projection data P1 (view, ch) to obtain a tomographic image G1.

  As described above, when the data corresponding to the X-ray superabsorber transmission region is estimated from the surrounding data and replaced by the processing in steps S103 and S104, and the tomographic image is reconstructed from the replacement projection data, the superabsorber is obtained. Information on tissues other than the X-ray superabsorber included in the data other than the transmission region can be carried on the tomographic image while suppressing the influence of the data obtained through the X-ray high absorber.

  For example, as shown in FIG. 4, the tomographic image G1 corresponds to a contour line 171 corresponding to the epidermis of the subject 71, a soft tissue image 174 corresponding to the soft tissue 74, and X-ray superabsorbers 72a and 72b. X-ray superabsorber images 172a and 172b, an object image 173 corresponding to the object 73, and streak-like artifacts 175 generated around the X-ray superabsorber images 172a and 172b are included. The X-ray superabsorber images 172a and 172b are reconstructed with pixel values close to the soft tissue image 174 due to the effect of data replacement in step S103, and have little distortion. The object image 173 includes information about the object 73 in the projection data of a view having a path where the X-ray superabsorbent body 72a or 72b and the object 73 overlap, such as 90-degree view replacement projection data P1 (90, ch). Although it is missing, the projection data of the other views includes the information of the object 73, so that the object image 173 appears distorted as an elliptical shape extending in the y-axis direction.

  In step S105, a parameter i for counting the number of iterations in the later-described iteration process is set to 1.

  In step S106, the tomographic image Gi is projected forward in a plurality of views to calculate projection data Pi + 1 (view, ch). Specifically, the forward projection unit 35 obtains projection data Pi + 1 (view, ch) by forwardly projecting each pixel constituting the tomographic image Gi for each view using a predetermined algorithm. At this time, the forward projection unit 35 forward-projects the tomographic image Gi only for the path passing through the X-ray superabsorber to obtain data corresponding to the X-ray superabsorber transmission region, and the projection data Pi (view, ch). The projection data Pi + 1 (view, ch) is calculated by overwriting the data corresponding to the X-ray superabsorber transmission region. In step S106, the tomographic image Gi to be processed in the first process (i = 1) is the tomographic image G1 obtained by the first reconstruction unit 34 in step S104.

  In step S107, the data value of the channel corresponding to the X-ray superabsorber transmission region in the projection data Pi + 1 (view, ch) obtained in step S106 is adjusted, and the projection data P′i + 1 (view , Ch). Specifically, for the data value of the channel corresponding to the X-ray superabsorber transmission region in the projection data Pi + 1 (view, ch), the data adjustment unit 36 determines the value of the data and the tomographic image Gi. Of the data obtained by multiplying a difference ΔPi + 1 by a predetermined weighting factor α from the data value of the channel corresponding to the X-ray superabsorber transmission region in the projection data Pi (view, ch) used for the image reconstruction of The projection data P′i + 1 (view, ch) is obtained by adding the values α · ΔPi + 1.

  FIG. 5 is a diagram showing a part of the projection data P′2 (view, ch) and a tomographic image G2 reconstructed from the projection data P′2 (view, ch). The projection data shown in the figure shows the data after fan-para conversion.

  As shown in FIG. 5, the projection data P′2 (0, ch) in the 0-degree direction has substantially the same profile as the replacement projection data P1 (0, ch). On the other hand, in the projection data P′2 (90, ch) in the 90-degree direction, the X-ray superabsorber transmission region P′2 (90, ch7) to P′2 (90, ch8) is shown in FIG. An elliptical shape extending in the y-axis direction of the object image 173 in the tomographic image G1 shown is fed back as information, and compared with the replacement projection data P1 (90, ch7) to P1 (90, ch8), The profile becomes concave in the path direction.

  In step S108, the tomographic image Gi + 1 is reconstructed based on the projection data P′i + 1 (view, ch) obtained by adjusting the data in step S107. Specifically, the second reconstruction unit 37 performs a three-dimensional image reconstruction process by the Feldkamp method on the projection data P′i + 1 (view, ch) to obtain a tomographic image Gi + 1.

  For example, as shown in FIG. 5, the tomogram G2 corresponds to the contour line 271 corresponding to the epidermis of the subject 71, the soft tissue image 274 corresponding to the soft tissue 74, and the X-ray superabsorbers 72a and 72b, respectively. X-ray superabsorber images 272a and 272b, an object image 273 corresponding to the object 73, and streak-like artifacts 275 generated around the X-ray superabsorber images 272a and 272b. The X-ray superabsorber images 272a and 272b are reconstructed with pixel values very close to the soft tissue image 274, like the tomographic image G1. The object image 273 appears as an elliptical shape that is closer to the shape of the object 73 than the object image 173. This is because data corresponding to the X-ray superabsorber transmission region in projection data P′2 (view, ch) by forward projection or the like, for example, P′2 (90, ch7) to P′2 (90 ° direction). 90, ch8) reflects the information of the object image 173, which is a result of the object 73 being restored more accurately. Further, the artifact 275 is further reduced as compared with the artifact 175.

  As described above, by the processing in steps S106 to S108, the information of the tissue other than the X-ray superabsorber carried by the tomographic image is reflected in the data corresponding to the X-ray superabsorber transmission region, and the image to be originally obtained. And inconsistency between projection data can be reduced.

  In step S109, the iterative control unit 38 determines whether or not the parameter i = n, whereby the processing of steps S106 to S108, that is, the processing by forward projection, data adjustment, and image reconstruction is set to n. It is determined whether or not it has been repeated a number of times. If it is determined in step S109 that the parameter i is not n, the process proceeds to step S110, and the iterative control unit 38 increments the parameter i. Thereafter, the process returns to step S106, and the processes of steps S106 to S108 are repeated again. On the other hand, if it is determined that the parameter i = n, the process proceeds to step S111, and the tomographic image Gn + 1 is set as the final tomographic image of the subject 71.

  In the second and subsequent steps S106, unlike the first process, the tomographic image Gi reconstructed in the previous step S108 is forward projected to obtain projection data Pi + 1 (view, ch). For example, since i = 2 in the second process, the tomographic image G2 obtained in step S108 in the first process is forward projected to obtain projection data P3 (view, ch).

  In the second and subsequent steps S107 and S108, similarly to the first time, Pi + 1 (view, ch) data adjustment is performed to obtain projection data P′i + 1 (view, ch). The tomographic image Gi + 1 is reconstructed based on the projection data P′i + 1 (view, ch). For example, since i = 2 in the second processing, the projection data P′3 (view, ch) is obtained by adjusting the data of P3 (view, ch), and this projection data P′3 (view, ch) ) To reconstruct the tomographic image G3.

  FIG. 6 is a diagram showing a part of the projection data P′3 (view, ch) and a tomographic image G2_3 reconstructed from the projection data P′3 (view, ch).

  As shown in FIG. 6, the projection data P′3 (0, ch) of the view in the 0 degree direction has substantially the same profile as the projection data P′2 (0, ch). On the other hand, in the projection data P′3 (90, ch) of the view in the direction of 90 degrees, in P′3 (90, ch7) to P′3 (90, ch8) which are X-ray superabsorber transmission regions, FIG. The ellipse shape slightly extended in the y-axis direction of the object image 273 in the tomographic image G2_2 shown in FIG. 6 is fed back as information, and compared with the projection data P′2 (90, ch7) to P′2 (90, ch8). Thus, a concave profile contracted in the y-axis direction is obtained.

  Further, for example, as shown in FIG. 6, the tomographic image G3 is similar to the previously obtained tomographic image G2, X-ray superabsorbent images 372a and 372b corresponding to the X-ray superabsorbers 72a and 72b, respectively, and an object 73. And an object image 373 corresponding to. However, the tomographic image G3 is reconstructed with pixel values closer to the soft tissue images 174 and 274 and less distorted than the tomographic image G3 obtained last time, the X-ray superabsorbent body images 372a and 372b. In addition, the object image 373 becomes more circular. Streaky artifacts 375 generated around the X-ray superabsorber images 372a and 372b are hardly seen.

  By repeating steps S106 to S108 n times in this way, the tissue around the X-ray superabsorber is restored with higher accuracy, and artifacts around the X-ray superabsorber are also reduced. A tomographic image can be obtained.

  As described above, according to the present embodiment, the tomographic image G1 is reconstructed by replacing the data of the path passing through the X-ray superabsorbers 72a and 72b in the measured projection data P0 with the data estimated from the surrounding data. Thereafter, the tomographic image Gi + 1 (i = 1, 2, 3,...) Is forward projected to obtain projection data Pi + 1, and the tomographic image Gi + 1 is reconstructed based on the projection data Pi + 1. The processing is performed n times (n ≧ 1), and the tomographic image Gn + 1 obtained at the end is obtained as the final tomographic image. Therefore, the X-ray high absorption possessed by the data in the region other than the superabsorbent transmission region Information on tissues other than the body is carried on a tomographic image while suppressing the influence of the data obtained through the X-ray high absorber, and information on tissues other than the X-ray superabsorber carried by the tomographic image is X Inconsistency between the image and projection data that should be originally obtained by reflecting the data corresponding to the transmission region of the linear superabsorber. Can be reduced, it is possible to obtain a tomographic image near the X-ray superabsorbent of the subject tissue is restored with higher accuracy.

  In addition, this invention is not limited to said embodiment, A various deformation | transformation is possible unless it deviates from the meaning of this invention.

  For example, the X-ray superabsorber specifying unit 32 determines the tomographic value based on the threshold value determination of the pixel value or the position designation by the operator in the tomographic image G0 obtained by image reconstruction from the measured projection data P0 (view, ch). The X-ray superabsorber region of the image G0 is extracted, and the data region corresponding to the path through which the X-ray passes through the X-ray superabsorber region is extracted from the measured projection data P0 (view, ch). The transmission region may be specified.

  For example, the forward projection unit 35 forward-projects a predetermined tomographic image Gi only for a path passing through the X-ray superabsorber, obtains data corresponding to the X-ray superabsorber transmission region, and obtains an image of the tomographic image Gi. The projection data Pi + 1 (view, ch) may be calculated by overwriting the projection data used for the reconstruction with data corresponding to the X-ray superabsorber transmission region. In the case of the method in which the predetermined tomographic image Gi is forward projected for all the paths, the restoration of the projection data only affects the X-ray superabsorbent transmission region, but there is an advantage that the calculation amount is reduced and the processing time is shortened. is there. On the contrary, in the case of the method in which the predetermined tomographic image Gi is forward projected only for the path passing through the X-ray superabsorber, the processing time becomes long, but the restoration operation of the projection data is not in the X-ray superabsorber transmission region. Therefore, the image quality of the entire tomographic image can be improved. In addition, taking the middle, the forward projection unit 35 forward-projects the predetermined tomographic image Gi with respect to a path passing through the X-ray high absorber and a part of the tissue other than the X-ray high absorber, and X-ray high absorption. Data corresponding to the region including the body transmissive region is obtained, and projection data Pi + 1 (view, ch) is calculated by overwriting the projection data used for image reconstruction of the tomographic image Gi with the data corresponding to the region. Also good.

  In addition, for example, the data adjustment unit 36 limits the data adjustment to data corresponding to the X-ray high-absorber transmission region. However, the data adjustment unit 36 expands this to the data of all regions and performs the same adjustment. You may do it. In this case, the processing time becomes long, but it is considered that the image quality is further improved because the restoring action covers the entire region.

  Further, for example, the data adjustment unit 36 may be configured to set the weighting coefficient α = 0 in Equation 1. This is equivalent to the case where no data adjustment is performed or the case where the data adjustment unit 36 is not provided. Of course, even if the data is not adjusted, there is an effect of restoring the image in the tomographic image, but if the data is adjusted, a faster restoration can be expected.

  Further, the data adjustment unit 36 may adjust the channel data of the X-ray superabsorber transmission region in the projection data Pi + 1 (view, ch) by another method.

  For example, for each channel data corresponding to the X-ray high-absorber transmission region in the projection data Pi + 1 (view, ch), the data adjustment unit 36 performs the data of the channel with respect to the data value of the channel. Of the data obtained by multiplying the difference ΔP′i + 1 by the predetermined weighting coefficient α between the value of and the value of the data of the channel corresponding to the channel in the replaced projection data P1 (view, ch) Adjusted projection data P′i + 1 (view, ch) may be obtained by adding ′ i + 1.

  That is, adjusted projection data P′i + 1 (view, ch) is obtained according to the following formula 2.

P′i + 1 (view, ch) = Pi + 1 (view, ch) + α · ΔP′i + 1 (view, chz)
However, ΔP′i + 1 (view, chz) = Pi + 1 (view, chz) −P1 (view, chz)
... (Formula 2)

  Further, for example, the data adjustment unit 36, for each channel data corresponding to the X-ray high-absorber transmission region in the projection data Pi + 1 (view, ch), for each channel data value, The difference between the data value and the data value of the channel corresponding to the channel in the projection data Pi (view, ch) used for image reconstruction of the replacement projection data P1 (view, ch) or the predetermined tomographic image Gi The adjusted projection data P ′ is obtained by adding the value f (ΔP ″ i + 1) (view, chz) of data derived by a predetermined function f (ΔP ″ i + 1) depending on ΔP ″ i + 1. i + 1 (view, ch) may be obtained.

  That is, adjusted projection data P′i + 1 (view, ch) is obtained according to the following Equation 3.

P′i + 1 (view, ch) = Pi + 1 (view, ch) + f (ΔP ″ i + 1) (view, chz)
However, ΔP ″ i + 1 (view, chz) = Pi + 1 (view, chz) −P1 (view, chz)
or Pi + 1 (view, chz)-Pi (view, chz)
... (Formula 3)

  Here, the function f (ΔP ″ i + 1) is defined by obtaining a suitable function experimentally or theoretically.

  In addition, the image which has the X-ray superabsorber specific part 32, the data replacement part 33, the 1st reconstruction part 34, the forward projection part 35, the data adjustment part 36, the 2nd reconstruction part 37, and the repetition control part 38 The processing device is also an embodiment of the present invention.

  In addition, the computer includes an X-ray superabsorber specifying unit 32, a data replacement unit 33, a first reconstruction unit 34, a forward projection unit 35, a data adjustment unit 36, a second reconstruction unit 37, and a repetition control unit 38. A program for causing the program to function as is also an embodiment of the present invention.

1 is a configuration block diagram of an X-ray CT apparatus according to an embodiment of the present invention. It is a flowchart showing the flow of operation | movement of the X-ray CT apparatus by this embodiment. It is a figure which shows the predetermined cross section in the sample model of a test object, and its projection data. It is a figure which shows a part of substituted projection data, and a tomographic image reconstructed from the substituted projection data. It is a figure which shows a part of adjusted projection data in the process of the 1st time, and the tomographic image reconstructed from adjusted projection data. It is a figure which shows a part of adjusted projection data in the process of the 2nd time, and the tomographic image reconstructed from adjusted projection data.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Central processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Gantry rotation part 20 Scanning gantry (imaging means)
21 X-ray tube 22 X-ray controller 23 Collimator 24 X-ray detector (X-ray detector)
24a X-ray detection element 25 Data acquisition device 26 Rotation control unit 29 Gantry control unit 31 Scan control unit 32 X-ray high absorber specifying unit (X-ray high absorber specifying unit)
33 Data replacement part (data replacement means)
34 1st reconstruction part (1st reconstruction means)
35 Forward projection unit (forward projection means)
36 Data adjustment unit (data adjustment means)
37 Second reconstruction unit (second reconstruction unit)
38 Repeat control unit (control means)
100 X-ray CT system (X-ray CT system)
71 Subject 72a, 72b X-ray superabsorber 73 Object 74 Soft tissue 81 X-ray beam 171, 271, 371 Contour line 172a, 172b, 272a, 272b, 372a, 372b X-ray superabsorber image 173, 273, 373 Object Image 174, 274, 374 Soft tissue image 175, 275, 375 Artifact

Claims (7)

Imaging means for irradiating a subject including an X-ray superabsorber with a plurality of views and detecting transmitted X-rays of the subject with an X-ray detector having a plurality of channels to obtain measured projection data of the subject When,
Superabsorber region specifying means for specifying an X-ray superabsorber transmission region based on the measured projection data;
Data replacement means for replacing the data corresponding to the X-ray superabsorber transmission region in the measured projection data with data estimated from the data corresponding to the vicinity region of the region to obtain replaced projection data;
First reconstruction means for reconstructing a tomographic image based on the replaced projection data;
Forward projection means for forwardly projecting predetermined tomographic images in the plurality of views and calculating forward projection data;
The data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data is set to the value of the data and the channel corresponding to the X-ray high absorber transmission region in the replaced projection data. Data adjustment means for obtaining adjusted projection data adjusted to change based on the difference between the data value;
Second reconstruction means for reconstructing a tomographic image based on the adjusted projection data;
A control unit that repeatedly performs the process by the forward projection unit, the data adjustment unit, and the second reconstruction unit a plurality of times, and obtains a finally reconstructed tomographic image,
The predetermined tomographic image is a tomographic image obtained by the first reconstruction unit in the first process, and is obtained by the second reconstruction unit in the previous process in the second and subsequent processes. tomographic image der is,
The data adjustment unit is configured to output the data value and the X-ray superabsorber in the replaced projection data with respect to the data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data. An X-ray CT apparatus , wherein the adjusted projection data is obtained by adding a value derived by a predetermined function depending on a difference from the data value of the channel corresponding to a transmission region . The X-ray CT apparatus according to claim 1, wherein the predetermined function is a function that multiplies the difference by a predetermined weight . 3. The X-ray according to claim 1, wherein the data replacement unit replaces data corresponding to the X-ray superabsorber transmission region with data having a value substantially equal to the data of the neighboring region. CT device. The forward projection means forward-projects the predetermined tomographic image only for a path passing through the X-ray superabsorber to obtain data corresponding to the X-ray superabsorber transmission region, and an image of the predetermined tomographic image The X-ray CT apparatus according to any one of claims 1 to 3 , wherein the forward projection data is calculated by overwriting projection data used for reconstruction with data corresponding to the X-ray superabsorber transmission region. . The high absorbent region specifying means, X according to any one of claims 1 to 4 for identifying the X-ray superabsorbent transmissive area based on a threshold judgment value of the data in the measured projection data Line CT placement. The superabsorbent region specifying means is configured to determine a threshold value of a pixel value in a tomographic image obtained by reconstructing an image from the measured projection data, or based on position designation by an operator on the tomographic image. The X-ray CT apparatus according to claim 1, wherein an X-ray superabsorber region is extracted and the X-ray superabsorber transmission region is specified. Computer
Based on the measured projection data of the subject obtained by irradiating the subject including the X-ray superabsorber with a plurality of views and detecting the transmitted X-rays of the subject with an X-ray detector having a plurality of channels. A high-absorber region specifying means for specifying the X-ray high-absorber transmission region;
Data replacement means for replacing the data corresponding to the X-ray superabsorber transmission region in the measured projection data with data estimated from the data corresponding to the vicinity region of the region to obtain replaced projection data;
First reconstruction means for reconstructing a tomographic image based on the replaced projection data;
Forward projection means for forwardly projecting predetermined tomographic images in the plurality of views and calculating forward projection data;
The data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data is set to the value of the data and the channel corresponding to the X-ray high absorber transmission region in the replaced projection data. Data adjustment means for obtaining adjusted projection data adjusted to change based on the difference between the data value;
Second reconstruction means for reconstructing a tomographic image based on the adjusted projection data;
A program for causing the processing by the forward projection means, the data adjustment means, and the second reconstruction means to be repeated a plurality of times, and to function as a control means for obtaining a tomographic image reconstructed last ,
The predetermined tomographic image is a tomographic image obtained by the first reconstruction unit in the first process, and is obtained by the second reconstruction unit in the previous process in the second and subsequent processes. A tomogram,
The data adjustment unit is configured to output the data value and the X-ray superabsorber in the replaced projection data with respect to the data value of the channel corresponding to the X-ray superabsorber transmission region in the forward projection data. A program for obtaining the adjusted projection data by adding a value derived by a predetermined function depending on a difference from a data value of the channel corresponding to a transmission region .

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