JP2016101332A - Radiation tomography apparatus and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an image in which image noise is suppressed in radiation tomography in which the intensity of a radiation is reduced in a prescribed view angle range.SOLUTION: A radiation tomography apparatus comprises: control means which controls a data collection system so as to collect projection data by emitting a radiation at the first intensity in the first view angle range and emitting a radiation at the second intensity lower than the first intensity in the second view angle range other than the first view angle range of the view angle range of 360°; reconstitution means which reconstitutes an image on the basis of at least a part of the collected projection data of the plurality of views, and reconstitutes the image by overlapping the first reconstitution function on the projection data of the view angle at which the radiation is emitted at the first intensity, and overlapping the second reconstitution function in which the high frequency component is suppressed more than the first reconstitution function on the data of the view angle at which the radiation is emitted at the second intensity.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a technique for improving the image quality of an image obtained by radiation tomography in which the intensity of radiation is changed according to a view angle.

  Conventionally, as one of medical tests, an inspection by puncture has been performed. In this examination, it is necessary to accurately insert the tip of the puncture needle into a target portion in the body of the subject, and the body of the subject cannot be seen with the naked eye. Therefore, when performing puncture, generally, a fluoroscopic image around the target portion is displayed on the monitor screen in substantially real time by continuous scanning with an X-ray CT apparatus. The surgeon performs puncturing while checking the position of the puncture needle while viewing the fluoroscopic image.

  When a puncture is performed as usual with an X-ray CT apparatus, the operator's hand or arm holding the puncture needle is exposed to the irradiated X-rays. Therefore, conventionally, the gantry is tilted in the body axis direction of the subject so as not to directly irradiate the operator's hand with X-rays, thereby reducing the dose to the operator (Patent Document 1). , See summary etc.).

JP 2006-320468 A

  However, in recent years, since the weight of the data collection unit has increased due to the increase in the rotation speed of the gantry and the increase in the number of detectors, it is difficult to mechanically tilt the gantry at a large angle. .

  In view of this, a method has been proposed in which the intensity of X-rays is made lower than usual in a predetermined view angle range in which X-rays are directly irradiated onto an operator's hand instead of tilting the gantry.

  By the way, in order to display a substantially real-time fluoroscopic image of the target portion by the X-ray CT apparatus, usually, a scanning portion including the target portion is continuously scanned for a certain period to collect projection data. . In parallel with this, the tomographic image corresponding to each frame is reconstructed and displayed while shifting the use range of the projection data used for image reconstruction in terms of time and view angle. The image reconstruction at this time generally uses a half reconstruction method in which an image can be reconstructed after collecting less projection data in order to shorten the delay of image display. The half reconstruction method is a method of reconstruction based on projection data corresponding to a fan angle α of 180 degrees + X-ray beam. In addition, the frame rate is, for example, 6 frames / one gantry rotation by performing image reconstruction for each data collection for a view angle of 60 degrees, and image reconstruction for each data collection for a view angle of 36 degrees. By doing so, 10 frames / one gantry rotation can be realized.

  Here, as described above, it is considered that the X-ray intensity is lowered only in a predetermined view angle range. In this case, the projection data used for the reconstruction of the image of one frame is composed of only projection data when the X-ray intensity is normal according to the timing (timing), and the X-ray intensity is higher than normal. In some cases, low projection data is included. The projection data when the X-ray intensity is low has a smaller signal noise ratio (S / N) and noise than the projection data when the X-ray intensity is normal. In general, a reconstructed image using projection data when the X-ray intensity is lower than normal is compared with a reconstructed image using only projection data when the X-ray intensity is normal. , Image noise increases. Therefore, the tomographic image obtained by changing the X-ray intensity may increase image noise depending on the frame, and may be difficult to refer to.

  Under such circumstances, there is a demand for a technique capable of obtaining an image in which image noise is suppressed in radiation tomography for reducing the intensity of radiation in a predetermined view angle range.

The invention of the first aspect
Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstructing means for reconstructing an image based on at least a part of the collected projection data of a plurality of views, with respect to projection data of a view angle irradiated with radiation at the first radiation intensity Superimposes the first reconstruction function, and for the projection data at the view angle irradiated with radiation at the second radiation intensity, the second reconstruction function suppresses higher frequency components than the first reconstruction function. There is provided a radiation tomography apparatus comprising: reconstruction means for reconstructing an image by superimposing a configuration function.

The invention of the second aspect is
Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstructing means for reconstructing an image based on at least a part of the collected projection data of a plurality of views, based on projection data of only a view angle irradiated with radiation at the first radiation intensity When the image is reconstructed, the first reconstruction function is superimposed on the projection data, and the image is based on the projection data including the view angle irradiated with the radiation at the second radiation intensity. Reconstructing means for reconstructing an image by superimposing a second reconstruction function that suppresses higher frequency components than the first reconstruction function on the projection data in the case of reconstruction. A radiation tomography apparatus is provided.

The invention of the third aspect is
Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstruction means for reconstructing an image based on at least a part of the collected projection data of the plurality of views;
When an image is reconstructed based on projection data of only the view angle irradiated with radiation at the first radiation intensity, the first image filter is applied to the image, or the image filter is When the image is reconstructed based on the projection data including the view angle irradiated with the radiation at the second radiation intensity without applying the high-frequency component to the image from the first image filter. And a radiation tomography apparatus comprising: an image processing unit to which a second image filter that suppresses the image is applied.

The invention of the fourth aspect is
The second view angle range is included between +180 degrees and -180 degrees, with the view angle at which the radiation source is located at the highest point in the vertical direction being 0 degrees, from the first viewpoint to the third viewpoint. A radiation tomography apparatus according to any one of the above aspects is provided.

The invention of the fifth aspect is
The control means controls to continuously collect projection data of a desired slice for a certain period;
The reconstruction unit reconstructs an image of the desired slice at a predetermined frame rate in parallel with the collection of the projection data, according to any one of the first to fourth aspects. A radiation tomography apparatus is provided.

The invention of the sixth aspect is
The radiation tomography apparatus according to the fifth aspect, in which the reconstruction unit reconstructs images of two adjacent frames so that projection data used for reconstruction partially overlap each other.

The invention of the seventh aspect
The radiation tomography apparatus according to any one of the first to sixth aspects, wherein the reconstruction unit reconstructs an image based on projection data in a view angle range for a half scan.

The invention of the eighth aspect
Any one of the first to sixth aspects, wherein the reconstruction means reconstructs an image based on projection data in a view angle range that is greater than half scan and less than or equal to full scan. Provided is a radiation tomography apparatus according to an aspect.

The invention of the ninth aspect is
When the puncture mode is set, the control means controls to irradiate radiation with the first radiation intensity and the second radiation intensity. From the first aspect to the eighth aspect, A radiation tomography apparatus according to any one of the aspects is provided.

The invention of the tenth aspect is
The radiation tomography apparatus according to any one of the first to ninth aspects, wherein the first radiation intensity is an intensity determined by an automatic exposure mechanism.

The invention of the eleventh aspect is
A program for causing a computer to function as each means constituting the radiation tomography apparatus according to any one of the first to tenth aspects is provided.

  According to the invention of the above aspect, an image with suppressed image noise can be obtained in radiation tomography in which the intensity of radiation is reduced within a predetermined view angle range.

1 is a diagram schematically showing a configuration of an X-ray CT apparatus according to an embodiment of the invention. It is a functional block diagram (block diagram) of the X-ray CT apparatus which concerns on embodiment of invention. It is a flowchart (flow chart) which shows the flow of a process with the X-ray CT apparatus in 1st Example. It is a figure which shows the view angle range of the projection data used for the reconstruction of the image of each frame in 1st Example, and the reconstruction function applied with respect to the projection data of each view. It is a flowchart which shows the flow of a process with the X-ray CT apparatus in 2nd Example. It is a figure which shows the view angle range of the projection data used for the reconstruction of the image of each frame in 2nd Example, and the reconstruction function applied with respect to the projection data of each frame. It is a flowchart which shows the flow of a process with the X-ray CT apparatus in 3rd Example. It is a figure which shows the view angle range of the projection data used for the reconstruction of the image of each frame in 3rd Example, and the image filter applied with respect to the image of each frame.

  Embodiments of the invention will be described below. The invention is not limited thereby.

  FIG. 1 is a diagram schematically showing a configuration of an X-ray CT apparatus according to the present embodiment.

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

  The operation console 1 collects data acquired by the input device 2 that receives input from the operator 41, an arithmetic processing device 3 that controls various parts for scanning the subject 40 and performs various calculations, and the scanning gantry 20. A data collection buffer 5, a monitor 6 for displaying images, and a storage device 7 for storing programs and data.

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

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

  FIG. 2 is a functional block diagram of the X-ray CT apparatus according to the present embodiment.

  As shown in FIG. 2, the X-ray CT apparatus according to the present embodiment functionally includes a data acquisition system 101, an operation unit 102, a display unit 103, a storage unit 104, an image reconstruction unit 105, And a control unit 106.

  The data collection system 101 is a system that scans the subject 40 under the control of the control unit 106 and collects projection data thereof. The data collection system 101 mainly includes a scanning gantry 20 and an imaging table 10.

  The operation unit 102 is a part that receives an operation from the operator 41, and receives various information desired by the operator 41.

  The display unit 103 is a part that displays characters, images, and the like, and various information is output to the operator 41.

  The storage unit 104 is a part that stores various data, information, programs, and the like.

  The image reconstruction unit 105 performs image reconstruction under the control of the control unit 106, and obtains images of each frame constituting a tomographic image of the subject 40 and a moving image-like perspective image of the subject 40.

  The control unit 106 comprehensively controls each unit, and controls the data collection system 101 to execute scanning, and controls the image reconstruction unit 5 to reconstruct an image.

  The image reconstruction unit 105 and the control unit 106 are realized by causing a computer to execute a predetermined program, for example.

(First embodiment)
Hereafter, the flow of processing in the X-ray CT apparatus in the first embodiment will be described.

  FIG. 3 is a flowchart showing the flow of processing in the X-ray CT apparatus in the first embodiment.

  In step A1, a scout scan is performed. Specifically, the control unit 106 controls the data collection system 101 based on a scout scan protocol. Thereby, a scout scan is executed, and scout data (scout data) of the subject 40 is acquired.

  In step A2, the tube voltage of the X-ray tube 21, the scan region (one or a plurality of slices) of the subject 40, a reconstruction function used for image reconstruction, and the like are set. Specifically, the operator 41 inputs information such as a desired tube voltage, a scan region, a reconstruction function, and the like through the operation unit 102. Based on this input information, the control unit 106 sets a tube voltage at the time of photographing, a scan region, a type of reconstruction function to be applied at the time of image reconstruction, and the like. The reconstruction function is superimposed on the projection data in the frequency space. The image filter affects the quality of the reconstructed image, particularly sharpness, resolution, image noise, and the like. In general, the reconstruction function is prepared in a multi-step manner from the one in which the high-frequency component of the image is suppressed to the one in which the high-frequency component is emphasized.

  In step A3, a noise index value is set. Specifically, the operator 41 inputs information on an index value representing a desired noise level as image noise of a captured image through the operation unit 102. The control unit 106 sets a noise index value based on the input information.

  In step A4, the tube current is set. Specifically, the control unit 106 calculates a tube current for obtaining an image of a desired noise level by the automatic exposure mechanism based on the scout data, the set tube voltage, the scan region, and the noise index value, Set. The tube current may be modulated only in the body axis direction (slice direction) of the subject 40 with respect to the scan region, or may be further modulated in the view angle direction (tomographic plane direction). The tube current set here is the tube current when the X-ray intensity is at a normal level. When the X-ray intensity is changed, the tube current set here is raised and lowered as a reference.

  In step A5, the puncture mode is selected. Specifically, the operator 41 inputs information for selecting the puncture mode through the operation unit 102. The control unit 106 selects the puncture mode as the imaging mode based on the input information.

  In Step A6, in response to selection of the puncture mode, imaging for the puncture mode (CT fluoro) is started. Specifically, the control unit 106 executes a photographing process for the puncture mode. In shooting for the puncture mode, the scan area is continuously scanned, and images of each frame are reconstructed and displayed at a predetermined frame rate. As a result, a substantially real-time animated perspective image of the scan area can be seen. Here, the images of two adjacent frames are reconstructed so that projection data used for reconstruction partially overlap. For example, the frame rate is 10 frames / gantry rotation, and the gantry rotation speed is 1 second / gantry rotation. The scan may be an axial scan or a helical scan.

  In step A7, projection data collection is started. Specifically, the control unit 106 controls the data collection system 101 based on the puncture mode imaging protocol. As a result, the scan for the set scan area is continuously repeated, and projection data of a plurality of views are continuously collected. At this time, in order to suppress the exposure of the hand or arm of the doctor performing the puncture, control is performed to lower the X-ray intensity to a low level LX lower than the normal level NX in a predetermined view angle range including the hand or arm. Do. Here, this view angle range is referred to as a low exposure view angle range. The low exposure view angle range is basically a range in which the view angle θ is 0 ± 90 °, where the view angle θ at which the X-ray tube 21 corresponds to the vertical upward direction, that is, the highest point in the vertical direction is 0 °. Set within. In this example, the low exposure view angle range is a range of + 45 ° to −45 °. Further, the lowering of the X-ray intensity is realized by lowering the tube current value of the X-ray tube 21. In this example, in the low exposure view angle range, the tube current value is reduced to 60% or less of the set tube current value.

  In step A8, projection data used for image reconstruction is specified. Specifically, the image reconstruction unit 105 identifies a view angle range of projection data used to reconstruct an image of one frame among the collected projection data of a plurality of views. In this example, a half reconstruction method and a filtered back projection method are used for image reconstruction. That is, the image corresponding to the frame is reconstructed by superimposing the reconstruction function on the projection data in the view angle range corresponding to the fan angle α of the 180 ° + X-ray beam and performing back projection. In this example, projection data in the view angle range corresponding to the fan angle α of the 180 ° + X-ray beam collected immediately before the time corresponding to the target frame is specified.

  In step A9, a reconstruction function is applied to the projection data. Specifically, the projection obtained when the image reconstruction unit 105 has the X-ray intensity at the low level LX for the projection data of each view in the projection data in the predetermined view angle range specified in step A8. It is determined whether it is data. The projection data obtained when the X-ray intensity is at the low level LX has a reduced S / N, which increases the image noise of the reconstructed image. Therefore, a reconstruction function LR in which high frequency components are further suppressed as compared with the set reconstruction function NR is applied to the projection data. On the other hand, in the projection data obtained when the X-ray intensity is the normal level NX, S / N is secured at a certain level or more. Therefore, the set reconstruction function NR is applied to the projection data.

  FIG. 4 is a diagram illustrating a view angle range of projection data used for reconstruction of an image of each frame in the first embodiment and a reconstruction function applied to the projection data of each view.

  In the example of FIG. 4, the projection data used for image reconstruction of the frames f1 to f2 and f4 to f12 includes at least a part of data in the range of the exposure reduction view angle 0 ° ± 45 ° that lowers the X-ray intensity. Yes. Therefore, the reconstruction function LR in which the high-frequency component is suppressed from the normal set reconstruction function NR is applied to the projection data in the view angle range for reducing the X-ray intensity. The normal set reconstruction function NR is applied. On the other hand, the projection data used for the reconstruction of the images of the frames f3 and f13 does not include projection data in the range of the view angle that lowers the X-ray intensity. Therefore, the normal reconstruction function NR is applied to all of these projection data.

  In step A10, the projection data is backprojected. Specifically, the image reconstruction unit 105 reconstructs an image corresponding to one frame by back projecting projection data of each view to which the reconstruction function is applied. In this way, an image corresponding to one or more desired slices included in the scan region, that is, a tomographic image, is reconstructed at a predetermined frame rate in parallel with the collection of projection data.

  In step A11, a reconstructed image is displayed. Specifically, the control unit 106 controls the display unit 103 to display a reconstructed image corresponding to one frame on the monitor screen.

  In step A12, it is determined whether or not to end shooting. Specifically, the control unit 106 determines whether or not a shooting end command has been input. If it has been input, the shooting is terminated. If it has not been input, the process returns to step A8 to continue shooting.

  According to the first embodiment as described above, the normal set reconstruction function NR is applied to the projection data obtained when the X-ray intensity is the normal level NX, while the X-ray intensity is applied. For the projection data obtained when is reduced to the low level LX, the image is reconstructed by applying a reconstruction function LR that suppresses high-frequency components from the normal set reconstruction function NR. The S / N degradation of the projection data obtained when the X-ray intensity is relatively low can be supplemented by the reconstruction function. As a result, it is possible to obtain an image in which image noise is suppressed in X-ray CT imaging for reducing the X-ray intensity in a predetermined view angle range.

  In particular, in the first embodiment, the correction is performed directly only on the projection data having a deteriorated S / N. Therefore, excessive correction can be prevented, and the image is obtained while maintaining the spatial resolution of the reconstructed image as much as possible. Noise can be reduced.

  Further, according to the first embodiment, a frame reconstructed using projection data obtained when the X-ray intensity is lowered to the low level L for a plurality of frames of images generated as a perspective image of the subject 40. An increase in image noise in the images of the frames can be suppressed, and a fluoroscopic image that is easy to observe can be provided in which variation in image noise between images of each frame is suppressed.

  In the first embodiment, the half reconstruction method is used for image reconstruction. However, a reconstruction method may be used in which reconstruction is performed using projection data in a view angle range that is greater than half scan and less than full scan. Good. For example, in the case of the full reconstruction method that reconstructs using projection data for full scan, unlike the half reconstruction method, in the reconstruction of the image of all frames, the projection data used for image reconstruction In addition, the same amount of projection data when the X-ray intensity is at the low level LX is included. In this case, the variation in image noise between images of each frame is not large. However, by superimposing the reconstruction function LR that suppresses the high-frequency component only on the projection data when the X-ray intensity is the low level LX, only the projection data having a relatively small S / N (poor S / N). Can be corrected at a pin point, and unnecessary resolution degradation can be prevented.

(Second embodiment)
In the first embodiment, the reconstruction function applied to the projection data is determined in units of views, but in the second embodiment, it is determined in units of frames. That is, when the projection data used for the reconstruction of the image corresponding to the frame includes the projection data when the X-ray intensity is the low level LX, all the projection data used for the image reconstruction are Then, the reconstruction function LR that suppresses the high frequency component from the set reconstruction function NR is applied. On the other hand, the projection data used for the reconstruction of the image corresponding to the frame does not include the projection data when the X-ray intensity is the low level LX, that is, the projection data when the X-ray intensity is the normal level NX. When an image is reconstructed based only on this, a normal set reconstruction function NR is applied to all projection data used for the image reconstruction.

  FIG. 5 is a flowchart showing the flow of processing in the X-ray CT apparatus in the second embodiment.

  Steps B1 to B8 are the same processing as steps A1 to A8 in the first embodiment, and thus description thereof is omitted here. However, the exposure reduction view angle range is set to 0 ± 30 °.

  In step B9, it is determined whether or not the specified projection data includes data when the X-ray intensity is the low level LX. Specifically, whether or not the image reconstruction unit 105 includes the projection data obtained when the X-ray intensity is the low level LX for the projection data in the predetermined view angle range specified in step B8. Determine. That is, it is determined whether or not the predetermined view angle range includes at least a part of the exposure reduction view angle range. If included, the process proceeds to step B10. If not included, the process proceeds to step B11.

  In step B10, a reconstruction function LR that suppresses high-frequency components is applied to the projection data. Specifically, the image reconstruction unit 105 performs a reconstruction function in which high frequency components are suppressed from the set reconstruction function NR for all projection data in the predetermined view angle range specified in step B8. Apply LR. Then, it progresses to step B12.

  In step B11, the normal set reconstruction function NR is applied to the projection data. Specifically, the image reconstruction unit 105 applies the set reconstruction function NR to all the projection data in the predetermined view angle range specified in step B8. Then, it progresses to step B12.

  FIG. 6 is a diagram illustrating a view angle range of projection data used for reconstruction of an image corresponding to each frame in the second embodiment and a reconstruction function applied to the projection data of each frame.

  In the example of FIG. 6, the projection data used for the reconstruction of the images of the frames f1, f4 to f11 includes at least a part of data in the range of the exposure reduction view angle 0 ° ± 30 ° for reducing the X-ray intensity. Accordingly, the reconstruction function LR that suppresses higher-frequency components than usual is applied to all projection data used for image reconstruction corresponding to these frames. On the other hand, the projection data used to reconstruct the images of the frames f2 to f3 and f12 to f13 do not include projection data in the range of the view angle that lowers the X-ray intensity. Therefore, the normal reconstruction function NR is applied to all of these projection data.

  In step B12, the projection data is backprojected. Specifically, the image reconstruction unit 105 reconstructs an image corresponding to a frame by back projecting projection data of each view to which the reconstruction function is applied.

  In step B13, the reconstructed image is displayed. Specifically, the control unit 106 controls the display unit 103 to display an image corresponding to the frame on the monitor screen.

  In step B14, it is determined whether or not to end shooting. Specifically, the control unit 106 determines whether or not a shooting end command has been input. If it has been input, the shooting is terminated. If it has not been input, the process returns to step B8 to continue shooting.

  According to the second embodiment, an image reconstructed using projection data when the X-ray intensity is lowered to the low level LX for a plurality of frame images generated as a perspective image of the subject 40. Increase in image noise can be suppressed, and a fluoroscopic image easy to observe can be provided in which variation in image noise between images of each frame is suppressed.

  Further, in the second embodiment, the reconstruction function to be applied is changed in units of frames, so that the algorithm is simple, and the development cost and the load on the arithmetic processing unit 3 can be suppressed.

(Third embodiment)
In the first and second embodiments, the image noise of the image corresponding to the frame is adjusted by changing the reconstruction function applied to the projection data as necessary, but in the third embodiment, the reconstructed image is adjusted. The image noise is adjusted by applying an image filter to the image. That is, when projection data used for reconstruction of an image corresponding to a frame includes projection data when the X-ray intensity is a low level LX, high-frequency components are suppressed for the reconstructed image. Apply the image filter LF. On the other hand, when the projection data used for the reconstruction of the image corresponding to the frame does not include the projection data when the X-ray intensity is the low level LX, the normal level or the image filter is applied to the reconstructed image. The image filter NF that emphasizes the high frequency component more than the LF is applied, or the image filter is not applied. Note that the image filter LF can also be said to be an image filter that further suppresses high-frequency components as compared to the image filter NF.

  FIG. 7 is a flowchart showing the flow of processing in the X-ray CT apparatus in the third embodiment.

  Steps C1 to C8 are the same processes as steps A1 to A8 in the first embodiment, and thus description thereof is omitted here. However, the exposure reduction view angle range is set to 0 ± 30 °.

  In step C9, a reconstruction function is applied to the projection data. Specifically, the image reconstruction unit 105 applies the normal set reconstruction function NR to the projection data in the predetermined view angle range specified in step C8.

  In step C10, back projection of the projection data is performed. Specifically, the image reconstruction unit 105 reconstructs an image by back projecting projection data of each view to which the reconstruction function is applied.

  In Step C11, it is determined whether or not the specified projection data includes data when the X-ray intensity is the low level LX. Specifically, whether or not the image reconstruction unit 105 includes the projection data obtained when the X-ray intensity is at the low level LX, in the predetermined view angle range identified in step C8. Determine. That is, it is determined whether or not the predetermined view angle range includes at least a part of the exposure reduction view angle range. If included, the process proceeds to step C12. If not included, the process proceeds to step C13.

  In Step C12, an image filter LF that suppresses high frequency components is applied to the reconstructed image. Specifically, the image reconstruction unit 105 applies an image filter LF that suppresses high-frequency components, such as a smoothing filter or an adaptive smoothing filter, to the image reconstructed in step C10. Thereafter, the process proceeds to Step C14.

  In Step C13, the image filter NF that enhances the normal level or higher frequency components is applied to the reconstructed image, or no image filter is applied. Specifically, the image reconstruction unit 105 applies the image filter NF in which the normal level or higher frequency components are emphasized to the image reconstructed in Step C10, or does not apply the image filter. Thereafter, the process proceeds to Step C14.

  FIG. 8 is a diagram showing a view angle range of projection data used for reconstruction of each frame image in the third embodiment and an image filter applied to the image of each frame.

  In the example of FIG. 8, the projection data used for the reconstruction of the images of the frames f1, f4 to f11 includes at least a part of data in the range of the view angle 0 ° ± 30 ° for reducing the X-ray intensity. Therefore, the normal reconstruction function NR is applied to all projection data used for image reconstruction corresponding to these frames, and high-frequency components are suppressed more than usual for the reconstructed image. The image filter LF is applied. On the other hand, the projection data used to reconstruct the images of the frames f2 to f3 and f12 to f13 do not include projection data in the range of the view angle that lowers the X-ray intensity. Accordingly, the normal reconstruction function NR is applied to all projection data used for image reconstruction corresponding to these frames, and the normal image filter NF is applied to the reconstructed image. Or no image filter is applied.

  In step C14, the reconstructed image is displayed. Specifically, the control unit 106 controls the display unit 103 to display an image corresponding to the frame on the monitor screen.

  In step C15, it is determined whether or not to end shooting. Specifically, the control unit 106 determines whether or not a shooting end command has been input. If it has been input, the shooting is terminated. If it has not been input, the process returns to step C8 to continue shooting.

  According to the third embodiment, a plurality of frames of images generated as a perspective image of the subject 40 are reconstructed using projection data when the X-ray intensity is lowered to the low level LX. An increase in image noise in an image can be suppressed, and a fluoroscopic image that is easy to observe can be provided in which variation in image noise between images in each frame is suppressed.

  In the third embodiment, the applied image filter is changed in units of frames, so that the algorithm is simple and the development cost and the load on the arithmetic processing unit 3 can be suppressed.

  Further, in the third embodiment, since the adjustment of the image filter is used as a means for performing the correction, not the adjustment of the reconstruction function, a variety of correction arrangements are possible as compared with the reconstruction function. Therefore, it is easy to obtain a desired image quality by adjustment.

  In the second and third embodiments, the half reconstruction method is used for image reconstruction. However, reconstruction is performed by reconstructing an image using projection data in a view angle range that exceeds half scan and less than full scan. For example, a full reconstruction method may be used.

(Fourth embodiment)
In the first to third embodiments, the S / N degradation of the projection data obtained when the X-ray intensity is relatively low is compensated by using a reconstruction function or an image filter. Then, it compensates by increasing the amount of projection data used for image reconstruction. That is, when the projection data used for the reconstruction of the image corresponding to the frame includes the projection data when the X-ray intensity is the low level LX, the view angle range of the projection data used for the image reconstruction To increase the amount of projection data (number of views) used for image reconstruction. The amount of data to be increased is larger as the amount of projection data when the X-ray intensity is the low level LX is larger, and is smaller as the amount is smaller. For example, normally, image reconstruction is performed using projection data for a half scan, and the maximum full scan is included depending on the amount of projection data contained in the X-ray intensity at a low level LX. The amount of data is increased until the projection data becomes.

  In the fourth embodiment as well, as in the second and third embodiments, the X-ray intensity is lowered to a low level L for images corresponding to a plurality of frames generated as a perspective image of the subject 40. The increase in image noise in an image reconstructed using the projection data at the time can be suppressed, and a transparent image that is easy to observe can be provided in which variation in image noise between images of each frame is suppressed. .

  The invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

  For example, although the above embodiment is an X-ray CT apparatus, a program for causing a computer to function as control means in such an X-ray CT apparatus and a computer-readable storage medium storing the program are also provided. This is an embodiment of the invention.

DESCRIPTION OF SYMBOLS 1 Operation console 2 Input device 3 Processing unit 5 Data collection buffer 6 Monitor 7 Storage device 10 Imaging table 12 Cradle 15 Rotating part 16 Supporting part 20 Scanning gantry 21 X-ray tube 22 X-ray controller 23 Collimator 24 X-ray detector 25 DAS
26 Rotating part controller 29 Control controller 30 Slip ring 40 Subject 41 Operator 81 X-ray

Claims (11)

Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstructing means for reconstructing an image based on at least a part of the collected projection data of a plurality of views, with respect to projection data of a view angle irradiated with radiation at the first radiation intensity Superimposes the first reconstruction function, and for the projection data at the view angle irradiated with radiation at the second radiation intensity, the second reconstruction function suppresses higher frequency components than the first reconstruction function. A radiation tomography apparatus comprising: reconstruction means for reconstructing an image by superimposing a configuration function. Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstructing means for reconstructing an image based on at least a part of the collected projection data of the plurality of views, based on only projection data of a view angle irradiated with radiation at the first radiation intensity When the image is reconstructed, the first reconstruction function is superimposed on the projection data, and the image is based on the projection data including the view angle irradiated with the radiation at the second radiation intensity. Reconstructing means for reconstructing an image by superimposing a second reconstruction function that suppresses higher frequency components than the first reconstruction function on the projection data in the case of reconstruction. Radiation tomography equipment. Control means for controlling a data acquisition system including a radiation source and a detector so as to collect projection data of a plurality of views by irradiating a subject with radiation from a plurality of view angle directions, and having a view angle range of 360 degrees Among these, radiation is irradiated at a first radiation intensity in a first view angle range, and a second smaller than the first radiation intensity in a second view angle range other than the first view angle range. Control means for controlling to emit radiation at radiation intensity;
Reconstruction means for reconstructing an image based on at least a part of the collected projection data of the plurality of views;
When an image is reconstructed based on projection data of only the view angle irradiated with radiation at the first radiation intensity, the first image filter is applied to the image, or the image filter is When the image is reconstructed based on the projection data including the view angle irradiated with the radiation at the second radiation intensity without applying the high-frequency component to the image from the first image filter. An image processing means for applying a second image filter for suppressing the radiation tomography apparatus.   The second view angle range is included between +180 degrees and -180 degrees, with a view angle at which the radiation source is located at the highest point in the vertical direction being 0 degrees. The radiation tomography apparatus according to one item. The control means controls to continuously collect projection data of a desired slice for a certain period,
The radiation reconstruction unit according to claim 1, wherein the reconstruction unit reconstructs an image of the desired slice at a predetermined frame rate in parallel with the collection of the projection data. Shooting device.   The radiation tomography apparatus according to claim 5, wherein the reconstruction unit reconstructs images of two adjacent frames so that projection data used for reconstruction partially overlap each other.   The radiation tomography apparatus according to claim 1, wherein the reconstruction unit reconstructs an image based on projection data in a view angle range for a half scan.   The radiation tomography apparatus according to any one of claims 1 to 6, wherein the reconstruction unit reconstructs an image based on projection data in a view angle range exceeding half scans and less than full scans. .   9. The control unit according to claim 1, wherein when the puncture mode is set, the control unit performs control so as to perform radiation irradiation with the first radiation intensity and the second radiation intensity. 10. The radiation tomography apparatus according to item.   The radiation tomography apparatus according to any one of claims 1 to 9, wherein the first radiation intensity is an intensity determined by an automatic exposure mechanism.   The program for functioning a computer as each means which comprises the radiation tomography apparatus as described in any one of Claims 1-10.

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