JP2005334230A - Radiation ct equipment and image reconstruction method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide radiation CT equipment capable of controlling to uniformize the noise of whole images reconstructed in a cone beam back projection method and to provide an image reconstruction method using the same. <P>SOLUTION: This radiation CT equipment is provided with a radiation source and a two-dimensional X-ray detector detecting the radiation by a radiation detecting surface two-dimensionally disposed with a plurality of radiation detectors, and reconstructs a tomographic image in a predetermined reconstructed image based on projection data obtained by the two-dimensionally X-ray detector. This equipment is further provided with interpolation means 52 and 53 dividing the reconstructed image into a plurality of regions based on a distance from the center of the reconstructed image and interpolating the respective regions of the divided reconstructed image using the plurality of projection data, and image reconstruction means 54, 55 and 56 reconstructing the tomographic image using the projection data and the interpolation data. The interpolation means interpolate the divided regions in the circumference part side at a different degree from that of the center part side. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a radiation CT (Computed Tomography) apparatus that calculates a tomographic image of a subject from projection data of radiation on the subject, and an image reconstruction method using the same.

  As a radiation CT apparatus, for example, an X-ray CT apparatus using X-rays for radiation is known. As an X-ray CT apparatus, an apparatus having a radiation detector in which X-ray detectors of a plurality of channel arrays are two-dimensionally arranged is known. The plurality of X-ray detectors are arranged so as to have a width in a direction along a predetermined axis with respect to the subject. Since the rows of X-ray detectors are formed in a predetermined width along the axial direction, the X-ray detectors arranged in a two-dimensional matrix are called multi-row X-ray detectors.

As one of the tomographic reconstruction methods using a multi-row X-ray detector, a cone beam BP (Back Projection) method is known (see, for example, Patent Document 1).
When the cone beam BP method is applied, an X-ray CT apparatus having a multi-row X-ray detector has, for example, a cone shape in which an X-ray tube perpendicular to a predetermined axis (hereinafter referred to as z-axis) with respect to a subject is fan-shaped. While irradiating the X-rays, the multi-row X-ray detector is relatively moved to make one rotation around the axis. Then, X-rays that have passed through the subject are detected by a multi-row X-ray detector, and projection data of the subject site of the subject are collected.

  Since the above multi-row X-ray detector collects more projection data, when the X-ray CT apparatus reconstructs a predetermined reconstructed image, the projection data detected by the multi-row X-ray detector is A process of superimposing a predetermined reconstruction function is performed. For example, the conventional X-ray CT apparatus weights the entire reconstructed image using the same number of projection data by the same interpolation method, and interpolates the data. Thereafter, back projection is performed to reconstruct a tomographic image. In the cone beam BP method, the data density of the center portion of the reconstructed image is smaller than that of the peripheral portion due to back projection.

However, in the cone beam BP method as described above, since the interpolation is uniformly performed on the entire image, there is a characteristic that noise is not uniform between the central portion and the peripheral portion in the reconstructed image. .
JP 2001-161678 A

  Accordingly, an object of the present invention is to provide a radiation CT apparatus capable of controlling and making uniform noise of the entire image to be reconstructed in the cone beam BP method, and an image reconstruction method using the same. is there.

  The radiation CT apparatus according to the present invention is a two-dimensional radiation detector surface in which a radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in first and second arrangement directions. And a means for reconstructing a tomographic image in a predetermined reconstructed image based on projection data obtained by the two-dimensional radiation detector Regarding the radiation CT apparatus, the reconstruction unit divides the reconstructed image into a plurality of regions based on the distance from the center of the reconstructed image in a predetermined reconstructed image reconstructed based on the projection data. Interpolating means for reconstructing each region of the reconstructed image by different interpolation methods, and image reconstructing means for reconstructing a tomographic image using the projection data and the interpolated data. Oite, the region of peripheral-portion side of the segmented reconstructed image, performs interpolation in different orders and its central portion.

  The radiation CT apparatus according to the present invention forms a one-dimensional radiation detector surface by arranging a radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors in one direction. A radiation CT apparatus comprising: a one-dimensional radiation detector for detecting radiation by a radiation detector surface; and means for reconstructing a tomographic image in a predetermined reconstructed image based on projection data obtained by the one-dimensional radiation detector The reconstructing means divides the reconstructed image into a plurality of regions based on the distance from the center of the reconstructed image in a predetermined reconstructed image reconstructed based on the projection data, Interpolation means for interpolating each region of the image using a plurality of projection data, and image reconstruction means for reconstructing a tomographic image using the projection data and the interpolated data. In the area of the peripheral-portion side of the segmented reconstructed image, performs interpolation in different orders and the center portion side of the area.

  According to the above-described radiation CT apparatus, the region on the peripheral part side of the divided reconstructed image is interpolated with a different order from the central part side.

  An image reconstruction method according to the present invention is a two-dimensional radiation detector in which a radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in first and second arrangement directions. A radiation that reconstructs a tomographic image in a predetermined reconstructed image based on projection data obtained by the two-dimensional radiation detector. Regarding an image reconstruction method of a CT apparatus, a reconstructed image is divided into a plurality of regions based on a distance from the center of a predetermined reconstructed image reconstructed based on projection data, and each region of the reconstructed image divided Reconstructing an image with different interpolation methods, and reconstructing a tomographic image using the projection data and the interpolated data. In the interpolation step, the peripheral part side of the segmented reconstructed image Performing interpolation regions in different orders and its central portion.

  In addition, the image reconstruction method according to the present invention forms a one-dimensional radiation detector surface by arranging a radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors arranged in one direction. A radiation CT apparatus for reconstructing a tomographic image in a predetermined reconstructed image based on projection data obtained by the one-dimensional radiation detector. Regarding the image reconstruction method, the reconstructed image is divided into a plurality of regions based on the distance from the center of a predetermined reconstructed image reconstructed based on projection data, and each region of the reconstructed image thus divided is interpolated differently. A method of reconstructing an image by the method, and a step of reconstructing a tomographic image using the projection data and the interpolated data. Perform interpolation in different orders and its central portion.

According to the image reconstruction method described above, the projection data is interpolated by different interpolation methods and divided into a plurality of regions to be reconstructed.
Next, in each divided area, the projection data corresponding to the area on the peripheral part side of the reconstructed image is interpolated with a different order from the central part side.
Next, a tomographic image is reconstructed using the interpolated projection data.

According to the radiation CT apparatus of the present invention, the noise of the entire image to be reconstructed can be controlled and made uniform in the cone beam BP method.
According to the image reconstruction method of the present invention, the noise of the entire image to be reconstructed can be controlled and made uniform in the cone beam BP method.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. The radiation in the present invention includes X-rays. In the following embodiments, an X-ray CT apparatus is taken as an example of a radiation CT apparatus.

  FIG. 1 is a diagram showing a configuration of an X-ray CT apparatus according to the present invention, and FIG. 2 is a diagram showing a configuration of a main part of the X-ray CT apparatus shown in FIG. An X-ray CT apparatus 1 shown in FIG. 1 is an embodiment of a radiation CT apparatus in the present invention.

  As shown in FIG. 1, the X-ray CT apparatus 1 includes a scanning gantry 2, an operation console 3, and an imaging table 4.

  The scanning gantry 2 includes an X-ray tube 20, a collimator 22, a detector 23, a data acquisition system (DAS) 24, an X-ray controller 25, a collimator controller 26, a rotating unit 27, and a rotation. And a controller 28. One embodiment of the radiation source in the present invention corresponds to the X-ray tube 20, and one embodiment of the two-dimensional radiation detector in the present invention corresponds to the detector 23.

  As shown in FIG. 2, the X-ray controller 25 is connected to the X-ray tube 20, and the collimator controller 26 is connected to the collimator 22. Although not shown, the detector 23 is connected to the DAS 24, and the rotation controller 28 is connected to the rotating unit 27.

  The X-ray tube 20 emits X-rays having a predetermined intensity toward the collimator 22 based on a control signal CTL 251 from the X-ray controller 25.

  The collimator 22 adjusts the irradiation range of the X-ray emitted from the X-ray tube 20 in the z-axis direction by adjusting the opening degree of the opening 221 based on the control signal CTL 261 from the collimator controller 26.

The detector 23 is configured in a two-dimensional matrix of i rows and j columns using a plurality of X-ray detectors. Each X-ray detector is constituted by a combination of a scintillator and a photodiode, for example.
In the row direction, for example, about i = 1000 X-ray detectors are arranged to form one detector row. In the column direction, for example, an X-ray detector column of about j = 16 is configured. As described above, the two-dimensional X-ray detector surface 23S is formed as a whole by arranging the X-ray detectors in a matrix.

As shown in FIGS. 1 and 2, the detector 23 is disposed at a predetermined interval from the collimator 22. An X-ray cone beam 29 is formed between the collimator 22 and the detector 23. The subject 6 is moved into the X-ray cone beam 29.
The detector 23 detects the intensity of the X-ray beam 5 irradiated from the opening 221 portion of the collimator 22 and passed through the X-ray cone beam 29 to which the subject 6 is moved by the X-ray detector surface 23S.

Based on the control signal CTL 303 from the operation console 3, the DAS 24 collects X-ray intensity projection data from individual X-ray detectors constituting the X-ray detector surface 23 </ b> S.
The DAS 24 performs AD (Analog to Digital) conversion on the collected projection data and transmits the converted projection data to the operation console 3. Data transmitted from the DAS 24 is called raw data.

The rotating unit 27 rotates around a predetermined rotation axis AX in the X-ray cone beam 29 based on a control signal from the rotation controller 28.
An X-ray tube 20, a collimator 22, a detector 23, a DAS 24, an X-ray controller 25, and a collimator controller 26 are mounted on the rotating unit 27, and these components are relatively positioned by rotation of the rotating unit 27. Rotate around the rotation axis AX while maintaining

  In the present embodiment, the row direction of each detector of the detector 23 is arranged along the direction of the rotation axis AX. The direction of the rotation axis AX, that is, the column direction of the detectors of the detector 23 is defined as the z-axis direction.

The imaging table 4 is moved by driving means such as a motor (not shown), for example. A subject 6 is placed on the imaging table 4. The imaging table 4 receives the control signal CTL 30b from the operation console 3 and changes the relative position when the subject 6 is carried into the X-ray cone beam 29 or when the subject 6 is imaged.
The imaging table 4 carries the subject 6 in the X-ray cone beam 29 so that, for example, the direction of the body axis from the head to the leg substantially matches the z-axis direction. As a result, the X-ray tube 20, the detector 23, and the subject 6 move relatively along the z-axis direction.

  The X-ray controller 25 transmits a control signal CTL 251 for controlling the intensity of X-rays emitted from the X-ray tube 20 to the X-ray tube 20 based on a control signal CTL 301 from the central processing unit 30 described later of the operation console 3. To do.

  The collimator controller 26 controls the width of the opening 221 by the control signal CTL 261 transmitted to the collimator 22 in accordance with the control signal CTL 302 from the operation console 3.

  The rotation controller 28 transmits a control signal to the rotating unit 27 based on the control signal CTL 304 from the central processing unit 30 of the operation console 3 to rotate around the rotation axis AX.

Next, the operation console 3 will be described.
As shown in FIG. 1, the operation console 3 includes a central processing unit 30, an input device 31, a display device 32, and a storage device 33.

The input device 31 receives a command input from the operator and moves it to the central processing unit 30 in order to move the X-ray CT apparatus 1.
The display device 32 displays a CT image obtained as a result of calculation by the central processing unit 30.
The storage device 33 stores various programs and parameters for moving the X-ray CT apparatus 1 via the central processing unit 30 and data such as CT image images.

As shown in FIG. 2, the central processing unit 30 includes a control unit 34, a reconstruction unit 36, and a display unit 38.
The central processing unit 30 may be configured using only one piece of hardware, or may be configured as a system in which hardware is appropriately assigned to each of the above-described components.

The control unit 34 is connected to the DAS 24, the input device 31, the X-ray controller 25, the collimator controller 26, the rotation controller 28, and the imaging table 4. A reconfiguration unit 36 and a display unit 38 are further connected to the control unit 34.
The storage device 33 is appropriately accessed from the reconfiguration unit 36 and the display unit 38 via the control unit 34.

The control unit 34 transmits a control signal CTL 30b to the imaging table 4 to move the subject 6 along the z-axis direction.
Further, the control unit 34 transmits a control signal CTL 30 a to the operation gantry 2 based on a command input from the operator via the input device 31 in order to obtain projection data used for image reconstruction of the tomographic image of the subject 6. Then, the subject 6 is scanned. A scanning method of the subject 6 will be described later. The control signal CTL 30a from the control unit 34 includes the control signals CTL 301, 302, 303, and 304 described above.
Further, the control unit 34 transmits the projection data of the subject collected by the DAS 24 by scanning to the control unit 34. The control unit 34 transmits the received projection data to the reconstruction unit 36.

  The reconstruction unit 36 receives projection data via the control unit 34. Then, the reconstruction unit 36 performs arithmetic processing such as image reconstruction on the received projection data to reconstruct a tomographic image of the region to be examined of the subject 6. Details of the reconstruction unit 36 will be described later.

  The display unit 38 causes the display device 32 to display the tomographic image reconstructed by the reconstruction unit 36 in response to a command signal from the control unit 34.

Here, the reconstruction unit 36 will be described in detail.
FIG. 3 is a block diagram schematically showing the configuration of the reconstruction unit 36 described above.

  As shown in FIG. 3, the reconstruction unit 36 includes a preprocessing unit 51, a segmented region setting unit 52, an interpolation data generation unit 53, a filter processing unit 54, a back projection unit 55, and a postprocessing unit 56. Have One embodiment of the interpolation means of the present invention corresponds to the segmented region setting section 52 and the interpolation data generation section 53, and one embodiment of the image reconstruction means of the present invention is a filter processing section 54, a back projection section 55, and a post-processing section. 56.

The preprocessing unit 51 receives projection data from the control unit 34. The preprocessing unit 51 performs offset correction, reference correction, and the like on the received projection data. Processes such as offset correction and reference correction are called pre-processing because they are executed before the image is generated.
The offset correction is to correct an offset value mixed in the projection data mainly due to a drift included in the data collection system. The reference correction is for correcting fluctuations in the intensity of X-rays emitted from the X-ray tube 20.
The preprocessing unit 51 transmits the projection data subjected to the above correction to the interpolation data generation unit 53 and the filter processing unit 54.

  The sorting area setting unit 52 receives the sorting area information transmitted through the control unit 36 based on the command input from the operator input to the input device 31. The segmented area setting unit 52 classifies the reconstruction area based on the received information and sets it as a segmented area. Further, the segmented area setting unit 52 transmits the set segmented area to the interpolation data generating unit 53.

The back projection unit 55 receives the segment area transmitted from the segment area setting unit 52. Further, the interpolation data generation unit 53 receives the preprocessed projection data from the preprocessing unit 51.
The interpolation data generation unit 53 generates a weighting factor on a predetermined reconstructed image based on the segment area received from the segment area setting unit 52. The interpolation data generation unit 53 selects backprojected data for each of the divided reconstruction areas, and performs a product-sum operation on the selected backprojection data to obtain final projection data.

The filter processing unit 54 receives the interpolation data transmitted from the interpolation data generating unit 53 and the projection data transmitted from the preprocessing unit 51. The filter processing unit 54 performs the Fourier transform on the received data and multiplies the reconstruction function to perform the existing inverse Fourier transform.
The back projection unit 55 receives the data transmitted from the filter processing unit 54. The back projection unit 55 interpolates and selects the filtered projection data using the weighting coefficient generated by the interpolation data generation unit 53, and generates back projection data.
The post-processing unit 56 receives the backprojection data transmitted from the backprojection unit 55. The post-processing unit 56 reconstructs a tomographic image from the received backprojection data.

Interpolation data generation Next, interpolation data generation will be described with reference to the drawings.

  4 and 5 are schematic cross-sectional views schematically showing a predetermined reconstructed image to be reconstructed in the X-ray CT apparatus 1 according to the present invention. FIG. 4 shows an arbitrary j column of the detector 23 in the predetermined view direction. Note that a reconstruction area Pxy reconstructed based on projection data detected from the detector 23 including the detectors in j columns is shown as the reconstructed image.

As shown in FIG. 4, the X-ray beam bm1 irradiated from the X-ray tube 20 and passing through a part of the subject indicated by coordinates (x ₁ , y ₁ ) is absorbed by a region passing through the subject. Incident between the detector i + n channel and the detector i + n + 1 channel. Similarly, the X-ray beam bm2 that passes through a part of the subject indicated by coordinates (x ₂ , y ₂ ) is incident between the detector i channel and the detector i + 1 channel. At this time, other x-ray beams (not shown) are incident on the respective channels of the detectors i, i + 1, i + n, and i + n + 1.
Therefore, in order to reconstruct the pixel data indicated by the coordinates (x ₁ , y ₁ ) and coordinates (x ₂ , y ₂ ) in the reconstruction area, the projection data by the X-ray beams bm1 and bm2 are interpolated. There is a need to.

Similarly, in the yz plane shown in FIG. 5, the X-ray beam bm3 irradiated from the X-ray tube 20 and passing through a part of the subject indicated by coordinates (x ₁ , y ₁ , z ₁ ) It is absorbed by the region passing through the specimen and enters between the detector j + n channel and the detector j + n + 1 channel. Similarly, the X-ray beam bm2 that passes through a part of the subject indicated by coordinates (x ₂ , y ₂ , y ₂ ) is incident between the detector j channel and the detector j + 1 channel. At this time, other X-ray beams (not shown) are incident on the respective channels of the detectors j, j + 1, j + n, and j + n + 1.
Therefore, in order to generate pixel data indicated by coordinates (x ₁ , y ₁ , z ₁ ) and coordinates (x ₂ , y ₂ , y ₂ ) in the reconstructed image, X-ray beams bm2, bm3 It is necessary to interpolate the projection data by.

  In the reconstructed image, projection data reconstructed by the X-ray beams bm1 to bm3 is interpolated.

  FIG. 6 is a flowchart showing an interpolation process of the X-ray CT apparatus 1 according to the present invention.

First, a segment area is set in the segment area setting unit 52 of the reconstruction unit 36 (ST11).
The operator inputs, for example, a distance r from the rotation center AX to the divided region setting unit 52 of the control unit 34 by operating the input device 31. The segmented region setting unit 52 inputs the distance r as the contour information of the segmented region, sets the segmented region as a circle with a radius r, and transmits the segmented region to the interpolation data generating unit 53. In the present embodiment, the rotation center AX and the center of the reconstructed image are set to the same coordinate.

Next, the reconstruction area is partitioned based on the partition area set by the partition area setting unit 52 (ST12).
As shown in FIGS. 4 and 5, the interpolation data generation unit 53 divides the reconstruction region into a first region R1 and a second region R2 set at a radius r from the center (axis AX). For example, the interpolation data generation unit 53 generates the coordinates in the reconstruction area and the interpolation weight coefficient for the transition area from the contour information of the segmented area.
Here, the segmented image generation region in the vicinity of the center corresponds to the first region R1, and the surrounding region corresponds to the second region R2. As shown in FIG. 4, the coordinates (x ₁ , y ₁ ) are included in the first region R1 and the coordinates (x ₂ , y ₂ ) are included in the second region R2. Further, as shown in FIG. 5, the coordinates (y ₁ , z ₁ ) are included in the first region R1, and the coordinates (y ₂ , z ₂ ) are included in the second region R2.

  Next, the interpolation data generation unit 53 performs interpolation of the preprocessed data corresponding to the divided regions R1 and R2 on the projection data. Alternatively, all types of interpolation are performed on all regions of the preprocessed projection data (ST13).

First, generation of interpolation data in the channel direction will be described with reference to FIG.
Specifically, in order to interpolate the data D (g (x ₁ , y ₁ )) based on the X-ray beam bm1 passing through the coordinates (x ₁ , y ₁ ) in the first region R1, the interpolation data generation unit 53 Selects two points of projection data D (i + n) and D (i + n + 1). The number of projection data to be used is set in the interpolation data generation unit 53 by the operator via the input device 31. Projection data D (i + n) and D (i + n + 1) are detected by an i + n channel detector and an i + n + 1 channel detector, and indicate projection data indicated by an address i + n channel and an i + n + 1 channel.

  For example, when the projection data is stored in the storage device 23, the interpolation data generation unit 53 stores the projection data D detected by the detector closest to the position where the data is interpolated via the control unit 34. 23.

Next, the interpolation data D (g (x ₁ , y ₁ )) is calculated by averaging the two projection data as described above (ST14).
Specifically, the interpolation data D (g (x ₁ , y ₁ )) can be expressed by the following equation (1) in the case of linear interpolation.

(Equation 1)
_{D (g (x 1, y} 1)) = (g (x 1, y 1) - (i + n)) × D (i + n + 1) + (i + n + 1-g (x 1, y 1)) × D (i + n) ... (1)

  In the present invention, since the reconstruction area Pxy is divided into two areas, interpolation is similarly performed for the second area R2. At this time, the projection data of the second region R2 which is the peripheral portion is subjected to higher-order interpolation than the first region R1 on the center side.

In order to interpolate the projection data D (g (x ₂ , y ₂ )) based on the X-ray beam bm2 passing through the coordinates (x ₂ , y ₂ ) in the second region R2, the interpolation data generation unit 53 has the projection data D Three points (i-1), D (i), and D (i + 1) are selected. Here, an image reconstructed by the cone beam BP method has a higher data density near the center of the rotation axis AX, and a lower data density as it moves away from the center. Therefore, the second region R2 is more accurately interpolated using more projection data than the first region R1 on the center side.

In the same manner as described above, the interpolation data generation unit 53 takes out the projection data D from the storage device 33 via the control unit 34. Then, interpolation data D (g (x ₂ , y ₂ )) is calculated by performing a weighted product-sum operation on the projection data D.
For example, if the weights are w1, w2, and w3, the interpolation data D (g (x ₂ , y ₂ )) can be expressed by the following equation (2). The weights w1, w2, and w3 are determined based on the fractional part of the address g (x ₂ , y ₂ ) where the interpolation coefficient by Hanning is generated.

(Equation 2)
D (g (x ₂ , y ₂ )) = w1 × D (i−1) + w2 × D (i) + w3 × D (i + 1) (2)

Next, interpolation in the column direction will be described with reference to FIG. This is substantially the same as the interpolation in the row direction shown in FIG.
Specifically, in order to interpolate the data D (h (x ₁ , y ₁ , z ₁ )) based on the X-ray beam bm3 passing through the coordinates (x ₁ , y ₁ , z ₁ ) of the first region R1. The interpolation data generation unit 53 selects two points of projection data D (j + n) and D (j + n + 1). The projection data D (j + n) and D (j + n + 1) are detected by the detectors in the j + n column and the detectors in the j + n + 1 column, and indicate the projection data indicated by the addresses j + n and j + n + 1.

The interpolation data D (h (x ₁ , y ₁ , z ₁ )) is calculated by performing a weighted product-sum operation on the projection data at the two points as described above. The weights w1 and w2 loaded on the projection data are determined based on the decimal part of the address h (x ₁ , y ₁ , z ₁ ) at which the interpolation data is generated.

Next, in order to interpolate the data of the coordinates h (x ₂ , y ₂ , z ₂ ) of the second region R2, the interpolation data generation unit 53 performs projection data D (j−1), D (j), D Select three points (j + 1). Here, the image reconstructed by the cone beam BP method has a higher data density near the center of the rotation axis AX and a lower data density as it moves away from the center, so the second region R2 is the first region R1. Interpolate with a different order.

In the same manner as described above, the interpolation data generation unit 53 extracts projection data from the storage device 33 via the control unit 34. Then, interpolation data D (h (x ₂ , y ₂ , z ₂ )) is calculated by performing a weighted product-sum operation on the projection data. For example, the weights w1, w2, and w3 loaded on the projection data are determined based on the decimal part of the address h (x ₂ , y ₂ , z ₂ ) at which the interpolation data is generated, and their sum is, for example, 1 and To do.

By repeating the above process, the interpolation data generation unit 53 generates necessary interpolation data. The number of projection data used for the interpolation is only an example, and the central portion may be interpolated at three points and the peripheral portion may be interpolated at five points. Further, the reconstruction area may be divided into three or more areas.
Further, the number of projection data used for interpolation may be set by a program of the interpolation data generation unit 53 instead of being set by an input from the operator.

  As described above, the center portion of the reconstruction area where the data density is increased by back projection is interpolated with a different order from the peripheral portion.

Next, the operation of the X-ray CT apparatus 1 will be briefly described with reference to FIG.
FIG. 7 is a flowchart showing the operation of the X-ray CT apparatus 1.

First, the X-ray tube 20 and the detector 23 are rotated around the subject, the imaging table 4 is moved in the z-axis direction, and projection data is collected (ST1).
The control unit 34 transmits control signals CTL30b and CTL304 to the imaging table 4 and the rotation controller 28 to acquire projection data used for image reconstruction, and moves the imaging table 4 along the z-axis direction. The rotating unit 27 is rotated around the rotation axis AX.
When viewed from the subject 6 on the imaging table 4, the X-ray tube 20, the collimator 22, and the detector 23 mounted on the rotating unit 27 appear to move spirally around the rotation axis AX. The DAS 24 collects projection data at a predetermined sampling interval while the rotating unit 27 is rotating. The DAS 24 transmits the collected projection data to the control unit 34.

Next, pre-processing is performed on the collected projection data (ST2).
The preprocessing unit 51 of the reconstruction unit 36 receives the projection data D from the control unit 34. The received projection data is subjected to existing processing such as offset correction and reference correction by the preprocessing unit 51. The processed projection data D is transmitted from the preprocessing unit 52 to the interpolation data generating unit 53 and the filter processing unit 54.

Next, an interpolation process is performed on the preprocessed projection data D (ST3).
The projection data D is subjected to the above interpolation processing in the segmented region setting unit 52 and the interpolation data generating unit 53. The interpolated data D is transmitted to the filter processing unit 54.

Next, filter processing is performed on the interpolated data D (ST4).
The received projection data D is filtered by the filter processing unit 54. For example, after performing fast Fourier transform on the projection data D to frequency domain data, it is multiplied by a reconstruction function for reducing blur and noise, and further converted into the original projection data format by fast inverse Fourier transform. return. Thereafter, the filtered projection data D is transmitted to the back projection unit 55.

Next, back projection processing is performed on the filtered projection data D (ST5).
The received projection data D is subjected to existing three-dimensional backprojection processing in the backprojection unit 55 and converted to backprojection data D ′. The obtained backprojection data D ′ is transmitted to the post-processing unit 56. Specifically, back projection data after interpolation of segmented areas is obtained by weighting two backprojection data interpolated with different orders and reconstructing images in each segmented area.

Next, post-processing is performed on the backprojection data D ′ (ST6).
The post-processing unit 56 reconstructs a tomographic image by combining the backprojection data D ′. The reconstructed tomographic image is transmitted to the display unit 38 and the storage device 23. The post-processing unit 56 also performs processing such as conversion of tomographic image colors and switching between two-dimensional display and three-dimensional display.

  The display unit 38 receives the reconstruction unit 36 in response to a command from the control unit 34, and then transfers it to the display device 32. The display device 32 displays the received tomographic image. The storage device 23 stores the received tomographic image.

According to the present embodiment, when reconstructing by the cone beam BP method, a weighting factor is obtained based on the distance from the central portion of the reconstructed image, and the weighted product with the peripheral region in the segmented region. Interpolate by sum operation.
As a result, higher-order interpolation can be performed in a peripheral portion where the number of projection data is originally small due to reconstruction. Therefore, noise and artifacts are reduced in the reconstructed image, and a uniform S / N ratio can be obtained in the entire image. In addition, it is possible to easily interpret an image by obtaining a uniform image.

Further, in the present embodiment, as an example, the divided data is divided into two divided regions, the center portion uses two projection data, and the peripheral portion uses three projection data, and the interpolation data of each region is generated. For example, the center portion may be interpolated using two projection data and the peripheral portion may be interpolated using four projection data. Hereinafter, calculating interpolation data using two projection data is also referred to as two-point interpolation, and calculating interpolation data using three projection data is also referred to as three-point interpolation.
Further, for example, three divided areas are set, and the first area, the second area, the third area, and the fourth area are divided from the center, and the first area is divided into two points by interpolation, The order used for interpolation may be increased from the central portion to the outer peripheral portion, such as four-point interpolation for the region, five-point interpolation for the third region, and seven-point interpolation for the fourth region.

[Modification 1]
FIG. 8 is a graph showing the distance from the center and the number of interpolation points at a predetermined distance.
As shown in FIG. 8 (a), at the boundary of the circle to generate a threshold r ₁ as the radius, the transition area width r ₀ is formed. That is, the transition area is an area set from the radius r _a to the radius r _b shown in FIG.

In other words, in the present modification, for example, two-point interpolation is performed in the central region divided by the radius r ₁ as described above, and three-point interpolation is performed in the peripheral region. Further, a transition region is provided at the boundary portion by the segment region setting unit 52 based on an input from the operator.
In areas in the region of the central portion which is divided by the radius r _a from the center, the interpolation data generator 53 performs a two-point interpolation.
In the transition area, the interpolation data generation unit 53 gradually decreases the area where the two-point interpolation is performed, and gradually increases the area where the three-point interpolation is performed.
Outside the region _divided by the distance rb, the interpolation data generation unit 53 performs only three-point interpolation.

  The present modification can also be applied when a plurality of transition areas are set as shown in FIG. Interpolation is performed by the interpolation data generation unit 53 in substantially the same manner as in FIG.

  According to the first modification, since the number of interpolations gradually changes, even if the number of interpolation points is rapidly increased for each divided area, the generated image is not affected. As a result, a more uniform image can be reconstructed.

[Modification 2]
FIG. 9 is a schematic diagram schematically showing a reconstructed image according to this modification.
In this modification, for example, two segmented areas are set, and the number of interpolation points selected from the channel direction and the column direction of the detector 23 is changed in the segmented areas.
Specifically, the interpolation data generation unit 53 performs two-point interpolation from the channel direction and the column direction in the most central first region, and the channel direction and the column in the outermost third region. Three-point interpolation is performed from each direction. In the second region sandwiched between the first region and the third region, the interpolation data generation unit 53 performs three-point interpolation in the channel direction and two-point interpolation in the column direction.
Increasing the order of interpolation reduces the artifacts of the backprojected reconstruction screen, but increases the slice thickness. Therefore, multipoint interpolation is used in the channel direction that has little contribution to slice thickness. As a result, noise and artifacts are reduced in the generated image.

  According to the second modification, since the number of interpolations gradually changes, even if the number of interpolation points is rapidly increased, the generated image is not affected. Further, by increasing the interpolation order only in the channel direction that has little influence on the slice thickness in the second region, it is possible to prevent the slice thickness of the tomographic image to be reconstructed from being increased, and noise is generated at a lower order interpolation point. A reduced uniform image can be generated.

The present invention is not limited to the above embodiment. Modifications can be made as appropriate within the scope of the claims.
For example, the radiation used for scanning is not limited to X-rays, but may be other radiation such as γ rays. Further, the number of projection data used for the interpolation is not limited to the above example as long as the central part and the peripheral part are interpolated with different data numbers. Further, the number of interpolation points may be gradually increased from the central portion by setting the divided areas finely. Alternatively, even if a one-dimensional line sensor detector is used instead of a two-dimensional X-ray detector, the same effect can be obtained by changing the number of interpolation points in each segmented region.
Various other changes are possible.

It is a block diagram which shows the apparatus structure of the X-ray CT apparatus which concerns on this invention. It is a figure which shows the structure of the principal part of the X-ray CT apparatus shown in FIG. It is a block diagram which shows the structure of the reconstruction part shown in FIG. 1 and FIG. It is a schematic sectional drawing which shows typically the process of the interpolation which concerns on this invention. It is a schematic sectional drawing which shows typically the process of the interpolation which concerns on this invention. It is a flowchart which shows the interpolation process of the X-ray CT apparatus which concerns on this invention. It is a flowchart which shows operation | movement of the X-ray CT apparatus based on this invention. 10 is a graph showing the distance from the center and the number of interpolation points in a reconstructed image provided with an interference region according to Modification Example 1. It is the schematic which shows typically the reconstruction image which concerns on the modification 2. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... X-ray CT apparatus 2 ... Scanning gantry 3 ... Operation console 4 ... Imaging table 6 ... Subject 20 ... X-ray tube 22 ... Collimator 23 ... Detector 23S ... Detector surface 24 ... Data acquisition system (DAS)
DESCRIPTION OF SYMBOLS 25 ... X-ray controller 26 ... Collimator controller 27 ... Rotation part 28 ... Rotation controller 29 ... X-ray cone beam 30 ... Central processing unit 31 ... Input device 32 ... Display device 33 ... Memory | storage device 34 ... Control part 36 ... Reconfiguration part 38 ... Display unit 51 ... Pre-processing unit 52 ... Division area setting unit 53 ... Interpolation data setting unit 54 ... Filter processing unit 55 ... Back projection unit 56 ... Post-processing unit

Claims (14)

A radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in first and second arrangement directions to form a two-dimensional radiation detector surface, and the radiation detector surface A radiation CT apparatus comprising: a two-dimensional radiation detector for detecting radiation by means; and means for reconstructing a tomogram in a predetermined reconstructed image based on projection data obtained by the two-dimensional radiation detector,
The reconstruction means includes
In a predetermined reconstructed image reconstructed based on the projection data, the reconstructed image is divided into a plurality of regions based on a distance from the center of the reconstructed image, and each of the divided reconstructed images Interpolation means for interpolating a region using a plurality of the projection data;
Image reconstruction means for reconstructing the tomographic image using the projection data and the interpolated data, and
In the interpolation means, the region on the peripheral side of the divided reconstructed image is interpolated with a different order from the region on the central side. Radiation CT apparatus. In the interpolating means, a transition area is set at a boundary where the reconstructed image is divided, and the center side of the transition area is interpolated using n (n ≧ 2) pieces of projection data, and the peripheral section The side is interpolated using m (m ≧ 2) pieces of projection data, and in the transition region, the n pieces of projection data and the m pieces of projections are moved from the central part side toward the peripheral part side. The radiation CT apparatus according to claim 1, wherein data is transferred from the former to the latter while weighting the data. The radiation CT apparatus according to claim 2, wherein two or more transition regions are set in the interpolation unit. In the interpolating means, two or more transition regions are set at a boundary for partitioning the reconstructed image, and the p-th region from the center side partitioned by the transition region is the first region of the two-dimensional radiation detector. Interpolation is performed using n (n ≧ 2) projection data from the array direction and the second array direction, respectively, and the (p + 2) -th region is the first array direction of the two-dimensional radiation detector and the Interpolation is performed using m (m ≧ 2) pieces of projection data from the second array direction, and the (p + 1) th region is q (n ≦ q ≦) from the first array direction of the two-dimensional radiation detector. Interpolated using m or m ≦ q ≦ n) projection data, and interpolated using (q + 1) projection data from the second array direction of the two-dimensional radiation detector. The radiation CT apparatus according to 1. The radiation CT apparatus according to claim 4, wherein the first arrangement direction of the two-dimensional radiation detector is a column direction, and the second arrangement direction of the two-dimensional radiation detector is a channel direction. A radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in first and second arrangement directions to form a two-dimensional radiation detector surface, and the radiation detector surface The present invention relates to an image reconstruction method for a radiation CT apparatus, comprising: a two-dimensional radiation detector for detecting radiation by means of: and reconstructing a tomographic image in a predetermined reconstruction image based on projection data obtained by the two-dimensional radiation detector. ,
The reconstructed image is divided into a plurality of regions based on a distance from the center of a predetermined reconstructed image reconstructed based on the projection data, and each region of the reconstructed image is divided into a plurality of projection data. Interpolating using, and
Reconstructing the tomographic image using the projection data and the interpolated data, and
An image reconstruction method in which, in the interpolation step, an area on the peripheral part side of the divided reconstructed image is interpolated with a different order from an area on the central part side. In the interpolation step, a transition region is set at a boundary where the reconstructed image is divided, and the center side of the transition region is interpolated using n (n ≧ 2) projection data, and the peripheral portion The side is interpolated using m (m ≧ 2) pieces of projection data, and in the transition region, n pieces of projection data and m pieces of projections are directed from the central part side toward the peripheral part side. The image reconstruction method according to claim 6, wherein the data is shifted from the former to the latter while weighting the data. The image reconstruction method according to claim 7, wherein two or more transition regions are set in the interpolation step. In the interpolation step, two or more transition regions are set at a boundary for partitioning the reconstructed image, and the p-th region from the center side partitioned by the transition region is the first region of the two-dimensional radiation detector. Are interpolated using n (n ≧ 2) projection data respectively from the array direction and the second array direction, and the (p + 2) -th region is the first array direction of the two-dimensional radiation detector Then, interpolation is performed using m (m ≧ 2) pieces of projection data from the second arrangement direction, and the (p + 1) -th region is q (n) from the first arrangement direction of the two-dimensional radiation detector. .Ltoreq.q.ltoreq.m or n.ltoreq.q.ltoreq.m) projection data, and interpolate using (q + 1) projection data from the second array direction of the two-dimensional radiation detector. Item 7. The image reconstruction method according to Item 6. The image reconstruction method according to claim 9, wherein the first arrangement direction of the two-dimensional radiation detectors is a column direction, and the second arrangement direction of the two-dimensional radiation detectors is a channel direction. A radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in one direction to form a one-dimensional radiation detector surface, and the radiation is detected by the radiation detector surface. A radiation CT apparatus comprising a one-dimensional radiation detector and means for reconstructing a tomographic image in a predetermined reconstructed image based on projection data obtained by the one-dimensional radiation detector;
The reconstruction means includes
In a predetermined reconstructed image reconstructed based on the projection data, the reconstructed image is divided into a plurality of regions based on a distance from the center of the reconstructed image, and each of the divided reconstructed images Interpolation means for interpolating a region using a plurality of the projection data;
Image generating means for reconstructing the tomographic image using the projection data and the interpolated data, and
In the interpolation means, the region on the peripheral side of the divided reconstructed image performs higher-order interpolation than the region on the central side. Radiation CT apparatus. In the interpolating means, a transition area is set at a boundary where the reconstructed image is divided, and the center side of the transition area is interpolated using n (n ≧ 2) pieces of projection data, and the peripheral section The side is interpolated using m (m ≧ 2) pieces of projection data, and in the transition region, the n pieces of projection data and the m pieces of projections are moved from the central part side toward the peripheral part side. The radiation CT apparatus according to claim 11, wherein data is transferred from the former to the latter while weighting the data. A radiation source that emits radiation having a predetermined spread to a subject and a plurality of radiation detectors are arranged in one direction to form a one-dimensional radiation detector surface, and the radiation is detected by the radiation detector surface. A radiation CT apparatus for reconstructing a tomographic image in a predetermined reconstruction image based on projection data obtained by the one-dimensional radiation detector,
The reconstructed image is divided into a plurality of regions based on a distance from the center of a predetermined reconstructed image reconstructed based on the projection data, and each region of the reconstructed image is divided into a plurality of projection data. Interpolating using, and
Reconstructing the tomographic image using the projection data and the interpolated data, and
An image reconstruction method in which, in the interpolation step, a region on a peripheral portion side of the divided reconstructed image is subjected to higher-order interpolation than a central portion thereof. In the interpolation step, a transition region is set at a boundary where the reconstructed image is divided, and the center side of the transition region is interpolated using n (n ≧ 2) projection data, and the peripheral portion The side is interpolated using m (m ≧ 2) pieces of projection data, and in the transition region, n pieces of projection data and m pieces of projections are directed from the central part side toward the peripheral part side. The image reconstruction method according to claim 13, wherein data is shifted from the former to the latter while weighting the data.

Images