JP2010279427A - X-ray computed tomographic system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To contract a mask region to the utmost to expand a reconstitution region in the mask region reconstitution method for an X-ray CT. <P>SOLUTION: The X-ray CT includes an X-ray tube 101, a multi-row type X-ray detector 103, a support mechanism 102, a memory part 112 for storing a plurality of projection data sets detected by the multi-row type X-ray detector, a first reconstitution processing part 201 for reconstituting first image data according to a Feldkamp reconstitution method on the basis of a plurality of the projection data sets, a second reconstitution processing part 203 for reconstituting second image data according to a mask region reconstitution method on the basis of a plurality of the projection data sets, and a mask region setting part 209 for setting a mask region out of the target for the reconstitution processing according to the mask region reconstitution method on the basis of the absolute value sum or square sum of the difference between the first and second image data. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an X-ray computed tomography apparatus that reconstructs three-dimensional data by scanning a wide range with cone beam X-rays.

  In Feldkamp (FDK) reconstruction, which calculates approximate solutions, a method is adopted in which both end portions in the body axis direction where projection data are not distributed over the entire circumference are set as mask regions and the mask regions are excluded from reconstruction processing. It has been.

  There is a mask area reconstruction method as a method for reducing the mask area and enlarging the reconstruction area. In this mask area reconstruction method, as shown in FIG. 8, both ends where projection data are not aligned over the entire circumference are complemented by extrapolation data from projection data actually measured with X-ray irradiation. Have realized it.

  However, since the ratio of using extrapolated data is higher in the end row images that have not been collected, the image quality decreases. Therefore, even in the mask area reconstruction method, the mask area is currently set within a certain range.

Japanese Patent No. 378846

  An object of the present invention is to reduce the mask area as much as possible and enlarge the reconstruction area in the mask area reconstruction method of the X-ray computed tomography apparatus.

  An aspect of the present invention includes an X-ray tube that generates X-rays, a multi-row X-ray detector that detects X-rays transmitted through a subject, the X-ray tube and the multi-row X-ray detector, A support mechanism that rotatably supports a rotation axis around the rotation axis, a storage unit that stores a plurality of projection data sets detected by the multi-row X-ray detector, and a Feldkamp reconstruction based on the plurality of projection data sets. A first reconstruction processing unit for reconstructing first image data according to a construction method; a second reconstruction processing unit for reconstructing second image data according to a mask region reconstruction method based on the plurality of projection data sets; A mask area setting unit for setting a mask area to be excluded from the reconstruction process by the mask area reconstruction method based on the absolute value sum or the square sum of the difference between the first image data and the second image data. X-ray computed tomography To provide a shadow device.

  According to the present invention, in the mask area reconstruction method of the X-ray computed tomography apparatus, the mask area can be reduced as much as possible to enlarge the reconstruction area.

It is a figure which shows the structure of the X-ray computed tomography apparatus which concerns on embodiment of this invention. In this embodiment, it is a flowchart which shows the 1st process sequence for setting a mask area | region. It is supplementary explanatory drawing of process S17 of FIG. In this embodiment, it is a figure which shows an example when the absolute value sum total of a difference image is higher than a threshold value. In this embodiment, it is a figure which shows an example when the absolute value sum total of a difference image is lower than a threshold value. In this embodiment, it is a flowchart which shows the 2nd process sequence for setting a mask area | region. It is a figure which shows an example of the sum total value profile produced | generated by process S18 of FIG. In this embodiment, it is a figure which shows the extrapolation interpolation range in a mask area | region reconstruction method. In this embodiment, it is a figure which shows the reduced mask area | region.

  Hereinafter, an X-ray computed tomography apparatus according to the present invention will be described in detail with reference to the drawings by a preferred embodiment. In the X-ray computed tomography system, ROTATE / ROTATE-TYPE, in which an X-ray tube and an X-ray detector are combined as one body, rotates around the subject, and a large number of detection elements arrayed in a ring shape are fixed. There are various types such as STATIONARY / ROTATE-TYPE in which only the X-ray tube rotates around the subject, and the present invention can be applied to any type. Here, it is described as ROTATE / ROTATE-TYPE. Further, in order to reconstruct image data, projection data for one rotation around the subject and 360 ° is required, and projection data for 180 ° + fan angle is also required in the half scan method. The present invention can be applied to any reconstruction method. Here, the 360 ° method will be described as an example. In addition, the mechanism for converting incident X-rays into electric charges is based on an indirect conversion type in which X-rays are converted into light by a phosphor such as a scintillator and the light is further converted into electric charges by a photoelectric conversion element such as a photodiode. The generation of electron-hole pairs in a semiconductor such as selenium and the transfer to the electrodes, that is, the direct conversion type utilizing a photoconductive phenomenon, are the mainstream. Any of these methods may be adopted as the detection element.

  FIG. 1 is a diagram showing a configuration of an X-ray computed tomography apparatus according to the present embodiment. The gantry 100 accommodates a rotation support mechanism 102. The rotation support mechanism 102 includes a rotation ring and a ring support mechanism that rotatably supports the rotation ring about the rotation axis Z. An X-ray tube 101 is mounted on the rotating ring. The X-ray tube 101 radiates X-rays from the focal point upon receiving application of tube voltage and supply of tube current from the high voltage generator 109 via the slip ring 108. A collimator unit 111 is attached to the X-ray emission window of the X-ray tube 101. The collimator unit 111 limits the X-rays from the X-ray tube 101, and together with the X-ray divergence angle (fan angle) in the XY plane orthogonal to the rotation axis Z, the divergence angle (cone angle) in the rotation axis Z direction. A cone-beam X-ray having

  An X-ray tube 101 and an X-ray detector (also referred to as an area detector) 103 called a two-dimensional array type or a multi-row type are mounted on the rotating ring. The X-ray detector 103 is attached at a position and an angle facing the X-ray tube 101 across the rotation axis Z. The X-ray detector 103 has a plurality of X-ray detection elements. In the following description, it is assumed that a single X-ray detection element constitutes a single channel. The plurality of channels are two-dimensional in two directions: a direction parallel to the rotation axis Z (slice direction) and an arc direction (channel direction) that is orthogonal to the rotation axis Z and gently curves around the X-ray focal point. Arranged. The X-ray detector 103 having such a two-dimensional detection element arrangement may be configured by arranging a plurality of detection elements arranged in one row with respect to the channel direction, with respect to the slice direction. May be configured by arranging a plurality of modules arranged in an M × N matrix. During imaging or scanning, the subject is inserted into a cylindrical imaging area between the X-ray tube 101 and the X-ray detector 103.

  A data acquisition circuit 104 generally called a DAS (Data Acquisition System) is connected to the output of the X-ray detector 103. The data acquisition circuit 104 includes an IV converter that converts a current signal of each channel of the X-ray detector 103 into a voltage, and periodically integrates the voltage signal in synchronization with an X-ray exposure cycle. An integrator, an amplifier that amplifies the output signal of the integrator, and an analog / digital converter that converts the output signal of the preamplifier into a digital signal are provided for each channel.

  Data (pure raw data) output from the data collection circuit 104 is transmitted to the preprocessing device 106 via the non-contact data transmission device 105 using magnetic transmission / reception or optical transmission / reception. The preprocessing device 106 performs preprocessing on the pure raw data. Preprocessing includes, for example, sensitivity non-uniformity correction processing between channels, X-ray strong absorber, processing for correcting an extremely low signal strength drop or signal drop mainly due to a metal part, and the like. The data immediately before reconstruction processing output from the pre-processing device 106 (referred to as raw data or projection data, here referred to as projection data) is the channel number, detection element array number, and data collected. Are stored in the projection data storage unit 112 including a magnetic disk, a magneto-optical disk, or a semiconductor memory.

  The projection data is a set of data values according to the intensity of the X-rays transmitted through the subject. Here, for convenience of explanation, all the viewing angles collected at the same time in one shot for each viewpoint are the same. A set of projection data across channels is called a projection data set. Further, the view angle represents each position on the circular orbit around which the X-ray tube 101 circulates around the rotation center axis Z as an angle in a range of 360 ° with the top position of the rotation orbit being zero °. is there.

  Here, two image reconstruction units 201 and 203 having different reconstruction methods are provided as reconstruction processing units. The two image reconstruction units 201 and 203 may be physically provided as separate circuit units, or are actually a single circuit unit that is equipped with two types of programs and can be arbitrarily selected. You may make it use for. As a preferable example of the two kinds of reconstruction methods, a Feldkamp image reconstruction method and a mask region image reconstruction method are adopted.

  In the present embodiment, the mask region image reconstruction method is applied within the mask region on the Feldkamp reconstruction method, but the application range of the mask region image reconstruction method is not uniform but is expanded as much as possible. As shown in FIG. 8, the mask area on the Feldkamp reconstruction method is set to a range where projection data is not collected on the slice plane (XY plane) orthogonal to the rotation axis Z. Therefore, the XY reconstruction field of view is reduced at both ends of the reconstruction area in the Z direction. As shown in FIG. 9, the mask region image reconstruction method is applied to a part of the mask region, but the application range is expanded by this embodiment as long as there is little clinical influence.

  The Feldkamp image reconstruction unit 201 reconstructs three-dimensional image data (volume data) according to the Feldkamp reconstruction method based on a plurality of projection data sets having a view angle in the range of 360 °. As is well known, the Feldkamp reconstruction method is an approximate reconstruction method based on the fan beam convolution and back projection method, and the convolution process is based on the assumption that the cone angle is relatively small. This is done by considering it as projection data. However, the back projection process is performed along an actual ray. (1) The projection data is weighted depending on the Z coordinate, (2) the same reconstruction function as the fan beam reconstruction is convoluted with the data of (1), and the data of (3) (2) is The image is reconstructed by the procedure of back-projecting along a diagonal actual ray having a corner.

  The mask area image reconstruction unit 203 reconstructs three-dimensional image data (volume data) according to a mask area image reconstruction method also called Pixel Based Sector reconstruction. Unlike the Feldkamp image reconstruction method, the mask area image reconstruction method does not assume a fan beam, and the CT value of each pixel is determined from all projection data passing through each pixel. It is processing. In the mask area image reconstruction method, projection data of a missing portion that is not actually collected is interpolated from the projection data that is actually collected by an extrapolation method.

  The difference processing unit 207 is a three-dimensional image obtained by reconstructing the two-dimensional image data related to the slice in the mask area generated from the three-dimensional image data reconstructed by the Feldkamp image reconstruction unit 201 by the mask area image reconstruction unit 203. A difference image is generated by subtracting from the two-dimensional image data regarding the slice at the same position in the mask region generated from the data. The mask area setting unit 209 calculates the sum of absolute values or the sum of squares (described here as the sum of absolute values) for the difference image of each slice. It is selected by the user via the input device 115 which of the absolute value sum and the square sum is adopted.

  The mask area setting unit 209 compares a predetermined threshold with the sum of absolute values in each slice. The mask area setting unit 209 searches sequentially from the innermost slice of the mask area on the Feldkamp reconstruction method toward the outside, and specifies a slice position where the sum of absolute values exceeds a threshold value for the first time. The mask area image reconstruction method is applied to the range from the inside of the slice position specified by the mask area setting unit 209 to the outside of the area where the projection data is uniform over the entire area. A range on the solo side from the slice position specified by the mask area setting unit 209 is set as a mask area.

  The total profile generating unit 211 generates a total profile in which the sum of absolute values of the difference image calculated for each slice by the difference processing unit 207 is distributed with respect to the rotation axis Z. The display unit 116 is provided to display a two-dimensional image, a rendering image of three-dimensional image data, a summation profile, and the like. In the above description, the mask area setting unit 209 sets the application range of the mask area image reconstruction method by comparing the sum of absolute values with the threshold value. However, the mask area setting unit 209 uses the input device 115 on the displayed sum profile. It may be specified by the user.

  FIG. 2 shows a first processing procedure for setting the application range of the mask area image reconstruction method according to the present embodiment. First, a relatively wide range of projection data over a plurality of detection element arrays with respect to the rotation axis Z is collected by circular orbit scanning and stored in the storage unit 112.

  In step S11, the Feldkamp image reconstruction unit 201 reconstructs three-dimensional image data based on a plurality of projection data sets within a range of view angles of 360 °. In step S12, the mask area image reconstruction unit 203 reconstructs three-dimensional image data. The first slice is set at the innermost position of the mask area, and the slice position to be searched is a predetermined distance corresponding to the slice pitch every time S13 to S16 are repeated until it becomes yes in step S15. Each is moved outward (S16).

  Next, in step S13, the difference processing unit 207 generates two-dimensional image data related to the slice in the mask region from the three-dimensional image data reconstructed by the Feldkamp image reconstruction unit 201, and the same as the two-dimensional image data. The difference between the two-dimensional image data generated from the three-dimensional image data reconstructed by the mask area image reconstruction unit 203 relating to the slice is generated. In step S14, the mask area setting unit 209 calculates the sum ST of the absolute values of the pixel values for all the pixels in the difference image. As illustrated in FIG. 3, a predetermined threshold value TH is compared with the sum ST of absolute values (S15). The loop of S13 to S16 is repeated until the sum total ST of absolute values exceeds the threshold value TH.

  Here, when the imaging region of the subject is a complicated shoulder or the like, the difference between the image by the Feldkamp reconstruction method and the image by the mask region reconstruction method is significant as illustrated in FIG. Become. On the other hand, in the case of an uncomplicated abdomen, the difference tends to be small as illustrated in FIG. In this embodiment, this tendency is used to reduce the range to which the mask region reconstruction method is applied in the case of a complex region, and conversely to expand the range to which the mask region reconstruction method is applied in a region that is not complicated. To reduce the mask area.

  That is, as illustrated in FIG. 9, the mask region setting unit 209 sequentially searches from the innermost slice of the mask region on the Feldkamp reconstruction method toward the outside, and the sum of absolute values exceeds the threshold value for the first time. The mask region image reconstruction method is applied to a range outside the region where the slice position is specified and inside the specified slice position and the projection data is uniform over the entire area. A range outside the slice position specified by the mask area setting unit 209 is set as a mask area (S17).

  FIG. 6 shows a procedure for manually setting as a mask area. In step S14, the mask region setting unit 209 calculates a plurality of sums ST for a plurality of slice positions in the mask region by the Feldkamp reconstruction method. A plurality of sums ST having different slice positions are distributed with respect to the rotation axis Z by the sum profile generating unit 211, whereby a sum profile is generated and displayed on the display unit 116 (S18). As shown in FIG. 7, the user operates the input device 115 to designate an arbitrary position. Typically, a position where the total sum ST increases rapidly is designated. The mask area setting unit 209 sets a mask area for a range outside the slice position designated by the user via the input device 115 on the displayed sum profile (S19).

  Until now, the area to be masked has always been a constant value without depending on the shape of the imaging region. In this case, the masking process is performed on the region that can be imaged monotonously and clinically acceptable, and information is wasted. That is, a region that cannot be imaged in spite of the region irradiated with X-rays is set wide, and it is considered that the exposure is wasted. However, the present embodiment can estimate subject shape information from images obtained by two types of reconstruction methods, and manually or automatically set an area to be masked to provide all clinically acceptable areas.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

  The present invention can be used to reduce the mask area as much as possible and expand the reconstruction area in the mask area reconstruction method of the X-ray computed tomography apparatus.

  DESCRIPTION OF SYMBOLS 100 ... Gantry, 101 ... X-ray tube, 102 ... Rotation support mechanism, 103 ... X-ray detector, 104 ... Data acquisition circuit, 105 ... Non-contact data transmission device, 106 ... Pre-processing device, 108 ... Slip ring, 109 ... High voltage generator, 111 ... collimator unit, 112 ... projection data storage unit, 201 ... Feldkamp image reconstruction unit, 203 ... mask region image reconstruction unit, 207 ... difference processing unit, 209 ... mask region setting unit, 115 ... Input device 211... Sum profile generation unit.

Claims (5)

An X-ray tube that generates X-rays;
A multi-row X-ray detector for detecting X-rays transmitted through the subject;
A support mechanism for rotatably supporting the X-ray tube and the multi-row X-ray detector around a rotation axis;
A storage unit for storing a plurality of projection data sets detected by the multi-row X-ray detector;
A first reconstruction processing unit for reconstructing first image data based on the Feldkamp reconstruction method based on the plurality of projection data sets;
A second reconstruction processing unit for reconstructing second image data according to a mask region reconstruction method based on the plurality of projection data sets;
A mask area setting unit for setting a mask area to be excluded from the reconstruction process by the mask area reconstruction method based on the absolute value sum or the square sum of the difference between the first image data and the second image data. An X-ray computed tomography apparatus comprising: 2. The X-ray computed tomography apparatus according to claim 1, wherein the mask area setting unit sets the mask area outside a position where the absolute sum or square sum exceeds a predetermined threshold value. An X-ray tube that generates X-rays;
A multi-row X-ray detector for detecting X-rays transmitted through the subject;
A support mechanism for rotatably supporting the X-ray tube and the multi-row X-ray detector around a rotation axis;
A storage unit for storing a plurality of projection data sets detected by the multi-row X-ray detector;
A first reconstruction processing unit for reconstructing first image data based on the Feldkamp reconstruction method based on the plurality of projection data sets;
A second reconstruction processing unit for reconstructing second image data according to a mask region reconstruction method based on the plurality of projection data sets;
A profile generator for generating a profile relating to the rotational axis direction of the sum of absolute values or sum of squares of differences between the first image data and the second image data;
An X-ray computed tomography apparatus comprising: a mask region setting unit that sets a range designated by the user on the profile as a mask region that is not subject to reconstruction processing by the mask region reconstruction method . An X-ray tube that generates X-rays;
A multi-row X-ray detector for detecting X-rays transmitted through the subject;
A support mechanism for rotatably supporting the X-ray tube and the multi-row X-ray detector around a rotation axis;
A storage unit for storing a plurality of projection data sets detected by the multi-row X-ray detector;
A reconstruction processing unit that reconstructs the first and second image data according to the first and second reconstruction methods based on the plurality of projection data sets;
A mask area setting unit for setting a mask area to be excluded from a reconstruction process by the mask area reconstruction method based on a sum of absolute values or a sum of squares of differences between the first image data and the second image data. An X-ray computed tomography apparatus comprising: An X-ray tube that generates X-rays;
A multi-row X-ray detector for detecting X-rays transmitted through the subject;
A support mechanism for rotatably supporting the X-ray tube and the multi-row X-ray detector around a rotation axis;
A storage unit for storing a plurality of projection data sets detected by the multi-row X-ray detector;
A reconstruction processing unit that reconstructs the first and second image data according to the first and second reconstruction methods based on the plurality of projection data sets;
A profile generator for generating a profile relating to the rotational axis direction of the sum of absolute values or sum of squares of differences between the first image data and the second image data;
An X-ray computed tomography apparatus comprising: a mask region setting unit that sets a range designated by the user on the profile as a mask region that is not subject to reconstruction processing by the mask region reconstruction method .

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