JP2010042096A - X-ray computed tomography apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reconstruct a mask region while suppressing a possibility of deteriorating strain-shape reproducibility in a circular-orbit cone-beam reconstruction method. <P>SOLUTION: This X-ray computed tomography apparatus includes an X-ray tube 101 generating cone-beam-shaped X rays, a two-dimensional array-type X-ray detector 103 detecting the X rays transmitting through a subject to generate projection data, a rotary mechanism 102 rotating the X-ray tube around the circumference of the subject along with the X-ray detector, a reconstruction processing section 119 reconstructing image data using a reconstruction area as an object based on the projection data, and a projection data selection section 117 which selects the projection data for use in the reconstruction for changing a view number of the projection data used for the reconstruction of the image data in both ends of the reconstruction area according to the reconstruction position on the rotary shaft. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an X-ray computed tomography (CT) having a circular orbit cone beam reconstruction function.

  The circular orbit cone beam reconstruction method has been proposed by (Feldkamp) et al. In this reconstruction method, there are regions where the projection data for 360 ° are not aligned (called mask regions) at both ends of the reconstruction region. Even if the mask area is reconstructed, sufficient image quality cannot be obtained. At present, it is masked by saying that sufficient image quality cannot be obtained, and the mask area is excluded from image reconstruction. Even if the mask area is forcibly reconfigured, there is a high possibility that distortion and shape reproducibility will deteriorate.

  An object of the present invention is to reconstruct a mask region while suppressing the possibility of distortion and deterioration of shape reproducibility in a circular orbit cone beam reconstruction method.

  An aspect of the present invention includes an X-ray tube that generates cone-beam X-rays, a two-dimensional array type X-ray detector that detects X-rays transmitted through a subject and generates projection data, and the X-ray detector A rotation mechanism that rotates the tube around the subject together with the X-ray detector; a reconstruction processing unit that reconstructs image data based on the projection data; A projection data selection unit that selects projection data used for the reconstruction in order to change the number of views of the projection data used for the reconstruction of the image data at both ends according to the reconstruction position on the rotation axis; An X-ray computed tomography apparatus is provided.

  According to the present invention, in the circular orbit cone beam reconstruction method, the mask region can be reconstructed while suppressing the possibility of distortion and deterioration of shape reproducibility.

  Embodiments of an X-ray computed tomography apparatus according to the present invention will be described below with reference to the drawings. The X-ray computed tomography apparatus includes a rotation / rotation method in which an X-ray tube and an X-ray detector are rotated as one body, and a large number of X-ray detectors are arranged on a ring. There is a fixed / rotation method in which only the X-ray tube rotates around the subject, and the present invention can be applied to any method. Regarding the rotation / rotation method, a so-called single-tube type in which a pair of X-ray tubes and X-ray detectors are mounted on a rotating frame, and a so-called multiple pairs of X-ray tubes and X-ray detectors are mounted on a rotating frame. There are multitubular bulb types, but the present invention can be applied to any type. Regarding the X-ray detector, an indirect conversion type in which X-rays transmitted through a subject are converted into light by a phosphor such as a scintillator and then converted into electric charge by a photoelectric conversion element such as a photodiode, and electrons in a semiconductor by X-rays There is a direct conversion type using the generation of a hole pair and its movement to the electrode, that is, a photoconductive phenomenon, but the present invention may adopt any form.

  FIG. 1 shows the configuration of an X-ray computed tomography apparatus according to this embodiment. The gantry 100 includes an X-ray tube 101. The X-ray tube 101 receives a tube voltage and a filament current from the high voltage generator via the slip ring mechanism 108 and generates cone beam X-rays. The X-ray tube 101 is mounted together with the X-ray detector 105 on a rotating frame 102 that is supported rotatably about a rotation axis (Z axis). The rotation angle detection unit 107 is provided to detect the rotation angle of the rotation frame 102. Normally, when the X-ray tube 101 is at the top, the rotation angle of the rotating frame 102 is set to zero degrees. The X-ray detector 103 faces the X-ray tube 101 across the rotation axis RA. A cylindrical reconstruction area around the rotation axis RA is set via the input device 115.

  The X-ray detector 103 detects X-rays that have passed through the subject from the X-ray tube 101. The X-ray detector 103 is a multi-slice type or a two-dimensional array type corresponding to a cone beam. That is, the X-ray detector 103 has a plurality of X-ray detection element arrays arranged in parallel along the rotation axis RA. Each X-ray detection element array has a plurality of X-ray detection elements arranged in a line along a direction orthogonal to the rotation axis RA.

  The output of the X-ray detector 103 is amplified for each channel by a data acquisition circuit (DAS) 104, converted into a digital signal, and sent to a preprocessing device 106 via, for example, a contactless data transmission device 105, Accordingly, correction processing such as sensitivity correction is performed and stored in the projection data storage unit 116 as so-called projection data immediately before the reconstruction processing. The scan controller 110 controls the rotation drive unit, the high voltage generator 109, the data collection circuit 104, and the like for data collection (scan).

  The reconstruction processing unit 119 reconstructs image data based on the projection data under the cone beam image reconstruction method. Here, the cone beam image reconstruction method is a Feldkamp method. As is well known, the Feldkamp 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, and the data is fan projected. This is done as data. However, the back projection process is performed along an actual ray. In other words, a weight depending on the Z coordinate is applied to the projection data, and the same reconstruction function as the fan beam reconstruction is convoluted to the projection data to which the weight is applied, and the data is converted into an oblique actual ray having a cone angle. The image is reconstructed by the procedure of back projection along.

The projection data selection unit 117 uses the projection data stored in the projection data storage unit 116 for the projection data used for the reconstruction process by the reconstruction processing unit 119 according to the slice position (reconstruction position) on the Z axis. To select.
As shown in FIG. 2, at both end portions (Z1 to Z2 and Z3 to Z4) of the cylindrical reconstruction area, the number of views of the projection data used for the image reconstruction process, particularly the back projection process, is changed. In order to change according to the configuration position, projection data used for reconstruction is selected. Both ends are
As illustrated in FIG. 3, the number of views is changed continuously or stepwise from the innermost positions Z2, Z3 to the outermost positions Z1, Z4 of both end portions. When the number of views is continuously changed, it is changed linearly (defined by an arbitrary linear expression) or curved (defined by an arbitrary multi-order expression). The innermost positions Z2 and Z3 are defined as the Z positions where the outer periphery of the reconstruction area intersects with the edge of the cone beam X-ray.

  Actually, the projection data selection unit 117 determines the number of views of projection data to be collected by one rotation N in the central portion (Z2 to Z3) of the reconstruction area where the mask area does not exist. Set to. In addition, at both end portions (Z1 to Z2, Z3 to Z4) of the reconstruction area where the mask area partially exists, the number of views used for reconstruction is changed from the view number N for one rotation according to the Z position. , It is reduced to N · ((180 ° + α) / 360 °) equivalent to the number of views required for half reconstruction. Α is a fan angle. The change pattern of the number of views can be selected by the operator via the input device 115.

  In the central part (Z2 to Z3) of the reconstruction area in which no mask area exists, the projection data selection unit 117 reconstructs the collected projection data of all views N for each of all slices therein. On the other hand, at both end portions (Z1 to Z2, Z3 to Z4) of the reconstruction area where the mask area partially exists, the projection data of the number of views corresponding to the Z position smaller than N is as uniform as possible. This is thinned out and supplied to the reconstruction processing unit 119. The reconstruction processing unit 119 reconstructs image data in the reconstruction area based on the projection data supplied from the projection data selection unit 117 according to the Feldkamp reconstruction method.

  As described above, in the present embodiment, in the circular orbit cone beam reconstruction method, the number of views used for reconstruction is gradually reduced in accordance with the Z position at both ends of the reconstruction area where the mask area partially exists. As a result, it is possible to suppress the possibility that the mask region is distorted and the shape reproducibility is deteriorated.

  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 components 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 figure which shows the structure of the X-ray computed tomography apparatus by embodiment of this invention. The figure which shows the relationship between Z position and the number of views used for a reconstruction in this embodiment. The figure which shows the variation of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... X-ray tube, 102 ... Rotating frame, 103 ... X-ray detector, 116 ... Projection data storage part, 117 ... Projection data selection part, 119 ... Reconstruction process part.

Claims (4)

An X-ray tube that generates cone-beam X-rays;
A two-dimensional array type X-ray detector that detects X-rays transmitted through the subject and generates projection data;
A rotation mechanism for rotating the X-ray tube around the subject together with the X-ray detector;
A reconstruction processing unit for reconstructing image data for a reconstruction area based on the projection data;
Projection data selection for selecting projection data used for reconstruction in order to change the number of views of projection data used for reconstruction of the image data at both ends of the reconstruction area in accordance with the reconstruction position on the rotation axis An X-ray computed tomography apparatus.   The X-ray computed tomography apparatus according to claim 1, wherein the projection data selection unit linearly changes the number of views at both end portions of the reconstruction area.   The X-ray computed tomography apparatus according to claim 1, wherein the projection data selection unit changes the number of views in a curved manner at both end portions of the reconstruction area.   The projection data selection unit sets the number of views N for one rotation at the central portion of the reconstruction area, and N · ( The X-ray computed tomography apparatus according to claim 1, wherein the X-ray computed tomography apparatus is lowered to (180 ° + α) / 360 °).

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