JP2006000222A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably capture a high-resolution CT image even when a geometric condition of imaging is changed correspondingly to an object region of the imaging. <P>SOLUTION: First, in a step S1, an imaging condition is set up, and in a step S2, the geometric condition of the imaging which comprises a distance between an X-ray source and a subject, a distance between the subject and an X-ray detector, or the like, is checked in the imaging condition. In the case that the geometric condition of the imaging is changed after the previous imaging, a reconstruction function is selected from a table corresponding to the changed geometric condition of the imaging in a step S3, and after the imaging in a step S4, the processing is executed using the changed reconstruction function in the reconstruction processing in a step S5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an X-ray CT apparatus capable of arbitrarily changing imaging geometric conditions such as a distance to a subject.

  Conventionally, in an X-ray CT (Computer Tomography) apparatus as shown in FIG. 6, the X-ray source 1 and the sensor 2 are arranged to face each other with the subject S interposed therebetween. Shooting is performed while rotating around. In this case, the geometric positional relationship between the X-ray source 1, the sensor 2, and the center of the subject S is usually mechanically fixed.

  However, considering the region to be imaged, for example, the chest SB and the head SH are different in size as shown in FIG. 7, and therefore the X-ray source 1 and the sensor 2 are simultaneously moved by the distance d. In addition, a photographing method for photographing in accordance with the size of the chest SB and the head SH to be photographed is also employed.

  Also, in recent years, a projection image of a necessary part is acquired at once using cone beam X-rays similar to simple imaging using a two-dimensional sensor, and then reconstructed, and the same high-definition as a tomographic image in the body axis direction. A cone beam X-ray CT apparatus capable of obtaining three-dimensional image data having a high resolution is also being put into practical use. In such a cone beam X-ray CT apparatus, there are a method of photographing while rotating the X-ray source 1 and the sensor 2 around the subject S and a method of rotating the subject S.

  In general, the length in the body axis direction necessary for diagnosis is not always constant and varies depending on the imaging region. Therefore, for an imaging region having a small length in the body axis direction necessary for diagnosis, even if the distance between the X-ray source 1 and the sensor 2 is small, it is possible to obtain sufficient image quality within the range necessary for diagnosis.

  In order to reduce the influence of scattered radiation, it is effective to increase the distance between the subject S and the sensor 2 due to the Gradel effect. Therefore, it is necessary to determine the distance between the X-ray source 1 and the sensor 2 while taking a large distance between the subject S and the sensor 2 for a part with a large scattered radiation such as the chest and abdomen.

  However, in the case of an X-ray CT apparatus using a one-dimensional sensor, it is difficult to deal with various imaging parts only by simultaneous shifting of the X-ray source 1 and the sensor 2.

  The Feldkamp method is used as a reconstruction algorithm in an X-ray CT apparatus using cone beam X-rays. The Feldkamp method uses a convolution back projection method, which is a two-dimensional reconstruction method of a fan beam. It is an approximate reconstruction method extended to three dimensions.

  In this case, in the case of an X-ray CT apparatus using cone beam X-rays, not all of the projection data necessary for reconstruction can be obtained only by imaging by the rotation, but the missing data Since so-called missing data exists, there is no exact reconstruction algorithm.

  Therefore, if reconstruction is performed with higher accuracy, it is necessary to increase the distance between the X-ray source 1 and the sensor 2 as much as possible. This is because the larger the distance between the X-ray source 1 and the sensor 2 is, the closer to the parallel beam, the missing data is reduced and the reconstruction accuracy is improved. For this reason, also in the cone beam X-ray CT apparatus, the distance between the X-ray source 1 and the sensor 2 that ensures the accuracy required for reconstruction is used, and imaging is performed with this distance fixed.

  In addition, since the cone beam X-ray CT apparatus uses a two-dimensional sensor, the influence of scattered radiation cannot be ignored. In a one-dimensional sensor or a sensor in which a small number of one-dimensional sensors are arranged, a scattered radiation absorber is arranged for each pixel of the sensor to prevent invasion of scattered radiation, or scattered radiation is detected in a free area in the body axis direction of the sensor. Although the scattered radiation correction can be performed by arranging a detector for use, such a case cannot be performed with a two-dimensional sensor.

  An object of the present invention is to provide an X-ray CT apparatus capable of solving the above-described problems and always reconstructing using an optimal reconstruction function corresponding to imaging geometric conditions and obtaining a high-quality CT image. There is to do.

  In order to achieve the above object, an X-ray CT apparatus according to the present invention can set a first distance between an X-ray source and a subject and a second distance between a subject and a sensor to arbitrary distances. The imaging geometric condition can be changed in accordance with the imaging target region.

  According to the X-ray CT apparatus of the present invention, an optimal imaging geometric condition can be selected according to a region to be imaged, and reconstruction is always performed with an optimal reconstruction function corresponding to the imaging geometric condition. Therefore, a stable and high-quality CT image can be obtained.

The present invention will be described in detail based on the illustrated embodiments.
FIG. 1 shows a configuration diagram of an X-ray CT system. An X-ray source 11 and an X-ray detector 12 are arranged with a subject S as a patient in between. The distance D1 between the X-ray source 11 and the subject and the distance D2 between the subject S and the X-ray detector 12 are arbitrarily set. It can be changed.

  The output of the imaging system control unit 13 is connected to the X-ray source 11 via the X-ray generation control unit 14, and the output of the X-ray detector 12 is sent to the imaging system control unit 13 via the image input unit 15. It is connected. Further, an image processing unit 16, an image storage unit 17, a diagnostic monitor 18, an operation unit 19, and a network 20 are connected to the imaging system control unit 13, and a printer 21, a diagnostic workstation 22, and an image database are connected to the network 20. 23 is connected.

  X-rays generated from the X-ray source 11 controlled by the X-ray generation control unit 14 pass through the subject S and are detected by the X-ray detector 12, and the detected X-rays are projected to the image input unit 15 as projection images. Entered. The X-ray source 11 and the X-ray detector 12 collect projection images in the image storage unit 17 via the imaging system control unit 13 at every predetermined rotation angle while rotating around the subject S as a rotation center. Alternatively, as shown in FIG. 2, the subject S fixed on the rotary table may be rotated while maintaining the positional relationship between the X-ray source 11 and the X-ray detector 12.

  The projection image of each rotation angle input to the image input unit 15 is corrected by the image processing unit 16 via the imaging system control unit 13 and subjected to image processing such as preprocessing including objective conversion and reconstruction processing. A tomographic image group is created. The created tomographic image group is displayed on the diagnostic monitor 18, stored in the image storage unit 17, and output to the printer 21, diagnostic workstation 22, and image database 23 via the network 20. Various operations such as a display window operation, a tomographic image switching display operation in the body axis direction, a cross-sectional conversion operation, and a three-dimensional surface display operation are performed by the operation unit 19.

  FIG. 3 is a flowchart showing the photographing operation in the above-described system. First, photographing conditions are set by the operation unit 19 in step S1. Next, in step S2, the imaging system control unit 13 sets the imaging geometric conditions such as the distance D1 between the X-ray source 11 and the subject S and the distance D2 between the subject S and the X-ray detector 12 among the set imaging conditions. Check.

  If the shooting geometric condition has been changed since the previous shooting, the reconstruction function is selected in accordance with the changed shooting geometric condition in step S3, and shooting is performed in step S4.

  At the time of imaging in step S4, an imaging request is input from a hospital system (HIS), a radiation system (RIS), or the like, and an imaging site is selected. When an imaging region is selected, an optimal imaging distance for the selected imaging region is displayed on the operation screen of the diagnostic monitor 18, and the operator adjusts the distance D1 and the distance D2 according to this distance.

  For example, if the overhead traveling X-ray source 11 is used, a scale is attached to the ceiling traveling rail, and the distances D1 and D2 are adjusted to this scale. When the X-ray source 11 and the X-ray detector 12 are integrated, a mechanism that can arbitrarily change the distance D1 or D2 is adopted.

  That is, as shown in FIG. 4, the distances D1 and D2 can be set to arbitrary distances, and the distances D1 and D2 are changed from each other in accordance with the target region for imaging. The distances D1 and D2 determined in advance corresponding to the imaging region may be adjusted according to the body size and body thickness of each subject S. The adjusted distances D <b> 1 and D <b> 2 are photographed automatically as the distance at the time of photographing in the system or via the operation unit 19 and input to the system control unit 13. The distances D1 and D2 may be automatically adjusted.

  After the shooting, in the reconstruction process in step S5, the imaging system control unit 13 and the image processing unit 16 perform the reconstruction process using the changed reconstruction function. Then, after the reconstructed image is displayed in step S6, it is transferred to the output device as necessary, and the process proceeds to the next shooting in step S7.

  Thus, even if the distances D1 and D2 are changed and projection data captured under different imaging geometric conditions are reconstructed using the same reconstruction function, an optimum image quality cannot be obtained. Therefore, as shown in FIG. 4, the reconstruction function of the photographing geometric condition closest to the photographing geometric condition at the time of photographing is obtained from a table prepared in advance as shown in the following table, for example, and the reconstruction function to be used is selected. . When imaging is performed and projection data is obtained, reconstruction is performed using this reconstruction function to create a tomographic image.

Table Distance D1 Distance D2 Reconstruction function used 200 50 Reconstruction function 1
40 Reconstruction function 2
30 Reconstruction function 3
20 Reconstruction function 4
180 50 reconstruction function 5
40 Reconstruction function 6
30 Reconstruction function 7
.....

  In this shooting distance condition-reconstruction function table, for example, when the sum of the distances D1 and D2 is large, the defocus is increased. Therefore, as shown in the graph of FIG. To do. Alternatively, the distance D2 is also a reconstruction function that emphasizes high-frequency components as the distance D2 increases because the scattered radiation content increases as the distance D2 increases.

1 is a configuration diagram of an X-ray CT system. It is explanatory drawing when a to-be-photographed object rotates. It is a flowchart figure of imaging | photography operation | movement. It is explanatory drawing of the reconstruction function table for every imaging | photography geometric condition. It is a graph figure of the frequency characteristic of the reconstruction function by imaging | photography geometric conditions. It is explanatory drawing of the geometric arrangement | positioning of a X-ray CT apparatus. It is explanatory drawing of the conventional shift mechanism.

Explanation of symbols

11 X-ray source 12 X-ray detector 13 Imaging system control unit 14 X-ray generation control unit 15 Image input unit 16 Image processing unit 17 Image storage unit 18 Diagnostic monitor 19 Operation unit 20 Network S Subject

Claims (4)

  The first distance between the X-ray source and the subject and the second distance between the subject and the sensor can be set to arbitrary distances, and these imaging geometric conditions can be changed according to the imaging target region. X-ray CT apparatus.   The X-ray CT apparatus according to claim 1, wherein a reconstruction function is selected according to the imaging geometric condition, and a reconstruction process is performed on an image obtained by the sensor.   The X-ray CT apparatus according to claim 2, wherein the reconstruction function is selected from the imaging geometric condition and the reconstruction function table.   The X-ray CT apparatus according to claim 1, wherein the reconstruction function emphasizes a high-frequency component as the second distance increases.

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