JP3368676B2 - X-ray CT system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus. 2. Description of the Related Art In an X-ray CT apparatus, an X-ray
An image is reconstructed by back-projecting the line projection data. In order to obtain a CT image for one slice plane, 360 ° or 180 ° projection data for the slice plane is required. As a scanning method for collecting projection data from each angle direction in these necessary angle ranges, a rotation / rotation type in which both an X-ray tube and a fan-shaped X-ray detector rotate around the subject, X
A rotation / stationary type, in which the X-ray detector is fixed while the X-ray detector is fixed and the X-ray tube only rotates,
There is also known a stationary / stationary type in which the X-ray detector is arranged around the entire periphery and fixed, with only the X-ray focal point rotating around the subject and the X-ray detector being fixed. In these cases, in order to obtain projection data from each angular direction, it is necessary to rotate at least only the X-ray focal point around the subject. In these, at least only the X-ray focal point is continuously rotated, that is, only the X-ray focal point is continuously rotated in the stationary / stationary type, and the entire X-ray tube is continuously rotated in the rotation / stationary type. To rotate the X-ray focal point continuously, and in the rotation / rotation type, continuously rotate the entire X-ray tube to continuously rotate the X-ray focal point and continuously rotate the X-ray detector to obtain an image of one slice plane. If projection data in an angle range necessary for reconstruction is sequentially obtained in a time series, it is possible to obtain a large number of images for displaying a moving image style that changes in a time series with respect to one slice plane. Thereby, for example, a moving image display showing the movement of the contrast agent injected into the blood vessel can be performed. In this case, conventionally, one image is re-produced by sequentially back-projecting projection data in an angle range necessary for reconstructing one image into a back-projection memory for each angle. After the configuration, this memory is initialized to prepare for the next back projection. [0005] However, if the memory for back projection is initialized each time one image is reconstructed as in the prior art, it is necessary to obtain one CT image. There is a problem that processing time is long.
Therefore, even if the speed of continuous rotation for collecting projection data in each angular direction is increased to increase the time resolution, a large number of CT images cannot be reconstructed in accordance therewith, and real-time This makes it impossible to display a moving image. For this reason, when the continuous rotation speed is increased and the time resolution is increased, the projection data is temporarily stored in a memory, a magnetic disk, or the like, and read out later to reconstruct an image. However, even in that case, since the time required for one CT image is long, it takes a lot of time to obtain a large number of CT images. SUMMARY OF THE INVENTION In view of the above, the present invention provides a method for continuously collecting at least 360 ° of projection data in each angular direction by continuously rotating at least the X-ray focus.
It is an object of the present invention to provide an X-ray CT apparatus in which the time required for reconstructing each CT image is reduced and the time resolution of a large number of reconstructed CT images can be improved. And In order to achieve the above object, in an X-ray CT apparatus according to the present invention, at least the X-ray focal point is continuously rotated so that projection data in each angular direction can be extended over 360 °. Means for continuously collecting, memory means for back-projection, comprising a plurality of planes corresponding to respective ranges obtained by dividing the angle range of projection data necessary for reconstructing one image; The projection data sequentially collected for each area is added to the plane of the memory means corresponding to the divided area, the plane is initialized, and then the means for selectively performing back projection and the same pixels of each plane are added. And an adding means for forming one complete image. For example, assume that projection data of a 360 ° angle range is used to reconstruct one image, and the 360 ° angle range is divided into four equal parts. Then, the memory means makes four planes corresponding to the four equally divided angle ranges. Since the projection data for each angular direction is continuously collected over 360 °,
Data of 0 ° to 90 ° is stored in the first plane, and 90 ° to 18 °.
Data of 0 ゜ is selectively back-projected on the second plane, data of 180 ゜ -270 ゜ is on the third plane, data of 270 ゜ -360 ゜ is on the fourth plane, and so on. When the same pixel of the first to fourth planes is added at the end of one rotation (360 °), one image reconstructed by the back projection data of 0 ° to 360 ° is obtained. . Next 1/4 rotation (360-450 degree rotation)
Starts, first, the first plane is initialized, and then data of 0 ° to 90 ° is back-projected on the first plane. And 1 (1/4) rotation (45
At the end of rotation to 0 °), these first to fourth
Adding the same pixels of the plane, 90 ° -450
One image reconstructed by the back projection data of ゜ is obtained. Therefore, a large number of images having a time resolution of 1/4 rotation can be sequentially obtained. Further, since initialization of each plane at the time of rewriting only needs to be performed for the corresponding plane, it does not take much time for rewriting, and the other three planes can be used without rewriting. Therefore, the time required to obtain one image for each quarter turn is extremely short. Therefore, it is possible to reconstruct a large number of images in real time. Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows an embodiment in which the present invention is applied to a rotation / rotation type X-ray CT apparatus. In FIG. 1, X
The X-ray tube 1 and the X-ray detector 2 are arranged to face each other, and the subject 21 can be arranged therebetween.
The X-ray detector 2 is integrally rotated around the subject 21 by a rotation mechanism (not shown). The X-ray transmitted through the subject 21 is transmitted to the X-ray detector 2
And a signal representing the transmitted X-ray intensity is obtained. As a result, data indicating the X-ray absorption in the subject 21 is obtained. The X-ray detector 2 is formed in a fan shape, and a number of detection elements are arranged in an arc direction. Then, X
The data obtained from the X-ray detector 2 is profile data indicating X-ray absorption by X-rays emitted from the X-ray tube 1 in a fan shape. The profile data is sent to the projection data creation device 3 and converted into projection data for back projection. When the X-ray tube 1 and the X-ray detector 2 rotate from a reference position (an angle of 0 °), projection data for each angle is sequentially obtained, and this is designated by the selector 5 by the back projector 4 by the selector 5. Memory 6, 7, 8,
9 is back-projected. The four memories 6, 7, 8, and 9 are memories for back projection, each having one plane. That is, each of the four memories 6, 7, 8, 9 has a 256 × 256 address corresponding to the pixel if the reconstructed image has a pixel represented by a 256 × 256 matrix. I have. Here, when the X-ray tube 1 and the X-ray detector 2 rotate continuously, the angle range of 0 ° to 90 ° is set as the range A, and the angle range of 90 ° to 180 ° is set as shown in the figure. B,
180 ° to 270 ° angle range C, 270 ° to 3
An angle range of 60 ° is defined as a range D. As shown in FIG. 2, the projection data is changed into the ranges A, B, C, D in the first rotation and the ranges A, B, C, D,
... When the projection data of the range A is obtained, the selector 5 specifies the memory 6 so that the back projection of the projection data of the range A is performed on the memory 6. 7, 8, and 9 are designated, and the back projection of the projection data in the ranges B, C, and D is performed on them. When the first rotation is completed in this way, 90 ° projection data is back-projected into each of the memories 6, 7, 8, and 9, and a total of 36
Since the back projection of the projection data of 0 ° is made, if the corresponding pixels of these 6, 7, 8, and 9 are added by the adder 10, 0
This means that the first image # 1 based on the projection data of 360 to 360 degrees has been obtained (see FIG. 2). This is a process called back projection,
This is because projection data is added to pixels on the projection line. Next, the first rotation is completed, and the subsequent 1
In the fourth rotation, after the memory 6 is initialized, the projection data in the range A is back-projected into the memory 6. 1 from the beginning
When the (４) rotation is completed, the same pixels of the memories 6, 7, 8, and 9 are added by the adder 10 to obtain
As shown in FIG. 2, a second image # 2 based on 360 ° projection data of 90 ° to 450 ° is obtained. In this case, the initialization of the back-projection memory and the rewriting thereof need only be performed for the memory 6 corresponding to １ ／ of the entire memory of 360 °, and corresponds to the other ／. No operation is performed on the memories 7, 8 and 9 to be executed, and the contents stored up to that time may be simply read as it is and added by the adder 10. That is, since the back-projection operation only needs to correspond to 1/4 of the whole, the time from obtaining the first image # 1 to obtaining the second image # 2 is extremely short. When the next 1/4 rotation is completed, a third image # 3 is obtained, and when the next 1/4 rotation is completed, a fourth image is obtained. Then, an image based on 360 ° projection data is obtained one after another. These images are sequentially sent to the display device 12 via the image memory 11 and displayed.
Therefore, on the display screen, with the continuous rotation of the imaging system of the X-ray tube 1 and the X-ray detector 2, a large number of time-series images are sequentially displayed in real time every quarter of the rotation. Will be. In this example, the adder 10 includes memories 6, 7,
Although the same pixels 8 and 9 are added to each other, the memories 6, 7, 8 and 9 can be selectively added by the adder 10. For example, if the same pixel is added only in the memories (that is, two memories) in which the back projection of the preceding 1/4 turn and the preceding 1/4 turn from the present time are performed, FIG. As shown in the figure, the memories 6 and 7 are added when 回 転 rotation is completed from the beginning, and the memories 7 and 8 are added when the next 回 転 rotation (3/4 rotation from the beginning) is completed.
Is added, and the next 1/4 rotation (4
When the） rotation is completed, the memories 8 and 9 are added, and when the next 回 転 rotation (5/4 rotation from the beginning) is completed, the memories 9 and 6 are added. , Images # 1, # 2, # by back projection of projection data of 180 ° each
,... Can be sequentially obtained every quarter rotation. In this case, since only a memory capable of performing back projection of 180 ° is required, only two of the four memories 6, 7, 8, and 9 are required. If four memories 6, 7, 8, and 9 are provided, a CT image of 360 ° projection data as in the first example described above and a CT image of 180 ° projection data as in this example will be described. It is also possible to obtain both with the image. The back-projection memory is an independent memory 6 for each plane.
Although 7, 8, and 9 are used, it goes without saying that the area of each plane can be provided in one memory by managing the addresses. Further, if the number of planes of the back projection memory is further increased to, for example, eight planes, it is possible to perform back projection of デ ー タ rotation (45 °) of projection data on each plane. An image based on 360 ° or 180 ° projection data can be sequentially obtained every / 8 rotation. As described above, by increasing the number of planes of the back projection memory, the time resolution of a large number of images obtained in time series can be arbitrarily increased. In addition, even if such a configuration as to increase the time resolution is employed, by controlling the timing of addition by the adder 10 and the selection of the memory to be added, one rotation can be performed similarly to the normal CT image reconstruction method. It is also possible to lower the time resolution, for example, to obtain one image sequentially for each rotation or to obtain one image for each half rotation. Further, in the above embodiment, the present invention is applied to a rotation / rotation type X-ray CT apparatus. However, the present invention is applied to other rotation / stationary type or stationery / stationary type X-ray CT apparatuses. It goes without saying that the present invention can be applied. As described above, according to the X-ray CT apparatus of the present invention, at least the X-ray focal point is continuously rotated and the projection data in each angular direction is continuously converted over 360 °. When collecting on CT
It is possible to obtain a large number of time-series CT images at high speed by greatly shortening the image reconstruction time for each image, and to arbitrarily increase the time resolution of the multiple CT images. It is.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an X-ray CT apparatus according to one embodiment of the present invention. FIG. 2 is a diagram for explaining an operation example in the embodiment. FIG. 3 is a view for explaining another operation example in the embodiment. [Description of Signs] 1 X-ray tube 2 X-ray detector 3 Projection data creation device 4 Back projector 5 Selector 6, 7, 8, 9 Memory 10 Adder 11 Image memory 12 Display device 21 Subject

Claims (1)

(57) [Claim 1] A means for continuously collecting at least 360 ° projection data in each angular direction by continuously rotating at least the X-ray focal point, and reconstructing one image. A plurality of planes corresponding to each range obtained by dividing the angle range of the projection data necessary for the back projection memory means, and the projection data sequentially collected for each divided range correspond to the divided range. Means for selectively back-projecting the plane of the memory means after initializing the plane, and adding means for adding the same pixels of each plane to form a complete image. An X-ray CT apparatus characterized by the above-mentioned.