JPH1043177A - X-ray CT system - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To improve the S / N ratio of data in the direction where X-ray absorption is large without increasing the X-ray output and improve the image quality of a reconstructed image. SOLUTION: Data of one view is read (51),
After offset processing (52) and Log conversion (53),
The low input data is marked (54), the marked data is low-pass filtered (55), and the calibration (56), the convolution (57),
The back projection (58) and the image reconstruction processing are advanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an X-ray CT apparatus for reconstructing an image by processing data obtained by scanning a subject with an X-ray beam.

[0002]

2. Description of the Related Art In order to reconstruct an image using an X-ray CT apparatus, projection data from a 360 ° (or 180 °) direction around a subject is required. However, in an object having a flat shape such as a human shoulder or a subject having two bones having a large X-ray absorption such as a pelvis, the direction of the X-ray transmission path of the flat object is long or two. The X-ray absorption increases in the direction in which the bones overlap, and the input from the X-ray detector decreases. Therefore, the S / N ratio of the projection data in this direction deteriorates, and when this is reconstructed together with other data having a good S / N ratio, an image having artifacts as shown in FIG. 4 is obtained. That is,
FIG. 4 shows a tomographic image 71 of the shoulder, but an artifact 72 that is streaked in a direction in which the X-ray transmission path length is long (horizontal direction in the figure) occurs.

In order to prevent this, it is conceivable to increase the X-ray output in all directions so that a large output can be obtained from the X-ray detector even in the direction where the X-ray absorption is large. It is also conceivable to change the X-ray output according to the X-ray absorption such that the X-ray output is increased only in the direction in which the X-ray absorption is large.

[0004]

However, increasing the X-ray output in all directions causes overheating of the X-ray tube and requires a cooling waiting time.
Above all, there is the problem that the exposure dose to patients and the like increases.
Further, in order to increase the X-ray output only in the direction in which the X-ray absorption is large, it is necessary to know in advance which direction has the large X-ray absorption, and the labor and time required for that purpose become considerable. The line generation system also needs to be changed to a variable output type, which is not easy.

[0005] In view of the above, the present invention does not cause overheating of the X-ray tube or increase of the exposure dose to the patient or the like, does not require labor and time, has a simple structure, and has a large X-ray absorption direction. X-ray CT improved so as to eliminate the artifact of the reconstructed image by increasing the S / N ratio of the data obtained in
It is intended to provide a device.

[0006]

In order to achieve the above object, in an X-ray CT apparatus according to the present invention, a means for scanning a subject with an X-ray beam, and a low-input data of data obtained by the scanning are provided. It is characterized in that there are provided means for applying a low-pass filter to the level data, and means for performing image reconstruction processing using the data.

In the direction in which the thickness of the subject is large or the direction in which the bones overlap, X-rays are absorbed more than in other directions.
Only low input level data can be obtained. This low input level data has a poor S / N ratio, and if such data is mixed, artifacts occur in the reconstructed image.
Therefore, the S / N ratio is improved by applying a low-pass filter to the constant input level data. As described above, since the S / N ratio of the constant input level data increases, an excellent image free of artifacts can be obtained even if the image reconstruction processing is performed using data including the data.

[0008]

Next, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a process performed by the present invention, and FIG. 2 shows an entire X-ray CT apparatus for performing the process. For convenience of explanation, the explanation will be given with reference to FIG. 2. The X-ray CT apparatus includes a gantry 10, a console 20, and a display device 30.
It is composed of The gantry 10 has an X-ray tube 1
1, an X-ray detector 12, and a data collection device 13, which are fixed to a rotating frame 14, and the rotating frame 14 is rotated by a rotating device 15 including a motor, gears, and the like. The tube 11 and the X-ray detector 12 rotate while maintaining the facing state.

An object 40 is inserted into the space between the X-ray tube 11 and the X-ray detector 12, and the X-ray generated from the X-ray tube 11 is
The ray beam passes through the subject 40 and enters the X-ray detector 12. The X-ray detector 12 has detectors for each channel arranged in a fan shape as shown in the figure. The output of each channel is sent to the data collection device 13 to obtain view data as shown in FIG.

[0010] By rotating the rotating frame 14, view data in each direction around the subject 40 is sequentially obtained. The view data is sent to the console 20 and stored in the memory 22. The console 20 includes, in addition to the memory 22, a CPU 21, a high-speed operation device 23, a display circuit 24
Etc. are provided.

When the view data starts to be stored in the memory 22 in this manner, the memory 22 is controlled under the control of the CPU 21.
Is read from the data (step 5 in FIG. 1).
1) Normal offset processing (subtraction processing) and Log conversion are performed in the high-speed arithmetic device 23 (steps 52 and 53 in FIG. 1). Thereafter, the low input data in the data of one view is marked, and the marked data is subjected to low-pass filtering (steps 54 and 5 in FIG. 1).
5). That is, the view data in the horizontal direction in FIG.
Since the line transmission path length is in the long direction, the input becomes small as shown in FIG. The low input portion 61 is marked, and the data in this portion 61 is low-pass filtered.

Since the low-pass filter is applied to the low-input data, the S / N ratio is improved. Thereafter, the low-pass filtered data and other high-input data (to which the low-pass filter is not applied) are subjected to the calibration (detector sensitivity correction) of steps 56, 57 and 58 in FIG. After the convolution and the back projection, the image is reconstructed.

Each view data is sequentially read from the memory 22 and the above processing is performed in the high-speed arithmetic unit 23. When an image is reconstructed, the image signal is transmitted to the display device 30 by the display circuit 24. The reconstructed image is displayed.

As described above, the low-input data having a poor S / N ratio is subjected to a low-pass filter so as to increase the S / N ratio. Artifacts (72 in FIG. 4) that are caused by the presence of a mixture of. Moreover, since the X-ray output is not increased to increase the S / N ratio of the data, it is possible to avoid an increase in the load on the X-ray tube 11 or the like and an increase in the exposure dose of the subject (patient) 40. be able to. In addition, since the X-ray output is not changed during the scan, but is based on software-based data processing, the configuration is simple.

In the above description, the object 4 by the X-ray beam
0 scan method is a so-called rotation-rotation type, but the X-ray C
Of course, it can be applied to the T device. In the above description, the low-pass filter is applied after the Log conversion (step 53). However, the step 55 for applying the low-pass filter is performed after the calibration (step 56) or the convolution (step 57). It may be next to Further, in the above description, the processing of each step in FIG. 1 is described as being performed by software, but it is of course possible to perform processing by dedicated hardware. In addition, various changes can be made without departing from the spirit of the present invention.

[0016]

As described above, since the S / N ratio of data in the direction where X-rays are absorbed is improved without increasing the X-ray output, artifacts can be avoided while avoiding the problem of patient exposure. A reconstructed image with excellent image quality can be obtained. Also, since this is realized by data processing, the configuration is simple.

[Brief description of the drawings]

FIG. 1 is a flowchart for explaining an embodiment of the present invention.

FIG. 2 is an overall schematic diagram of the embodiment.

FIG. 3 is a graph showing view data.

FIG. 4 is a diagram showing a reconstructed image in which a conventional artifact has occurred.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Gantry 11 X-ray tube 12 X-ray detector 13 Data acquisition device 14 Rotating frame 15 Rotation device 20 Console 21 CPU 22 Memory 23 High-speed operation device 24 Display circuit 30 Display device 40 Subject 51 1 View data reading step 52 Offset processing step 53 Log conversion step 54 Marking low input data 55 Applying low-pass filter to mark data 56 Calibration step 57 Convolution step 58 Back projection step 61 Low input data part 71 Shoulder tomographic image 72 Artifact

Claims (1)

[Claims] A means for scanning an object with an X-ray beam; a means for applying a low-pass filter to low input level data among data obtained by the scanning;
Means for performing an image reconstruction process using the data.