JPH1099321A - X-ray ct device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent pseudo images, blurs and blots from appearing near a valid view field and a background area on tomographic images by setting a threshold value for selecting a weighting coefficient by pixel data extracted from the pixel data of the four corners of a small matrix and selecting the different weighting coefficient by the threshold value. SOLUTION: A filter mask data selection part 24 switches first filter mask data 22 storing an image matrix for a normal area and second filter mask data 23 storing the image matrix for a boundary area. A selection condition input part 25 sets and inputs the threshold value for deciding to which of the normal area and the boundary area the filter mask data selection part 24 is to be controlled. Then, a pixel data detection control part 26 reads the pixel data of the four corners of the image matrix and controls the filter mask data selection part 24 so as to perform calculation as the normal area when the pixel data are less than the threshold value and as the boundary area when they are more than the threshold value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus for detecting X-rays from a rotating X-ray source and obtaining a tomographic image corresponding to the rotating tomographic plane. To improve the filtering process.

[0002]

2. Description of the Related Art A conventional X-ray CT apparatus has a scanner unit and an image reconstruction unit. The scanner section has an X-ray source and an X-ray detector arranged opposite to each other, and is rotatable while maintaining their positional relationship. A subject lying on a bed is inserted between the X-ray source and the X-ray detector into this scanner unit. The X-ray detector obtains a detection signal for X-rays including a desired cross section of the subject and the subject and air around the subject. The image reconstruction unit obtains a tomographic image including the subject and the air around the subject from the detection signal, and obtains m × n (m, n) smaller than the matrix size of the tomographic image.
(Integer of 3 or more) is read out, and a filter process of performing a calculation process of multiplying each pixel data by a desired weighting coefficient by updating the address of the tomographic image is performed. This type of filter processing corrects image blur caused by various factors such as X-ray irradiation unevenness.

In displaying the tomographic image on a screen, a circular area called an effective field of view (hereinafter referred to as "FOV") is set. This FOV is defined as the image of the subject from the image center point corresponding to the central axis of the rotational motion in the region where the tomographic plane of the subject and the air around the subject is displayed when displayed on the screen in the tomographic image. The tomographic image area is a circular area in which the radius is set for each part of the subject in accordance with the area where diagnosis is most easily performed. The pixel value of this air portion was uniformly set to, for example, “−1000”. Then, a portion deviating from the FOV in the display area of the screen (hereinafter referred to as “background area”)
), The same value as that of the air portion was uniformly set.

[0004] In the filter processing, the address of the small matrix is updated at each pixel point of the entire tomographic image.

In displaying the tomographic image, there are some diagnostic parts of the subject, for example, a pixel value of a bronchial part of a lung field, etc., slightly lower than "-1000". Since the air portion and the background region are set to the same pixel value, the two cannot be distinguished. Therefore, in order to distinguish them, the pixel value of the background area is uniformly set to, for example, “−2000”.

[0006]

However, in the filtering process, since the address update of the small matrix is performed at each pixel point of the entire tomographic image, the filtering process is performed after distinguishing the background region. , The false image is displayed near the FOV and the background area, or bleeding or blurring may appear in the image even though the filtering process is performed.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to prevent a false image, blur, or blur from appearing on a tomographic image and in the vicinity of a background area. An object of the present invention is to provide an X-ray CT apparatus capable of displaying a tomographic image without a false image, blur, or blur.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide an X-ray source which emits fan-shaped X-rays by performing a rotary motion, and which is arranged opposite to the X-ray source to detect X-rays from the X-ray source and to generate a detection signal. An X-ray detector for outputting, and an image reconstruction calculator for reconstructing a tomographic image corresponding to the rotated tomographic plane from the detection signal, wherein the image reconstruction calculator is smaller than a matrix size of the tomographic image. m × n (m and n are integers of 3 or more)
In the X-ray CT apparatus provided with filter processing means for reading out the pixel data of the small matrix and multiplying each pixel data by a desired weighting coefficient by updating the address of the tomographic image, the image reconstruction Means for setting at least one pixel data among the pixel data at the four corners of the small matrix as extracted pixel data, and setting a threshold value for selecting the weighting coefficient based on the extracted pixel data; This is achieved by providing means for selecting the different weighting factors depending on the threshold value.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the X-ray CT apparatus according to the present invention will be described with reference to the drawings. 1 is a block diagram illustrating a configuration example of an X-ray CT apparatus according to the present invention, FIG. 2 is a block diagram illustrating a configuration example of an image reconstruction operation unit in FIG. 1, and FIG. 3 is a filtering process of the image reconstruction unit in FIG. 4 is a flowchart showing an example of the operation of FIG. 4. FIG. 4 shows the principle of filter processing in one pixel, and shows an example of use in a region other than the boundary region between the background region and the FOV (hereinafter referred to as a “normal region”). FIG. 4 is a diagram illustrating the principle of filter processing in one pixel and illustrating an example in which a background region and an FOV are used as a boundary region (hereinafter, simply referred to as a “boundary region”).

As shown in FIG. 1, the X-ray CT apparatus according to the present invention has a scanner unit 1, an image reconstruction unit 2, and an image display unit 3. The scanner unit 1 is arranged along a tomographic plane that virtually cuts the subject while the X-ray source and the X-ray detector are arranged to face each other.
The X-ray source can rotate while emitting X-rays, and the X-ray detector can rotate while detecting X-rays from the X-ray source, and both the subject and the surrounding air can detect X-rays corresponding to the tomographic plane. The signal is obtained until a tomographic image can be created.

The image reconstructing section 2 reconstructs a tomographic image from the detection signal and performs a filtering process to display the tomographic image sharply. The image display unit 3 includes an image display monitor, and displays a filtered tomographic image on a screen of the monitor.

As shown in FIG. 2, the image reconstruction unit 2 includes an image reconstruction operation unit 21, a first filter mask data storage unit 22, a second filter mask data storage unit 23, and a filter mask data selection unit 24. And a selection condition input unit 25, a pixel data detection control unit 26, and a filter operation unit 27. The image reconstruction calculation unit 21 reconstructs a tomographic image,
A pixel value “−1000” is uniformly set in a circular region corresponding to the FOV determined by the size of the subject and the region of the subject, and a pixel value “−2000” is uniformly set in a background region outside the FOV. As shown in FIG. 4, the first filter mask data 22 is, for example, a 5 × 5 for a normal area multiplied by each pixel value of the read 5 × 5 image matrix.
(Shown on the right side in the figure) is stored. Second
As shown in FIG. 5, the filter mask data 23 stores, for example, a 5 × 5 image matrix (shown on the right side in the figure) for a boundary area to be multiplied by each pixel value of the read 5 × 5 image matrix. I do. The filter mask data selection unit 24 switches between first filter mask data 22 storing an image matrix for a normal area and second filter mask data 23 storing an image matrix for a boundary area. The selection condition input unit 25 sets and inputs a threshold value that determines whether the filter mask data selection unit 24 is controlled to be in a normal region or a boundary region at a certain threshold. The pixel data detection control unit 26 reads pixel data at four corners of the image matrix, determines whether at least one of the pixel data is equal to or smaller than the threshold, and determines whether the pixel data is smaller than the threshold. The filter mask data selection unit 24 is controlled so as to calculate as a normal area when is smaller than the threshold value and as a boundary area when less than the threshold value. The filter operation unit 27 performs a filter operation on the selected filter mask data based on the control of the pixel data detection control unit 26. Here, the threshold is “−2000”.

Next, the principle of the filter processing of the present invention will be described with reference to FIGS. For example, 512 × 51
M × n operation set in advance for a 2-matrix CT image (pixel values (corresponding to CT values in many cases): +4000 to −1999, pixel values of the background region: −2000) The center of the matrix is assigned to the coordinate point (i, j) on the tomographic image, an m × n image matrix is defined at the center, and pixel data of the image matrix is read. Here, since an example of a 5 × 5 image matrix is given, if the center pixel data is Di, j, 25 pixel data of Di−2, j−2 to Di + 2, j + 2 will be read. . In the example shown in FIG. 4, the filter mask data is in the normal region (pixel value: +4000 to-
In 1999), the pixel data of the central point and the other 24 points are appropriately weighted to calculate the calculated pixel values D′ i, j.
It is for seeking. In the example shown in FIG. 5, the filter mask data is in the boundary area (pixel value: +4000 to -20).
00), the pixel data at the center point is used as it is as a calculated pixel value D′ i, j. The filter operation unit 27 calculates the filter mask data (W0,0 to
W4, 4) is selected depending on whether the area is a normal area or a boundary area, and the selected filter mask data is multiplied by the small matrix. That is, the calculated pixel value D'i, j of the normal area is (Equation 1)
, And the calculated pixel value D′ i, j in the boundary region is as shown in (Equation 2).

If such filter mask data is selected, if the data at the four corners of the calculated pixel value D'i, j exceeds the threshold value, the filter processing is performed as a normal area,
If the threshold value is less than the threshold value, the pixel value at the center point is output as it is without performing the filtering process as the boundary region, so that a circular frame-like false image, blur, or blur appearing between the FOV and the background region is obtained. The present inventor has verified that does not appear on the tomographic image.

Next, the operation of the X-ray CT apparatus filter processing of the present invention will be described with reference to FIG. Here, for the sake of simplicity, attention is focused on processing of only one pixel, and the small matrix is 5 × 5. Step. 31 Each pixel data of a 5 × 5 small matrix centered on a coordinate point (i, j) on a tomographic image of a 512 × 512 matrix is read (data read). Step. 32 It is determined whether or not the condition that the pixel data at the upper left corner of the small matrix is -2000 or less is satisfied. If the condition is not satisfied, step 33 is executed.
(Di-2, j-2 ≦ −2000?). Step. 33. It is determined whether or not the condition that the pixel data at the upper right corner of the small matrix is -2000 or less is satisfied. If the condition is not satisfied, step 34 is executed. If the condition is satisfied, step 37 is executed.
(Di−2, j + 2 ≦ −2000?). Step. 34 It is determined whether or not the condition that the pixel data at the lower left corner of the small matrix is -2000 or less is satisfied. If the condition is not satisfied, step 35 is executed. If the condition is satisfied, step 37 is executed.
(Di + 2, j−2 ≦ −2000?). Step. 35 It is determined whether or not the condition that the pixel data at the lower right corner of the small matrix is -2000 or less is satisfied. If the condition is not satisfied, step 36 is executed. If the condition is satisfied, step 37 is executed.
(Di + 2, j + 2 ≦ −2000?). Step. 36 Selects filter mask data for a normal image and executes step 38 (selection of data for a normal area). Step. 37. The filter mask data for the boundary image is selected, and Step 38 is executed (selection of the data for the boundary area). Step. 38 A filter process is performed based on the filter mask data selected in step 36 or 37 (filter process execution).

In the present embodiment described above, if the pixel value serving as the threshold value corresponds to at least one of the four corners of the small matrix, the filtering process is not substantially performed, so that the False image, blur, and bleeding appearing on the image can be prevented.

Further, each example such as the number of small matrices and the numerical value of the threshold value assists the understanding of the present invention, and it goes without saying that the present invention is not limited to these. Further, although the example of the small matrix has been described as an odd number, even in the case of an even number, any of the four pixels surrounding the image center may be set as the central pixel data. Also, the description has been given of the case where the filter mask data is selected according to the threshold value. However, since the above-described false image, bleeding, and blurring are caused by a large amount of change in pixel data, a change is performed instead of the threshold value. The amount may be set.

[0018]

The present invention has the above-described structures, and since these structures operate as described above, false images, blurs, and bleeding appear near the FOV on the tomographic image and the background area. And an X-ray CT apparatus which displays a tomographic image without such a false image, blur, or bleeding can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of an X-ray CT apparatus according to the present invention.

FIG. 2 is a block diagram illustrating a configuration example of an image reconstruction calculation unit in FIG. 1;

FIG. 3 is a flowchart illustrating an example of an operation of a filter process of the image reconstruction unit in FIG. 2;

FIG. 4 is a diagram showing an example of the principle of filter processing of a normal region in one pixel.

FIG. 5 is a diagram showing an example of the principle of filter processing of a boundary region in one pixel.

[Explanation of symbols]

 22 first filter mask data storage unit 23 second filter mask data storage unit 24 filter mask data selection unit 25 selection condition input unit 26 pixel data detection unit 27 filter calculation unit

(Equation 1)

(Equation 2)

Claims (1)

[Claims] An X-ray source that performs a rotary motion and emits fan-shaped X-rays, an X-ray detector that is arranged opposite to the X-ray source, detects X-rays from the X-ray source, and outputs a detection signal, An image reconstruction calculator for reconstructing a tomographic image corresponding to the rotated tomographic plane from the detection signal, wherein the image reconstruction calculator has a size of m × n smaller than the matrix size of the tomographic image;
A filter processing means for reading out pixel data of a small matrix (m and n are integers of 3 or more) and performing a calculation process of multiplying each pixel data by a desired weighting coefficient by updating an address of the tomographic image; In the X-ray CT apparatus, the image reconstruction calculator should use at least one of the pixel data at the four corners of the small matrix as extracted pixel data, and select the weighting coefficient based on the extracted pixel data. An X-ray CT apparatus comprising: means for setting a threshold value; and means for selecting the different weighting coefficients depending on the threshold value.