JP3770669B2 - X-ray CT system - Google Patents

Description

【数２】

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an X-ray CT apparatus that detects X-rays of a rotating X-ray source and obtains a tomographic image corresponding to a rotating tomographic plane, and more particularly to an improvement in filter processing for correcting image blur.
[0002]
[Prior art]
A conventional X-ray CT apparatus has a scanner unit and an image reconstruction unit. In the scanner unit, an X-ray source and an X-ray detector are arranged to face each other, and can be rotated while maintaining the positional relationship therebetween. A subject lying on a bed is inserted between the X-ray source and the X-ray detector into the scanner unit. The X-ray detector obtains a detection signal for an X-ray including a cross section desired by the subject and air around the subject and the subject. The image reconstruction unit obtains a tomographic image including the subject and air around the subject from the detection signal, and has an m × n (m, n is an integer of 3 or more) smaller than the matrix size of the tomographic image. A filter process is performed in which pixel data of a small matrix is read out and a calculation process in which each pixel data is multiplied by a desired weighting coefficient is performed by updating the address of the tomographic image. This type of filter processing corrects image blur caused by various factors such as X-ray irradiation unevenness.
[0003]
When displaying the tomographic image on the screen, a circular area called an effective field of view (hereinafter referred to as “FOV”) is set. This FOV refers to the subject from the image center point corresponding to the central axis of the rotational movement in the region where the tomographic plane of the air surrounding the subject and the subject is displayed when displayed on the screen as a tomographic image. The tomographic image region is a circular region in which the radius is set for each part of the subject according to the region that is most easily diagnosed. The pixel value of the air portion is uniformly set, for example, “−1000”. Then, the same value as that of the air portion is uniformly set for the pixel value of the portion outside the FOV (hereinafter referred to as “background region”) in the display region of the screen.
[0004]
In the filtering process, the address of the small matrix is updated at each pixel point of the entire tomographic image.
[0005]
By the way, when displaying the tomographic image, there are cases where the pixel value of the diagnosis part of the subject, for example, the bronchial part of the lung field, is slightly below “−1000”, and the gradation is adjusted to this part. Since the background area is set to the same pixel value, the two cannot be distinguished from each other. Therefore, in order to distinguish these, the pixel value of the background area is uniformly set to, for example, “−2000”.
[0006]
[Problems to be solved by the invention]
However, in the filtering process, the address of the small matrix is updated at each pixel point of the entire tomographic image. Therefore, when the filtering process is executed after distinguishing the background area, a fake image is formed near the FOV and the background area. However, there is a problem that blurs and blurs may appear in an image despite the fact that is displayed.
[0007]
The present invention has been made to solve the above-described problems, and its purpose is to prevent the appearance of fake images, blurs and blurs in the vicinity of the FOV and the background area on the tomographic image. An object of the present invention is to provide an X-ray CT apparatus capable of displaying a tomographic image without blur or blur.
[0008]
[Means for Solving the Problems]
The purpose is to detect an X-ray source that emits a fan-like X-ray while rotating, and to detect the X-ray from the X-ray source that is arranged opposite to this X-ray source and outputs a detection signal. And an image reconstruction calculating means for reconstructing a tomographic image from the detection signal, the image reconstruction calculating means being m × n (m and n are integers of 3 or more) smaller than the matrix size of the tomographic image. In an X-ray CT apparatus equipped with means for reading out pixel data of a small matrix , when all the pixel data values of the image data at the four corners of the small matrix are larger than a predetermined threshold, a weight is selected for each pixel In this case, the calculation pixel value is output, and in other cases, the value of the image data at the center of the small matrix is output as the calculation pixel value to reconstruct the image.
Further, the X-ray CT apparatus includes selection condition input means capable of setting and inputting the threshold value, and the weight for each pixel is selected from a plurality of pre-stored filter mask data and applied. Is achieved.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the X-ray CT apparatus of the present invention will be described with reference to the drawings.
1 is a block diagram showing a configuration example of an X-ray CT apparatus of the present invention, FIG. 2 is a block diagram showing a configuration example of an image reconstruction calculation unit in FIG. 1, and FIG. 3 is a filter 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 excluding the boundary region between the background region and the FOV (hereinafter referred to as “normal region”), FIG. It is a figure which shows the principle of the filter process in one pixel, and shows the example which uses a background area | region and FOV for a boundary area | region (henceforth only a "boundary area | region").
[0010]
As shown in FIG. 1, the X-ray CT apparatus of the present invention includes a scanner unit 1, an image reconstruction unit 2, and an image display unit 3. While the X-ray source emits X-rays along the tomographic plane that virtually cuts the subject while the X-ray source and X-ray detector are opposed to each other, the scanner unit 1 The X-ray can be rotated while detecting X-rays from the source, and an X-ray detection signal corresponding to the tomographic plane is obtained for both the subject and the surrounding air until a tomographic image can be created.
[0011]
The image reconstruction unit 2 reconstructs a tomographic image from the detection signal and performs a filter process to display the tomographic image sharply. The image display unit 3 includes an image display monitor, and displays the filtered tomographic image on the screen of the monitor.
[0012]
As shown in FIG. 2, the image reconstruction unit 2 includes an image reconstruction calculation unit 21, a first filter mask data storage unit 22, a second filter mask data storage unit 23, a filter mask data selection unit 24, and selection conditions. An input unit 25, a pixel data detection control unit 26, and a filter calculation unit 27 are provided. The image reconstruction calculation unit 21 reconstructs a tomographic image, sets a pixel value “−1000” in a circular area corresponding to the FOV determined by the subject and the size of the part of the subject, and the like in the background area outside the FOV. Pixel values “−2000” are set uniformly. As shown in FIG. 4, the first filter mask data 22 is, for example, a 5 × 5 image matrix for a normal region multiplied by each pixel value of the read 5 × 5 image matrix (shown on the right side in the figure). Remember. As shown in FIG. 5, the second filter mask data 23 is, for example, a 5 × 5 image matrix for a boundary region to be multiplied for each pixel value of the read 5 × 5 image matrix (shown on the right side in the figure). Remember. The filter mask data selection unit 24 switches between the first filter mask data 22 in which the image matrix for the normal region is stored and the second filter mask data 23 in which the image matrix for the boundary region is stored. 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 a normal region or a boundary region with a certain threshold value as a boundary. The pixel data detection control unit 26 reads out pixel data at the four corners of the image matrix, determines whether all of the pixel data is equal to or less than the threshold value, and all the pixel data are When the threshold value is exceeded , the filter mask data selection unit 24 is controlled so as to calculate as a normal region, and when at least one pixel data is equal to or less than the threshold value as a boundary region. The filter calculation unit 27 performs a filter calculation with the filter mask data selected based on the control of the pixel data detection control unit 26. Here, the threshold value is “−2000”.
[0013]
Next, the principle of the filter processing of the present invention will be described with reference to FIGS. For example, a 512 × 512 matrix CT image (pixel value (in many cases, associated with a CT value): +4000 to −1999, pixel value of background region: −2000) m set in advance The center of the xn calculation matrix is applied to the coordinate point (i, j) on the tomographic image, an m × n image matrix is defined at the center, and pixel data for this image matrix is read out. Here, since an example of a 5 × 5 image matrix is given, when the central pixel data is Di, j, 25 pixel data of Di−2, j−2 to Di + 2, j + 2 are read out. . In the example shown in FIG. 4, the filter mask data is a normal region (pixel value: +4000 to −1999), and the pixel data of the center point and the other 24 points are appropriately weighted to calculate the calculated pixel value D′ i, This is for obtaining j. In the example shown in FIG. 5, the filter mask data is a boundary region (pixel value: +4000 to −2000), and the pixel data at the center point is used as it is as the calculated pixel value D′ i, j. The filter calculation unit 27 selects the filter mask data (W0, 0 to W4, 4) according to the normal region or the boundary region, and multiplies the selected filter mask data by the small matrix. That is, the calculated pixel value D′ i, j in the normal region is as shown in (Equation 1), and the calculated pixel value D′ i, j in the boundary region is as shown in (Equation 2).
[0014]
If such filter mask data is selected, if the data at the four corners of the calculated pixel value D′ i, j exceed the threshold value, the filtering process is performed as a normal area, and if the data is below the threshold value, the boundary area Since the pixel value of the center point is output as it is without performing the filtering process, it is confirmed that the circular frame-like fake image, blur, or blur that appears between the FOV and the background area does not appear on the tomographic image. The inventor of this application verified.
[0015]
Next, the operation of the X-ray CT apparatus filter processing of the present invention will be described with reference to FIG. Here, in order to simplify the explanation, attention is paid to processing of only one pixel, and the small matrix is assumed to be 5 × 5.
Step. 31
Each pixel data of a 5 × 5 small matrix centering on a certain coordinate point (i, j) on the tomographic image of the 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, and if the condition is satisfied, step 37 is executed (Di-2, j-2 ≦ −2000?).
Step. 33
It is determined whether or not the condition that the pixel data in the upper right corner of the small matrix is −2000 or less is satisfied. If the condition is not satisfied, step 34 is executed, and 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 in the lower left corner of the small matrix is −2000 or less is satisfied. If the condition is not satisfied, step 35 is executed, and 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 in the lower right corner of the small matrix is −2000 or less is satisfied. If the condition is not satisfied, step 36 is executed, and if the condition is satisfied, step 37 is executed (Di + 2, j + 2 ≦ -2000?)
Step. 36
Filter mask data for normal images is selected, and step 38 is executed (selection of data for normal regions).
Step. 37
The filter mask data for the boundary image is selected, and step 38 is executed (selection of boundary area data).
Step. 38
Filter processing is performed based on the filter mask data selected in step 36 or step 37 (filter processing execution).
[0016]
In the present embodiment described above, if at least one of the four corners of the small matrix corresponds to the pixel value serving as the threshold value, the filtering process is not substantially performed. Images, blurs and blurring can be prevented.
[0017]
In addition, each example such as the number of small matrices and the numerical value of the threshold value assists understanding of the present invention, and needless to say, it is not limited thereto.
Further, although an example of a small matrix has been described with an odd number, even in the case of an even number, an arbitrary pixel among four pixels surrounding the center of the image may be set as the central pixel data.
In addition, the filter mask data is selected based on the threshold value. However, since the above-described fake image, blur, blur, and the like are caused by a large change amount of pixel data, the change occurs instead of the threshold value. An amount may be set.
[0018]
【The invention's effect】
The present invention has the configuration as described above, and these configurations operate as described above. Therefore, it is possible to prevent the appearance of fake images, blurs, and blurs in the vicinity of the FOV on the tomographic image and the background region. It is possible to provide an X-ray CT apparatus that displays a false image, a tomographic image with no blur or blur.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration example of an X-ray CT apparatus of the present invention.
FIG. 2 is a block diagram showing a configuration example of an image reconstruction calculation unit in FIG.
FIG. 3 is a flowchart showing an example of an operation of filter processing of the image reconstruction unit in FIG. 2;
FIG. 4 is a diagram showing an example of the principle of normal area filtering 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

[Expression 2]

Claims (2)

An X-ray source that emits fan-like X-rays while rotating, an X-ray detector that is arranged opposite to the X-ray source and detects X-rays from the X-ray source while rotating, and outputs a detection signal; Image reconstruction calculation means for reconstructing a tomographic image from a detection signal, and the image reconstruction calculation means is a small matrix pixel of m × n (m and n are integers of 3 or more) smaller than the matrix size of the tomographic image. In the X-ray CT apparatus provided with a means for reading data, the image reconstruction calculation means is configured to detect each pixel when all pixel data values of the image data at the four corners of the small matrix are larger than a predetermined threshold value. An X-ray CT apparatus characterized in that a weight is selected and a calculated pixel value is output, and in other cases, the image data value at the center of the small matrix is output as a calculated pixel value to reconstruct an image. 2. The selection condition input means capable of setting and inputting the threshold value, wherein the weight for each pixel is selected from a plurality of pre-stored filter mask data and applied. X-ray CT system.

Images