A method for removing filter line patterns from X-ray images

Abstract

The present invention provides a method for removing filter line grid patterns from X-ray images. First, the difference between the frequency spectrum of grid pattern direction data and non-grid pattern direction data is compared, the two direction data are subtracted in the frequency domain to separate the grid patterns, the frequency domain inverse transformation is used to reconstruct the grid pattern image, and finally the grid pattern image is subtracted from the grid pattern image to obtain the non-grid image. The method for removing filter line grid patterns from X-ray images provided by the present invention can effectively remove the grid patterns in the X-ray image after the filter line grid is filtered by using the image processing method while ensuring that the detector is normally loaded, and obtain a relatively ideal grid removal effect; in addition, in the process of grid removal, there is no need to find the exact position of the peak of the grid pattern in the frequency domain, and there is no need to design a corresponding complex filter, which greatly saves costs and improves work efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of image processing, and in particular to a method for removing filter line grid patterns from an X-ray image. Background Art

[0002] For X-ray flat-panel detectors, Compton scattering effect will occur when X-rays pass through thick targets. At this time, the scattered rays will make the details of the collected X-ray images blurred and the contrast reduced. Therefore, in digital X-ray imaging equipment, a grid is usually used to filter scattered rays, thereby improving the detail contrast of the X-ray image and thus improving the image quality. During the sampling process of the flat-panel detector of the DR system, if the grid frequency is twice higher than the sampling frequency of the flat-panel detector, the Nyquist sampling theorem cannot be satisfied, and the signal aliasing effect will inevitably occur, resulting in the generation of new striped artifacts in the image - grid patterns.

[0003] Currently, the main method to eliminate grating patterns is to convert the image into the frequency domain and use a one-dimensional band-stop or notch filter to suppress the peaks of the grating image in the frequency domain. This method can effectively remove some grating patterns, but it also has the following disadvantages:

[0004] 1. The effect is not ideal when processing complex images. The main reasons are: (1) For the case where the spectrum image fluctuates and there are multiple peaks connected, it is not realistic to design multiple filters for processing, and it is difficult to ensure that each peak of the spectrum image can be effectively suppressed; (2) Most filters use Gaussian filters, but the peak shape is uncertain, and the main peak is likely to be accompanied by a secondary peak. As a result, although the filter can suppress the main peak, it will also produce small peaks near the main peak, and the grating pattern cannot be removed completely.

[0005] 2. It is difficult to design the height and width of the filter and determine where to use it. The spectrum of each line of the image is somewhat different. If the same filter is used, it will be difficult to remove all the gratings. If a filter is designed for each line, the processing process will be too complicated. Summary of the invention

[0006] In view of the shortcomings of the prior art described above, the present invention provides a method for removing filter line grid patterns in X-ray images, firstly comparing the difference in the frequency spectrum of the grid pattern direction data and the non-grid pattern direction data, using the two-directional data to separate the grid patterns in the frequency domain by subtracting them, reconstructing the grid pattern image by inverse transformation in the frequency domain, and finally subtracting the grid pattern image from the grid-bearing image to obtain the non-grid pattern image. The method for removing filter line grid patterns in X-ray images provided by the present invention can effectively remove the grid patterns in the X-ray image after the filter line grid is filtered by using the image processing method while ensuring that the detector is normally loaded, and obtain a more ideal grid removal effect; in addition, there is no need to find the exact position of the peak of the grid pattern in the frequency domain during the grid removal process, and there is no need to design a corresponding complex filter, which greatly saves costs and improves work efficiency.

[0007] To achieve the above-mentioned and other related purposes, the present invention provides a method for removing filter line grid patterns in an X-ray image, comprising the following steps:

[0008] S1: Acquire X-ray images;

[0009] S2: Acquire grating information from the X-ray image;

[0010] S3: Processing the grating information to obtain a sub-band image of the grating information;

[0011] S4: Processing the sub-band image to obtain a grating image;

[0012] S5: Subtract the grating image from the X-ray image to obtain a grating-free X-ray image.

[0013] Optionally, step S1 further includes determining whether it is necessary to perform partition processing on the X-ray image according to the following steps:

[0014] If the row and column sizes of the X-ray image are equal, no partitioning is required;

[0015] If the row and column sizes of the X-ray image are not equal, the X-ray image is divided into a plurality of square areas of the same size with overlapping areas, and steps S1 to S5 are performed on the plurality of square areas respectively, and finally they are assembled into a complete image.

[0016] Optionally, step S2 includes:

[0017] S21: Obtaining a frequency domain image of the X-ray image by Fourier transform;

[0018] S22: Determine the direction of the grating according to the frequency domain image, and define the direction with the grating as a first direction, and the direction without the grating as a second direction.

[0019] Optionally, step S3 includes: performing wavelet multi-level decomposition on the X-ray image to extract a first direction sub-band image and a second direction sub-band image, wherein the first direction sub-band image contains the grating information.

[0020] Optionally, step S4 includes: subtracting the subband image in the second direction with a weight from the subband image in the first direction to obtain subband data, and performing wavelet reconstruction on the subband data to obtain a grating image.

[0021] Optionally, step S2 further includes recording the number, position and shape of peaks in the frequency domain image.

[0022] Optionally, in step S3, the level of the wavelet multi-level decomposition is determined according to the number of the peaks.

[0023] Optionally, in step S3, the position of the grating in the first direction sub-band image is determined according to the position of the peak.

[0024] Optionally, in step S3, a wavelet basis used in the wavelet multi-level decomposition is determined according to the shape of the wave peak.

[0025] The method for removing X-ray image filter line grid patterns provided by the present invention has at least the following technical effects:

[0026] The method for removing X-ray image filter line grid patterns provided by the present invention can effectively remove the grid patterns in the X-ray image after the filter line grid is filtered by using the image processing method while ensuring the normal image loading of the detector, thereby obtaining a relatively ideal grid removal effect; in addition, during the grid removal process, there is no need to find the precise position of the peak of the grid pattern in the frequency domain, and there is no need to design a corresponding complex filter, which greatly saves costs and improves work efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A flowchart for removing X-ray image grid lines provided by an embodiment is shown.

[0028] Figure 2a Shown is a frequency domain image with the orientation of the grating in an embodiment.

[0029] Figure 2b Shown is a frequency domain image of the direction without moiré in the embodiment. DETAILED DESCRIPTION

[0030] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0031] It should be noted that the illustrations provided in this embodiment only illustrate the basic concept of the present invention in a schematic manner. Although the illustrations only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation, the form, quantity, positional relationship and proportion of each component in actual implementation can be changed at will under the premise of realizing the technical solution of this party, and the component layout form may also be more complicated.

[0032] Example

[0033] This embodiment provides a method for removing filter line patterns of an X-ray image. Figure 1 As shown, the following steps are included:

[0034] S1: Acquire X-ray images;

[0035] First, an X-ray image is acquired. Since the row and column information of the image will be used for calculations later, it is first determined whether the X-ray image needs to be partitioned, which specifically includes the following steps: if the row and column sizes of the X-ray image are equal, there is no need to partition it, and the original X-ray image can be used for subsequent steps; if the row and column sizes of the X-ray image are not equal, the X-ray image is divided into multiple square areas of the same size with overlapping areas, and the multiple square areas are processed in subsequent steps respectively, and finally assembled into a complete image. In this embodiment, the X-ray image can be divided into four square areas of the same size with overlapping areas. In other optional embodiments, the specific number of square areas to be divided into is determined according to the row and column sizes of the X-ray image, as long as the multiple square areas can cover the X-ray image and have overlapping areas.

[0036] S2: Acquire grating information from the X-ray image;

[0037] Next, a frequency domain image of the X-ray image is obtained by Fourier transform, including a frequency domain image in the row direction and a frequency domain image in the column direction, so as to convert the image from the spatial domain to the frequency domain. Grating information, such as the direction of the grating, can be obtained from the frequency domain image, and the parameters used in processing the grating information in step S3 can also be obtained according to the number, position, and shape of the peaks in the frequency domain image.

[0038] The direction of the grating is determined based on the above frequency domain image. The frequency domain image in the direction with grating has fluctuating peaks, while the frequency domain image in the direction without grating has only one peak. The frequency domain images in the two directions are quite different, and the direction of the grating can be determined based on this. As an example, in the frequency domain image of the X-ray image of this embodiment, the frequency domain image in the row direction is as follows: Figure 2a As shown in Figure 1, there are multiple undulating peaks, while the frequency domain image in the column direction is as follows: Figure 2b There is only one peak shown, which means that the row direction has grating patterns, but the column direction does not have grating patterns. In this embodiment, the direction with grating patterns is defined as the first direction, and the direction without grating patterns is defined as the second direction.

[0039] Next, the number, position and shape of the peaks in the frequency domain image are obtained by analyzing the maximum and minimum values ​​of the region and other algorithms, and recorded. As an example, the peak position is high frequency, medium-high frequency, medium frequency, low frequency, etc. The position where the peak appears indicates that there is a grating at this frequency, and the peak at this location needs to be processed in step S3; the peak shape can be divided into single peak, double peak, multi-peak, etc., and the peak shape is recorded to facilitate the determination of the wavelet basis used in the wavelet decomposition in step S3; the number of peaks is recorded to facilitate the determination of the order of wavelet decomposition in step S3.

[0040] S3: Processing the grating information to obtain a sub-band image of the grating information;

[0041] When processing grating information, the minimum number of sub-bands should be used as much as possible to reduce the possibility of extracting the original image information. Therefore, we choose to perform wavelet multi-level decomposition on the X-ray image to extract the first direction sub-band image and the second direction sub-band image, where the first direction sub-band image contains the grating information.

[0042] In this embodiment, the number of wavelet multi-level decomposition is determined according to the number of peaks recorded in step S2. Generally, the more peaks there are, the higher the number of wavelet decomposition should be, so that the grating information of different peaks in the frequency domain image can be effectively separated into corresponding wavelet sub-bands. The position of the grating in the first direction sub-band image is determined according to the position of the peak recorded in step S2, wherein the peaks located in the low-frequency part are not processed, because the low-frequency area mainly contains the information of the original image and is easy to be extracted. The wavelet basis used in the wavelet multi-level decomposition is determined according to the shape of the peak recorded in step S2. Most wavelet bases have good effects on single-peak gratings. For peaks with complex shapes, it is necessary to select the best wavelet basis according to the final effect. Reasonable selection of wavelet basis can concentrate the energy of wavelet transform space, which is conducive to selecting the main components as features.

[0043] S4: Processing the sub-band image to obtain a grating image;

[0044] Next, the sub-band image in the second direction with weights is subtracted from the sub-band image in the first direction to obtain sub-band data. The reason for subtracting the sub-band image in the second direction with weights is to minimize the small differences in the frequency domain images caused by the differences in information in different directions contained in the obtained grating information. In this embodiment, the selection of weights mainly considers two parts: 1. The overall difference between the frequency domain images in the first direction and the second direction; 2. The position of the peak. The closer the peak is to the low frequency, the easier it is to extract the original image information into the grating image.

[0045] Next, the sub-band data is reconstructed by wavelet to obtain a grating image.

[0046] S5: Subtract the grating image from the X-ray image to obtain a grating-free X-ray image.

[0047] Finally, the grating image obtained in step S4 is subtracted from the X-ray image obtained in step S1 to obtain a non-grating X-ray image.

[0048] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for removing filter line patterns of X-ray images, characterized in that: The steps include: S1: Acquire X-ray images; S2: Obtaining grating information from the X-ray image, first obtaining a frequency domain image of the X-ray image by Fourier transform, then determining the direction of the grating according to the frequency domain image, and defining the direction with the grating as a first direction, and the direction without the grating as a second direction; S3: Processing the grating information to obtain a sub-band image of the grating information, including performing wavelet multi-level decomposition on the X-ray image to extract a first direction sub-band image and a second direction sub-band image, wherein the first direction sub-band image contains the grating information; S4: Processing the sub-band image to obtain a grating image, first subtracting the sub-band image in the second direction with a weight from the sub-band image in the first direction to obtain sub-band data, and then performing wavelet reconstruction on the sub-band data to obtain a grating image; S5: Subtract the grating image from the X-ray image to obtain a grating-free X-ray image.

2. The method for removing X-ray image grid patterns according to claim 1, characterized in that: Step S1 also includes determining whether the X-ray image needs to be partitioned according to the following steps: If the row and column sizes of the X-ray image are equal, no partitioning is required; If the row and column sizes of the X-ray image are not equal, the X-ray image is divided into a plurality of square areas of the same size with overlapping areas, and steps S1 to S5 are performed on the plurality of square areas respectively, and finally they are assembled into a complete image.

3. The method for removing X-ray image grid patterns according to claim 1, characterized in that: Step S2 also includes recording the number, position and shape of peaks in the frequency domain image.

4. The method for removing X-ray image grid patterns according to claim 3, characterized in that: In step S3, the level of the wavelet multi-level decomposition is determined according to the number of the wave peaks.

5. The method for removing X-ray image grid patterns according to claim 3, characterized in that: In step S3, the position of the grating pattern in the first directional sub-band image is determined according to the position of the wave peak.

6. The method for removing X-ray image grid patterns according to claim 3, characterized in that: In step S3, the wavelet basis used in the wavelet multi-level decomposition is determined according to the shape of the wave peak.

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