A X-ray grating imaging method based on bidirectional staggered absorption grating

Abstract

The present invention discloses an X-ray grating imaging method based on a bidirectional staggered absorption grating. The present invention belongs to the technical field of X-ray phase contrast imaging. A bidirectional staggered absorption grating is used to arrange an X-ray grating interference imaging device, and an X-ray two-dimensional intensity image is obtained; the X-ray two-dimensional intensity image is extracted by combining the nine-square grid neighbor term approximation method with the Fourier analysis method, and three images of X-ray absorption contrast, differential phase contrast and dark field contrast are extracted; the contrast image is processed by an image post-processing algorithm based on pixel value similarity to improve the image quality. The present invention does not require moving the grating, and a variety of contrast images can be obtained under one exposure, which greatly reduces the imaging time, reduces the imaging dose, and improves the imaging efficiency and stability of the system. In addition, the method of the present invention has little effect on the spatial resolution, can reduce the structural stripes caused by the staggered grating in the contrast image, and can maximize the image quality.

Description

Technical Field

[0001] The invention relates to the technical field of X-ray phase contrast imaging, and in particular to an X-ray grating imaging method based on a bidirectional staggered absorption grating. Background Art

[0002] In the X-ray grating interference contrast imaging method, the most commonly used traditional method is to use the Talbot-Lau interferometer to simultaneously obtain three images: X-ray absorption contrast, differential phase contrast, and dark field contrast. The Talbot-Lau interferometer consists of six parts: X-ray source, source grating G0, test object, phase grating G1, absorption grating G2, and detector. The G0 grating is arranged behind the X-ray source, which can make the X-rays emitted by the ordinary X-ray tube produce local quasi-coherence, thereby producing an interference effect. The grating part of the G1 grating is made of pure silicon, which can cause the X-rays to produce phase shifts, while the non-grating part has no effect on the X-rays. Its function is to introduce phase shifts, thereby forming interference fringes; and the G2 grating is made of aluminum etched and electroplated on a silicon wafer, in which the aluminum area absorbs all X-rays passing through, while the silicon area does not attenuate the X-rays at all. Since the period of the interference fringes formed by the G1 grating is much smaller than the spatial resolution of common detectors, the interference fringes cannot be measured directly. The fractional Talbot distance placed behind the G1 grating is made to step equidistantly within one period of the G2 grating itself, and the light intensity of the detector at each step position is recorded. A curve of light intensity following the step of the G2 grating can be obtained, from which the phase information of the X-ray can be analyzed. Therefore, the G2 grating is also called an analyzer grating.

[0003] The traditional phase contrast imaging method based on Talbot-Lau interferometer requires multiple steps of the G2 grating, which results in long imaging time, high dose, low stability and low efficiency, thus restricting its further engineering and clinical applications. In recent years, through structural improvements on the absorption grating, there are X-ray grating differential phase contrast imaging methods based on longitudinally displaced gratings and transversely displaced gratings, which can achieve the analysis of X-ray phase information without the need for G2 grating steps.

[0004] For X-ray grating differential phase contrast imaging methods based on longitudinal and transverse staggered absorption gratings, these two methods can only approximate the G2 step curve in one direction using adjacent pixels. The error distribution between the pixels involved in the approximation and the calculated pixels is biased, and the errors will be formed in the contrast image in an orderly manner, which will produce obvious structural stripes in the three extracted contrast images. At the same time, when approximating, these two methods select 4 adjacent pixels in the longitudinal and transverse directions respectively, which will cause a significant decrease in the spatial resolution in this direction, and ultimately cause distortion of the contrast image.

[0005] At present, no X-ray grating differential phase contrast imaging method and device based on a bidirectional staggered absorption grating has been found. Summary of the invention

[0006] In order to achieve the analysis of X-ray phase information without G2 grating stepping and solve the problems in the current method, the present invention provides an X-ray grating imaging device and method based on a bidirectional staggered absorption grating.

[0007] The present invention proposes an X-ray grating imaging device based on a bidirectional staggered absorption grating, the device comprising an imaging acquisition module and a calculation module; wherein:

[0008] The imaging acquisition module is composed of a Talbot-Lau interferometer, which includes six parts: X-ray source, source grating G0, test object, phase grating G1, absorption grating G2, and detector; it is used to obtain X-ray two-dimensional intensity images. The parameters of the Talbot-Lau interferometer should satisfy the following formulas (1)-(4):

[0009] (1)

[0010] (2)

[0011] (3)

[0012] (4)

[0013] in, represents the distance between the phase grating G1 and the absorption grating G2; is the magnification ratio, is the distance between the source grating G0 and the phase grating G1; Indicates Fractional Talbot distance; is the period of the phase grating G1, is the wavelength of the X-rays used, is the period of the absorption grating G2, is the period of the source grating G0, is the width of the source grating that allows X-rays to pass through in each period;

[0014] The computing module, composed of a computer, is used to control the on and off of the X-ray source and the adjustment of various parameters, control the acquisition of the detector, and apply the nine-square grid adjacent term approximation method and Fourier analysis method to the X-ray two-dimensional intensity image obtained by the imaging acquisition module to obtain three contrast images: X-ray absorption contrast, differential phase contrast and dark field contrast. It can also apply the image post-processing algorithm based on pixel similarity to the three contrast images to finally obtain a high-quality contrast image.

[0015] The present invention proposes an X-ray grating imaging method based on a bidirectional staggered absorption grating, the method comprising the following steps:

[0016] Step 1: Arrange an X-ray grating interference imaging device using a bidirectional staggered absorption grating, and obtain an X-ray two-dimensional intensity image;

[0017] Step 2, using the nine-square grid neighbor term approximation method combined with the Fourier analysis method to separate three contrast images, namely, X-ray absorption contrast, differential phase contrast and dark field contrast, from the two-dimensional intensity image;

[0018] Step 3: Use the image post-processing algorithm based on pixel similarity to process the three contrast images to further improve the image quality.

[0019] The beneficial effects of the present invention are as follows: compared with the existing X-ray grating differential phase contrast imaging method based on grating improvement and without absorption grating stepping, the present invention can select adjacent term pixel points for approximation in two directions, and combined with the nine-square grid adjacent term approximation method proposed in the present invention, it can minimize the adjacent term error and eliminate the grating structural stripes generated in the contrast image to a large extent; the contrast image obtained in the above manner is processed using the image post-processing algorithm based on pixel similarity specially designed for it by the present invention, which can further improve the image quality and promote the engineering and clinical application of the phase contrast imaging method based on Talbot-Lau interferometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A flow chart of an X-ray grating imaging method based on a bidirectional staggered absorption grating provided in an embodiment of the present invention;

[0021] Figure 2 A schematic diagram of an X-ray grating imaging device based on a bidirectional staggered absorption grating provided in an embodiment of the present invention;

[0022] Figure 3 A schematic diagram of the structure of a bidirectional staggered absorption grating provided in an embodiment of the present invention;

[0023] Figure 4 is a two-dimensional intensity image of an X-ray grating imaging method based on a bidirectional staggered absorption grating; wherein, Figure 4 (a) is the reference image, Figure 4 (b) is the experimental object;

[0024] Figure 5 The present invention is used to compare the absorption contrast, differential phase contrast and dark field contrast images of the test object after being processed by the image post-processing algorithm based on pixel similarity provided by the present invention with the three contrast images before processing; wherein Figure 5 (a) is the absorption contrast image without algorithm processing; Figure 5 (b) is the absorption contrast image processed by the algorithm; Figure 5 (c) is the differential phase contrast image without algorithm processing; Figure 5 (d) is the differential phase contrast image processed by the algorithm; Figure 5 (e) is a dark field contrast image without algorithm processing; Figure 5 (f) is the dark field contrast image processed by the algorithm.

[0025] In the figure, 20 is an X-ray source, 21 is a source grating G0, 22 is a test object, 23 is a phase grating G1, 24 is an absorption grating G2, 25 is a detector, and 26 is a computer. DETAILED DESCRIPTION

[0026] The present invention is further described below with reference to the accompanying drawings and specific embodiments.

[0027] Figure 1 A flow chart of an X-ray grating differential phase contrast imaging method based on a bidirectional staggered absorption grating provided in an embodiment of the present invention; compared with the existing X-ray grating differential phase contrast imaging method based on grating improvement that does not require absorption grating stepping, the imaging method provided by the present invention can select neighboring item pixel points for approximation in two directions, and combined with the nine-square grid neighboring item approximation method proposed in the present invention, it can minimize the neighboring item error and eliminate the grating structural stripes generated in the contrast image to a large extent; the contrast image obtained in the above manner is processed using the image post-processing algorithm based on pixel similarity specially designed for it by the present invention, which can further improve the image quality and promote the engineering and clinical application of the phase contrast imaging method based on Talbot-Lau interferometer. The specific steps of the method are as follows:

[0028] Step S1, arranging an X-ray grating interference imaging device using a bidirectional staggered absorption grating, and acquiring a two-dimensional X-ray intensity image.

[0029] Figure 2 is a schematic diagram of the X-ray grating interferometry imaging device described in step S1; Figure 2 As shown, the X-ray grating interference imaging device includes an imaging acquisition module and a calculation module:

[0030] The imaging acquisition module consists of a Talbot-Lau interferometer, such as Figure 2 As shown, it includes 6 parts in total: X-ray source 20, source grating G0 21, test object 22, phase grating G1 23, absorption grating G2 24, and detector 25, which are used to obtain X-ray two-dimensional intensity images. The Talbot-Lau interferometer parameters should satisfy the following formulas (1)-(4):

[0031] (1)

[0032] (2)

[0033] (3)

[0034] (4)

[0035] in, represents the distance between the phase grating G1 and the absorption grating G2; is the magnification ratio, is the distance between the source grating G0 and the phase grating G1; Indicates Fractional Talbot distance; is the period of the phase grating G1, is the wavelength of the X-rays used, is the period of the absorption grating G2, is the period of the source grating G0, is the width of the source grating that allows X-rays to pass through in each period;

[0036] Computational modules such as Figure 2 As shown, it is composed of a computer 26, which is used to control the opening and closing of the X-ray source and the adjustment of various parameters, control the acquisition of the detector, and apply the nine-square grid adjacent term approximation method, Fourier analysis method and image post-processing algorithm based on pixel similarity to the X-ray two-dimensional intensity image acquired by the imaging acquisition module.

[0037] Figure 3 The schematic diagram of the structure of the bidirectional staggered absorption grating provided in the embodiment of the present invention. The relative position between the bidirectional staggered absorption grating and the detector probe is periodically staggered in both the horizontal and vertical directions, so that the intensity signal obtained by a detector probe on its adjacent probes in both the horizontal and vertical directions can be approximately equivalent to the intensity signal obtained by a detector probe in the traditional imaging method when the absorption grating is at different positions.

[0038] For a detector element, it is marked as , the three adjacent detectors on the right and the three adjacent detectors below will be marked as , , as well as , , Each detector ( ; ) Width is , the height is , in the bidirectional staggered absorption grating, corresponds to a width of , the height is The small raster block is marked as , the grating period in each grating block is There is a distance of The position difference of adjacent detectors in the horizontal direction is and Corresponding grating segment and The grating inside exists The position difference of the adjacent detectors in the longitudinal direction and Corresponding grating segment and The grating inside also exists The position difference of the detector and Corresponding grating end and The grating positions in the .

[0039] The specific operation steps for obtaining the X-ray two-dimensional intensity image are as follows: using a bidirectional staggered absorption grating arrangement to complete the X-ray grating interference imaging device, first moving the test object out of the imaging area, using a computer to control the X-ray source to emit rays and control the detector to collect a two-dimensional intensity image for reference; then moving the test object into the imaging area, using a computer to control the X-ray source to emit rays and control the detector to collect the two-dimensional intensity image of the test object.

[0040] Step S2, using a nine-square grid neighbor term approximation method combined with a Fourier analysis method to separate three images of X-ray absorption contrast, differential phase contrast and dark field contrast from the two-dimensional intensity image;

[0041] For the acquired 2D intensity image, except for a circle of pixels at the edge of the image, each pixel has a complete 3 3 neighborhood, based on the 3 3 neighborhood, using the nine-square grid neighboring term approximation method in step 2 combined with the Fourier analysis separation method, the two-dimensional intensity image is separated into four images equivalent to the traditional method absorption grating at four step positions, and then the X-ray absorption contrast, differential phase contrast, and dark field contrast are extracted from the four images by Fourier analysis, including:

[0042] According to formulas (6)-(9), the pixel with a complete 3x3 neighborhood is analyzed:

[0043] (6)

[0044] (7)

[0045] (8)

[0046] (9)

[0047] in, are the horizontal and vertical coordinates of the pixel points of the two-dimensional intensity image; is the pixel point in the two-dimensional intensity image The strength value of They represent the image at the pixel point equivalent to the traditional method with the absorption grating located at 4 step positions. The intensity value at .

[0048] According to formulas (10)-(15), three images of X-ray absorption contrast, differential phase contrast and dark field contrast are extracted:

[0049] (10)

[0050] (11)

[0051] (12)

[0052] (13)

[0053] (14)

[0054] (15)

[0055] in, Represents a point The number of different intensity values ​​in , ; For point The mean of the sine curves fitted at four different intensity values; For point The amplitude of the sine curve fitted by 4 different intensity values; For point In projection angle The phase value of the sine curve fitted with 4 different intensity values; Indicates when no test object is placed value, Indicates when the test object is placed value; Indicates when no test object is placed value, Indicates when the test object is placed value; Indicates when no test object is placed value, Indicates when the test object is placed value; is the absorption contrast image pixel The value of is the pixel point of the differential phase contrast image The value of is the pixel point of dark field contrast image The value of represent

[0056] Step S3: Use an image post-processing algorithm based on pixel similarity to process the three contrast images to further improve image quality.

[0057] The image post-processing algorithm based on pixel similarity in step S3 calculates the similarity between a pixel in the contrast image and all pixels in a certain range of the neighborhood of the pixel according to formulas (16)-(18), and obtains the contrast image processed by the above algorithm:

[0058] (16)

[0059] (17)

[0060] (18)

[0061] in, Represents the contrast image being processed at The pixel value of the pixel at ; Represents the contrast image being processed at The pixel value of the pixel at ; Represents the processed image at pixel point The pixel value at ; Represents the area in the processed contrast image as well as The similarity between two pixels; Representative The larger number among them; represents a natural constant; Represents the control and Parameters that determine how much the relative position of two points affects similarity; Represents the control and Parameters that measure the effect of the difference between two pixel values ​​on similarity; Represents the neighborhood range involved in the similarity calculation ; Represents a symbolic function in mathematics; It is the parameter that controls the similarity standard; Represents the pixel in the processed contrast image The number of pixels that meet the similarity standard in the above neighborhood.

[0062] The following scheme is used as an example to illustrate the effectiveness of the embodiment of the present invention:

[0063] According to step S1, two two-dimensional intensity images can be obtained, which are a reference image without a test object and an image with a test object (guinea pig paw). Figure 4 are the two above-mentioned two-dimensional intensity images (without test object and with test object), where: Figure 4 (a) is the reference image, Figure 4 (b) is the experimental object. The nine-square grid adjacent term approximation method provided by the present invention is applied to the two images ( , , , ) Combined with Fourier analysis, the absorption contrast, differential phase contrast and dark field contrast images of the test object can be obtained respectively.

[0064] For the three contrast images, the image post-processing algorithm based on pixel similarity provided by the present invention is applied to obtain the three improved contrast images. Figure 5 This is the comparison between the improved absorption contrast, differential phase contrast and dark field contrast images of the test object and those before processing. Figure 5 (a) is the absorption contrast image without algorithm processing; Figure 5 (b) is the absorption contrast image processed by the algorithm; Figure 5 (c) is the differential phase contrast image without algorithm processing; Figure 5 (d) is the differential phase contrast image processed by the algorithm; Figure 5 (e) is a dark field contrast image without algorithm processing; Figure 5(f) is the dark field contrast image processed by the algorithm.

[0065] In summary, according to the present invention, compared with the traditional X-ray grating differential phase contrast imaging method, the present invention does not need to perform a phase stepping step, while increasing the stability of the system, reducing the imaging time, reducing the radiation dose, and being able to obtain three types of contrast images; compared with the existing X-ray grating differential phase contrast imaging method based on longitudinal and transverse misaligned gratings, the present invention greatly reduces the contrast resolution error and improves the contrast image quality. The present invention can also be combined with CT technology to further obtain tomographic images inside the test object, and has considerable practical application value in the fields of non-destructive testing, clinical applications, etc.

[0066] The above specific implementations further describe the purpose, technical solutions and beneficial effects of the present invention in detail. Those skilled in the art can clearly understand that for the convenience and simplicity of description, only the division of the above-mentioned functional modules is used as an example. In actual applications, the above-mentioned functions can be assigned to different functional modules as needed, that is, the internal structure of the device is divided into different functional modules to complete all or part of the functions described above. The specific working process of the device described above can refer to the corresponding process in the aforementioned method embodiment, which will not be repeated here.

[0067] It should be understood that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or make equivalent replacements for some or all of the technical features and parameters therein. However, these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An imaging method of an X-ray grating imaging device based on a bidirectional staggered absorption grating, characterized in that: The method comprises the following steps: Step 1: Arrange an X-ray grating interference imaging device using a bidirectional staggered absorption grating, and obtain an X-ray two-dimensional intensity image; Step 2, using the nine-square grid neighbor term approximation method combined with the Fourier analysis method to separate three contrast images, namely, X-ray absorption contrast, differential phase contrast and dark field contrast, from the two-dimensional intensity image; Step 3: Use an image post-processing algorithm based on pixel similarity to process the three contrast images to further improve image quality; For the acquired two-dimensional intensity image, except for a circle of pixels at the edge of the image, each pixel has a complete 3x3 neighborhood. Based on the 3x3 neighborhood of these pixels, the two-dimensional intensity image is separated into four images with the absorption grating located at four step positions. Then, the X-ray absorption contrast, differential phase contrast, and dark field contrast are extracted from the four images by Fourier analysis, including: According to formulas (6)-(9), the pixels with a complete 3x3 neighborhood are analyzed: , （6） （7） （8） （9） in, are the horizontal and vertical coordinates of the pixel points in the two-dimensional intensity image; is the pixel point in the two-dimensional intensity image The strength value of They represent the image at the pixel point when the absorption grating is located at 4 step positions. The intensity value at ; According to formulas (10)-(15), three contrast images, namely X-ray absorption contrast, differential phase contrast and dark field contrast, are extracted: （10） （11） （12） （13） （14） （15） in, Represents a pixel The number of different intensity values ​​in , ; Pixel The mean of the sine curves fitted at four different intensity values; Pixel The amplitude of the sine curve fitted by 4 different intensity values; Pixel In projection angle The phase value of the sine curve fitted with 4 different intensity values; Indicates when no test object is placed value, Indicates when the test object is placed value; Indicates when no test object is placed value, Indicates when the test object is placed value; Indicates when no test object is placed value, Indicates when the test object is placed value; is the absorption contrast image pixel The value of is the pixel point of the differential phase contrast image The value of is the pixel point of dark field contrast image The value of represent ; According to formulas (16)-(18), the similarity between a pixel in the contrast image and all pixels in a certain range of the neighborhood of the pixel is calculated, and the processed contrast image is obtained: （16） （17） （18） in, Represents the contrast image being processed at The pixel value of the pixel at ; Represents the contrast image being processed at The pixel value of the pixel at ; Represents the processed image at pixel point The pixel value at ; Represents the area in the processed contrast image as well as The similarity between two pixels; Representative The larger number among them; represents a natural constant; Represents the control and Parameters that determine how much the relative position of two points affects similarity; Represents the control and Parameters that measure the effect of the difference between two pixel values ​​on similarity; Represents the neighborhood range involved in the similarity calculation ; Represents a symbolic function in mathematics; It is the parameter that controls the similarity standard; Represents the pixel point in the processed contrast image The number of pixels that meet the similarity standard in the above neighborhood; The X-ray grating imaging device based on the bidirectional staggered absorption grating includes an imaging acquisition module and a calculation module; wherein, The imaging acquisition module is composed of a Talbot-Lau interferometer, which includes six parts: X-ray source, source grating G0, test object, phase grating G1, absorption grating G2, and detector; it is used to obtain X-ray two-dimensional intensity images. The parameters of the Talbot-Lau interferometer should satisfy the following formulas (1)-(4): （1） （2） （3） （4） in, represents the distance between the phase grating G1 and the absorption grating G2; is the magnification ratio, is the distance between the source grating G0 and the phase grating G1; Indicates Fractional Talbot distance; is the period of the phase grating G1, is the wavelength of the X-rays used, is the period of the absorption grating G2, is the period of the source grating G0, is the width of the source grating G0 that allows X-rays to pass through in each period; The computing module, which is composed of a computer, is used to control the on and off of the X-ray source and the adjustment of various parameters, control the acquisition of the detector, and apply the nine-square grid adjacent term approximation method and Fourier analysis method to the X-ray two-dimensional intensity image obtained by the imaging acquisition module to obtain three types of contrast images: X-ray absorption contrast, differential phase contrast, and dark field contrast. The image post-processing algorithm based on pixel similarity can be applied to the three types of contrast images to finally obtain a high-quality contrast image; The bidirectional misaligned absorption grating refers to the absorption grating G2 in the Talbot-Lau interferometer. The relative position of the absorption grating G2 and the detector probe is periodically misaligned in both the horizontal and vertical directions, so that the intensity signal obtained by a detector probe on its adjacent probes in both the horizontal and vertical directions can be equivalent to the intensity signal obtained by a detector probe when the absorption grating is in different positions. For a detector element, it is marked as , the three adjacent detectors on the right and the three adjacent detectors below will be marked as , , as well as , , , each detector , ; , the width is , the height is , in the bidirectional staggered absorption grating, corresponds to a width of , the height is , marking it as , the grating period in each grating block is , the gratings in the grating blocks corresponding to the detector elements directly adjacent in the horizontal and vertical directions have a distance of The position difference of adjacent detectors in the lateral direction and Corresponding grating segment and The grating inside exists The position difference of the adjacent detectors in the longitudinal direction and Corresponding grating segment and The grating inside also exists The position difference of the detector and Corresponding grating end and The grating positions in the , assuming it is marked as The grating position in the grating block is equivalent to the first step position of the absorption grating, which is marked as The grating position in the grating block is equivalent to the absorption grating Step positions, and Satisfies the following formula (5): , ; ； （5） in, represents the step position of the absorption grating, Represents the lateral and longitudinal positions of the grating block relative to the grating block of the bidirectional offset grating with a step position of 1, at which , represents the modulo operation; Therefore, any four grating blocks corresponding to four detector elements adjacent in the horizontal or vertical direction are equivalent to the situation where the absorption grating has moved to four different positions. This grating is called a bidirectional staggered grating.

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