A method for testing the penetration capability of an industrial CT system

Abstract

This invention relates to a method for testing the penetration capability of an industrial CT system, comprising: designing a test block for testing the industrial CT system under test, the test block being a cylinder with a through hole arranged coaxially with the cylinder, the longitudinal section of the through hole being trapezoidal; placing the test block at the center of a turntable, and using the industrial CT system under test to perform CT scanning imaging on the test block to obtain multiple CT images of the test block; obtaining the material center position and penetration thickness of each CT image of the test block; calculating the point spread function of the CT images; obtaining the gray values ​​of the internal void portion and the material portion in each CT image of the test block; calculating the contrast value corresponding to different spatial resolutions at different penetration thicknesses; and when the MTF... k If the penetration rate is ≥10%, then the current industrial CT system's equipment and process conditions are considered to allow penetration of a thickness of D. n Products.

Description

Technical Field

[0001] This invention relates to the field of optical system performance testing technology, and in particular to a method for testing the penetration capability of an industrial CT system. Background Technology

[0002] Industrial CT inspection technology is a practical non-destructive testing method developed from X-ray inspection technology. It is a special optical system with intuitive imaging and accurate quantitative, localization, and qualitative analysis, and is widely used in industrial non-destructive testing, medical and health fields, and other areas. Its imaging principle involves irradiating the object being inspected with X-rays of a certain energy intensity, collecting the attenuation patterns and distribution of the X-rays after they pass through the object, and then displaying these attenuation patterns and distributions in image form through computer information processing and image reconstruction technology. Therefore, traditional CT technology is a transmission-based imaging inspection technology.

[0003] The penetration capability of industrial CT technology and equipment characterizes the density and thickness range of products that the equipment can detect, directly affecting whether the equipment can perform product inspection, and is an important performance indicator. The evaluation of the penetration capability of industrial CT systems is necessary throughout the research and development, production, acceptance, use, commissioning, and maintenance of CT equipment. Therefore, manufacturers and users urgently need a convenient, accurate, and practical method for testing the penetration capability of industrial CT systems.

[0004] Currently, two national standards, GB / T 37158 "Test Method for Maximum Detectable Steel Thickness by Industrial Computed Tomography (CT) in Nondestructive Testing" and GB / T 37122 "Test Card for Maximum Detectable Steel Thickness by Industrial Computed Tomography (CT) in Nondestructive Testing", have provided test methods and test block processing for the maximum detectable steel thickness index of industrial CT systems with X-ray energy of 225keV and above. The main technical principle of the method is to first specify the required spatial resolution, make a corresponding stripe (circular hole) spatial resolution phantom, embed the phantom into a circular disk of different diameters, and perform CT reconstruction on it for a specific CT system. When the imaging result of the spatial resolution phantom at a certain diameter meets the requirements and the contrast signal-to-noise ratio of the detection feature is greater than or equal to 3, that diameter is the maximum thickness that can be penetrated. The advantage of this method is that the penetration capability test results are intuitive, and quantitative results can be obtained without excessive calculations. However, it has the following disadvantages: it requires high precision and capability in the processing of the phantom. When the radiation energy is less than 225 keV, it is difficult to process the corresponding test phantom for micro / nano-focus CT. Therefore, both of the above-mentioned national standards specify CT systems applicable to radiation energies greater than 225 keV. It is evident that there is a lack of a widely applicable method for testing the penetration capability of industrial CT systems.

[0005] Therefore, further improvements to existing technologies are needed. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for testing the penetration capability of industrial CT systems with low sample block processing difficulty, high test accuracy and speed, and applicable to the full energy range, in contrast to the above-mentioned prior art.

[0007] The technical solution adopted by this invention to solve the above-mentioned technical problem is as follows: a method for testing the penetration capability of an industrial CT system, wherein the industrial CT system includes an X-ray source, a turntable, and a detector arranged sequentially at intervals, characterized by comprising the following steps:

[0008] Step 1: Design a test block for testing the industrial CT system to be tested according to the penetration capability range of the industrial CT system to be tested. The test block is a cylinder with through holes arranged coaxially with the cylinder inside. The longitudinal section of the through holes is trapezoidal.

[0009] Step 2: Place the test block from Step 1 at the center of the turntable, so that the centerline of the test block is basically coincident with the centerline of the turntable, and make the reference plane of the test block parallel to the CT scanning plane. Use the industrial CT system under test to perform CT scanning imaging on the test block to obtain multiple CT images of the test block.

[0010] Each CT image of the test block includes a centrally located, circular internal void portion and a material portion located around the internal void portion and arranged concentrically with the internal void portion, wherein the material portion is annular;

[0011] Step 3: Obtain the material center position of each test block CT image, and obtain the penetration thickness of each test block CT image based on the material center position. The penetration thickness of the nth test block CT image is denoted as D. n ;

[0012] Step 4: Calculate the point spread function of the CT image;

[0013] Step 5: Obtain the grayscale values ​​of the internal void portion and the material portion in each CT image of the test block;

[0014] Let A be the gray value of the internal void portion of the nth test block CT image. n The grayscale value of the material part is denoted as B. n ;

[0015] Step 6: Calculate the contrast values ​​corresponding to different spatial resolutions under different penetration thicknesses;

[0016] Specifically:

[0017] Determine the line-pair density, i.e., k line pairs / mm; where k is a positive integer;

[0018] Establish an ideal line-pair card grayscale value distribution map g n (x), where g n (x) is a rectangular wave, g n The peak value of (x) is B n The trough value is A n ;

[0019] The ideal line-to-card grayscale value distribution map g n (x) is convolved with the point spread function of the CT image in step 4 to obtain the grayscale distribution map of the degraded line pair card; where the grayscale value of the material in the grayscale distribution map of the degraded line pair card is denoted as B′. n The gray value of the internal gap in the gray value distribution map of the degraded line pair card is denoted as A′. n ;

[0020] The contrast value at the current spatial resolution

[0021] Step 7, when MTF k If the penetration rate is ≥10%, then the current industrial CT system's equipment and process conditions are considered to allow penetration of a thickness of D. n Products.

[0022] Preferably, in step 1, the cross-sectional diameter of the cylindrical test block is c, the length of the upper base of the longitudinal section of the through hole is b, the length of the lower base of the longitudinal section of the through hole is a, and the penetration capability of the industrial CT system being tested is x. Then, x, c, b, and a satisfy the following relationship:

[0023] ca <x<c-b。

[0024] Specifically, the steps for calculating the material center position of each test block CT image in step 3 are as follows:

[0025] Histogram statistics were performed on the grayscale value distribution of all test block CT images, and threshold segmentation was performed on the histogram to distinguish between materials and internal voids;

[0026] Using the threshold used in threshold segmentation to analyze the CT image of the test block n (x,y) is transformed into a binary image J n (x,y), the centroid method is used to calculate the material center position (x,y) of each CT image of the test block. c ,y c ).

[0027] Furthermore, the specific steps for determining the penetration thickness of each test block CT image in step 3 are as follows:

[0028] Using the material center of the test block CT image as the center, multiple radially distributed search lines are extended along the diameter of the internal void portion. The number of points with a gray value of 1 on each search line is counted in the corresponding binary image of the test block CT image. The average number of points with a gray value of 1 on all search lines is calculated, and this average is multiplied by the pixel size of the binary image to obtain the penetration thickness of the current test block CT image.

[0029] Furthermore, the point spread function calculation steps for the CT image in step 4 are as follows:

[0030] Extract the grayscale values ​​of the CT image near the outer circle of the material part in each CT image of the test block, calculate the distance from each point in the annular region to the center of the corresponding material in the CT image of the test block, group the pixels in the annular region according to the distance unit from the center of the material in the CT image of the test block, and take the average pixel value of the pixels in each group to establish a correspondence graph between distance and average pixel value. This graph is the edge response function curve.

[0031] The point spread function is obtained by differentiating the edge response function curve and then by fitting the point spread function with a Gaussian function to obtain the point spread function of the CT image.

[0032] Furthermore, in step 5, the grayscale values ​​of the internal void portion and the material portion in each test block CT image are obtained as follows:

[0033] Calculate the average gray value of the internal void portion in each CT image of the test block, and then use this average gray value as the gray value of the internal void portion in the current CT image of the test block.

[0034] Calculate the average gray value of the material portion in each CT image of the test block, and then use this average gray value as the gray value of the material portion in the current CT image of the test block.

[0035] Compared with existing technologies, the advantages of this invention are: the test block used for testing the penetration capability of industrial CT systems is simple to process, low in cost, and widely applicable, without the hardware limitation stipulated by standards that only CT systems with a voltage greater than 225 keV can be used. Therefore, this method is easy to automate, and the measurement results are fast and stable. Attached Figure Description

[0036] Figure 1 This is a longitudinal cross-sectional view of the test block in an embodiment of the present invention;

[0037] Figure 2 These are two CT images of test blocks in an embodiment of the present invention, wherein 2(a) is a CT image of the test block in the area with a larger thickness, and 2(b) is a CT image of the test block in the area with a smaller thickness;

[0038] Figure 3 This is a schematic diagram of the search line drawn on the CT image of the test block in an embodiment of the present invention;

[0039] Figure 4 This is a schematic diagram of the area near the outer circle of the material portion during point diffusion function calculation in an embodiment of the present invention;

[0040] Figure 5 This is a graph of the edge response function in an embodiment of the present invention;

[0041] Figure 6 This is a point diffusion function curve diagram in an embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram illustrating the acquisition of grayscale values ​​of the internal void portion and the material portion in each CT image of the test block in an embodiment of the present invention.

[0043] Figure 8 This is a grayscale value distribution diagram of an ideal line pair card in an embodiment of the present invention;

[0044] Figure 9 This is a grayscale value distribution diagram of the degraded line pair card in an embodiment of the present invention;

[0045] Figure 10 This is a curve showing the relationship between the grayscale values ​​of the material under different penetration thicknesses in an embodiment of the present invention;

[0046] Figure 11 This is a curve showing the relationship between the grayscale values ​​of the voids under different penetration thicknesses in an embodiment of the present invention;

[0047] Figure 12 This is the ideal 10-line pair CT value distribution curve in this embodiment of the invention;

[0048] Figure 13 for Figure 12 The grayscale value distribution curve of the degraded line pair card obtained by convolving the CT value distribution map of the ideal 10 line pair card with the PSF function;

[0049] Figure 14 This is the contrast value curve corresponding to 10 line pairs / mm for each penetration thickness in this embodiment of the invention;

[0050] Figure 15 This is an image of the internal line relative to the phantom in an embodiment of the present invention when the penetration thickness is 16mm;

[0051] Figure 16 This is an image of the internal line on the model when the penetration thickness is 8mm in an embodiment of the present invention. Detailed Implementation

[0052] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0053] An industrial CT system includes an X-ray source, a turntable, and a detector arranged sequentially at intervals. The penetration capability testing method for the industrial CT system in this embodiment includes the following steps:

[0054] Step 1: Design a test block for testing the industrial CT system to be tested according to the penetration capability range of the industrial CT system to be tested. The test block is a cylinder with through holes arranged coaxially with the cylinder inside. The longitudinal section of the through holes is trapezoidal.

[0055] like Figure 1 As shown, in step 1, the diameter of the cylindrical test block is c, the length of the upper base of the longitudinal section of the through hole is b, the length of the lower base of the longitudinal section of the through hole is a, and the penetration capability of the industrial CT system being tested is x, where x is in mm. Then, x, c, b, and a satisfy the following relationship:

[0056] ca <x<c-b;

[0057] Furthermore, the height of the test block is determined according to the industrial CT scan height range, and the height of the through hole is less than the height of the cylindrical part of the test block;

[0058] In this embodiment, the test block material is aluminum alloy, with c = 40mm, b = 10mm, a = 35mm, and a height of 40mm. The distance from the X-ray source of the industrial CT system to the test block is 200mm, and the distance from the X-ray source to the detector is 450mm. The X-ray source is a micro-focus X-ray machine, with an experimental tube voltage of 80kV, a tube current of 50uA, and an integration time of 500ms. Under this scanning process, the CT image pixel size is 0.14mm.

[0059] Step 2: Place the test block from Step 1 at the center of the turntable, so that the centerline of the test block is basically coincident with the centerline of the turntable, and make the reference plane of the test block parallel to the CT scanning plane. Use the industrial CT system under test to perform CT scanning imaging on the test block to obtain multiple CT images of the test block.

[0060] The reference plane of the aforementioned test block corresponds to the upper or lower surface of the cylindrical portion of the test block. The CT scanning plane is parallel to the turntable. After determining typical scanning parameters (scanning mode, scan time, X-ray source tube voltage / current, etc.), CT scanning imaging is performed on the test block; as follows... Figure 2 The image shown is two CT images of the test blocks;

[0061] Each CT image of the test block includes a centrally located, circular internal void and a material portion located around the internal void and arranged concentrically with it. The material portion is ring-shaped.

[0062] Step 3: Obtain the material center position of each test block CT image, and obtain the penetration thickness of each test block CT image based on the material center position. The penetration thickness of the nth test block CT image is denoted as D. n ;

[0063] The specific steps for calculating the material center position in each CT image of the test block are as follows:

[0064] Histogram statistics were performed on the gray value distribution of all test block CT images, and threshold segmentation was performed on the histogram using methods such as the maximum inter-class variance method to distinguish between materials and internal voids.

[0065] Using the threshold used in threshold segmentation to analyze the CT image of the test block n (x,y) is transformed into a binary image J n (x,y), the centroid method is used to calculate the material center position (x,y) of each CT image of the test block. c ,y c ).

[0066]

[0067]

[0068] Where i and j correspond to the x and y coordinates of the binary image, respectively, and u and p correspond to the length and width of the binary image; i|J n (i,j)==1 indicates that the binary image J n (x,y) is the x-coordinate I when the pixel value is equal to 1;

[0069] The specific steps for achieving penetration thickness in each CT image of a test block are as follows:

[0070] Taking the material center position of the test block CT image as the center, multiple search lines are extended radially along the diameter direction of the internal void portion. The number of points with a gray value of 1 on each search line is counted in the binarized image corresponding to the test block CT image. The average number of points with a gray value of 1 on all search lines is calculated, and the average value is multiplied by the pixel size of the binarized image to obtain the penetration thickness of the current test block CT image.

[0071] like Figure 3 As shown, the dashed lines represent the drawn search lines. In this embodiment, the number of white pixels on each search line is counted.

[0072] Step 4: Calculate the point spread function of the CT image;

[0073] The steps for calculating the point spread function of a CT image are as follows:

[0074] Extract the grayscale values ​​of the CT image near the outer circle of the material part in each CT image of the test block, calculate the distance from each point in the annular region to the center of the corresponding material in the CT image of the test block, group the pixels in the annular region according to the distance unit from the center of the material in the CT image of the test block, and take the average pixel value of the pixels in each group to establish a correspondence graph between distance and average pixel value. This graph is the edge response function curve.

[0075] The edge response function curve (ERF) is differentiated to obtain the point spread function (PSF), and the PSF is then fitted with a Gaussian function. Fitting methods such as least squares method and minimum residual method can be used to obtain the point spread function of the CT image.

[0076] In this embodiment, the region near the outer circumference of the aforementioned material portion corresponds to the following: Figure 4 The edge response function curve of the annular region enclosed by the two dashed lines shown is as follows. Figure 5 As shown, the point spread function curve is as follows: Figure 6 As shown;

[0077] Step 5: Obtain the grayscale values ​​of the internal void portion and the material portion in each CT image of the test block;

[0078] The grayscale values ​​of the internal void portion and the material portion in each CT image of the test block are obtained as follows:

[0079] Calculate the average gray value of the internal void portion in each CT image of the test block, and then use this average gray value as the gray value of the internal void portion in the current CT image of the test block.

[0080] Calculate the average gray value of the material portion in each CT image of the test block, and then use this average gray value as the gray value of the material portion in the current CT image of the test block.

[0081] Let A be the gray value of the internal void portion of the nth test block CT image. n The grayscale value of the material part is denoted as B. n In this embodiment, for example... Figure 7 As shown, the mean (median) grayscale value of the CT image in region 1 (circular region) is taken as the CT value of the internal gap, denoted as A. n The mean (median) grayscale value of the CT image in region 2 (circular region) is taken as the CT value of the material, denoted as B. n ;

[0082] Step 6: Calculate the contrast values ​​corresponding to different spatial resolutions under different penetration thicknesses;

[0083] Specifically:

[0084] Determine the line-pair density, i.e., k line pairs / mm; where k is a positive integer;

[0085] Establish an ideal line-pair card grayscale value distribution map g n (x), where g n (x) is a rectangular wave, g n The peak value of (x) is B n The trough value is A n Its image is as follows Figure 8 As shown, the periods of the rectangular wave are 1, 2, ... k, with a total period of 1 mm;

[0086] The ideal line-to-card grayscale value distribution map g n (x) is convolved with the point spread function of the CT image in step 4 to obtain the degraded line-pair card grayscale value distribution map, as shown below. Figure 9 As shown; where the grayscale value of the material in the degraded line-pair card grayscale value distribution map is denoted as B′. n The gray value of the internal gap in the gray value distribution map of the degraded line pair card is denoted as A′. n ;

[0087] The contrast value at the current spatial resolution

[0088] Step 7, when MTF k If the penetration rate is ≥10%, then the current industrial CT system's equipment and process conditions are considered to allow penetration of a thickness of D. n Products.

[0089] In this embodiment, as shown Figure 10 The figure shows the relationship curves of material grayscale values ​​under different penetration thicknesses, as shown in the figure. Figure 11 The figure shows the relationship curves of void grayscale values ​​under different penetration thicknesses. The density of a single-line card is determined, i.e., k = 10 line pairs / mm (lp / mm), and the ideal grayscale value distribution g of the single-line card is established. n (x). Taking a penetration thickness of 5mm as an example, the material CT value is 9805 and the background CT value is 1415. An ideal 10-line pair CT value distribution curve is established, as follows: Figure 12 As shown; the ideal line-pair card grayscale value distribution g n (x) is convolved with the PSF function in the CT image to form the degraded line-pair card grayscale value distribution, as shown below. Figure 13 As shown; calculate the contrast value at the current spatial resolution (10 line pairs / mm). This value corresponds to a penetration thickness of 5mm. When this MTF... k When the percentage is ≥10%, it can be considered that under the current CT equipment and process conditions, it can penetrate a product with a thickness of 5mm.

[0090] Calculate the contrast value corresponding to 10 line pairs / mm for each penetration thickness, forming a result like... Figure 14 The curve shown. From Figure 14 As can be seen from the data, under the current equipment and process conditions, the maximum penetration thickness of aluminum alloy meeting the requirement of 10 line pairs / mm is 8mm. To verify the effectiveness of this method, GB / T 37158 and GB / T 37122 were used for verification. Line pair molds meeting the requirements were fabricated, and aluminum alloy cylinders of different thicknesses were fitted onto the outer side. When the penetration thickness was 16mm, the imaging effect of the internal line pair mold was as follows: Figure 15 As shown, the phantom with 10 line pairs / mm is quite blurry and indistinguishable; when the penetration thickness is 8mm, the imaging effect of the internal line pairs phantom is as follows: Figure 16 As shown, the phantom with 10 line pairs / mm is relatively clear, verifying the reliability of this method.

[0091] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for testing the penetration capability of an industrial CT system, the industrial CT system comprising an X-ray source, a turntable, and a detector arranged sequentially at intervals, characterized in that... Includes the following steps: Step 1: Design a test block for testing the industrial CT system to be tested according to the penetration capability range of the industrial CT system to be tested. The test block is a cylinder with through holes arranged coaxially with the cylinder inside. The longitudinal section of the through holes is trapezoidal. Step 2: Place the test block from Step 1 at the center of the turntable, so that the centerline of the test block is basically coincident with the centerline of the turntable, and make the reference plane of the test block parallel to the CT scanning plane. Use the industrial CT system under test to perform CT scanning imaging on the test block to obtain multiple CT images of the test block. Each CT image of the test block includes a centrally located, circular internal void portion and a material portion located around the internal void portion and arranged concentrically with the internal void portion, wherein the material portion is annular; Step 3: Obtain the material center position of each test block CT image, and obtain the penetration thickness of each test block CT image based on the material center position. The penetration thickness of the nth test block CT image is denoted as D. n ; Step 4: Calculate the point spread function of the CT image; Step 5: Obtain the grayscale values ​​of the internal void portion and the material portion in each CT image of the test block; Let A be the gray value of the internal void portion of the nth test block CT image. n The grayscale value of the material part is denoted as B. n ; Step 6: Calculate the contrast values ​​corresponding to different spatial resolutions under different penetration thicknesses; Specifically: Determine the line-pair density, i.e., k line pairs / mm; where k is a positive integer; Establish an ideal line-pair card grayscale value distribution map g n (x), where g n (x) is a rectangular wave, g n The peak value of (x) is B n The trough value is A n ; The ideal line-to-card grayscale value distribution map g n (x) is convolved with the point spread function of the CT image in step 4 to obtain the grayscale distribution map of the degraded line pair card; where the grayscale value of the material in the grayscale distribution map of the degraded line pair card is denoted as B′. n The gray value of the internal gap in the gray value distribution map of the degraded line pair card is denoted as A′. n ; The contrast value at the current spatial resolution Step 7, when MTF k If the penetration rate is ≥10%, then the current industrial CT system's equipment and process conditions are considered to allow penetration of a thickness of D. n Products.

2. The method for testing the penetration capability of an industrial CT system according to claim 1, characterized in that: In step 1, the diameter of the cylindrical test block is c, the length of the upper base of the longitudinal section of the through hole is b, the length of the lower base of the longitudinal section of the through hole is a, and the penetration capability of the industrial CT system being tested is x. Then, x, c, b, and a satisfy the following relationship: ca <x<c-b。 3. The method for testing the penetration capability of an industrial CT system according to claim 1, characterized in that: The specific steps for calculating the material center position of each CT image of the test block in step 3 are as follows: Histogram statistics were performed on the grayscale value distribution of all test block CT images, and threshold segmentation was performed on the histogram to distinguish between materials and internal voids; Using the threshold used in threshold segmentation to analyze the CT image of the test block n (x,y) is transformed into a binary image J n (x,y), the centroid method is used to calculate the material center position (x,y) of each CT image of the test block. c ,y c ).

4. The method for testing the penetration capability of an industrial CT system according to claim 1, characterized in that: The specific steps for achieving the penetration thickness of each CT image in step 3 are as follows: Using the material center of the test block CT image as the center, multiple radially distributed search lines are extended along the diameter of the internal void portion. The number of points with a gray value of 1 on each search line is counted in the corresponding binary image of the test block CT image. The average number of points with a gray value of 1 on all search lines is calculated, and this average is multiplied by the pixel size of the binary image to obtain the penetration thickness of the current test block CT image.

5. The method for testing the penetration capability of an industrial CT system according to claim 1, characterized in that: The point spread function calculation steps for the CT image in step 4 are as follows: Extract the grayscale values ​​of the CT image near the outer circle of the material part in each CT image of the test block, calculate the distance from each point in the annular region to the center of the corresponding material in the CT image of the test block, group the pixels in the annular region according to the distance unit from the center of the material in the CT image of the test block, and take the average pixel value of the pixels in each group to establish a correspondence graph between distance and average pixel value. This graph is the edge response function curve. The point spread function is obtained by differentiating the edge response function curve and then by fitting the point spread function with a Gaussian function to obtain the point spread function of the CT image.

6. The method for testing the penetration capability of an industrial CT system according to claim 1, characterized in that: In step 5, the gray values ​​of the internal void portion and the material portion in each CT image of the test block are obtained as follows: Calculate the average gray value of the internal void portion in each CT image of the test block, and then use this average gray value as the gray value of the internal void portion in the current CT image of the test block. Calculate the average gray value of the material portion in each CT image of the test block, and then use this average gray value as the gray value of the material portion in the current CT image of the test block.

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