CN111307841B - Method suitable for measuring small gap of cone beam CT - Google Patents
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Abstract
The invention discloses a method suitable for measuring a small gap of cone beam CT, which comprises the steps of manufacturing a reference piece according to a piece to be measured, and establishing a mapping curve related to a gap measured value and an actual value by utilizing the reference piece; scanning a piece to be detected, and carrying out CT reconstruction imaging; and determining small-gap imaging through contrast searching based on the CT image, calculating a gap measurement value by using the determined small-gap calculation formula, and obtaining an actual value of the gap according to the mapping curve. The invention can be applied to a cone beam CT image based on a panel to obtain the size of the small gap under the imaging condition, design a similar reference piece according to the piece to be measured to obtain a mapping curve, measure the small gap of the piece to be measured, allow the design of the reference piece to have larger deviation with the piece to be measured, or directly use the value calculated by a small gap calculation formula as the actual value result of measurement, and does not need to design a similar reference piece, thereby reducing the cost.
Description
Technical Field
The invention belongs to the field of nondestructive testing application, and particularly relates to a method suitable for measuring a small gap of cone beam CT.
Background
Industrial CT is mainly used for structure detection and density detection, and when measuring the structure size of an object, the edge of the object in an image is generally extracted, and then the structure size of the object is measured by measuring the edge distance. The most commonly used dimension measurement in CT images is the full width at half maximum measurement method. In practice, it can be found that the conventional full width at half maximum measurement method can cause the measured value to be much larger than the actual value due to the CT broadening effect occurring at the small gap edge. When the size of the small gap is smaller than the size corresponding to one pixel of the image, ideally, the imaging width of such a small gap on the image is at most one pixel. In practice, the imaging width of such a small gap is much larger than one pixel. The measurement of small gaps is a field in which the field of nondestructive testing has been neglected for a long time, and has a great influence on the test results.
Regarding the measurement of small gap, the current research is mainly performed in the institute of materials of chinese institute of engineering and physics, which has conducted a lot of research in the fields of small gap imaging simulation, small gap measurement, etc. over the past decade, and has formed some effective theories and measurement methods, and written the software for small gap measurement. The achievement is in patents (CN104596449B, 2017.07.07), which discloses a small gap accurate measurement method based on CT image, and the technology has some limitations:
1) the method is basically applied to a second-generation or third-generation linear array CT system, in recent years, the application of area-array-based cone-beam CT is more and more extensive, the imaging result of an area array is different from that of the linear array, for example, reconstruction is not accurate, a reconstruction cone angle effect exists, stronger scattering artifacts exist, hardening correction is more complicated, and meaningful measurement cannot be carried out simply by adopting the method disclosed by the patent CN 104596449B.
2) The measurement method described in patent CN104596449B has too many limitations and no universality, and the use experience cannot meet the requirements. If the patent requires that the gap is adjusted to the vertical direction before measurement, and then the gray value distribution of one line of data is analyzed, this is a strong, not reasonable constraint, and may bring bad use experience. The patent judges the size of the gap by calculating the pixel mean value in a rectangular area containing the gap with a specified size, and when the imaging quality of the CT image is not particularly good, the pixel mean values of the CT image in the rectangular area containing the gap image with the specified size in the images formed by the gap with the same size are greatly different. The mean values of the CT image pixels in a rectangular area of the same size containing the gap images in the images of the same size gaps in different materials are also very different. These all limit the application of this patent.
Therefore, the prior art is subject to further improvement and development.
Disclosure of Invention
In order to solve the above problems, a method suitable for small gap measurement in cone beam CT is proposed.
The invention provides the following technical scheme:
a method suitable for cone beam CT small gap measurement, comprising the following steps:
and manufacturing a reference part according to the to-be-measured part, and establishing a mapping curve about the measured value and the actual value of the clearance by using the reference part.
And scanning the piece to be detected, and carrying out CT reconstruction imaging.
And determining small-gap imaging through contrast searching based on the CT image, calculating a gap measurement value by using the determined small-gap measurement formula, and obtaining an actual value of the gap according to a mapping curve.
The method is suitable for measuring the small gap of the cone beam CT, wherein the reference piece and the piece to be measured are made of the same or similar materials in density, structure and external dimension.
A method suitable for measuring small gaps in cone beam CT is disclosed, in which the gaps in the reference member are distributed along non-crossed straight lines, and the widths of the gaps are in an arithmetic progression.
A method suitable for measuring small gaps of cone beam CT is disclosed, wherein a mapping curve about the measured values and the actual values of the gaps is established by using a reference piece, and the mapping curve comprises the following steps:
making a series of straight line segments along the normal direction of the Nth gap, wherein the middle points of the straight line segments are positioned on the middle line of the gap;
sampling the image to obtain CT image values on the straight line segments, respectively calculating the width of a concave curve on each straight line segment, and solving the maximum width value;
determining the width of the integration area by using the maximum width value, determining the integration area gapro by combining a length value gapconcrete along the length direction of the gap, and calculating the sum of the number of pixels gapconcrete pixel Nr of the gap in the integration area gapconcrete and the pixel value of the integration area gapconcrete; calculating to obtain a zero-gap pixel mean value gaprovgnogap according to a surrounding area of the integration area, and selecting an internal air area or an external area of the reference piece to calculate to obtain an air area pixel mean value gaprovgievgair; the gap width measurement is further calculated using a small gap calculation formula:
gapLM＝(gapRoiPixelNr*(gapRoiAvgNoGap-gapRoiAvgAir)-(gapRoiSum-gapRoiPixelNr*gapRoiAvgAir))/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))＝(gapRoiPixelNr*gapRoiAvgNoGap-gapRoiSum)/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))。
and carrying out curve fitting by utilizing the clearance measured value and the clearance actual value to obtain a mapping curve.
A method suitable for cone beam CT small gap measurement is provided, wherein the width of an integration region is obtained by adding a maximum width value to a width pre-adding value, and the integration region is determined through calculation.
When the condition for manufacturing a reference piece is not met, a result is measured by using a small gap calculation formula to serve as an actual value of the gap.
A method suitable for cone beam CT small gap measurement is provided, wherein after CT reconstruction imaging is carried out on a piece to be measured or a reference piece, a mapping curve is calculated or an image used for measurement is an arbitrary slice on three-dimensional data.
Advantageous effects
The invention provides a method suitable for measuring a small gap of cone beam CT, which has the following beneficial effects:
(1) the method has wide application, can be applied to cone beam CT which is difficult to perform small gap measurement by using a conventional half-height-width method, can also be applied to second-generation or third-generation linear array CT, and can perform meaningful measurement;
(2) the method can be applied to the cone beam and CT images based on the panel to obtain the small gap size under the imaging condition;
(3) the method allows the design of the reference part to deviate greatly from the part to be measured, or directly uses the value calculated by the clearance reference value calculation formula as the measurement result, so that a similar reference part does not need to be designed, and the cost is reduced.
Drawings
FIG. 1 is a flowchart of a method for measuring small gaps in cone beam CT according to an embodiment of the present invention;
FIG. 2 is a schematic diagram illustrating the distribution of CT values along a line perpendicular to a gap of different sizes in accordance with an embodiment of the present invention;
FIG. 3 is a graph showing the distribution of CT values along a line perpendicular to the gap and the results of the full width at half maximum measurement according to one embodiment of the present invention;
FIG. 4 is a schematic diagram of the gap distribution of the small gap defect on the circle near the distance R2 from the center of the cylinder of the DUT in embodiment 1 of the present invention;
fig. 5 is a schematic diagram of two region distributions of the reference piece used for calculating gaprovgagnogap on a circle at a distance of about R2 from the center of the cylinder in embodiment 1 of the present invention.
Detailed Description
In order to make the technical solutions of the present invention better understood, the technical solutions of the present invention are described below clearly and completely with reference to the accompanying drawings of the present invention, and based on the embodiments in the present application, other similar embodiments obtained by a person of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present application. In addition, directional terms such as "upper", "lower", "left", "right", etc. in the following embodiments are directions with reference to the drawings only, and thus, the directional terms are used for illustrating the present invention and not for limiting the present invention.
A method suitable for cone beam CT small gap measurement, wherein the flowchart is shown in fig. 1, and comprises the following steps:
s1: and manufacturing a reference part according to the to-be-measured part, and establishing a mapping curve about the measured value and the actual value of the clearance by using the reference part.
Furthermore, the reference piece and the piece to be detected have the same or similar material density, the same or similar structure and the same or similar external dimension.
In particular, in CT imaging, the CT imaging quality of two similar objects has great similarity. The similarity of the two objects means that the densities of the materials of the objects are the same or similar, the lengths of the rays passing through the objects are not greatly different, further similarity also comprises similarity of object structures, the outline dimensions of the objects are not greatly different, and if the position of the measuring position from the outer surface of the object is fixed, the similarity of the objects also comprises similarity of the measuring positions. Therefore, a reference piece with the same or similar material density, the same or similar structure and the same or similar external dimension as the to-be-measured piece can be designed, a mapping curve of the reference piece is obtained, a relation between a gap measurement value and an actual value is established by using the reference piece, and then the small gap in the to-be-measured piece is measured. In the face of a CT imaging task, reference parts in the similar sense above are designed according to a measured part, and under the condition that the similarity is not available or unknown, the reference parts with low similarity can also be designed, if the condition is met, the similar reference parts are designed as far as possible.
Further, the gaps in the reference part are distributed along non-intersecting straight lines, and the widths of the gaps form an arithmetic progression without limiting the distribution rule.
Specifically, the size range of the gap is gapWMin-gapWMax, gapWMin is the minimum value of the gap, gapWMax is the maximum value of the gap, the width of the nth gap from small to large is gapWN, and the number of the gaps is Nr.
S2: and scanning the reference piece, and carrying out CT reconstruction imaging.
Further, the size of the actual object corresponding to one voxel of the CT reconstruction is VoxelSize, the size of the gap is designed to be VoxelSize, CT scan imaging is performed, the gap size gapSize is measured by using a full width at half maximum, the maximum value gapWMax of the gap is less than or equal to the gapSize in principle, and the gapWMax can be defined as VoxelSize.
Further, the gapWMin is determined such that a gap of the size of gapWMin is visible to the naked eye. The gap in the reference member is of sufficient length. The Nr gaps are distributed on the reference part according to a certain rule by the principle that the Nr gaps are similar to the part to be measured as much as possible.
S3: and determining small-gap imaging through contrast searching based on the CT image, calculating a gap measurement value by using the determined small-gap measurement formula, and establishing a mapping curve according to the measurement value and the actual value of the gap.
The research result of the accurate CT measuring method (Yangguan; 11.2008, volume 28, 6 th period, nuclear electronics and detection technology) for the curved surface type micro-gap of different materials proves that the average gap width and the integral value of the CT value in an integral area are in a linear relation. Thus, the gap value can be measured by integrating the specific area.
Further, the mapping curve established by the reference member with respect to the measured value and the actual value of the clearance is specifically:
(1) a series of straight line segments are made along the normal direction of the Nth gap, the middle points of the straight line segments are positioned on the middle line of the gap, and the process can be manually or automatically completed.
(2) Sampling the image to obtain CT image values on a straight line segment, wherein the distribution of the values is shown in FIGS. 2-3, FIG. 2 is a schematic diagram of the distribution of the CT values on the straight line perpendicular to gaps with different sizes, the gaps correspond to different curves 1, 2, 3 and 4, the smaller the gap is, the higher the curve is, the larger the CT value is represented, and the curve 1 represents the curve corresponding to the minimum gap; FIG. 3 is a graph showing the distribution of CT values along a line perpendicular to the gap and the full width at half maximum measurement, and shows that the actual value of the gap with a measured length L of 4.677mm is much smaller than the full width at half maximum measurement of the CT image.
(3) The maximum width value is added to the width pre-value to obtain the width of the integration area, the length of the integration area gapRoi is smaller than the length of the gap, and the integration area gapRoi is determined through calculation.
Specifically, as shown in fig. 2, the width of the concave curve on each straight line segment is calculated, and the maximum width values IntegralWidthMax and IntegralWidthMax are obtained, and the appropriate distance marginDis is added to obtain the BC segment on fig. 2, where the width Lbc is IntegralWidthMax + marginDis, and this region is the width of the integration region gapro.
Further, the midpoint of the integration region gapro is on the center line of the gap.
Further, the number of pixels gaprotixelnr of the integration area gapro and the sum of the pixel values gaprisum of the integration area gapro are calculated using the length value gapRoi of the integration area gapro in the gap length direction; the zero-gap pixel mean value gaprovgnogap is calculated according to the pixels of the area around the integral area gaprov, and the area where the reference piece contains air is selected to calculate the air area pixel total number mean value gaprovavgair.
Further, the gapriovg air can be calculated by using a cavity inside the object, and can also be calculated by using an air area outside the object. When the condition for calculating gaproavg air is not met, the value may be set to a constant, which is ideally 0.
The clearance measurement is further calculated using the formula:
gapLM＝(gapRoiPixelNr*(gapRoiAvgNoGap-gapRoiAvgAir)-(gapRoiSum-gapRoiPixelNr*gapRoiAvgAir))/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))＝(gapRoiPixelNr*gapRoiAvgNoGap-gapRoiSum)/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))。
further, if there is no small gap inside gapRoi, gapLM is close to 0; if all are gaps, the value is close to gapconcrete PixelNr/gapRol, which is the width of gapRol. As a result, Nr data pairs, where gapNL represents the actual width of the Nth gap and the data pairs are represented by (gapLM, gapNL), where N is 0 to Nr-1, can be obtained.
(4) Curve fitting is performed using the calculated gap measurement values and the actual gap values, i.e. the data are curve fitted to (gapLM, gapNL), where the curve contains straight lines or the points are directly connected in a coordinate system, and the resulting curve is the mapping curve mapCurve for the measurement.
Furthermore, the measured piece is scanned and reconstructed by CT for imaging. And (4) judging which target is the image formed by the small gap on the image, acquiring gapro in a similar way of the reference piece, calculating gapLM, and then obtaining the measured value of the small gap of the measured piece according to the mapping curve mapCurve obtained in the step (4).
Further, when the condition for manufacturing the reference piece is not met, the actual value of the clearance on the piece to be measured is calculated by using a small clearance calculation formula of the reference piece.
Furthermore, after the CT reconstruction imaging of the to-be-measured or reference object, the image for calculating the mapping curve or for measuring is not limited to the directly reconstructed slice of the CT, and may be any slice on the three-dimensional data. Images formed due to small gaps are only visible in certain cross-sections. The small gap is typically an air gap, but may be another material within the other material. The latter case is handled in a similar manner to the former case.
Further, it is judged whether or not it is the small gap imaging. The small gaps are gaps with the width below the actual size of an object represented by one pixel on the CT image, the imaging size of the gaps on the CT image is larger than the imaging size of the gaps, the imaging of the gaps on the CT image is visually lighter than the CT value of the large gaps, and the corresponding pixel values are smaller, so that whether an object on the CT image is a small gap or not can be judged. Before the small gap measurement work is started, the CT imaging quality is improved as much as possible. Hardening artifacts, FDK reconstruction cone angle effects, noise, metal artifacts, scattering and the like have great influence on the measurement result, and the negative influence factors should be eliminated as much as possible, and low-density inclusions are excluded from the target to be measured.
The cone beam CT comprises cone beam spiral CT, and the method provided by the invention can be applied to the cone beam CT which is difficult to perform small gap measurement by using a conventional full width at half maximum method, and can also be applied to second generation or third generation linear array CT.
Detailed description of the preferred embodiment 1
The following describes an embodiment of the patented method of the invention, taking as an example the detection of small gaps in a steel cylinder with a section of a circle of 100mm diameter:
(1) the small gap defect to be measured is on a circle near the distance R2 from the center of the piece cylinder as shown in fig. 4, where gap is the gap. The reference piece is designed to be the same in material, shape and size as the piece to be measured, and the gaps are distributed on a circle near the distance R2 from the center of the cylinder, as shown in FIG. 5, wherein rect1 is the region outside the circle of R2, rect2 is the region inside the circle of R2, and the two regions are used for estimating the average value of the region near R2 sandwiched between rect1 and rect 2.
(2) A CT system with a detector of 400mmx300mm and an accelerator of 9MV and a probe element of 0.4mm is selected to scan the object and the reference element, the magnification ratio is 2, and the size of the object corresponding to the reconstructed image element is 0.2 mm. Gaps below 0.2mm are conventionally not imaged, but actually have a reconstructed image on the CT image, and are typically larger than 0.2mm in size. Therefore, the size range of the gaps on the design reference member is 0.02mm to 0.2mm, the sizes of the adjacent gaps are distributed in an arithmetic progression mode, and the size difference between the adjacent gaps is 0.02 mm. For ease of calculation, these gaps are designed as straight lines. Thus there are 11 gaps on the reference.
(3) The width Lbc of the integration zones is calculated, gapro is determined, and the mean value of these gaps in these zones gaprosum is calculated. Then, the mean value of the regions near the gap is calculated as gaprioavgnnogap, such as rect1 and rect2 regions in fig. 5, and this region selection mode can effectively reduce the influence caused by hardening. The area outside the reference is selected to calculate gaproavg air. And calculating gapLM according to a small gap calculation formula through the values, adding prior information about the size of the gap, selecting a polynomial curve as a fitting target curve, and fitting the polynomial curve.
(4) Scanning a piece to be measured to obtain a CT sectional image, selecting a proper section, determining which target is a small gap through contrast, and obtaining a measurement result through calculating gapLM and mapping of a mapping curve.
In this embodiment, a mapping curve is obtained by designing a similar reference member, and then a small gap in the to-be-measured member is measured. In many cases, similar references are designed either without such conditions or at such a cost. The method allows the design of the reference part and the part to be measured to have larger deviation or directly uses the value calculated by a small clearance calculation formula as the measurement result.
The present invention has been described in detail, and it should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
Claims (5)
1. A method suitable for cone beam CT small gap measurement is characterized by comprising the following steps:
manufacturing a reference piece according to the piece to be measured, and establishing a mapping curve about a clearance measurement value and an actual value by using the reference piece;
scanning a piece to be detected, and carrying out CT reconstruction imaging;
determining small-gap imaging through contrast searching based on the CT image, calculating a gap measurement value by using the determined small-gap measurement formula, and obtaining an actual value of the gap according to a mapping curve;
the gaps in the reference piece are distributed along non-crossed straight lines, and the width of the gaps forms an arithmetic progression;
the use of the reference piece to establish a mapping curve with respect to the measured values and the actual values of the clearance is specified by:
making a series of straight line segments along the normal direction of the Nth gap, wherein the middle points of the straight line segments are positioned on the middle line of the gap;
sampling the image to obtain CT image values on the straight line segments, respectively calculating the width of a concave curve on each straight line segment, and solving the maximum width value;
determining the width of the integration area by using the maximum width value, determining the integration area gapro by combining a length value gapconcrete along the length direction of the gap, and calculating the sum of the number of pixels gapconcrete pixel Nr of the gap in the integration area gapconcrete and the pixel value of the integration area gapconcrete; calculating to obtain a zero-gap pixel mean value gaprovgnogap according to a surrounding area of the integration area, and selecting an internal air area or an external area of the reference piece to calculate to obtain an air area pixel mean value gaprovgievgair; the gap width measurement is further calculated using a small gap measurement formula:
gapLM＝(gapRoiPixelNr*(gapRoiAvgNoGap-gapRoiAvgAir)-(gapRoiSum-gapRoiPixelNr*gapRoiAvgAir))/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))＝(gapRoiPixelNr*gapRoiAvgNoGap-gapRoiSum)/(gapRoiL*(gapRoiAvgNoGap-gapRoiAvgAir))；
and carrying out curve fitting by utilizing the clearance measurement value and the actual value to obtain a mapping curve.
2. The method for measuring the small gap in cone beam CT according to claim 1, wherein the reference member and the object to be measured have the same or similar material density, structure and external dimension.
3. The method of claim 1, wherein the integration zone is computationally determined by adding a maximum width value to a width pre-addition value to obtain an integration zone width.
4. The method for measuring the small gap in cone beam CT according to claim 3, wherein when the condition for making the reference member is not satisfied, the result is measured by a small gap calculation formula as the actual value of the gap.
5. The method for cone-beam CT small gap measurement as claimed in claim 4, wherein the image used for calculating the mapping curve or for measurement is an arbitrary slice on the three-dimensional data after CT reconstruction imaging of the object or the reference object.
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