Method for characterizing optical focus of x-ray source

Abstract

The invention relates to a method for characterizing an optical focus of a source (11) emitting an X-ray beam (13) towards a detector (12), comprising the steps of evaluating ambiguity in a radiograph of a reference object (14) and determining the size of the optical focus. Evaluating ambiguity in a radiograph includes determining a grayscale curve of a section of interest in the radiograph, determining a point of interest of the grayscale curve from a second derivative of the grayscale curve, and determining deviations between intersection points between straight lines passing through the point of interest and upper and lower bounds of the grayscale curve, respectively.

Description

Technical Field

[0001] This invention relates to the field of radiology, and more specifically, to X-ray tomography.

[0002] This invention has significant advantages in the field of non-destructive testing of industrial components, especially those in the aerospace industry. Background Technology

[0003] Non-destructive testing (NDT) of components refers to a set of methods that can characterize the health status of a component without damaging or altering it.

[0004] This testing method is applied in various industrial sectors, including aerospace, automotive, and nuclear industries. In particular, non-destructive testing of aircraft components is a fundamental element of aircraft operational safety, aiming to prevent any defects that could lead to malfunctions during flight.

[0005] Among non-destructive testing methods, X-ray radiography stands out for its ability to observe the interior of parts with minimal invasiveness and resolve details down to the micrometer level. Standardized X-ray images are interpreted as images of the attenuation of X-rays as they pass through a part.

[0006] Tomography involves acquiring one thousand or more X-ray images of a component (which may have a highly geometrically complex structure) in order to calculate a complete three-dimensional image of the component.

[0007] To perform X-ray imaging on components, an X-ray imaging system is required. This system includes an X-ray generator that emits X-ray radiation and an X-ray-sensitive detector capable of capturing the radiation emitted by the generator. The detector converts the captured physical signal into an electrical signal that can be used to obtain an image or radiograph.

[0008] The detector can be a planar detector, i.e., a surface array of photodiodes, or a linear detector, i.e., multiple photodiodes arranged together. A planar detector can capture radiation emitted by the generator and partially absorbed by the component, as well as radiation scattered by the component under test (which is positioned between the detector and the generator). Conversely, a linear detector only captures radiation emitted by the generator and partially absorbed by the component, while radiation scattered by the component under test is filtered out. Unlike a planar detector, which is a surface array of photodiodes and can acquire large amounts of data in a short time, a linear detector acquires data line by line, which is more time-consuming.

[0009] On the other hand, in the high-energy domain, that is, when the emitted X-rays have energies greater than 1 MeV, the generator includes a linear particle accelerator, which itself includes a target.

[0010] A linear particle accelerator is configured to deliver electrons to a target, such that collisions between the target and the electrons produce X-ray emission. A linear particle accelerator is characterized in particular by the optical focus, or focal spot, corresponding to the imprint of the electron beam on the target, which is directly related to the spatial resolution of the acquisition. Therefore, characterizing this optical focus is crucial, especially in specifically determining its vertical and horizontal dimensions.

[0011] The IEC 62976 standard proposes a method for measuring the optical focal length of a linear particle accelerator using X-ray photographs taken from a reference component. The reference component is a copper block characterized by a hyperbolic paraboloid shape on its upper left surface.

[0012] The method for measuring the optical focal size of a linear particle accelerator according to the above standard includes the following steps.

[0013] First, take a radiograph of the reference component.

[0014] Secondly, the geometric magnification of the radiograph is determined by calculating the ratio between the size of the reference part captured on the radiograph and the actual size of the reference part.

[0015] Third, determine the blur in the ray photograph. More specifically, determine the blur based on the grayscale curve of the region of interest (ROI) depicting the transition between the interior and exterior of the reference component in the ray photograph. More specifically, the ROI extends between points on the ray photograph depicting the exterior of the reference component and points on the ray photograph depicting the interior of the reference component.

[0016] refer to Figure 1 The grayscale curve therefore includes a high grayscale H portion, a low grayscale B portion, and a transition region T connecting the high grayscale H portion and the low grayscale B portion. The distance between two points in the grayscale curve, corresponding to an upper limit L1 and a lower limit L2, is determined. The upper limit L1 is equal to 84% of the difference between the maximum and minimum values ​​of the grayscale curve, and the lower limit L2 is equal to 16% of the difference between the maximum and minimum values ​​of the grayscale curve. This distance is then multiplied by a predetermined coefficient of 1.47 to determine the blur value. Since the blur is directly related to the size of the optical focus of the linear particle accelerator, the size of the optical focus can be determined, specifically using a previously determined geometric magnification. The values ​​of 84%, 16%, and 1.47 are predetermined and defined in the IEC 62976 standard.

[0017] However, this standard and method were established when the vast majority of detectors used were linear detectors. Therefore, the method does not account for the effect of radiation scattered by the reference component, which can cause measurement distortion, such as... Figure 1 and Figure 2 As shown. More specifically, Figure 1The grayscale curves of the region of interest obtained from the X-ray image constructed using a linear detector are shown. Figure 2 The image shows a grayscale curve of the region of interest, depicting the same portion of a reference component, obtained from a ray photograph constructed using a planar detector. Figure 1 In the middle, the high grayscale H part and the low grayscale B part are basically horizontal, while... Figure 2 In this case, because the radiation scattered by the reference component is captured, it is not horizontal, making the methods recommended in the IEC 62976 standard, especially the predetermined values, unsuitable for characterizing planar detectors. Summary of the Invention

[0018] One object of the present invention is to provide a method for characterizing the optical focus of an X-ray source applicable to any type of X-ray detector, thereby overcoming the above-mentioned disadvantages.

[0019] To this end, according to a first aspect, a method for characterizing the optical focus of a source capable of emitting an X-ray beam toward a detector capable of generating a radiograph of an object positioned between the source and the detector is proposed. The method includes the steps of: evaluating the blurriness in the radiograph of a reference object positioned between the source and the detector; and determining the size of the optical focus based on the evaluation of the blurriness.

[0020] Evaluating blur in a ray image includes: determining the grayscale curve of a region of interest in the ray image, which continuously depicts a portion of the interior of a reference object, a portion of the outline of a reference object, and a portion of the exterior of a reference object; determining the point of interest (POI) in the transition region of the grayscale curve between the high and low grayscale portions of the grayscale curve based on the second derivative of the grayscale curve; and determining the positional deviation between the locations of the intersections of the line passing through the POI with the upper and lower boundaries of the grayscale curve, respectively.

[0021] Advantageously, the point of interest is a point in the grayscale curve located at the extremum of the second derivative of the grayscale curve.

[0022] Advantageously, prior to the step of determining the point of interest, the method includes: smoothing the second derivative of the grayscale curve using a moving average, and then determining the location of the extrema of the second derivative based on the smoothing.

[0023] Preferably, determining the size of the optical focal point includes: determining the geometric magnification of the X-ray photograph; and dividing the determined positional deviation by the geometric magnification of the X-ray photograph.

[0024] Advantageously, determining the geometric magnification of a radiograph includes the following steps: determining the pixel size of a reference object on the radiograph; and dividing the determined pixel size on the radiograph by the actual size of the reference object.

[0025] Advantageously, the pixel size of the reference object on the determined radiograph and the actual size of the reference object are the width of the reference object.

[0026] Preferably, the upper bound of the grayscale curve corresponds to 84% of the difference between the maximum and minimum values ​​of the grayscale curve, and the lower bound of the grayscale curve corresponds to 16% of the difference between the maximum and minimum values ​​of the grayscale curve.

[0027] Advantageously, the section of interest is perpendicular to the upper surface of the reference object.

[0028] Advantageously, the upper surface is a hyperbolic paraboloid type left surface.

[0029] The present invention also relates to a computer program product comprising program code instructions for performing, when the program is executed on a computer, steps of a method as defined above for characterizing the optical focus of a source capable of emitting an X-ray beam. Attached Figure Description

[0030] Other features, objects, and advantages of the invention will become apparent from the following description, which is illustrative only and not restrictive, and should be read in conjunction with the accompanying drawings, in which: Figure 1 The grayscale curves of the region of interest obtained from the ray photographs created using a linear detector are shown.

[0031] Figure 2 The grayscale curve of the region of interest obtained from the ray photograph created using a planar detector is shown.

[0032] Figure 3 An X-ray radiographic system capable of being used to implement the method according to the invention is illustrated schematically.

[0033] Figure 4 The steps of the method according to the invention are illustrated schematically.

[0034] Figure 5A The reference object is shown in perspective view with a top left surface as described in the IEC 62976 standard.

[0035] Figure 5B It shows Figure 5A The front view of the reference object.

[0036] Figure 6 It shows the result of Figure 3 The system takes a ray photograph of the reference object, on which two regions of interest are plotted.

[0037] Figure 7 It shows along Figure 6The first grayscale curve of one of the regions of interest in the ray photograph has a clear indication of its correlation with the assessment of the blurriness of the ray photograph.

[0038] Figure 8 It shows Figure 7 The second derivative of the first grayscale curve in the image.

[0039] Figure 9 It shows along Figure 6 The second grayscale curve of another segment of interest in the image.

[0040] In all the accompanying drawings, similar elements have the same reference numerals. Detailed Implementation

[0041] Figure 3 An X-ray imaging system 10 capable of being used in tomographic applications is schematically shown.

[0042] The X-ray imaging system 10 includes an X-ray source 11 that emits X-rays along the emission direction and an X-ray sensitive detector 12. The X-ray sensitive detector 12 is capable of acquiring the rays emitted by the source 11 and generating a radiographic image of the object positioned between the source 11 and the detector 12 based on the acquired rays. The detector 12 converts the acquired physical signals into electrical signals that can be used to obtain an image or radiographic image.

[0043] X-ray source 11 is capable of emitting an X-ray beam 13 known as a high-energy X-ray beam 13, which has an energy greater than 1 MeV. Therefore, X-ray source 11 includes a linear particle accelerator 11a, which includes a target 11b.

[0044] A linear particle accelerator 11a is configured to send electrons to a target 11b, such that a collision between the target 11b and the electrons produces the emission of an X-ray beam 13. The X-ray beam 13 emitted by the source 11 propagates along the emission direction indicated by arrow 13.

[0045] Detector 12 can be: a planar detector including a surface array of photodiodes, a linear detector including multiple photodiodes arranged together, or a unit detector including a single photodiode.

[0046] The X-ray imaging system 10 enables methods for characterizing the optical focus (or focal spot) of the source 11, and more specifically, the optical focus (or focal spot) of the linear particle accelerator 11a. Figure 4 Some steps of the method are illustrated schematically and will now be described.

[0047] In the first step E1, a reference object 14 is positioned on the trajectory of the X-ray beam 13 emitted by the source 11, between the source 11 and the detector 12.

[0048] Reference object 14 is preferably a copper block as described in IEC 62976. Alternatively, reference object 14 can be a block of different suitable materials, such as tungsten or any other material dense enough to provide significant X-ray absorption. Reference object may also have different shapes, but must have sharp edges perfectly aligned with the emission direction of X-ray beam 13.

[0049] In the following text, reference object 14 will be considered as the copper block as described in the IEC 62976 standard, and reference will now be made to... Figure 5A and Figure 5B A description will be provided. Specifically, reference object 14 is... Figure 5A The perspective view shows that, in Figure 5B The previous view is shown in the middle.

[0050] More specifically, reference object 14 includes a rectangular base 15, a rear surface 16, a front surface 17, a first side surface 18, and a second side surface 19. The first side surface 18, the second side surface 19, the front surface 17, and the rear surface 16 each extend perpendicularly from one side of the rectangular base 15. The front surface 17 and the rear surface 16 extend perpendicularly from the long side of the rectangular base 15. The front surface 17 includes a first upper ridge 20a opposite to the rectangular base 15 and extending between the first side surface 18 and the second side surface 19. Similarly, the rear surface 16 includes a second upper ridge 20b opposite to the rectangular base 15 and extending between the first side surface 18 and the second side surface 19. A plane P perpendicular to the plane of the front surface 17 and including the first upper ridge 20a forms a 15° angle with a plane P' perpendicular to the plane of the rear surface 16 and including the second upper ridge 20b. Reference object 14 also includes an upper surface 20 opposite to the rectangular base 15 and extending between the four ends of the first upper ridge 20a and the second upper ridge 20b. The upper surface 20 is configured such that the ray beam 13, which orthogonally passes through the front surface 17, passes through more of the material of the reference object 14 at the middle of the upper surface 20 than it does between the middle and the first side surface 18 and the second side surface 19. For example, the upper surface 20 is a hyperbolic paraboloid left surface.

[0051] Reference object 14 is located in the middle of the straight path between one end of the emitted beam 13 of source 11 and detector 12. More specifically, reference object 14 is oriented such that its front face 17 faces source 11 and is orthogonal to the direction of the beam 13 emitted by source 11. In any case, reference object 14 is oriented such that its sharp edge is perfectly aligned with the emission direction of beam 13.

[0052] In the second step E2, a radiograph of reference object 14 is taken.

[0053] This type of X-ray photograph, such as Figure 6 As shown.

[0054] In the third step E3, the blurriness of the X-ray photograph taken in step E2 is evaluated.

[0055] To this end, a first grayscale curve 21 is plotted for the first region of interest S1 (sub-step E31). The first grayscale curve 21 more precisely shows the grayscale of each point plotted on the first region S1 on the ray photograph, which is a function of the position of the point in the first region S1 (in pixels).

[0056] The first section S1 depicts the transition between the interior and exterior of the reference object 14. More specifically, the first section S1 continuously depicts a portion of the interior of the reference object 14, a portion of the outline of the reference object 14, and a portion of the exterior of the object 14.

[0057] Preferably, the first segment S1 extends vertically on the radiograph, that is, it extends along a direction perpendicular to the rectangular base 15. Furthermore, the first segment S1 is preferably located at the center of the upper surface 20 in the plane of the radiograph.

[0058] Figure 7 The first grayscale curve 21 is shown in the figure.

[0059] Therefore, the first grayscale curve 21 includes a high grayscale portion H (corresponding to a portion outside the reference object 14), a low grayscale portion B (corresponding to a portion inside the reference object 14), and a transition region T (corresponding to a portion of the outline of the reference object 14) between the high grayscale portion H and the low grayscale portion B.

[0060] Next, the transition region T is characterized (sub-step E32). In fact, the slope of the transition region T is related to the ambiguity; the steeper the slope, the lower the ambiguity. More specifically, in sub-step E32, points of interest belonging to the transition region T are identified. The first segment S1 is oriented and located as described above to constrain the transition region and facilitate its characterization.

[0061] These points of interest are from Figure 8 The second derivative 21a of the first grayscale curve 21 shown is determined. Specifically, the positions of the extrema of the second derivative 21a of the first grayscale curve 21 are determined and transferred to the first grayscale curve 21. More specifically, the points of interest are points P1 and P2 on the first grayscale curve 21 located at the said positions of the extrema of the second derivative 21a of the first grayscale curve 21. The points of interest P1 and P2 can therefore characterize the linear region of the transition region T, and more specifically, define the region with the steepest slope of the transition region T in terms of location.

[0062] Optionally, before determining the points of interest P1 and P2 in the transition region T, smoothing of the second derivative 21a of the first grayscale curve 21 can be performed, for example by performing a moving average or sliding average on the second derivative 21a, to facilitate the location of extrema. Alternatively, smoothing can be simply pre-performed by filtering out noise during acquisition when taking the radiograph.

[0063] When smoothing is performed on the second derivative 21a of the first grayscale curve 21, the extreme value position is determined based on the smoothing.

[0064] Based on the determined points of interest P1 and P2, perform extrapolation of the linear region defined by points of interest P1 and P2 (sub-step E33). More specifically, in sub-step E33, determine two intersection points I1 and I2.

[0065] The first intersection point I1 is the intersection between the straight line D through the previously determined points of interest P1 and P2 and the upper bound N1 of the first grayscale curve 21. For example, the upper bound N1 of the first grayscale curve corresponds to 84% of the difference between the maximum and minimum values ​​of the first grayscale curve 21.

[0066] Similarly, the second intersection point I2 is the intersection point between the straight line D and the lower bound N2 of the first grayscale curve 21. For example, the lower bound N2 of the first grayscale curve 21 corresponds to 16% of the difference between the maximum and minimum values ​​of the first grayscale curve 21.

[0067] Then the positional deviation between the first intersection point I1 and the second intersection point I2 is measured (sub-step E34). This positional deviation is an evaluation of blur in pixels (the first grayscale curve 21 is plotted as a function of the pixel position of the points in the first segment S1).

[0068] In the final step E4 of the method, the size of the optical focal point of source 11 is determined based on the ambiguity evaluated in step E3.

[0069] More specifically, in the first sub-step E41, the geometric magnification of the ray photograph taken in step E1 is calculated.

[0070] To this end, a portion of the reference object 14 is measured in pixels on the ray photograph and then divided by its known actual size. More specifically, a second grayscale curve 22 of the second region of interest S2 is plotted.

[0071] A second region of interest, S2, is defined on the radiograph. This second region of interest, S2, depicts a portion of the object, for example, and preferably, the width of the reference object 14. Preferably, as... Figure 6As shown, the second region of interest (ROI) S2 extends perpendicularly to the first side 18 and the second side 19 between a point on the exterior of the reference object 14 facing the first side 18 and a point on the exterior of the reference object 14 facing the second side 19. Therefore, the second ROI S2 completely passes through the reference object 14 across its width. Furthermore, the second ROI S2 passes through the middle of the first side 18 and the middle of the second side 19. Thus, the second ROI S2 continuously depicts the first portion of the exterior of the reference object 14, the outline portion of the first side 18, the interior portion of the reference object 14, the outline portion of the second side, and the second portion of the exterior of the reference object 14.

[0072] The second grayscale curve 22 is in Figure 9 As shown, and therefore includes a first high grayscale region H1 (corresponding to the first part outside the reference object 14), a first transition region T1 (corresponding to the outline portion of the first side 18), a low grayscale region B (corresponding to the inner part of the reference object 14), a second transition region T2 (corresponding to the outline portion of the second side 19), and a second high grayscale region H2 (corresponding to the second part outside the reference object 14).

[0073] To measure the width of the reference object 14, the deviation between two points P3 and P4 on the second grayscale curve 22 is measured. Points P3 and P4 are points in the first transition region T1 and the second transition region T2, respectively, both located at the same distance from the low grayscale region B, such that they are taken at the same height. Preferably, point P3 is located at the midpoint of the first transition region T1 between the first high grayscale region H1 and the low grayscale region B, and point P4 is located at the midpoint of the second transition region T2 between the second high grayscale region H2 and the low grayscale region B.

[0074] The width of reference object 14 in pixels is the distance between points P3 and P4 on the second grayscale curve 22.

[0075] Divide the width of reference object 14 (in pixels) by the actual width of reference object 14 (in metric units) taken at the midpoint between the first side 18 and the second side 19. The result of this division is the geometric magnification of the ray photograph.

[0076] Finally, in the second sub-step E42, the size of the optical focus of source 11 is determined by dividing the ambiguity evaluated in step E3 by the geometric magnification of the X-ray image determined in step E41. More specifically, the embodiment just described, particularly regarding the orientation of reference object 14 (front 16 orthogonal to the direction of X-ray beam 13), allows the size of the optical focus in the vertical direction to be determined during step E42. To determine the size of the optical focus in the horizontal direction, reference object 14 needs to be oriented such that the upper surface 20 is located at one of the sides 18 or 19, and then the method is repeated accordingly. By determining the ambiguity using the second derivative 21a of the first grayscale curve, the transition region can be characterized more accurately as appropriate, whereas the method according to IEC 62976 only considers transition regions with fixed boundaries, without considering detector 12.

[0077] Therefore, the method according to the invention can more accurately characterize the optical focus of an X-ray source, including a linear particle accelerator, regardless of the nature of the detector used, which in particular can be a planar detector.

Claims

1. A method for characterizing the optical focus of a source (11), the source being capable of emitting an X-ray beam (13) toward a detector (12), the detector (12) being capable of generating a radiograph of an object disposed between the source (11) and the detector (12), the method comprising the steps of: The evaluation (E3) is set to the blurring in the X-ray image of the reference object (14) between the source (11) and the detector (12); as well as The size of the optical focus (E4) is determined based on the assessment of the ambiguity. The assessment of blurriness in the X-ray images described in (E3) includes: Determine (E31) the grayscale curve (21) of the region of interest (S1) in the radiograph, wherein the region of interest (S1) continuously depicts a portion of the interior of the reference object (14), a portion of the outline of the reference object (14), and a portion of the exterior of the reference object (14); Based on the second derivative (21a) of the grayscale curve (21), determine (E32) the points of interest (P1, P2) in the transition region (T) of the grayscale curve (21) between the high grayscale portion (H) and the low grayscale portion (B) of the grayscale curve (21); and Determine the positional deviation between the positions of the intersection points (I1, I2) of (E34), where the intersection points (I1, I2) are the points where the straight line (D) through the points of interest (P1, P2) intersects the upper boundary (N1) and lower boundary (N2) of the grayscale curve (21), respectively.

2. The method according to claim 1, wherein, The points of interest (P1, P2) are points in the grayscale curve (21) located at the extreme values ​​of the second derivative (21a) of the grayscale curve (21).

3. The method according to claim 2, wherein, Before the step (E32) of determining the points of interest (P1, P2), the method includes the step of smoothing the second derivative (21a) of the grayscale curve (21) using a moving average, and then determining the extreme positions of the second derivative (21a) based on the smoothing.

4. The method according to any one of claims 1 to 3, wherein, Determining the size of the optical focal point (E4) includes: Determine the geometric magnification of the radiograph described in (E41); Divide the determined positional deviation by (E42) the geometric magnification of the radiograph.

5. The method according to claim 4, wherein, Determining the geometric magnification of the radiograph (E41) includes the following steps: Determine the pixel size of the reference object (14) on the radiograph; and Divide the pixel size on the determined ray photograph by the actual size of the reference object (14).

6. The method according to claim 5, wherein, The pixel size of the reference object (14) on the determined radiograph and the actual size of the reference object (14) are the width of the reference object (14).

7. The method according to any one of claims 1 to 6, wherein, The upper bound of the grayscale curve (21) corresponds to 84% of the difference between the maximum and minimum values ​​of the grayscale curve (21), and the lower bound of the grayscale curve (21) corresponds to 16% of the difference between the maximum and minimum values ​​of the grayscale curve (21).

8. The method according to any one of claims 1 to 7, wherein, The region of interest (S1) is perpendicular to the upper surface (20) of the reference object (14).

9. The method according to claim 8, wherein, The upper surface (20) is a left surface of the hyperbolic paraboloid type.

10. A computer program product comprising program code instructions for performing, when the program is executed on a computer, the steps of a method for characterizing the optical focus of a source (11) capable of emitting an X-ray beam (13) according to any one of claims 1 to 9.

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