DEVICE AND METHOD FOR MEASURING ANGLES OF ORIENTATION OF AN X-RAY IMAGING SYSTEM

Abstract

A device and method for measuring angles of orientation of an x-ray imaging system including an x-ray source, an x-ray detector and a sample holder arranged to receive a sample to be analysed. The method includes: emitting a polychromatic x-ray beam through a reference sample installed on the sample holder in order to form a diffraction pattern on the sensing area of the x-ray detector, generating, with the x-ray detector, an observed image comprising the diffraction pattern, and determining the orientation of the x-ray source and the orientation of the x-ray detector by comparing the observed image with at least one target image comprising a diffraction pattern obtained for the reference sample with preset orientations of the x-ray source and of the x-ray detector.

Claims

1. Method for measuring angles of orientation of an x-ray imaging system, the x-ray imaging system comprising an x-ray source, an x-ray detector and a sample holder arranged to receive a sample to be analysed, the x-ray imaging system being arranged so that the x-ray source can emit a polychromatic x-ray beam along a main axis of propagation passing through the sample to be analysed and reaching a sensing area of the x-ray detector, an optical axis of the x-ray imaging system being defined between a centre of the x-ray source and the x-ray detector, the method comprising the steps of: emitting, from the x-ray source, a polychromatic x-ray beam along the main axis of propagation, so that the x-ray beam passes through a reference sample installed on the sample holder and forms a diffraction pattern on the sensing area of the x-ray detector, generating, with the x-ray detector, an observed image, the observed image comprising the diffraction pattern, and determining the orientation of the main axis of propagation and the orientation of the sensing area of the x-ray detector by comparing the observed image with at least one target image, the target image including a diffraction pattern obtained for the reference sample with a preset orientation of the main axis of propagation and a preset orientation of the sensing area of the x-ray detector.

2. Method according to claim 1, wherein the step of determining the orientation of the main axis of propagation and the orientation of the sensing area of the x-ray detector comprises comparing the observed image with a set of target images, each target image showing a diffraction pattern for a preset orientation of the main axis of propagation and a preset orientation of the sensing area of the x-ray detector, said orientations being different for each target image.

3. Method according to claim 1, wherein the step of determining the orientation of the main axis of propagation and the orientation of the sensing area of the x-ray detector comprises the substeps of: determining a similarity parameter representative of a degree of similarity between the diffraction pattern of the observed image and the diffraction pattern of a current target image for a given orientation of the main axis of propagation and a given orientation of the sensing area of the x-ray detector, comparing the similarity parameter with a similarity threshold, when the similarity parameter is less than the similarity threshold, identifying the given orientation of the main axis of propagation and the given orientation of the sensing area of the x-ray detector as effective orientations of the main axis of propagation and of the sensing area of the x-ray detector, and when the similarity parameter is greater than or equal to the similarity threshold, generating a new current target image, at least one of the given orientations for the current target image being different from the corresponding given orientation for the preceding target image, and reiterating the preceding substeps until the similarity parameter is less than the similarity threshold.

4. Method according to claim 2, wherein each target image is determined by a digital modelling of the diffraction generated by the reference sample installed on the sample holder, for a given orientation of the main axis of propagation and a given orientation of the sensing area of the x-ray detector.

5. Method according to claim 1 further comprising prior to the step of emitting the x-ray beam, a step of installing the reference sample on the sample holder.

6. Device for measuring angles of orientation of an x-ray imaging system, the x-ray imaging system comprising an x-ray source, an x-ray detector and a sample holder arranged to receive a sample to be analysed, the x-ray imaging system being arranged so that the x-ray source can emit a polychromatic x-ray beam along a main axis of propagation passing through the sample to be analysed and reaching a sensing area of the x-ray detector, an optical axis of the x-ray imaging system being defined between a centre of the x-ray source and the x-ray detector, the device comprising a processing unit configured to: emit, by the x-ray source, a polychromatic x-ray beam along the main axis of propagation, so that the x-ray beam passes through a reference sample installed on the sample holder and forms a diffraction pattern on the sensing area of the x-ray detector, generate, with the x-ray detector, an observed image, the observed image comprising the diffraction pattern, and determine the orientation of the main axis of propagation of the sensing area of the x-ray detector by comparing the observed image with at least one target image, the target image including a diffraction pattern obtained for the reference sample with a preset orientation of the main axis of propagation and a preset orientation of the sensing area of the x-ray detector.

7. Device according to claim 6 further comprising a first diaphragm placed upstream of the sample holder, and arranged to limit a transverse area of the x-ray beam.

8. Device according to claim 7, wherein the first diaphragm includes an opening a transverse area of which is smaller than a transverse area of the reference sample.

9. Device according to claim 7 further comprising a second diaphragm and/or a collimator placed between the x-ray source and the first diaphragm, arranged to obtain a collimated x-ray beam at the sample holder.

10. Device according to claim 6 further comprising a shutter element placed downstream of the sample holder, and arranged to prevent a portion of the x-ray beam having passed through the reference sample without being diffracted, from reaching the sensing area of the x-ray detector.

11. Device according to claim 6 further comprising the reference sample.

12. Method according to claim 1, wherein the reference sample is made of silicon.

13. Computer program comprising instructions that, when the program is executed by a computer, lead it to implement the method according to claim 1.

14. Device according to claim 6, wherein the reference sample is made of silicon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Other features, details and advantages of the invention will become apparent upon reading the following description, given only by way of example and made with reference to the appended drawings wherein:

[0040] FIG. 1 schematically shows an example of x-ray imaging system and of device for measuring angles of orientation of this system;

[0041] FIG. 2 shows the reference orthogonal mark and the orientation of the main axis of propagation of the x-ray source of the x-ray imaging system in relation to this orthogonal mark;

[0042] FIG. 3 shows the reference orthogonal mark and the orientation of the orthogonal mark related to the x-ray detector of the x-ray imaging system in relation to this orthogonal mark;

[0043] FIG. 4 shows an example of method for measuring angles of orientation of the x-ray imaging system according to the invention;

[0044] FIG. 5 shows a particular embodiment of a step of determining the orientation of the main axis of propagation of the x-ray source and of the orientation of the orthogonal mark related to the x-ray detector in the method shown in FIG. 4.

DETAILED DESCRIPTION

[0045] FIG. 1 schematically shows an example of x-ray imaging system and of device for measuring angles of orientation of an x-ray imaging system according to the invention. The x-ray imaging system 10 comprises an x-ray source 11, a sample holder 12 and an x-ray detector 13. The x-ray source 11 is arranged to emit a polychromatic x-ray beam 14 along a main axis of propagation X.sub.s. The x-ray beam 14 has a conical shape, the centre of emission of the source 11 corresponding to the top of the cone and the main axis of propagation X.sub.s corresponding to the axis of the cone. The sample holder 12 is positioned between the source 11 and the detector 13. It is arranged to receive a sample to be analysed by the x-ray imaging system and to hold it in a position making it possible to be passed through by the x-ray beam 14. Bearing surfaces of the sample holder 12 form reference planes defining a direct orthogonal mark XYZ of axes X, Y and Z. The axis X, called “optical axis”, passes through a centre of emission of the source 11. The x-ray detector 13 is a plane detector. It comprises a plurality of x-ray sensing elements, the sensing elements being disposed in a plane Y.sub.dZ.sub.d substantially parallel to the plane YZ and forming a sensing area 131.

[0046] FIG. 2 shows the orthogonal mark XYZ related to the sample holder 12 and the main axis of propagation X.sub.s of the x-ray source 11. The main axis of propagation X.sub.s forms an angle ξ.sub.xy with the optical axis X in the plane XY, that is to say by rotation about the axis Z, and an angle ξ.sub.xz with the optical axis X in the plane XZ, that is to say by rotation about the axis Y.

[0047] FIG. 3 shows the orthogonal mark XYZ related to the sample holder 12 and the orthogonal mark X.sub.dY.sub.dZ.sub.d related to the x-ray detector 13. The passage of the mark XYZ to the mark X.sub.dY.sub.dZ.sub.d is performed by a rotation of axis Z with an angle α, a rotation of axis Y with an angle β and a rotation of axis X with an angle γ.

[0048] FIG. 1 is again considered. The device for measuring angles of orientation of an x-ray imaging system 20 comprises a processing unit 21, a reference sample 22, a first diaphragm 23, a second diaphragm 24 and a shutter element 25. The reference sample 22 is formed in a single crystal material, for example made of silicon. It is placed on the sample holder 12 so that one of its crystal surfaces is parallel to the plane YZ. When passing through the reference sample 22, the polychromatic x-ray beam 14 is partially diffracted and is called “diffracted beam 15”. Indeed, in accordance with Bragg's Law, the wavelengths of the x-ray beam 14 that are in Bragg condition with crystal planes of the reference sample 22, generate constructive and destructive interferences, so that the x-ray beam is diffracted. The shape and the position of the diffracted beam 15 depend on the orientation of the x-ray source 11 and on the orientation of the reference sample 22. Insofar as the sample holder 12 and the reference sample 22 are used by convention to define the reference mark, the shape and the position of the diffracted beam 15 depend in practice on the angles ξ.sub.xy and ξ.sub.xz. The projection of the diffracted beam 15 on the sensing area 131 of the detector 13 forms a diffraction pattern consisting of spots. The shape and the position of the spots of the diffraction pattern therefore depend on the orientation of the source 11, but also on the orientation of the detector 13, that is to say angles α, ρ and γ. The detector 13 is arranged to generate an image comprising the diffraction pattern thus formed on its sensing area 131. This image is called “observed image”.

[0049] The processing unit 21 is arranged to synchronise the source 11 and the detector 13. In particular, it is arranged to emit the x-ray beam 14 by the source 11 and generate the observed image with the aid of the detector 13. The processing unit 21 is further arranged to determine, by an image processing applied on the observed image, the orientation of the source 11 and that of the detector 13. The image processing may comprise a first step wherein the position of the centroids of the diffraction spots is determined. Segmentation approaches combined with mass centre calculations may be used, just like deep-learning techniques using neural networks to distinguish the diffraction spots in images. The orientation of the source 11 and that of the detector 13 is subsequently determined in a second step by comparing the positions of the centroids of the diffraction spots in the observed image with the positions of the centroids of the diffraction spots in at least one reference image, called “target image”. Said target image includes a diffraction pattern obtained for the reference sample 22 with a preset orientation (known) of the source 11 and a preset orientation (known) of the detector 13. The target image includes for example a diffraction pattern obtained with a source 11 and a detector 13 perfectly aligned with the reference sample 22, that is to say with a main axis of propagation X.sub.s parallel to the optical axis X and a mark X.sub.dY.sub.dZ.sub.d combined with the mark XYZ. The determination of the orientation of the source 11 and of that of the detector 13 is described more precisely below, with reference to FIG. 4.

[0050] The first diaphragm 23 is placed upstream of the sample holder 12, for example as close as possible to the sample holder 12. It is arranged to limit a transverse area of the x-ray beam 14 in the plane XY. This limitation of area makes it possible to ensure that the entire x-ray beam reaching the detector 13 has effectively passed through the reference sample 22. The first diaphragm 23 includes for example an opening the dimensions of which in the plane YZ are smaller than the corresponding dimensions of the reference sample 22.

[0051] The second diaphragm 24 is placed between the source 11 and the first diaphragm 23. It is for example placed as close as possible to the source 11. It is arranged to obtain a collimated x-ray beam at the sample holder 12. The second diaphragm 24 is mainly of interest when the distance between the source 11 and the detector 13 is greater than or equal to 30 cm. This diaphragm 24 may advantageously be replaced with a collimator positioned so that its focal point is located at the centre of the source 11.

[0052] The shutter element 25 is placed downstream of the sample holder 12. It is for example placed as close as possible to the detector 13. The shutter element 25 is arranged to prevent a portion of the x-ray beam 14 having passed through the reference sample 22 without undergoing diffraction, from reaching the sensing area 131 of the detector 13. This portion of the x-ray beam 14, if it is not blocked, would form a central spot of relatively high intensity on the detector 13. This central spot would involve a relatively high electronic noise and therefore a lower resolution of the intensity measurement of the observed image. The shutter element 25 is formed in an x-ray opaque material, for example made of lead. When the x-ray beam 14 is collimated, the dimensions of the shutter element 25 may be equal to or slightly larger than the corresponding dimensions of the opening of the first diaphragm 23.

[0053] The first diaphragm 23, the second diaphragm 24 (or the collimator) and the shutter element 25 may form a one-piece unit. The reference sample 22 may also be integral with these optical elements.

[0054] FIG. 4 shows an example of method for measuring angles of orientation of the x-ray imaging system 10 shown in FIG. 1. The method 40 comprises a step 41 of installing the reference sample 22, a step 42 of emitting the x-ray beam 14, a step 43 of generating the observed image and a step 44 of determining the orientation of the source and of the detector. During step 41, the reference sample 22 is installed on the sample holder 12 so that one of its crystal planes is parallel with the plane YZ. The radiation source 11 may then be controlled by the processing unit 21 in step 42 in order to emit the polychromatic x-ray beam 14 according to the main axis of propagation X.sub.s. At the same time, the processing unit 21 may control the x-ray detector 13 in step 43 so that it generates the observed image, said image comprising the diffraction pattern produced by the reference sample 22. Finally, during step 44, the processing unit 21 determines the orientation of the main axis of propagation X.sub.s, defined by the angles ξ.sub.xy and ξ.sub.xz as well as the orientation of the sensing area 131 of the x-ray detector 13, defined by the angles α, ρ and γ. As indicated above, these orientations are determined by comparing the observed image with at least one target image including a diffraction pattern obtained for the reference sample with a preset orientation of the main axis of propagation X.sub.s of the source 11 and a preset orientation of the sensing area 131 of the x-ray detector 13.

[0055] According to a first embodiment, the determination of the orientation of the source 11 and of the detector 13 comprises a first step of determining the position of the centroids of the spots of the diffraction pattern in the observed image and a second step of comparing the observed image with a predetermined set of target images. Each target image shows a diffraction pattern obtained with the reference sample 22 for a preset orientation of the main axis of propagation X.sub.s and for a preset orientation of the sensing area 131. The orientations are different for each target image, so that the set of target images provides a representative sample of the diffraction patterns likely to be obtained with the x-ray imaging system 10. The comparison of the observed image with a target image involves for example determining an average of the distances between the centroid of each spot in the observed image and the centroid of each corresponding spot in the target image. The angles ξ.sub.xy and ξ.sub.xz as well as the angles α, ρ and γ are then determined as being those associated with the target image for which the average of the distances is the lowest. This target image has the diffraction pattern that best corresponds to the diffraction pattern of the observed image.

[0056] The various target images may be obtained by a digital modelling of the diffraction generated by the reference sample 22 installed on the sample holder 12, for a preset orientation of the main axis of propagation X.sub.s (angles ξ.sub.xy and ξ.sub.xz) and a preset orientation of the sensing area 131 of the detector 13 (angles α, β and γ). This digital modelling is for example performed with the aid of Laue diffraction modelling software such as the PyMicro software. Alternatively, the target images may be generated with the aid of a calibrated x-ray imaging system and of which the orientations of the source and of the detector are accurately controlled and measured. The position of the centroids of the spots of the diffraction pattern in each target image may be determined as those of the centroids of the spots of the diffraction pattern in the observed image.

[0057] FIG. 5 shows a second embodiment of the step 44 of determining the orientation of the main axis of propagation X.sub.s and of the sensing area 131 of the x-ray detector 13. In this embodiment, an iterative process is applied from an initial target image. More specifically, in a first substep 441, the position of the centroids of the diffraction spots in the observed image is determined, in a similar way to the first embodiment. In a second substep 442, a distance between the centroid of each spot of the diffraction pattern in the observed image and the centroid of the corresponding spot in the target image considered—namely the initial target image during the first occurrence of substep 442—is calculated. In a third substep 443, an average distance is calculated, the average distance being an average of the distances between the various spots in the observed image and the corresponding spots in the target image considered. In a fourth substep 444, the average distance is compared with a threshold distance D.sub.th. The threshold distance D.sub.th is determined depending on the accuracy desired for measuring the angles ξ.sub.xy, ξ.sub.xz, α, β and γ. If the average distance is less than the threshold distance D.sub.th, the angles ξ.sub.xy, ξ.sub.xz, α, β and γ associated with the target image considered are identified, in a substep 445, as being those defining the orientation of the source 11 and of the detector 13. On the other hand, if during substep 444, it is determined that the average distance is greater than or equal to the threshold distance D.sub.th, a new target image is generated in a substep 446 and substeps 442, 443 and 444 are reiterated with this new target image. The new target image is generated with an orientation of the main axis of propagation of the source 11 different from the orientation of the current target image and/or with an orientation of the sensing area 131 of the detector 13 different from the orientation of the current target image. Preferably, the angles ξ.sub.xy, ξ.sub.xz, α, β and γ are selected in such a way as to generate a new target image different from all of the preceding target images. Moreover, the angles ξ.sub.xy, ξ.sub.xz, α, β and γ may be selected depending on the average distance calculated for the current target image or on the various average distances calculated for the preceding target images.

[0058] It should be noted that, in the description above, it has been considered that the reference sample 22 was aligned with the reference mark XYZ, and particularly with the optical axis X of the x-ray imaging system 10. Nevertheless, the invention also applies when this condition is not met. Additional angles defining the orientation of the reference sample then need to be determined. For the embodiment involving a comparison with the diffraction patterns of a set of target images, this involves having a larger library of images, taking as additional degrees of freedom the angle or angles defining the orientation of the reference sample 22.

[0059] On the other hand, in the two embodiments of the step 44 of determining the orientation of the x-ray source 11 and the orientation of the x-ray detector 13, it has been considered that the observed image was compared to a target image based on the position of the centroids of the spots of their diffraction pattern. Nevertheless, instead of centroids, it is possible to take into account another point of the diffraction spots. Furthermore, the comparison of the observed image with a target image may take into account other properties of the diffraction patterns, such as the shape of the spots.