Plenoptic x-ray imager with diffraction crystals

Abstract

The invention relates to a plenoptic imaging system for acquiring an image of an object, this system comprising: - a source for generating an X-ray beam towards the object, - a main optical device for receiving and reflecting the beam coming from the object, - a matrix of lenses or holes, - an array of photodetectors, each receiving an X-ray beam coming from a lens or a hole. According to the invention, the main optical device comprises at least one curved crystal.

Description

Title of the invention: Plenoptic imager for X-rays with diffraction crystals Technical field

[0001] The present invention relates to a plenoptic imaging system for acquiring an image of an object. It finds a particularly interesting application in the field of biological imaging ranging from cells to small animals, but also in materials science.

[0002] The invention aims at an improvement of so-called plenoptic or "lightfield" cameras in English. State of the prior art

[0003] Generally speaking, plenoptic imaging involves digitally refocusing an image from a single physical acquisition. This refocusing allows a scene to be reconstructed in three dimensions, and if the object is transparent (which is the case when using X-rays), the volume can be reconstructed in three dimensions.

[0004] In the visible, the plenoptic camera is composed of a lens or a set of lenses, for example a microscope objective, followed by a lens array and then a pixelated detector. This assembly creates numerous sub-images of the same scene or object which, after digital processing, can be digitally refocused to the desired distance.

[0005] We know the document T. Georgiev and C. Intwala, “Light field camera design for integral view photography,” Adobe System, Inc, Tech. Rep., 2003. This document describes the principle of a visible plenoptic camera.

[0006] The present invention aims at the design of a plenoptic camera for use with X-rays. This makes it possible to visualize certain objects in transparency and thus to carry out 3D reconstructions.

[0007] We know the document E. Longo et al., "Flexible Plenoptic X-ray Microscopy" Photonics, February 2022. This document describes the principle of an X-ray-based plenoptic camera. However, the lenses used do not allow 3D reconstructions. X-ray lenses, such as Fresnel lenses ("Fresnel Zone plate" in English) or refractive lenses have two intrinsic problems: very low numerical aperture and very high chromatism.

[0008] The first problem concerns the lateral and longitudinal resolutions which vary in 1 / NA and 1 / NA2 respectively, with NA the numerical aperture of the optical system. Optics for hard X-rays (i.e. for energies of photons above a few keV, which is equivalent to wavelengths below 1 nm) currently have numerical apertures of 10' 4typically. So the lateral resolution is equivalent to about 10,000 times the wavelength (i.e. 100 nm at 1 keV), which is very poor compared to visible optics which easily achieve a NA of 1 and even slightly higher than 1 with submerged microscopes. It should be noted here that the most powerful commercial visible microscopes have spatial resolutions much lower than 100 nm. As it stands, there is no point in moving into X-rays, which are technically much more complex to manage than visible.

[0009] The first problem is related to the aspect ratio between the lateral and longitudinal resolutions, which varies in 1 / NA and then produces voxels with an anamorphosis of 10,000. This anamorphosis prevents three-dimensional reconstruction. In such a case, performing plenoptics under X-rays is no longer of interest.

[0010] The second problem prevents the use of X-ray plenoptics with table sources that emit polychromatic radiation and therefore produce blurred images.

[0011] The present invention aims to provide a new X-ray plenoptic imaging system enabling the production of high-quality three-dimensional reconstructions.

[0012] Another object of the invention is a new high-resolution plenoptic camera. Statement of the invention

[0013] At least one of the objectives is achieved with a plenoptic imaging system for acquiring an image of an object, this system comprising: - a source to generate an X-ray beam towards the object, - a main optical device to receive and reflect the beam coming from the object, - a matrix of lenses or holes, - an array of photodetectors, each receiving a beam of X-rays from a lens or hole.

[0014] According to the invention, the main optical device comprises at least one curved crystal.

[0015] A photodetector array is, for example, any detector capable of capturing X-ray beams from the lens or hole matrix in a spatially distinct manner. Such a detector is, for example, a CCD sensor.

[0016] The present invention makes it possible to produce an X-ray camera capable of producing 3D views using the plenoptic technique. Indeed, the optical system for X-rays according to the invention proposes a main optical device with a numerical aperture greater than the systems of the prior art. Compared to X-ray systems using Fresnel lenses for example, the system according to the invention makes it possible to achieve a numerical aperture of the order of 0.3, which improves the lateral resolution by a factor of 3000 and the longitudinal resolution by a factor of 9 million. In addition, the anamorphosis of the voxels is only a factor of 3, which is easily compensated for numerically. These characteristics allow the production of true 3D reconstructions from a single acquisition.

[0017] The ratio of longitudinal resolution to lateral resolution varies as 1 / NA, with NA the numerical aperture.

[0018] Ideally, with the system according to the invention, we can ideally approach a numerical aperture equal to 1, in particular so as to have cubic voxels.

[0019] Preferably, the X-ray source and the curved crystal are determined so that the ratio 1 / NA is less than 10, therefore NA greater than 0.1

[0020] For example, one can define a useful surface (surface of the curved crystal capable of reflecting the incident X-ray beam) of the curved crystal large enough to ensure a strong NA greater than 0.1.

[0021] The X-ray beam reflected by the curved crystal forms an image of the object beyond the image focal plane of the curved crystal; the array of lenses or holes being placed beyond this image focal plane.

[0022] According to one embodiment of the invention, the source, the curved crystal and the orientation of the curved crystal can be determined so that the X-ray beam is reflected according to the diffraction law known as Bragg's law:

[0023] 2d sin( 9) =n. A

[0024] where 0 is the diffraction angle, X the wavelength of the X-rays and " " the lattice pitch, therefore the distance between two crystal planes of the curved crystal.

[0025] Compliance with this law ensures better reflectivity of the curved crystal.

[0026] According to a preferred embodiment, the source can be positioned on a first virtual circle, known as the Rowland circle, of radius R equal to half the radius of a second virtual circle in which a crystal plane of the curved crystal is inscribed.

[0027] The two virtual circles are concentric, the Rowland circle being adjacent to the central point of the curvature made on the curved crystal.

[0028] With such a circle, when the source is positioned on the circle, the focal point image of the curved crystal is also located on the same circle.

[0029] In addition to all of the above, the surface of the curved crystal can be polished to a curvature identical to the curvature of a crystal plane of the curved crystal according to a geometry known as Johann geometry.

[0030] In this case, the polishing carried out on the useful surface of the curved crystal makes it possible to obtain a surface said to be parallel to the crystallographic planes of the curved crystal. This achievement makes it possible to improve the numerical aperture, but only on a part of the surface presented by the curved crystal.

[0031] According to one embodiment, the surface of the curved crystal can be polished according to a curvature identical to the curvature of the Rowland circle according to a geometry called Johansson geometry.

[0032] In this case, the polishing carried out on the useful surface of the curved crystal makes it possible to obtain a surface which crosses crystallographic planes of the curved crystal. Such an achievement makes it possible to ensure a good numerical aperture on almost the entire polished surface presented by the curved crystal.

[0033] In Johan's or Johansson's geometry, the source is ideally positioned on the Rowland circle, but it is possible to consider positioning it inside or outside the circle.

[0034] According to one embodiment of the invention, the array of lenses or holes may be a one-dimensional array and each photodetector is a one-dimensional photodetector.

[0035] In such a case, one can consider the production of an image in one direction and the production of a spectrum in the other direction.

[0036] Alternatively, the lens or hole array can be a two-dimensional array and each photodetector is a two-dimensional photodetector.

[0037] According to a preferred embodiment of the invention, the curved crystal may be of the toric type.

[0038] In the system according to the invention, the toroidal crystal has two advantages: a large effective diffraction surface, leading to a high numerical aperture and good collection of the beam flux, beneficial for the image quality.

[0039] According to one embodiment of the invention, the curved crystal may be of the cylindrical or spherical type.

[0040] According to one embodiment of the invention, the curved crystal may be a single crystal with a single radius of curvature.

[0041] According to a preferred embodiment, the curved crystal may be a single crystal with double perpendicular radii of curvature.

[0042] According to one embodiment of the invention, the main optical device can comprise two crystals, each having a single radius of curvature, the two radii of curvature being perpendicular.

[0043] When there is only one radius of curvature, focusing is only in one direction. In such a case, two crystals can be used, placed at approximately 90° to each other, thus ensuring focusing in both directions. An example of such an implementation is the so-called Kirkpatrick-Baez assembly. This solution significantly reduces the constraints of producing curved crystals.

[0044] One can work with a 2D curved crystal, whatever the shape, or with two 1D curved crystals mounted together or with a curved crystal mounted alone.

[0045] According to an advantageous characteristic of the invention, the source can be a monochromatic source.

[0046] Any type of X-ray monochromator can be used, such as a crystal or diffraction grating monochromator.

[0047] Thus, with monochromatic radiation incident on the object, the reflectivity of the curved crystal can be very high, several tens of percent compared to a polychromatic source.

[0048]

[0049] According to a preferred embodiment, the monochromatic source may be a point source obtained from a polychromatic source illuminating a second curved crystal whose image focal plane constitutes said point source.

[0050] Such an arrangement allows the use of a second curved crystal to make the incident X-rays monochromatic. This second curved crystal can have identical or possibly different parameters from the first curved crystal. This second curved crystal is placed between the source and the object. This solution ensures that the X-rays passing through the object are reflected with very good efficiency by the curved crystal, greatly limiting the absorbed dose.

[0051] This is a considerable advantage over current X-ray tomography, which uses hundreds or thousands of views to form the three-dimensional image, producing a high dose on the person, animal or object as well as a long exposure time.

[0052] According to another aspect of the invention, there is provided a novel plenoptic camera comprising the system as described above and a processing unit for the digital reconstruction of images at different depths of field. Description of figures and embodiments.

[0053] Other advantages and features of the invention will appear on reading the detailed description of non-limiting implementations and embodiments, and the following attached drawings:

[0054] [Fig. 1]: Figure 1 is a schematic view of the system according to the invention,

[0055] [Fig. 2]: Figure 2 is a schematic top view of the system according to the invention illustrating the path of the X-ray radiation according to Johann geometry,

[0056] [Fig. 3]: Figure 3 is a schematic view illustrating Johann's geometry,

[0057] [Fig. 4]: Figure 4 is a schematic top view of the system according to the invention illustrating the path of the X-ray radiation according to Johansson geometry,

[0058] [Fig. 5]: Figure 5 is a schematic view illustrating Johansson's geometry, and

[0059] [Fig. 6]: Figure 6 is a schematic view of a curved crystal monochromator in the system according to the invention.

[0060] The embodiments which will be described below are in no way limiting; it will be possible in particular to implement variants of the invention comprising only a selection of characteristics described below isolated from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one preferably functional characteristic without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0061] In particular, all the variants and embodiments described are intended to be combined with each other in all combinations where there is no technical obstacle to this.

[0062] Although the invention is not limited thereto, a plenoptic X-ray imager based on a curved crystal having a toric surface will now be described.

[0063] In Figure 1 we can see a source 1 capable of emitting a beam of X-rays towards an object 2 to be imaged. The X-ray beam is then reflected by a curved crystal 3 suitably designed and positioned in alignment with the source 1 and the object 2. The reflected X-ray beam then reaches a lens matrix and then a detector 5. This is a plenoptic system in which the main lens has been advantageously replaced by a curved crystal having a high numerical aperture.

[0064] The beam passing through each lens of the matrix 4 is collected on a part of detector 5.

[0065] In Figure 1, an intermediate image 6 is simply illustrated for information purposes. It represents the image of the object, this image being contained in the reflected X-ray beam.

[0066] A single acquisition thus makes it possible to obtain several spatially separated flows at the detector level. Digital processing then makes it possible to carry out three-dimensional reconstructions in a conventional manner.

[0067] The X-ray source is a source at, for example, 11 keV. The invention can be applied to any type of X-ray. In particular, we speak of X-rays as soon as the optical index of the material is less than 1. Typically, this corresponds to around 20-30 eV. For the upper part, we can consider that gamma rays start at around 1 MeV when a photon can create an electron-positron pair.

[0068] Curved crystal 3 has a 3A surface that has been polished and positioned to serve as the working surface, i.e., the surface that receives and reflects the X-ray beam. Curved crystal geometry can be divided into two main aspects: the curvature of the internal crystal lattice planes and the curvature of the 3A working surface. Both can affect the imaging quality of the crystal, such as aberration, image fluence, and numerical aperture. Adjusting the curvature of the internal crystal lattice planes can optimize the focusing performance of the crystal. Polishing along a toric curve has been found to achieve excellent results.

[0069] Generally speaking, a crystal reflects X-rays by diffraction on the crystal planes which behave like a lattice. The diffraction law is the so-called Bragg law:

[0070] 2dsin(6)=n.

[0071] where 0 is the diffraction angle, X is the wavelength of the X-rays and ".d" is the lattice pitch, i.e. the distance between two crystal planes of the curved crystal. Therefore, each wavelength corresponds to a diffraction angle. Outside of this angle, the crystal does not reflect X-rays of this wavelength.

[0072] In reality, crystals are not perfect: the lattice planes are not all spaced the same distance apart, creating an angular broadening for which a crystal will reflect monochromatic radiation. This angular band is commonly called the oscillation curve or "rocking curve" in English, and obviously depends on each crystal. For example, for a germanium crystal cut along the (220) planes, the "rocking curve" has a width at half-height of 0.00383°, or 6.5 x 10' 5 rad. This is a very low numerical aperture.

[0073]

[0074] Due to Bragg diffraction, the curved crystal behaves like a curved mirror for X-rays.

[0075] The curved crystals according to the invention are advantageously concave curved.

[0076] A curved crystal has a "focus circle" in the horizontal plane tangent to its center point. The radius of this circle is equal to half the radius of curvature of the crystal (radius of curvature of the crystal planes). This circle is called the Rowland circle and was defined for a concave lattice in visible light. If a point source lies on the Rowland circle of a concave curved crystal, it will be focused at another point on this circle.

[0077] In Figure 1, Rowland circle 7 is tangent to the center of curved crystal 3. Source 1 is positioned on this circle. The object to be imaged is located between the source and the curved crystal, inside Rowland circle 7. Intermediate image 6 is located outside the Rowland circle in the optical axis of the reflected X-ray beam.

[0078] According to the invention, by bending the crystal and placing the source on a circle equivalent to twice the radius of curvature of the crystal, the rays arriving on the surface form an angle which remains within the oscillation curve ("rocking curve") for an area of ​​the crystal.

[0079] Figure 2 is a very schematic geometric representation in top view of the system according to the invention.

[0080] We distinguish a source A illuminating an object C to be imaged. Source A can be real or a point source created by a real remote source and whose image is relayed to this point.

[0081] The X-ray beam from A passing through C reaches an area of ​​the curved crystal B. The reflected beam is directed towards a lens array E before reaching the detector F.

[0082] The angle between the lines B1-B0-B2 and A-Bo satisfies the Bragg condition, Bo being the central bridge of the curved surface of the curved crystal.

[0083] Curved crystal B has a polished surface according to the so-called Johann geometry, that is to say the polishing is parallel to a crystal plane inside the curved crystal.

[0084] A Rowland circle G is represented. This circle has a radius equal to the half the radius of curvature of the polished surface of the curved crystal. With such a circle, the source point A placed on the circle has its focal point A' also placed on the same Rowland circle.

[0085] Since the crystal is curved in one or two dimensions, an intermediate image D, in one or two dimensions, is formed by respecting the formula for conjugation of thin lenses: l / CBo+l / BoD=l / f with f=RMsin(0) / 2 for the meridional plane and f=Rs / 2sin(0) for the sagittal plane, RM being the radius in the meridional plane and Rs being the radius in the sagittal plane. If the crystal is curved only in one dimension, then only the formula for the meridional plane is used.

[0086] The distances DE and EF respect the formula for thin lenses: l / DE+l / EF=l / f where here f is the focal length of the lenses placed in the E plane. In the case where a matrix of holes is used, there are no conditions to be met here.

[0087] Figure 3 illustrates the principle of Johann geometry. The rays from the crystal surface and the crystal plane P are the same, so only rays reaching an area near the center of the crystal satisfy the Bragg condition.

[0088] Even if the source divergence is large enough to illuminate the entire crystal, only the principal ray arriving at the center of the crystal satisfies the Bragg condition and can be intensely reflected. Although the crystal has a certain tolerance on the angle of incidence depending on its rocking curve, the width of the rocking curve is very narrow, 0.0029° for a nearly perfect crystal and 0.029° for an imperfect crystal. Therefore, the effective diffraction area is very small and the reflection efficiency of the crystal is quite low.

[0089] This can be improved by polishing the crystal in the P crystal planes and not parallel to them. This is the Johansson geometry as used in Figure 4.

[0090] Figure 4 also shows a very schematic geometric representation in top view of the system according to the invention. The principle and the elements are identical to those of Figure 2 except that the curved crystal is different.

[0091] The X-ray beam from A passing through C reaches the entire surface of the curved crystal B. The reflected beam is directed towards the lens array E before reaching the detector F.

[0092] Curved crystal B has a polished surface in accordance with the so-called Johansson geometry, that is to say the polishing has a curve identical to the curvature of the Rowland circle G.

[0093] Since the crystal is curved in one or two dimensions, an intermediate image D, in one or two dimensions, is formed by respecting the formula for conjugation of thin lenses: l / CBo+l / BoD=l / f with f=RMsin(0) / 2 for the meridional plane and f=Rs / 2sin(0) for the sagittal plane, RM being the radius in the meridional plane and Rs being the radius in the sagittal plane. If the crystal is curved only in one dimension, then only the formula for the meridional plane is used.

[0094] The distances DE and EF respect the formula for thin lenses: l / DE+l / EF=l / f where here f is the focal length of the lenses placed in the E plane. In the case where a matrix of holes is used, there are no conditions to be met here.

[0095] Figure 5 illustrates the principle of Johansson geometry. The radius of the inner surface of the crystal is equal to half the radius of its crystal plane P. Due to the inscribed angle theorem of a circle, such geometry allows the rays from the source to arrive on the entire surface of the crystal at the Bragg angle.

[0096] With the system according to the invention, the source can advantageously be positioned on the Rowland circle. In this case, the maximum numerical aperture NA can be determined by: max(height / 2; width / 2) / (source-crystal distance). In an exemplary embodiment with a useful surface of 2cm height and 4cm width and a source-crystal distance of 13cm, the maximum numerical aperture would be 2 / 13=0.15.

[0097] Figure 6 shows the geometry of a two-crystal plenoptic camera that can significantly reduce the dose received by the sample / animal / person. The elements of Figure 2 are found with a crystal B polished according to the Johann geometry. However, one or both crystals can be polished according to the Johann or Johansson geometry.

[0098] In Figure 6, source A is a point source formed from a real source A' offset. This real source A' is polychromatic and illuminates a second curved crystal B' so that the beam reflected by this second crystal is monochromatic. The focal point of the beam reflected by the second crystal B' is point source A.

[0099] Regarding the field of application of the invention, X-ray plenoptic imaging potentially affects all areas normally covered by X-ray tomography, ranging from medical to biology, materials science, metallurgy, mechanics, plasma physics, agri-food, cultural heritage etc.

[0100] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.

Claims

Claims

1. Plenoptic imaging system for acquiring an image of an object, this system comprising: - a source to generate an X-ray beam towards the object, - a main optical device comprising at least one curved crystal for receiving and reflecting the beam coming from the object, - a matrix of lenses or holes, - an array of photodetectors, each receiving a beam of X-rays from a lens or a hole, characterized in that the source is a monochromatic source.

2. System according to claim 1, characterized in that the source, the curved crystal and the orientation of the curved crystal are determined so that the X-ray beam is reflected according to the diffraction law known as Bragg's law: 2d sin( 9) =n. Â where 0 is the diffraction angle, X the wavelength of the X-rays and ".d" the reticular pitch, therefore the distance between two crystal planes of the curved crystal.

3. System according to any one of the preceding claims, characterized in that the source is positioned on a first virtual circle, known as the Rowland circle, of radius R equal to half the radius of a second virtual circle in which a crystal plane of the curved crystal is inscribed.

4. System according to any one of the preceding claims, characterized in that the surface of the curved crystal is polished according to a curvature identical to the curvature of a crystal plane of the curved crystal according to a geometry called Johann geometry.

5. System according to any one of claims 1 to 3, characterized in that the surface of the curved crystal is polished according to a curvature identical to the curvature of the Rowland circle according to a geometry called Johansson geometry.

6. A system according to any preceding claim, characterized in that the array of lenses or holes is a one-dimensional array and each photodetector is a one-dimensional photodetector.

7. System according to any one of claims 1 to 5, characterized in that the matrix of lenses or holes is a two-layer matrix dimensions and each photodetector is a two-dimensional photodetector.

8. System according to any one of the preceding claims, characterized in that the curved crystal is of the toric type.

9. System according to any one of claims 1 to 7, characterized in that the curved crystal is of the cylindrical type.

10. System according to any one of claims 1 to 7, characterized in that the curved crystal is of the spherical type.

11. System according to any one of claims 1 to 7, characterized in that the curved crystal is a single crystal with a single radius of curvature.

12. System according to any one of claims 1 to 7, characterized in that the curved crystal is a single crystal with double perpendicular radii of curvature.

13. System according to any one of claims 1 to 7, characterized in that the main optical device comprises two crystals, each having a single radius of curvature, the two radii of curvature being perpendicular.

14. System according to any one of the preceding claims, characterized in that the monochromatic source is a source point obtained from a polychromatic source illuminating another curved crystal whose image focal plane constitutes said source point.

15. A plenoptic camera comprising the system according to any one of the preceding claims and a processing unit for the digital reconstruction of images at different depths of field.

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