Hartmann type X-ray imaging system

Abstract

The invention relates to a Hartmann phase imaging system (1, 10, 100) comprising an X-ray source (2); a sensor (3) comprising elementary sensors (31); at least one mask (4) comprising holes (41) and arranged between the X-ray source and the detector, each portion (F41) of a beam emitted by the source (2) passing through a hole in the mask forming a spot (T) on the sensor; at least one diffusing filter (6) configured to homogenize each spot formed on the sensor; a processing unit (5) configured to determine, from the offsets (Δx, Δy) between the position of each spot and a reference position (x0, y0), a deflection image of said sample. Figure to be published with the abstract: Fig. 1

Description

Title of the invention: Hartmann type X-ray imaging system

[0001] The invention relates to the field of X-ray phase imaging. More specifically, the invention relates to a Hartmann type imaging system.

[0002] X-rays are electromagnetic waves capable of passing, depending on their energy, through a majority of materials over significant thicknesses, typically from several millimeters to several meters. It is thus possible to use these X-rays to produce, in a non-invasive manner, images containing information relating to the internal structure of a sample, taking into account the absorption, scattering and phase shift of the X-rays when they pass through this sample. These X-rays thus find numerous applications, in particular in the medical field in radiography, in the field of security for the analysis of suitcases or containers, in the food industry and more generally in applications requiring non-destructive testing.

[0003] When an X-ray beam passes through a sample, this beam can be refracted locally by a local change in refractive indices induced by the inhomogeneous structure of the sample, this refraction introducing a phase change of the beam. It is thus possible to resample the beam into sub-beams which are projected onto sensors to obtain an image of the variations, or contrast, in the phase of the sample, which makes it possible to visualize the internal structure of the sample, even when this structure has little impact on the absorption of the beam.

[0004] There are thus different families making it possible to generate an image representative of the phase variations of a sample, and in particular interferometry techniques based on the analysis of interference between a reference beam and a beam passing through the sample, and deflectometry techniques based on the analysis of the local deflection of the beam.

[0005] Hartmann systems are based on these deflectometry techniques. A Hartmann system typically comprises an X-ray source, a pixelated sensor, and a mask comprising a plurality of holes interposed between the source and the sensor and placed at a given distance from the sensor. The mask is also called a Hartmann plate. The beam emitted by the X-ray source will pass through each of the holes in the mask to create a set of spots on the sensor.

[0006] The system can thus be used initially to obtain a reference image of all the spots formed on the sensor, in order to record their intensities and positions. The positions of the spots in the reference image are given only by the distance between the mask and the sensor, by the size and dimensioning of the holes and the wavefront of the beam emitted by the source. Other techniques can be used to generate this reference measurement, in particular by diffracting the beam emitted by the X-ray source by a predetermined object in order to create a wave whose wavefront is known.

[0007] In a second step, a sample can be positioned between the X-ray source and the mask, or after the mask. This sample will locally absorb and / or refract the X-rays, depending on its internal structure, which will individually vary the intensity of the spots formed on the sensor and / or individually move these spots. More precisely, the horizontal and vertical shifts of each spot are functions of the local derivatives of the phase variation induced by the local deflections of the X-rays by the sample and of the sensor-mask distance. The measurement of the intensity variations, relative to the reference image, thus makes it possible to obtain an absorption or transmission map indicating the local absorption or transmission properties of the sample.The measurement of the displacements of the spots along axes orthogonal to the direction of propagation of the beam emitted by the source, relative to the reference image, makes it possible to obtain deflection or displacement maps and, by integration, a phase variation map indicating the local refraction or reflection properties of the sample.

[0008] In this context, it is necessary for a processing unit of the Hartmann system to be able to determine the position of each spot in the image acquired by the sensor. It is thus common for the processing unit to estimate a centroid of each spot, for example by calculating the barycenter of the intensities measured by groups of pixels or by interpolating these intensities by a given function. These techniques offer satisfactory results if there is complete decoupling between the local intensity measurement and the local displacement measurement, in particular so that the intensity distribution in each spot formed on the sensor is uniform or homogeneous.

[0009] However, it is common for this intensity distribution to be significantly modified by the local absorption or transmission capacities of the sample. Indeed, the internal structure of the sample can generate local absorption or transmission of only part of the portion of the X-ray beam entering or leaving a hole. This may be the case, in the field of medical imaging, for a blood vessel or another absorbent anatomical element, which obscures part of a hole. In this case, the intensity distribution in the spot formed on the sensor and corresponding to this hole is modified in a non-homogeneous manner, which leads to a displacement of the barycenter or an error in the interpolation of the spot. The modification of the intensity distribution can then be interpreted as a displacement of the spot, adding to the actual displacement of the spot generated by the local deflection of the portion of the X-ray beam entering this hole.

[0010] In other words, to the extent that the structure of a sample can locally be both absorbent and refracting / reflecting, it becomes complex to dissociate at the level of the displacement maps, the absorption and refraction / reflection phenomena, which then leads to errors when interpreting the phase variation map of the sample.

[0011] In order to solve this problem, it is known to reduce the size of the holes in the mask in order to diffract the portions of the X-ray beam passing through these holes and thus to homogenize the intensity distribution of the spots formed on the sensor by these diffracted portions.

[0012] This solution, however, has several drawbacks. On the one hand, it leads to signal losses at the sensor, which must be compensated by an increase in the X-ray dose to which the sample is exposed. However, in a medical imaging context or in applications in which the samples are radiosensitive, this increase in dose is not desirable.

[0013] On the other hand, the diffraction of a portion of the X-ray beam generates a spot on the sensor whose dimensions are inversely proportional to those of the hole. In order to be able to maintain sensors whose pixels have reasonable dimensions with respect to the normal dimensions of the samples conventionally studied, it is therefore necessary to increase the dimensions of the spots formed by the mask and therefore to further reduce the dimensions of the holes. This reduction then requires an additional increase in the X-ray dose to which the sample is exposed.

[0014] Finally, in order to guarantee the operation of the system, the mask must remain opaque to X-rays between the holes. Given the increases in the X-ray dose caused by the diffraction of the beam by the holes, it then becomes necessary to increase the thickness of the mask in these opaque areas, in order to prevent the propagation of the beam. The mask then becomes complex, or even impossible, to produce industrially. In addition, the holes become tunnels which cause a loss of angular resolution at the output.

[0015] There is thus a need for a Hartmann-type imaging system, which makes it possible to minimize, or even eliminate, the impact of the local absorption or transmission properties of a sample in the generation of the deflection image of the sample without reducing the size of the holes in the Hartmann mask.

[0016] The present invention is placed in this context and aims to meet this need.

[0017] For these purposes, the invention relates to a Hartmann phase imaging system of a sample including: has. an X-ray source for emitting a beam of X-rays; b. a sensor capable of receiving said light beam and comprising a plurality of elementary sensors; c. at least one mask comprising a plurality of holes, the mask being arranged between said X-ray source and the detector such that a beam emitted by the X-ray source passes through the holes of the mask, each portion of said beam passing through a hole of the mask forming a spot on the sensor; d. at least one diffusing filter configured to homogenize each spot formed on the sensor by each portion of the beam emitted by the X-ray source and passing through each hole in the mask; e. a processing unit configured to determine, from the offsets between the position of each spot formed on the sensor by a beam emitted by the X-ray source and passing through a sample arranged between the X-ray source and the detector and a reference position, a deflection image of said sample.

[0018] It is thus understood that the invention thus proposes to interpose a diffusing filter in a Hartmann system, between the X-ray source and the mask and / or between the mask and the sensor. The beam emitted by the X-ray source therefore successively passes through the sample, the diffusing filter and the mask, regardless of the order of crossing, to reach the sensor. Each portion of the beam emitted by the X-ray source therefore forms a spot on an elementary sensor or a group of elementary sensors, the intensity distribution in this spot depending mainly on the portion of the diffusing filter through which this portion of the beam has passed.

[0019] In other words, the diffusing filter thus makes it possible to form on the sensor, from each portion of the beam having passed through the sample, a spot whose intensity distribution is substantially identical to that of a spot formed by the same portion of the beam having passed through a sample whose local absorption or transmission properties would be different. The diffusing filter thus makes it possible to homogenize, over the entire spot, this intensity distribution in the same way as a hole of reduced dimensions which would diffract the X-ray beam, while overcoming the disadvantages of this reduction in the dimensions of the hole. In particular, it will be possible to consider that the intensity distribution of a spot is homogeneous if the gradient of the intensities of the spot is less than a given threshold value at any point of the spot.Therefore, the determination of the position of the centroid of a spot becomes robust to local variations in absorption or transmission of the sample. It is therefore possible to obtain a deflection image of the sample, and therefore ultimately a phase image, or phase variation, of the sample, the construction of which is substantially decoupled from the local absorption or transmission properties of . the sample.

[0020] In the present invention, the term "X-ray source" means any device capable of emitting a beam of X-rays, and in a non-limiting manner in a spectral range between 30 eV (electronvolts) and several MeV (megaelectronvolts). The X-ray source may be arranged to generate a monochromatic or polychromatic beam. It may, for example, be an X-ray tube, in particular a micro-focus X-ray source, a synchrotron, a particle accelerator, a free electron laser, a plasma source, a Compton source, or more generally any generator capable of producing X-rays by the interaction of particles with an emitting element.

[0021] Advantageously, it will be possible to interpose one or more filters or any element or combination of elements capable of modifying the spectrum of the X-ray beam, in particular in order to adapt the spectral band of the beam to the constraints of the desired applications.

[0022] In the present invention, the term "mask" means any element or combination of elements provided with holes, i.e. zones transparent to X-rays, distributed over a structure opaque to X-rays. The term "opaque structure" means a structure configured so that its residual transmission to X-rays emitted by the X-ray source, in particular in a useful spectral band of said X-rays, is lower than the detection sensitivity of the sensor of the system. The term "transparent zone" means any zone configured so that its transmission to X-rays emitted by the X-ray source, in particular in a useful spectral band of said X-rays, is typically greater than 80%. The term "useful spectral band" of X-rays means a band of wavelengths selected with regard to the intended application. For example, in mammography, a useful spectral band between 20 and 40 keV is usually used.

[0023] As a non-limiting example, it is possible to envisage materials composed of elements with a low atomic number, in particular less than 15, such as Beryllium, Carbon, Silicon or Aluminum, or vacuum to constitute a transparent zone. As a non-limiting example, it is possible to envisage, to constitute an opaque structure, materials composed of elements with an atomic number greater than 28, chosen according to the energy level of the X-rays considered with regard to the intended application. For example, for so-called hard X-rays, with an energy greater than 10 keV, it is possible to envisage using materials composed of Gold, Tantalum, Tungsten or Lead.

[0024] The mask may, for example, comprise a two-dimensional arrangement of holes, of given shapes and dimensions such that the spot formed by the portion of the beam passing through each hole covers, partially or totally, one or several elementary sensors of the sensor.

[0025] In the present invention, the sensor may be an indirect type sensor, comprising for example a plurality of elementary sensors, in particular photodetectors, capable of detecting radiation whose spectrum belongs to the visible range and a fluorescent conversion element capable of converting, by fluorescence, X-ray radiation into visible radiation. As a variant, the sensor may be a direct type sensor, comprising for example a plurality of elementary sensors capable of detecting X-ray radiation.

[0026] The sensor may, for example, comprise a two-dimensional arrangement of identical elementary sensors. Said elementary sensors may be combined with each other, electronically and / or digitally, to form a plurality of groups of juxtaposed elementary sensors, each group comprising, for example and in a non-limiting manner, four or nine elementary sensors arranged in a square.

[0027] In the present invention, the term "processing unit" means an electronic device and / or a computer system designed to determine in an analog and / or digital manner, from the signals generated by the elementary sensors responding to the reception of a beam emitted by the X-ray source and passing through a sample arranged between the X-ray source and the detector, one or more deflection images of said sample. It may be provided that the processing unit is equipped with one or more processors, arranged to execute instructions of one or more computer programs in order to implement calculation or processing steps making it possible to generate said deflection image. It may be provided that these steps are implemented centrally by the same processing unit or in a distributed manner by several processing units.It may also be provided that all or part of these steps are implemented by a processing unit connected by wire to the sensor and / or by a processing unit remote from the sensor with which it can exchange data via wireless communication means.

[0028] Advantageously, the processing unit can be arranged to estimate the position of each spot formed by a portion of the beam emitted by the X-ray source and passing through a sample arranged between the X-ray source and the detector and passing through a hole (or conversely the hole then the sample) and to estimate the offset, in one or more directions, between this estimated position relative to a reference position associated with this hole. It will be noted that these offsets are a function of the local gradients of the phase of the X-ray beam induced by the sample. The processing unit can thus be arranged to generate images or deflection maps along two axes orthogonal to the direction of emission of the X-ray beam from the estimated offsets and then to generate, from these deflection maps, said phase image of the sample. The processing unit could for example be arranged to generate a phase image by integrating the images or deflection maps.

[0029] In the present invention, the term "deflection image" or "deflection map" means an image comprising a plurality of image elements, each image element indicating the offset, in a given direction, between the position of a spot formed on a sensor by a portion of a beam emitted by an X-ray source and passing through a sample and a hole in a Hartmann mask, or conversely a hole in a Hartmann mask and a sample, and a reference position associated with this hole.

[0030] In the present invention, the term "phase image" or "phase variation image" or "phase contrast image" is understood to mean an image comprising a plurality of image elements, each image element being characteristic of a local phase shift or a local deflection introduced by an elementary region of a sample, resulting from an inhomogeneity present in said elementary region, the phase shift or the local deflection being determined with respect to a reference phase.

[0031] The reference positions associated with the holes in the mask may, for example, be obtained by estimating the positions of the spots formed on the sensor in the absence of a sample between the X-ray source and the detector. Alternatively, the reference positions may be calculated from a known wavefront, which may in particular be obtained by diffracting the beam emitted by the X-ray source upstream of the sample, using a predetermined object.

[0032] In one embodiment of the invention, the processing unit is arranged to estimate the position of each spot by determining a centroid of said spot.

[0033] For example, it may be envisaged that the processing unit is arranged to estimate the position of each spot by determining the barycenter of a spot formed on an elementary sensor or on a group of elementary sensors. Alternatively, the processing unit may be arranged to estimate the position of each spot by interpolating a spot formed on an elementary sensor or on a group of elementary sensors by a given function, in particular a Gaussian function or a bi-Gaussian function, said function making it possible to determine a centroid of this spot.

[0034] Advantageously, the processing unit may be configured to simultaneously determine, from the intensities of each spot formed on the sensor by a beam emitted by the X-ray source and passing through a sample arranged between the X-ray source and the detector and from a reference intensity, an absorption or transmission image of the sample. For example, the processing unit may be arranged to determine a ratio between the sum of the intensities measured by the elementary sensors of each group of elementary sensors with and without sample in order to obtain a map representing the transmission or absorption of the sample.

[0035] In one embodiment of the invention, the diffusing filter is arranged between the X-ray source and the mask. In this embodiment, a beam emitted by the X-ray source successively passes through the diffusing filter and the mask to then be received by the sensor.

[0036] Alternatively, the diffusing filter may be arranged between the mask and the sensor. In this embodiment, a beam emitted by the X-ray source successively passes through the mask and then the diffusing filter to then be received by the sensor.

[0037] As a further variant, the diffusing filter may comprise a plurality of sub-filters, each arranged at one of the holes in the mask. In this embodiment, each sub-filter may thus extend into one of the holes in the mask, and each portion of the beam emitted by the X-ray source is thus homogenized by a sub-filter when it passes through a hole.

[0038] In one embodiment of the invention, the diffusing filter extends along dimensions greater than or identical to those of the surface formed by all of the holes in the mask. In this embodiment, the diffusing filter may be a single filter, which has the advantage of being simple to design.

[0039] Alternatively, the diffusing filter may comprise several separate sub-filters, each positioned upstream of the mask and / or downstream of the mask and / or in a hole in the mask, each sub-filter thus being positioned to correspond to a hole in the mask with which it is associated, a portion of the beam emitted by the X-ray source thus passing through the hole and the associated sub-filter. It is thus possible to define a sub-filter specific to each hole.

[0040] As a further variant, the diffusing filter may be formed by a periodic repetition, vertical and / or horizontal, of the same diffusing sub-filter. Where appropriate, the dimensions of the sub-filter will be substantially smaller than those of the mask and substantially greater than or equal to those of a hole in the case where the diffusing filter is arranged between the X-ray source and the mask or of dimensions substantially less than or equal to those of a hole if the diffusing filter is arranged between the mask and the sensor. It may be envisaged that the sub-filters are juxtaposed continuously to cover the entire surface of the diffusing filter, the sub-filters being spaced from each other, each being positioned to correspond to a hole in the mask.

[0041] As a further variant, the system according to the invention may comprise a plurality of diffusing filters, juxtaposed according to the propagation of the beam emitted by the X-ray source. These filters may all be arranged between the X-ray source and the mask, or as a variant all arranged between the mask and the sensor, or as a further variant arranged on either side of the mask.

[0042] The different embodiments of the diffusing filter which have just been described may be combined without departing from the scope of the present invention.

[0043] In one embodiment of the invention, the diffusing filter comprises, for each hole, a plurality of diffusing structures associated with said hole, each diffusing structure of said plurality being arranged to diffract the portion of the beam emitted by the X-ray source and passing through said hole. These diffusing structures absorb, phase shift and / or diffuse this portion of the X-ray beam and thus make it possible to form a spot on the sensor by means of the beams diffracted from this portion of the X-ray beam, which makes it possible to homogenize the intensity distribution of the spot by minimizing the impact of the local absorption or transmission properties of the sample.

[0044] Advantageously, each diffusing structure of a plurality of diffusing structures associated with a hole may have dimensions substantially smaller than that of said hole. It is thus possible to obtain a diffracted beam whose size is close to, equal to or greater than that of the task which must be formed on the detector.

[0045] In one embodiment of the invention, each plurality of diffusing structures associated with a hole has determined characteristics such that the shape and / or the intensity distribution of the spot formed on the sensor by the portion of the beam emitted by the X-ray source and passing through said hole follows a Gaussian distribution. Alternatively, it may be provided that each plurality of diffusing structures associated with a hole has determined characteristics such that the shape and / or the intensity distribution of the spot formed on the sensor by the portion of the beam emitted by the X-ray source and passing through said hole follows a defined distribution making it possible to homogenize said spot.Said characteristics of a plurality of diffusing structures may be one of the following characteristics or any combination of the following characteristics: the types of diffusing structure, the shapes and dimensions of the diffusing structures, the positions and spatial distribution of the diffusing structures or the composition and materials of the diffusing structures. These characteristics may thus be predetermined so that the plurality of diffusing structures reproduce the homogenization or smoothing generated by the diffraction carried out by a small hole.

[0046] In particular, it will be possible to design the diffusing structures to be microholes or pads.

[0047] It will also be possible to conceive that the diffusing structures have square, circular, elliptical, hexagonal or triangular shapes.

[0048] It could also be conceived that the shapes and / or dimensions and / or composition and / or materials of the diffusing structures are determined so that the diffusing structures are totally absorbent or opaque, or partially absorbent or opaque, or are transparent.

[0049] It could also be conceived that the shapes and / or dimensions and / or composition and / or materials of the diffusing structures are determined so that the diffusing structures are capable of introducing phase shifts.

[0050] Advantageously, the types and / or positions and / or shapes and / or dimensions and / or absorption coefficients and / or diffusion coefficients of the diffusing structures of a plurality of diffusing structures associated with a hole are distributed pseudo-randomly over the whole of said plurality. As a variant, the types and / or positions and / or shapes and / or dimensions and / or absorption coefficients and / or diffusion coefficients of the diffusing structures of a plurality of diffusing structures associated with a hole may be distributed according to a predetermined function making it possible to obtain a homogeneous distribution of intensity of the spot.

[0051] In an exemplary embodiment, it may for example be provided that the diffusing structures of the same plurality of diffusing structures associated with a hole are identical and that these structures are distributed spatially in a pseudo-random manner with respect to the surface of the hole.

[0052] In another exemplary embodiment, it may be provided that the dimensions of the diffusing structures of the same plurality of diffusing structures associated with a hole, and in particular their thickness, are defined in a pseudo-random manner over the entirety of this plurality.

[0053] In the present invention, the term "pseudo-random distribution of a characteristic over a plurality of diffusing structures" means any predetermined characteristic distribution obtained by means of a pseudo-random generator. The term "pseudo-random generator" means any algorithm capable of generating a statistically random sequence, in particular uniformly distributed, over a given length. In particular, pseudo-random generators may be used whose periodicity is substantially greater, in particular ten times greater, than the total number of diffusing structures that can be distributed over a surface associated with a hole in the mask.

[0054] According to an exemplary embodiment of the invention, the diffusing filter comprises a substrate made of silicon nitride on which are formed a plurality of gold pads forming said diffusing structures. It will be possible to replace the silicon nitride substrate with a substrate made of a suitable material that is substantially transparent to X-rays, in particular silicon, silicon carbide or silicon boride. It will also be possible to replace the gold pads with pads made of nickel, tungsten, tantalum, iron, molybdenum or any other material capable of diffracting an X-ray beam.

[0055] According to an exemplary embodiment of the invention, the diffusing filter comprises a carbon layer filling the spaces separating the diffusing structures. It is possible to replace the carbon layer with a layer made of silicon, beryllium, boron or any other suitable material and having a low X-ray absorption capacity, or conversely an opaque material. This layer of material makes it possible to avoid vibration phenomena of the diffusing structures, which would be detrimental with regard to the precision required by the system according to the invention.

[0056] The invention also relates to an X-ray phase imaging method, implemented by means of a Hartmann system according to the invention.

[0057] The present invention is now described using examples which are purely illustrative and in no way limitative of the scope of the invention, and from the appended drawings, drawings in which the various figures represent:

[0058] [Fig-1] represents, schematically and partially, a view of a system Hartmann phase imaging according to one embodiment of the invention;

[0059] [Fig.2A] represents, schematically and partially, an example of a spot projected onto a sensor of a Hartmann system according to the state of the art, in the absence of a sample;

[0060] [Fig.2B] represents, schematically and partially, an example of a spot projected onto a sensor of a Hartmann system according to the state of the art, in the presence of a non-absorbent sample;

[0061] [Fig.2C] represents, schematically and partially, an example of a spot projected onto a sensor of a Hartmann system according to the state of the art, in the presence of an absorbent sample;

[0062] [Fig.2D] represents, schematically and partially, an example of a spot projected onto a sensor of the Hartmann system of [Fig.l], in the presence of an absorbent sample;

[0063] [Fig.3] represents, schematically and partially, a front view of a diffusing filter used by a Hartmann system according to an exemplary embodiment of the invention;

[0064] [Fig.4] represents, schematically and partially, a sectional view of a filter diffusing agent used by a Hartmann system according to another exemplary embodiment of the invention;

[0065] [Fig.5] represents, schematically and partially, a view of a Hartmann phase imaging system according to a second embodiment of the invention;

[0066] [Fig.6] represents, schematically and partially, a view of a Hartmann phase imaging system according to a third embodiment of the invention.

[0067] In the following description, elements which are identical, by structure or by function, appearing in different figures retain, unless otherwise specified, the same references.

[0068] [Fig.l] shows a Hartmann 1 system for phase imaging of a sample according to a first embodiment of the invention. This Hartmann system can be used for medical imaging applications, such as in mammography, or for other applications outside the medical field, in particular for analyzing the contents of a suitcase or a container.

[0069] The system 1 comprises an X-ray source 2 for emitting a beam F of X-rays in a global propagation direction Z. This source 2 may for example comprise an X-ray tube or alternatively any other X-ray source capable of emitting an X-ray beam of energy adapted to the application considered.

[0070] The system 1 also comprises an X-ray sensor 3, arranged downstream of the source 2 in the direction Z, to be able to receive the beam F emitted by this source 2.

[0071] The sensor 3 comprises a two-dimensional arrangement of identical elementary sensors 31. Each elementary sensor is thus capable of detecting incident X-rays and of emitting an analog or digital signal in response. The signals from several juxtaposed elementary sensors, in particular from four or nine elementary sensors 31 arranged in a square, can thus be combined with each other, electronically and / or digitally, to define groups of sensors.

[0072] By way of non-limiting example, the sensor 3 may for example comprise a flat panel detector, comprising a matrix of detectors, in particular a semiconductor photosensitive to visible light, on which is deposited a fluorescent conversion layer capable of converting X-rays into visible light rays. The sensor 3 may also comprise a photon counting camera comprising a matrix of electrically polarized semiconductor sensors for converting X-rays into electrical signals, or a direct detection camera comprising a matrix of CCD or CMOS sensors sensitive to X-rays.

[0073] The system 1 comprises a mask 4, arranged between the X-ray source 2 and the sensor 3.

[0074] The mask 4 is opaque to X-rays, for example due to the choice of the material composing it and / or its thickness, and comprises a plurality of holes 41 transparent to X-rays. The holes 41 may be formed by holes formed in the mask 4 and / or by areas of the mask 4 made transparent, by a reduction in their thickness and / or by a choice of a transparent material.

[0075] A beam F emitted by the X-ray source 2 therefore passes through the holes 41 of the mask 4, the sensor 3 thus receiving each portion F41 of the beam F passing through one of the holes 41, which thus forms a spot T on one or more elementary sensors 31 of the sensor 3. The sensor 3 can thus acquire an image formed by the intensities of the incident X-rays detected by the elementary sensors 31 and translated by the signals emitted by these elementary sensors 31.

[0076] In the example described, the mask 4 comprises a two-dimensional arrangement of holes 41, of given shapes and dimensions such that each spot T formed by the portion F41 of the beam F passing through each hole 41 is centered on a group of elementary sensors 31.

[0077] Alternatively, it may be provided that the holes 41 have a shape other than a square, that the holes 41 have dimensions and / or shapes distinct from one another, or that the holes 41 are arranged on the mask 4 according to a distribution other than a matrix or with a variable pitch along the dimensions of the mask 4.

[0078] [Fig.2A] thus represents a projection of a spot To onto a sensor 3 in the absence of a sample, when the Hartmann system is a system according to the state of the art, comprising only the source 2, the sensor 3 and the mask 4.

[0079] In the absence of a sample, each hole 41 of the mask 4 forms a spot To whose position on the sensor 4 depends solely on the dimensions of the hole 41 and the distance separating the mask 4 from the sensor 3.

[0080] A processing unit 5 can thus analyze the image acquired by the sensor 3 to estimate the positions of the spots To, which define reference positions. Each reference position could for example be defined in the form of a pair of coordinates x0, y0, along axes X and Y, orthogonal to the direction Z.

[0081] In the example described, the processing unit 5 estimates the position of each spot To by determining the position of a centroid of said spot, defined as the barycenter of the intensities detected by each group of elementary sensors 31.

[0082] Alternatively, the processing unit 5 may estimate the position of each spot To by interpolating this spot by a given function, such as a Gaussian function or a Bigaussian function.

[0083] Simultaneously, the processing unit 5 can also record a reference intensity for each spot To, for example by calculating the sum of the intensities detected by each group of elementary sensors 31.

[0084] In an exemplary embodiment not shown, the reference positions x0, yo may be calculated from a known wavefront, which may in particular be obtained by diffracting the beam F emitted by the X-ray source 2.

[0085] When a sample E is positioned between the X-ray source 2 and the mask 4 or between the mask 4 and the detector 3, the sample will locally absorb and / or refract the X-rays of the beam F, depending on its local absorption or refracting properties. transmission, and refraction or reflection. This can result in local angular deviations of the X-rays incident on the sample, defined respectively along the X and Y axes. These local angular deviations are directly linked to the local refraction or reflection properties of the sample E, and in particular to the variations in its refractive index and to the local derivatives of the phase variation induced by the sample E.

[0086] As shown in [Fig.2B], these angular deviations will thus individually move each spot T, formed by a portion F41 of the beam F passing through the sample E and the holes 41 of the mask 4, from their reference positions x0, yo according to offsets Ax and Ay along the X and Y axes.

[0087] The processing unit 5 can then again determine the position of the centroids of the spots T, by calculating the position of the barycenter, to estimate these shifts Ax and Ay in order to generate images or deflection maps along the X and Y axes. These maps can then be integrated, according to known methods, to generate a phase image of the sample E indicating the local phase shifts induced by the sample E and therefore the inhomogeneities present in this sample.

[0088] Simultaneously, the processing unit 5 can also determine, for each group of elementary sensors 41, a ratio between the sum of the intensities measured by these elementary sensors 41 and the reference intensity, in order to obtain a map representing the transmission of the sample E.

[0089] In the example of [Fig.2B], the sample E is assumed to be locally homogeneous with respect to its absorption or transmission properties. However, this assumption is rarely verified and the structure of a sample can locally be both absorbent and refracting / reflecting. In this case, the intensity distribution in a spot T can be significantly modified, as shown in [Fig.2C].

[0090] The calculation of the barycenter of the spot T by the processing unit 5 is then distorted, which leads to an error in the estimation of the offsets Ax and Ay and therefore in the deflection images and in the phase image.

[0091] In order to be able to dissociate the absorption and refraction / reflection phenomena at the level of the displacement maps, the Hartmann system 1 according to the invention comprises a diffusing filter 6 configured to homogenize each spot T formed on the sensor 3 by each portion F41 of the beam F emitted by the X-ray source and passing through each hole 41 of the mask 4. In the example of [Fig. 1], the filter 6 is arranged between the mask 4 and the sensor 3, so that the beam F emitted by the X-ray source 2 successively passes through the sample E, the mask 4 then the diffusing filter 6 to then be received by the sensor 3.

[0092] As shown in [Fig.2D], the diffusing filter 6 thus makes it possible to form on the sensor 3, from each portion F41 of the beam F having passed through the sample E, a spot T whose intensity distribution is substantially identical to that of a spot formed by the same portion of the beam having passed through a sample whose local absorption properties would be different. Therefore, the determination of the position of the centroid of a spot T by the processing unit 5 becomes substantially invariant to local absorption variations of the sample E. It will be noted that, in the context of the acquisition of the reference positions x0, y0, the diffusing filter 6 has little or no influence on the position of the barycenter of the reference spots To.

[0093] In connection with [Fig. 3], we will now describe an example of the embodiment of the filter 6 used by the Hartmann system 1 of [Fig. 1].

[0094] According to this example, the diffusing filter 6 comprises a substrate 61 made of a material transparent to X-rays, in particular silicon nitride. A plurality of sub-filters 62 are positioned to each correspond to a hole 41 of the mask 4 with which it is associated.

[0095] Each sub-filter 62 is formed by a plurality of diffusing structures 7 whose characteristics, and in particular their material and / or their thickness, enable them to diffract a beam of X-rays. [Fig. 3] shows a front view of the filter 6 as well as an enlargement of a sub-filter 62 highlighting the diffusing structures 7.

[0096] In the example of [Fig. 3], the diffusing structures 7 are gold pads, square in shape and with dimensions identical to and substantially smaller than those of the holes 41. As a non-limiting example, the thickness of the pads may be between 20 pm and 100 pm, and their surface area may be between 0.01 pm2 and 1 pm2. Each pad 7 makes it possible to obtain a diffracted beam whose size is close to, equal to or greater than that of the task T which must be formed on the sensor 3.

[0097] Furthermore, the pads 7 of the same sub-filter 62 are distributed spatially in a pseudo-random manner over the surface of the sub-filter. In other words, the pads 7 are distributed on a checkerboard in a pseudo-random manner, according to a given distribution function, the spaces created by this distribution between neighboring pads 7 then being left free.

[0098] Therefore, the intensity distribution of the spot T formed on the sensor 3 by the portion F41 of the beam F emitted by the X-ray source 2 and passing through a hole 41 then the associated sub-filter 62 follows a Gaussian distribution.

[0099] The dimensions of the pads 7, their material and their pseudo-random spatial distribution thus make it possible to ensure the homogeneity of the intensity distribution of each spot T formed on the sensor 3.

[0100] In connection with [Fig. 4], we will now describe another example of embodiment of the filter 6 used by the Hartmann system 1 of [Fig. 1]. [Fig. 4] shows a sectional view of a sub-filter 62 according to this other example of embodiment.

[0101] In this example, the entire surface of the sub-filter 62 is provided with diffusing structures 7, formed by gold pads distributed in a matrix manner according to a given pitch. On the other hand, the thickness of the pads 7 varies pseudo-randomly over the entire surface of the sub-filter 62, for example according to a distribution around a mean value of 60 pm with a standard deviation of 40 pm. In a similar manner to the example of [Fig.3], it is then possible to obtain a spot T whose intensity distribution is homogeneous.

[0102] In the example of [Fig.4], the sub-filter 62 also comprises a layer of substantially transparent material 63, for example carbon, deposited on the substrate 61 to fill the spaces separating the pads 7. Preferably, this layer of carbon has a thickness of at least a quarter of the thickness of the pads 7. This layer of material 63 makes it possible to avoid vibration phenomena of the pads 7. This layer of material 63 could also be used in the example of [Fig.3] to fill the spaces left free between the pads 7.

[0103] In examples not shown, it may be possible to vary other characteristics of the diffusing structures 7 in a pseudo-random manner so that the intensity distribution of the spot T formed on the sensor 3 by the portion F41 of the beam F emitted by the X-ray source 2 and passing through a hole 41 then the filter 6 is homogeneous.

[0104] In particular, the types, shapes, composition, materials or even dimensions of the diffusing structures 7 may be varied so that the composition of the beams diffracted by the diffusing structures of the same sub-filter 62 reproduces the homogenization or smoothing generated by the diffraction carried out by a small hole.

[0105] It may in particular be provided that some of the diffusing structures 7 are arranged to introduce a phase shift, for example of ji / 2, while others are arranged to have no influence on the phase, the distribution of the phase-shifting structures and the non-phase-shifting structures on the sub-filter 62 being pseudo-random. It may also be envisaged to use more than two types of phase-shifting structures 7.

[0106] It may also be provided that the diffusing structures 7 are absorbent or partially absorbent, the distribution of the absorbent and partially absorbent structures on the sub-filter 62 being pseudo-random.

[0107] Although the examples which have been described use square-shaped pads, it will be possible to use other types of diffusing structures, and in particular microholes, or other shapes of diffusing structures, and in particular circular, elliptical, hexagonal or triangular shapes.

[0108] With reference to [Fig.5] and [Fig.6], we will now describe other methods of rea- implementation of the Hartmann system according to the invention.

[0109] [Fig. 5] represents a Hartmann system 10 according to a second embodiment of the invention. In this embodiment, the diffusing filter 60 is arranged between the X-ray source 2 and the mask 4 so that the beam F emitted by the X-ray source 2 successively passes through the diffusing filter 60, and the mask 4 to then be received by the sensor 3.

[0110] Furthermore, the diffusing filter 60 is a single filter continuously covered with diffusing structures 7.

[0111] [Fig. 6] represents a Hartmann 100 system according to a third embodiment of the invention. In this embodiment, the filter 600 comprises a plurality of sub-filters each arranged at one of the holes 41 of the mask 4, in which it extends entirely. Each portion F41 of the beam F emitted by the X-ray source 2 and passing through a hole 41 is thus homogenized by the sub-filter extending into this hole 4L

[0112] In examples not shown, the characteristics of the embodiments of [Fig. 1], [Fig. 5] and [Fig. 6] may be interchanged or combined, in particular so that the structure of the filter 6 of the first embodiment is that of the filter 60 of the second embodiment, or vice versa, or so that the Hartmann system comprises a combination of several of the diffusing filters 6, 60 and / or 600, juxtaposed in the Z direction while being either all arranged between the X-ray source 2 and the mask 4, or alternatively all arranged between the mask 4 and the sensor 3, or alternatively arranged on either side of the mask 4.

[0113] It will also be noted that, in the examples shown, the structures of the sub-filters may vary from one sub-filter to another, the pseudo-random distribution of the diffusing structures 7 being in this case different from one sub-filter to another. Alternatively, the diffusing filter may be formed by a periodic, vertical and / or horizontal repetition of the same diffusing sub-filter.

[0114] The preceding description clearly explains how the invention makes it possible to achieve the objectives it has set itself, namely to propose a Hartmann-type imaging system, which makes it possible to minimize, or even eliminate, the impact of the local absorption properties of a sample in the generation of the phase image of the sample without reducing the size of the holes in the Hartmann mask. It is understood that these objectives are achieved using a diffusing filter which thus makes it possible to homogenize the intensity distribution over the whole of each spot formed on the sensor of the system, in the same way as a hole of reduced dimensions which would diffract the X-ray beam, while overcoming the drawbacks of this reduction in the dimensions of the hole.

[0115] In any event, the invention cannot be limited to the specific embodiments technically described in this document, and extends in particular to all equivalent means and to any technically effective combination of these means.

Claims

Claims

1. Hartmann system (1, 10, 100) for phase imaging of a sample comprising: a. an X-ray source (2) for emitting a beam (F) of X-rays; b. a sensor (3) capable of receiving said light beam and comprising a plurality of elementary sensors (31); c. at least one mask (4) comprising a plurality of holes (41), the mask being arranged between said X-ray source and the detector so that a beam (F) emitted by the X-ray source passes through the holes of the mask, each portion (F41) of said beam passing through a hole of the mask forming a spot (T) on the sensor; d. at least one diffusing filter (6) configured to homogenize each spot formed on the sensor by each portion of the beam emitted by the X-ray source and passing through each hole in the mask; e. a processing unit (5) configured to determine, from the offsets (Ax, Ay) between the position of each spot formed on the sensor by a beam emitted by the X-ray source and passing through a sample € arranged between the X-ray source and the detector and a reference position (x0, y0), a deflection image of said sample.

2. System according to the preceding claim (1, 10, 100), characterized in that the processing unit (5) is arranged to estimate the position of each spot (T) by determining a centroid of said spot.

3. System (1, 10, 100) according to one of the preceding claims, characterized in that the diffusing filter (6) is arranged between the X-ray source (2) and the mask (4).

4. System (1, 10, 100) according to one of claims 1 or 2, characterized in that the diffusing filter (6) is arranged between the mask (4) and the sensor (3).

5. System (1, 10, 100) according to one of claims 1 or 2, characterized in that the diffusing filter (6) comprises a plurality of sub-filters (62) each arranged at one of the holes (41) of the mask (4).

6. System (1, 10, 100) according to one of the preceding claims, characterized in that the diffusing filter (6) extends in dimensions greater than or identical to those of the surface formed by all the holes (41) of the mask (4).

7. System (1, 10, 100) according to one of the preceding claims, characterized in that the diffusing filter (6) comprises, for each hole (41), a plurality of diffusing structures (7) associated with said hole, each diffusing structure of said plurality being arranged to diffract the portion (F41) of the beam (F) emitted by the X-ray source (2) and passing through said hole.

8. System (1, 10, 100) according to the preceding claim, characterized in that each plurality of diffusing structures (7) associated with a hole (41) has characteristics determined so that the shape and / or the intensity distribution of the spot (T) formed on the sensor (3) by the portion (F41) of the beam (F) emitted by the X-ray source (2) and passing through said hole follows a Gaussian distribution.

9. System (1, 10, 100) according to the preceding claim, characterized in that the types and / or positions and / or shapes and / or dimensions and / or absorption coefficients and / or diffusion coefficients of the diffusing structures (7) of a plurality of diffusing structures associated with a hole (41) are distributed pseudo-randomly over the whole of said plurality.

10. System (1, 10, 100) according to one of claims 7 to 9, characterized in that the diffusing filter (6) comprises a substrate (61) made of silicon nitride on which a plurality of gold pads are formed forming said diffusing structures (7).

11. System (1, 10, 100) according to one of claims 7 to 10, characterized in that the diffusing filter (6) comprises a carbon layer (63) filling the spaces separating the diffusing structures (7).

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