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 penetrating, depending on their energy, most materials over significant thicknesses, typically from several millimeters to several meters. It is therefore possible to use these X-rays to produce, non-invasively, images containing information about the internal structure of a sample, taking into account the absorption, scattering, and phase shift of the X-rays as they pass through that sample. These X-rays thus have numerous applications, particularly in the medical field for radiography, in the security field for the analysis of suitcases or containers, in the agri-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 introduces a phase change in the beam. It is thus possible to resample the beam into sub-beams that are projected onto detectors to obtain an image of the phase variations, or contrast, of the sample, which allows visualization of 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 allowing to generate an image representative of the phase variations of a sample, and in particular interferometry techniques based on the analysis of the interferences 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 with a plurality of holes interposed between the source and the sensor, positioned at a given distance from the sensor. The mask is also called a Hartmann plate. The beam emitted by the X-ray source passes 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 dimensions of the holes, and the wavefront of the beam emitted by the source. Other techniques can be used to generate this reference measurement, notably by diffracting the beam emitted by the X-ray source by a predetermined object to create a wave with a known wavefront.

[0007] Subsequently, 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 shift 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. Measuring 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.Measuring the displacement of spots along axes orthogonal to the direction of propagation of the beam emitted by the source, relative to the reference image, allows us 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 therefore common for the processing unit to estimate a centroid for each spot, for example by calculating the centroid of the intensities measured by groups of pixels or by interpolating these intensities using a given function. These techniques provide satisfactory results if there is complete decoupling between the local intensity measurement and the local displacement measurement, particularly to ensure 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 a portion of the X-ray beam entering or exiting a hole. This can be the case, in the field of medical imaging, for a blood vessel or other absorbing anatomical structure that 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 inhomogeneously, which leads to a shift in the centroid or an error in the interpolation of the spot. The modification of the intensity distribution can then be interpreted as a shift in the spot, in addition to the actual displacement of the spot generated by the local deviation of the portion of the X-ray beam penetrating into this hole.

[0010] In other words, since the structure of a sample can locally be both absorbing and refracting / reflecting, it becomes complex to dissociate the absorption and refraction / reflection phenomena at the level of displacement maps, which then leads to errors in the interpretation of 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 level, which must be compensated for by an increase in the X-ray dose to which the sample is exposed. However, in a medical imaging context or in applications where the samples are radiosensitive, this dose increase is undesirable.

[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 maintain sensors with pixels of reasonable size compared to the normal dimensions of the samples typically studied, it is therefore necessary to increase the size of the spots formed by the mask and thus further reduce the size of the holes. This reduction then requires a further increase in the X-ray dose to which the sample is exposed.

[0014] Finally, to ensure the system functions correctly, the mask must remain opaque to X-rays between the holes. Given the increases in X-ray dose resulting from beam diffraction by the holes, it becomes necessary to increase the mask thickness in these opaque areas to prevent beam propagation. This makes the mask complex, if not impossible, to manufacture industrially. Furthermore, the holes become tunnels, leading to a loss of angular resolution at the output.

[0015] There is therefore 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 having to reduce the size of the holes in the Hartmann mask.

[0016] The present invention falls within this context and aims to meet this need.

[0017] To this end, the invention relates to a Hartmann phase imaging system of a sample including: has. an X-ray source for the emission of an X-ray beam; 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 in the mask, each portion of said beam passing through a hole in 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 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 passes successively through the sample, the diffusing filter, and the mask, regardless of the order of passage, to reach the sensor. Each portion of the beam emitted by the X-ray source thus 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 that has 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 with different local absorption or transmission properties. The diffusing filter thus makes it possible to homogenize this intensity distribution across the entire spot in the same way as a small hole that would diffract the X-ray beam, while avoiding the drawbacks of this reduction in hole size. In particular, the intensity distribution of a spot can be considered homogeneous if the intensity gradient of the spot is less than a given threshold value at every point of the spot.Therefore, determining the position of the centroid of a spot becomes robust to local variations in absorption or transmission of the sample. It is thus possible to obtain a deflection image of the sample, and therefore ultimately a phase image, or phase variation image, of the sample, the construction of which is substantially decoupled from the local absorption or transmission properties of the sample. the sample.

[0020] In the present invention, the term "X-ray source" means any device capable of emitting an X-ray beam, including, but not limited to, a spectral range from 30 eV (electronvolts) to several MeV (megaelectronvolts). The X-ray source may be configured to generate a monochromatic or polychromatic beam. Examples include an X-ray tube, in particular a microfocus 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 through the interaction of particles with an emitting element.

[0021] Advantageously, it may 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, "mask" means any element or combination of elements having holes, i.e., areas transparent to X-rays, distributed over a structure opaque to X-rays. "Opaque structure" means a structure configured such that its residual transmission to X-rays emitted by the X-ray source, particularly in a useful spectral band of said X-rays, is less than the detection sensitivity of the system's sensor. "Transparent area" means any area configured such that its transmission to X-rays emitted by the X-ray source, particularly in a useful spectral band of said X-rays, is typically greater than 80%. "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] By way of non-limiting example, materials composed of elements with a low atomic number, particularly less than 15, such as Beryllium, Carbon, Silicon, or Aluminum, or a vacuum, may be considered to create a transparent zone. As a further non-limiting example, materials composed of elements with an atomic number greater than 28 may be considered to create an opaque structure, selected according to the energy level of the X-rays considered in relation to the intended application. For example, for so-called hard X-rays with energies greater than 10 keV, materials composed of Gold, Tantalum, Tungsten, or Lead may be used.

[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 partially or totally covers 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-rays into visible radiation. Alternatively, the sensor may be a direct type sensor, comprising, for example, a plurality of elementary sensors capable of detecting X-rays.

[0026] The sensor may, for example, comprise a two-dimensional arrangement of identical elementary sensors. These elementary sensors may be combined electronically and / or digitally to form a plurality of juxtaposed groups of elementary sensors, each group comprising, for example and without limitation, 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 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. The processing unit may be equipped with one or more processors, arranged to execute instructions from one or more computer programs in order to implement calculation or processing steps enabling the generation of said deflection image. These steps may be implemented centrally by a single processing unit or distributed across several processing units.It may also be possible for all or part of these steps to be implemented by a processing unit connected by wire to the sensor and / or by a processing unit located remotely from the sensor with which it can exchange data via wireless communication.

[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, along one or more directions, between this estimated position and a reference position associated with this hole. It should be noted that these offsets are a function of the local phase gradients 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 emission direction of the X-ray beam from the estimated offsets, and then to generate, from these deflection maps, the phase image of the sample. The processing unit could, for example, be arranged to generate a phase image by integrating images or deflection maps.

[0029] In the present invention, "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 that hole.

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

[0031] The reference positions associated with the holes in the mask can, 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 can be calculated from a known wavefront, which can notably 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, the processing unit could be arranged to estimate the position of each spot by determining the centroid of a spot formed on a single sensor or on a group of single sensors. Alternatively, the processing unit could be arranged to estimate the position of each spot by interpolating a spot formed on a single sensor or on a group of single sensors by a given function, in particular a Gaussian function or a bi-Gaussian function, said function allowing the determination of a centroid of this spot.

[0034] Advantageously, the processing unit can 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 can be configured 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 passes successively through the diffusing filter and the mask before being received by the sensor.

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

[0037] Alternatively, 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 can 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 as it passes through a hole.

[0038] In one embodiment of the invention, the diffusing filter extends over dimensions greater than or equal to those of the surface formed by all 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 distinct 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 being positioned to correspond to a hole in the mask to 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 therefore possible to define a specific sub-filter for each hole.

[0040] Alternatively, the diffusing filter may be formed by a periodic, vertical and / or horizontal repetition of the same diffusing sub-filter. In this case, 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 if the diffusing filter is arranged between the X-ray source and the mask, or substantially smaller than or equal to those of a hole if the diffusing filter is arranged between the mask and the sensor. The sub-filters may be placed side-by-side continuously to cover the entire surface of the diffusing filter, or they may be spaced apart, each positioned to correspond to a hole in the mask.

[0041] Alternatively, 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 alternatively all arranged between the mask and the sensor, or alternatively arranged on either side of the mask.

[0042] The different embodiments of the diffusing filter that have just been described can 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 scatter 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 scattering structure of a plurality of scattering 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 spot to be formed on the detector.

[0045] In one embodiment of the invention, each plurality of diffusing structures associated with a hole has specific characteristics such that the shape and / or 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, each plurality of diffusing structures associated with a hole may have specific characteristics such that the shape and / or 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 that homogenizes said spot.The characteristics of a plurality of diffusing structures may be one of the following characteristics or any combination thereof: the types of diffusing structures, 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 be predetermined so that the plurality of diffusing structures reproduces the homogenization or smoothing generated by diffraction through a small hole.

[0046] It may be conceivable in particular that the diffusing structures are microholes or studs.

[0047] It may also be conceivable that the diffusing structures have square, circular, elliptical, hexagonal or triangular shapes.

[0048] It may 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 either totally absorbent or opaque, or partially absorbent or opaque, or are transparent.

[0049] It may 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 entire plurality. Alternatively, 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 allowing for a homogeneous distribution of spot intensity.

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

[0052] In another 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 way over the whole of this plurality.

[0053] In the present invention, the term "pseudo-random distribution of a feature 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 a uniformly distributed one, 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 one embodiment of the invention, the diffusing filter comprises a substrate made of silicon nitride on which a plurality of gold pads are formed, constituting said diffusing structures. The silicon nitride substrate may be replaced by a substrate made of a suitable material that is substantially transparent to X-rays, in particular silicon, silicon carbide, or silicon boride. The gold pads may also be replaced by pads made of nickel, tungsten, tantalum, iron, molybdenum, or any other material capable of diffracting an X-ray beam.

[0055] According to one embodiment of the invention, the diffusing filter comprises a carbon layer filling the spaces between the diffusing structures. The carbon layer could be replaced by a layer made of silicon, beryllium, boron, or any other suitable material exhibiting low X-ray absorption capacity, or conversely, by an opaque material. This layer of material prevents vibration of the diffusing structures, which would be detrimental to the precision required by the system according to the invention.

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

[0057] The present invention is now described by means of purely illustrative and in no way limiting examples of the scope of the invention, and from the accompanying drawings, in which the various figures represent:

[0058] [Fig-1] represents, schematically and partially, a view of a system Hartmann phase imaging according to an 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 prior 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 prior art, in the presence of a non-absorbing sample;

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

[0062] [Fig.2D] represents, schematically and partially, an example of a spot projected onto a sensor of the Hartmann system of [Fig.1], in the presence of an absorbing sample;

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

[0064] [Fig.4] represents, schematically and partially, a cross-sectional view of a filter diffusing employed by a Hartmann system according to another 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, identical elements, whether structurally or functionally, appearing on different figures retain, unless otherwise specified, the same references.

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

[0069] The system 1 includes an X-ray source 2 for the emission of an X-ray beam F along an overall propagation direction Z. This source 2 may, for example, include 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 includes an X-ray sensor 3, arranged downstream of the source 2 in the direction Z, so as 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 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 electronically and / or digitally to define groups of sensors.

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

[0073] The system 1 includes 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 drillings made in the mask 4 and / or by areas of the mask 4 made transparent, by a reduction of 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 sensor 3. 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 each other, or that the holes 41 are arranged on the mask 4 according to a distribution other than 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 prior 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 only 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 can, for example, be defined as a pair of coordinates x0, y0, along axes X and Y, orthogonal to the Z direction.

[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 can 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 task To, for example by calculating the sum of the intensities detected by each group of elementary sensors 31.

[0084] In an example of an embodiment not shown, the reference positions x0, yo can be calculated from a known wavefront, which can 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, according to its local absorption or refraction 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 related to the local refractive or reflective properties of the sample E, and in particular to variations in its refractive index and to the local derivatives of the phase change 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 shifts 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 T spots, 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 in [Fig. 2B], sample E is assumed to be locally homogeneous with respect to its absorption or transmission properties. However, this assumption is rarely true, and the structure of a sample can locally be both absorbing and refracting / reflecting. In this case, the intensity distribution in a spot T can be significantly altered, 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 shifts Ax and Ay and therefore in the deflection images and in the phase image.

[0091] In order to be able to separate the absorption and refraction / reflection phenomena at the displacement map level, the Hartmann system 1 according to the invention includes 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 passes successively through the sample E, the mask 4 and then the diffusing filter 6 to be subsequently 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 that has passed through the sample E, a spot T, whose intensity distribution is essentially identical to that of a spot formed by the same portion of the beam having passed through a sample with different local absorption properties. Therefore, the determination of the centroid position of a spot T by the processing unit 5 becomes essentially invariant to local absorption variations of the sample E. It should be noted that, in the context of acquiring the reference positions x0, y0, the diffusing filter 6 has little or no influence on the position of the centroid of the reference spots To.

[0093] In connection with [Fig.3], we will now describe an example of an 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 to 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 scattering structures 7 are square gold studs with dimensions identical to, and substantially smaller than, those of the holes 41. By way of non-limiting example, the thickness of the studs may be between 20 pm and 100 pm, and their surface area may be between 0.01 pm² and 1 pm². Each stud 7 produces a diffracted beam whose size is close to, equal to, or greater than that of the spot T to be formed on the sensor 3.

[0097] Furthermore, the plots 7 of the same sub-filter 62 are spatially distributed in a pseudo-random manner over the surface of the sub-filter. In other words, the plots 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 plots 7 being then 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 and then through 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 embodiment of the filter 6 used by the Hartmann system 1 of [Fig. 1]. [Fig. 4] shows a cross-sectional view of a sub-filter 62 according to this other embodiment.

[0101] In this example, the entire surface of the sub-filter 62 is provided with diffusing structures 7, formed by gold dots distributed in a matrix pattern according to a given spacing. However, the thickness of the dots 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. Similar to the example in [Fig. 3], it is then possible to obtain a spot T with a homogeneous intensity distribution.

[0102] In the example of [Fig. 4], the sub-filter 62 also includes a layer of substantially transparent material 63, for example carbon, deposited on the substrate 61 to fill the spaces between the pads 7. Preferably, this carbon layer has a thickness of at least one-quarter of the thickness of the pads 7. This layer of material 63 prevents vibration of the pads 7. This layer of material 63 could also be used in the example of [Fig. 3] to fill the spaces left 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 way 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 and then the filter 6 is homogeneous.

[0104] In particular, the types, shapes, composition, materials or dimensions of the diffusing structures 7 can 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 be envisaged 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 and 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 foreseen that the diffusing structures 7 are absorbing or partially absorbing, the distribution of absorbing and partially absorbing structures on the sub-filter 62 being pseudo-random.

[0107] Although the examples that have been described employ square-shaped studs, it may be conceivable to employ other types of diffusing structures, and in particular microholes, or other forms 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 modes of rea- Implementation of the Hartmann system according to the invention.

[0109] Figure 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 such that the beam F emitted by the X-ray source 2 passes successively 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 unique filter continuously covered with diffusing structures 7.

[0111] Figure 6 represents a Hartmann system 100 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 that hole 41.

[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 along the Z direction, 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 should 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 different from one sub-filter to another. Alternatively, the diffusing filter may be formed by a periodic repetition, vertical and / or horizontal, of the same diffusing sub-filter.

[0114] The preceding description clearly explains how the invention achieves its objectives, namely, to provide a Hartmann-type imaging system that minimizes, or even eliminates, the impact of a sample's local absorption properties on the generation of the sample's phase image without reducing the size of the holes in the Hartmann mask. It is understood that these objectives are achieved using a diffusing filter, which homogenizes the intensity distribution across each spot formed on the system's sensor, in the same way that a small hole would diffract the X-ray beam, while avoiding the drawbacks of reducing the hole size.

[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 operative combination of these means.

Claims

Demands

1. Hartmann (1, 10, 100) phase imaging system for a sample comprising: a. an X-ray source (2) for the emission of an X-ray beam (F); 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 such that a beam (F) emitted by the X-ray source passes through the holes in the mask, each portion (F41) of said beam passing through a hole in 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 E 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 any 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 any 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 any 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 any one of the preceding claims, characterized in that the diffusing filter (6) extends along dimensions greater than or identical to those of the surface formed by the set of holes (41) of the mask (4).

7. System (1, 10, 100) according to any 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 determined characteristics such that the shape and / or 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 in a pseudo-random manner over the whole of said plurality.

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

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

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