EP4189373A1

EP4189373A1 - Backscattered x-photon imaging device

Info

Publication number: EP4189373A1
Application number: EP21719636.9A
Authority: EP
Inventors: Thierry LEMOINE
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2020-07-30
Filing date: 2021-04-19
Publication date: 2023-06-07
Also published as: US20230293126A1; FR3113132A1; CN115997256A; FR3113132B1; WO2022022868A1; AU2021314651A1

Abstract

The invention relates to a backscattered X-photon imaging device comprising a plurality of X-ray sources (14, 16), all configured to illuminate an analysis area (12) in which an object to be imaged can be placed, and a pixelated X-ray detector (24) configured so as to detect X-photons that can be scattered by the object.

Description

Backscattered X photon imaging device

The invention relates to an X-ray photon imaging system. Most X-ray imaging systems operate in transmission, as is the case with conventional radiography. More precisely, a part of incident X photons illuminating an object to be imaged is absorbed by the object. The image is obtained from the part of the unabsorbed X photons having passed through the object which is placed between the X radiation source and the detector. In certain situations, this type of radiology does not allow obtaining an image, this is the case in particular for checking luggage abandoned along a wall. It is then impossible to place the object between the source and the detector. This is also the case in the presence of a substance opaque to X-rays which appears as a uniform zone in conventional radiography. Backscattered photon imaging can overcome these situations. This type of imaging takes advantage of the interaction between incident X-ray photons and the material constituting the objects to be imaged. Several phenomena result in a diffusion of photons in all directions and in particular in the direction of the source of incident radiation. Among the physical phenomena identified, we note mainly Rayleigh and Compton scattering.

However, the implementation of backscattered photon imaging is struggling to develop because the realization of an image is difficult. The main reason is that for photons whose energy is between 1 and 1000 keV, it is not possible to produce devices operating in a similar way to optical focusing devices. The refractive index is too low for the production of lenses, and the transparency of metals at these energies prevents the production of mirrors.

However, several techniques have been developed to produce an image from backscattered photons. A first technique consists in illuminating the object to be imaged by means of a fine beam of X photons and moving the beam to cover the entire object. This technique is known in Anglo-Saxon literature as: “flying spot”. At a given instant, only a narrow zone of the object is likely to emit backscattered photons. It then suffices to collect all the photons emitted without worrying about their origin, by a detector with a single pixel. The image is reconstructed by scanning the entire object by moving the beam of X photons. The resolution of the image obtained is given by the geometry of the beam.

A second technique consists of illuminating the object as a whole with X-ray photons and using a pixelated and collimated detector to collect the backscattered photons. The collimator placed in front of each pixel of the detector is sufficiently anisotropic for each pixel to receive photons from a zone of the object located opposite. The resolution of the image is then given by the detector and its collimator.

A third technique is also to illuminate the object as a whole and use a pixelated detector. Unlike the second technique, the third technique does not use a collimator but an absorbent plate pierced with a hole, hence the Anglo-Saxon name of this technique: "pin hole" for needle hole.

The so-called “pin hole” technique has the advantage of simplicity. The size of the hole forms the most important parameter to take into account for the quality of the image obtained. As a first approach, the hole diameter is of the same order of magnitude, or even smaller, than the size of the detector pixels. A larger hole would degrade image resolution. On the other hand, the flux of photons crossing a hole remains of low intensity, which results in a signal-to-noise ratio which may be too low to obtain a usable image. In other words, to improve the signal-to-noise ratio, the dimensions of the hole must be increased, which degrades the spatial resolution of the image. Image quality is the result of a compromise between resolution and signal-to-noise ratio.

To increase the signal to noise ratio, one solution consists in increasing the quantity of incident photons emitted by the source, which makes it possible to proportionally increase the number of backscattered photons. However, certain objects are subjected to maximum doses of irradiation, in particular in medical imaging. In addition, X-ray sources are also limited in the doses they can emit. The limitation of sources is essentially due to the thermal aspect. The more radiation the source emits, the greater its heating. In the case of self-powered portable imaging systems, the emission of radiation is also limited by the batteries that the system embeds.

Another limitation of the pinhole technique lies in the geometry of the imaging system. In the object to be imaged, the areas closest to the source receive more incident radiation than the areas furthest away. The amount of backscattered radiation is therefore a function of the distance from the source. In addition, the amount of backscattered radiation is also a function of an angle formed between a direction passing through the source and the point of the object impacted by the incident radiation and a direction passing through this point and the hole in the absorber plate. These two geometric characteristics lead to an intrinsic inhomogeneity in the distribution of backscattered photons on the surface of the detector, independently of the object to be imaged.

The invention aims to overcome all or part of the problems mentioned above by proposing a backscattered X photon imaging device making it possible to improve the quality of the images obtained by illuminating an imaged object by means of several distinct sources of X radiation.

The illumination of the object by several distinct sources makes it possible to improve the homogeneity of the flux of incident photons reaching the object both in intensity and in angle of illumination.

To this end, the subject of the invention is a backscattered X photon imaging device comprising:

• several X-ray sources, all configured to illuminate an analysis area in which an object to be imaged can be placed,

• a pixelated X-radiation detector configured to simultaneously collect several distinct data, an image delivered by the imaging device being formed by juxtaposing the distinct data, the pixelated detector being arranged so as to detect X photons which can be diffused by the object

• an absorbent plate pierced with at least one orifice allowing X photons which can be diffused by the object to pass through the orifice, the pixelated detector being arranged so as to detect the X photons passing through the orifice. the orifice(s) can each form a diaphragm through which X photons pass, between the object and the pixelated detector. Alternatively, the absorber plate may be pierced with several orifices and form a collimator adapted to the pixelated detector and allowing only X photons to pass through, moving substantially in a predefined direction.

In the particular embodiment, the different sources of X-radiation are advantageously evenly distributed around the orifice.

Each X-radiation source advantageously comprises a cold cathode emitting an electron beam by field effect. The imaging device can comprise one of the X-ray sources. The control module can be configured to cause several sources to be emitted simultaneously or to cause one or more sources to be sequentially emitted from among the X-control module radiation sources. Each radiation source X is advantageously configured to illuminate the entire analysis zone at a given instant.

In the particular embodiment of the invention, the absorbent plate may comprise several orifices. The imaging device then comprises a module for processing signals coming from the detector, the processing module being configured to extract the useful information representing the image of the object to be imaged.

The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, description illustrated by the attached drawing in which: FIG. 1 schematically represents an example of a device imaging according to the invention; FIG. 2 schematically represents a variant of the device of FIG. 1; FIG. 3 schematically represents another variant of the device of FIG. 1.

For the sake of clarity, the same elements will bear the same references in the different figures. FIG. 1 schematically represents an example of an imaging device 10 according to the invention and making it possible to detect backscattered X photons. The imaging device 10 comprises several X-ray sources all configured to illuminate an analysis zone 12 of the device 10. The objects to be imaged are placed in the analysis zone 12. The shape of the beam emitted by each source can be conical and cover the entire analysis area. Thus the sources do not need scanning to illuminate the analysis zone 12 and all the points of the analysis zone 12 are illuminated at the same time by all the X-ray sources. Alternatively, the different sources can use a sweep to illuminate the analysis zone 12. It is also possible to implement sources that cannot illuminate the entire analysis zone 12 with or without sweeping. The presence of several sources of X-radiation, even if each one cannot illuminate the whole of the analysis zone already makes it possible to improve the homogeneity of the illumination of the analysis zone 12. In the example represented, two sources 14 and 16 are shown. It is of course possible to implement the invention in a device comprising more than two X-radiation sources.

The sources 14 and 16 are represented in FIG. 1 schematically for a point forming the focal point from which the X-radiation originates. In practice, within the scope of the invention, any type of X-radiation source can be work, whether it has a focal point or not. By way of example, mention may be made of thermionic cathode tubes. Among these tubes, it is possible to use fixed anode tubes or rotating anode tubes. This last type of tube has the advantage of better dissipation of the heat emitted when the beam of electrons emitted by the cathode reaches the anode. It is also possible to use cold cathode tubes emitting an electron beam by field effect. This type of tube is for example described in patent application WO 2019/011980 A1 filed in the name of the applicant. Cold-cathode X-radiation sources have the advantage of their compactness, which makes it possible, for example, to implement them in a portable imaging device. Cold cathode X-ray sources are also smaller in size than cathode sources. thermionic which makes it easier to increase the number of sources present in the imaging device 10.

The imaging device 10 is based on the “pin hole” principle. To this end, the device 10 comprises an absorbent plate 20. More specifically, the plate is made of a material that absorbs X-radiation. The absorbent plate 20 is pierced with at least one orifice 22 allowing X-ray photons diffused by the object to be imaged to pass through the orifice 22. In the variant shown in FIG. 1, the orifice(s) can each form a diaphragm through which X photons pass. The diaphragm has a fixed aperture and can be likened to the so-called pin hole” for needle hole. In practice, the material of the absorbing plate 20 makes it possible to absorb a majority of the radiation reaching it. Materials with a high atomic number are chosen. In a portable device where one seeks to reduce the overall mass, one may be inclined to reduce the mass of the on-board components and in particular the mass of the absorbent plate 20 in particular by reducing its thickness, which leads to reducing the absorption of the plate 20. The material and the thickness of the plate 20 are defined to allow discrimination between the part of the radiation absorbed by the plate and the part passing through the orifice 22.

The imaging device 10 further comprises an X-radiation detector 24 arranged so as to detect the X-ray photons passing through the orifice 22. The detector 24 is pixelated so as to identify the zone of the object to be imaged from which the photons originate. broadcast. By pixelated detector is meant any type of detector capable of simultaneously collecting different information in at least one direction. The image delivered by the device 10 is formed by juxtaposing the different data. In other words, the number of distinct pieces of information collected by the detector defines the spatial resolution of the image delivered by the device 10. The image is formed by spatial juxtaposition of the different data collected by the detector 24. It may be an analog detector such as for example a photosensitive film or a digital detector having several discrete pixels. In practice, digital detectors typically comprise several thousand to several million pixels. Among the digital detectors, many families of exist and can be implemented within the scope of the invention. By way of example, mention may be made of the flat panels with indirect detection and having a scintillator transforming the X photons into photons in a wavelength is adapted to the technology of the detectors. Mention may also be made of flat panels with direct detection of X photons. The flat panel extends along two dimensions. It is also possible to implement a strip detector extending in a single direction. It is also possible to implement an optical camera associated with a scintillator. Memory radio-luminescent screens can also be used as a detector in the context of the invention. This type of screen is commonly used in a particular form of digital radiology often called by its English acronym: CR for “Computed Radiography”. The principle of this form of radiology consists of making an image on the screen and then scanning the screen with a dedicated device. The screen is then brightly lit so as to erase the image before further use. The invention can also be implemented without an absorbent plate and with a collimated detector. More precisely, a collimated detector makes it possible to receive only photons originating from one direction or having a small angular difference therewith. The photons coming from other directions are absorbed by a collimator arranged between the detector and the zone 12. This makes it possible to distinguish, for each pixel of the detector, the zone of the object from which it comes.

The fact of illuminating the zone 12 by means of several distinct X-ray sources makes it possible to improve the homogeneity of the illumination of the zone 12 as well as the homogeneity of the distribution of the photons backscattered on the detector 24. With a single source, two types of inhomogeneities can be noticed. First of all, the distance to the source leads to an inhomogeneity of the illumination in intensity due to the conical spreading of the incident beam. The closer a point of the object is to the source, the more incident photons it receives and consequently the more scattered photons it emits. Then, it is possible to define for each point of the object, an angle between a first direction passing through the point considered and the source and a second direction passing through the point considered and the orifice 22. For the same flux of photons incidents, the intensity of scattered photons depends on the angle between the two directions. In figure 1, an angle Q1 and an angle Q2 are represented for diffusions carried out from a point of the object respectively receiving X photons coming from the two sources 14 and 16. With several sources, the two types of inhomogeneities due to the distance to the source and due to the angle tend to compensate according to the origin of the incident photons.

In order to best reduce the two types of inhomogeneities, the various sources of X-ray radiation are evenly distributed around the orifice 22. More precisely, the various sources are distributed over a circle whose center passes through an axis passing through the orifice 22. In the example shown, the two sources 14 and 16 are diametrically opposed on the circle defined above. In Figure 1, the circle is seen from the edge.

The imaging device 10 comprises a module 26 for controlling the X-ray sources (14, 16). The control module 26 can be configured so that the various sources can transmit simultaneously. Simultaneous emission makes it possible to improve the signal-to-noise ratio of the image of the object obtained by the detector 24. Indeed, for a source taken in isolation, the maximum X-radiation flux that it can emit is mainly linked to its possibility of heat dissipation. By multiplying the number of sources, the flux of X photons reaching the object to be imaged is increased accordingly. If, on the contrary, the signal-to-noise ratio of a single source is considered sufficient, by multiplying the number of sources, to reach the same flux of incident X photons, the emission duration of the different sources can be reduced. This makes it possible to reduce the integration time at the level of each pixel of the detector 24. In a digital detector, the reduction of the integration time makes it possible to reduce the impact of the leakage current of each pixel and therefore makes it possible to improve the quality of the signals collected and consequently the quality of the image of the object.

Alternatively or even in addition, it is possible to configure control module 26 so that the various sources can transmit sequentially. Sequential emission may be of interest in particular for limiting the instantaneous consumption of the device by distributing the switching on of the various sources of X-radiation over time. In the device 10, the control module 26 may be configured to allow a user to choose between a simultaneous transmission and a sequential transmission. These two types of transmission can even be combined by allowing simultaneous transmission of P sources among N, N being the total number of sources and P being a natural integer strictly less than N. The choice of the P sources rotating sequentially among the N sources.

FIG. 2 represents a variant of imaging device 10 in which one finds the sources 14 and 16 as well as the detector 24. Unlike the device 10 of FIG. 1, the absorbing plate forms a collimator 28 adapted to the detector 24. In other words, the collimator 28 is pierced with several orifices allowing only X photons to pass through, moving substantially in a predefined direction 29. The collimator 28 has the same surface as the detector 24. The pitch of the orifices of the collimator 28 is equal or multiple of the pitch of the pixels of the detector 24. In the example represented the direction 29 is perpendicular to the plane of the detector 24.

The X photons deviating from the direction 29 are absorbed by the collimator 28. In FIG. 2, the pixels of the detector 24 receiving X photons having passed through the collimator 28 are shown in darker gray than the other pixels.

In the variant of Figure 1, the image formed on the detector 24 is inverted while that of the variant of Figure 2 is not.

FIG. 3 represents a variant of imaging device 30 in which one finds the sources 14 and 16 as well as the detector 24. Unlike the device 10, the device 30 comprises a plate 32 pierced with several orifices 34, 36, 38 , 40, 42, 44 and 46. In the example shown, the various orifices are distributed on the same axis of the plate 32, vertical axis in FIG. 3. In practice, in the case of a plane detector 24, the different orifices are distributed over a surface of the plate 32, for example with a circular outline. The orifices can be discreet. More generally, the absorption of plate 32 varies according to a two-variable function in a spatial frame of the plate. We note for example the function: f(x, y), x and y being two Cartesian coordinates of the surface of the plate 32. The image l(x, y) delivered by the detector depends both on the object whose useful information is sought in a coordinate system x, y: 0(x, y) and of the function f(x, y). The imaging device 30 comprises a module 50 for processing signals from the detector 24 and configured to extract the useful information 0(x, y) representing the image of the object. The signals coming from the detector 24 form a convolution of elementary signals coming from the photons having passed through each of the orifices 34 to 46. The processing module 50 advantageously implements a deconvolution-based algorithm to find the image of the object 0( x, y). Note that if the images projected on the detector by the different orifices do not overlap, the deconvolution algorithm approaches a simple superposition of the images, possibly with a processing making it possible to reduce the impact of parallax effects.

Compared to device 10, device 30 makes it possible to substantially increase the flux of photons reaching detector 24 and therefore the signal-to-noise ratio of the image. This improvement is however obtained at the expense of a slight loss of spatial resolution which may remain acceptable with regard to the gain in image quality due to the improvement in the signal-to-noise ratio.

In the different variants, the processing module 50 is configured to deliver the image coming from the device 10 or 30. More precisely, the processing module 50 recovers the data coming from the detector 24 and assembles them by juxtaposing them to form an image of the object located in the analysis zone 12. When the detector 24 is digital, the processing module 50 receives the data from the different pixels, for example in the form of a charge or a voltage. The processing module 50 can comprise one or more analog-digital converters and a multiplexer making it possible to deliver the image in the form of a digital frame. In the case of the detector 30 equipped with the plate 32, the deconvolution processing can be performed on the digital information downstream of the analog-to-digital converter.

Claims

1. Backscattered X photon imaging device comprising:

• several X-radiation sources (14, 16), all configured to illuminate an analysis zone (12) in which an object to be imaged can be placed,

• a pixelated detector (24) of X-radiation configured to simultaneously collect several distinct data, an image delivered by the imaging device (10) being formed by juxtaposing the distinct data, the pixelated detector (24) being arranged so as to detect X photons that can be scattered by the object,

• an absorbent plate (20; 32) pierced with at least one orifice (22; 34, 36, 38, 40, 42, 44, 46) allowing X-ray photons which can be diffused by the object to pass through the orifice , the pixelated detector (24) being arranged to detect X-ray photons passing through the orifice.

2. Imaging device according to claim 1, in which the orifice(s) (22; 34, 36, 38, 40, 42, 44, 46) each form a diaphragm through which X photons pass between the object and the pixelated detector (24).

3. Imaging device according to claim 1, in which the absorbent plate is pierced with several orifices and forms a collimator (28) adapted to the pixelated detector (24) and allowing only X photons to pass through, moving substantially in a predefined direction. (29).

4. Imaging device according to one of the preceding claims, in which the various sources of X-radiation (14, 16) are evenly distributed around the orifice (22; 34, 36, 38, 40, 42, 44, 46 ).

5. Imaging device according to one of the preceding claims, in which each X-radiation source (14, 16) comprises a cold cathode emitting an electron beam by field effect.

6. Imaging device according to one of the preceding claims, comprising a module for controlling the sources of x-radiation (14, 16) configured to simultaneously emit multiple ones of the x-radiation sources (14, 16).

7. Imaging device according to one of the preceding claims, comprising an X-ray source control module (14, 16) configured to sequentially emit one or more sources from among the X-ray sources (14, 16).

8. Imaging device according to one of claims 6 or 7, wherein each X-ray source (14, 16) is configured to illuminate the entire analysis area (12) at a given time.

9. Device according to one of the preceding claims, in which the absorbent plate comprises several orifices (34, 36, 38, 40, 42, 44, 46), the imaging device comprising a signal processing module (50) from the detector (24), the processing module (50) being configured to extract the useful information representing the image of the object to be imaged.