Patent Application
20200400593
The present application relates to the field of imaging and more particularly imaging of gamma-ray sources. In particular, the present application concerns a device, system and methods of imaging by detector of the Compton camera type.
More precisely, the present application concerns a device, system and methods of imaging by so-called “multi-capture” (at least “bi-capture”) Compton camera, that is, using at least two Compton imaging captures carried out from at least two different sites (whether different sites of the same Compton camera or different sites of a plurality of separate Compton cameras). The present application therefore details a new technology of detection of gamma rays combining a plurality of Compton image captures and improvements made to the cameras and/or methods of image reconstruction using this type of multiple captures. The invention further concerns the use of such imaging and/or detection in the fields in particular of astronomy, the nuclear industry and medicine.
In the prior art concerning Compton camera imaging, it is known that the photons emitted by at least one scintillator (P1, P2) can be measured with the aid of a photodetector and a dedicated reading device under the effect of gamma radiation. The scintillator, the photodetector and the dedicated reading device are here referred to as the “capture centre” (CC in
Thus, as for example shown in
The common practice in the state of the art is to superimpose a gamma image on a visible image in order to identify the location of the source. This facilitates interpretation of the images, but does not make it possible to know whether the source is in front of a wall, behind a wall or in the wall, particularly in situations of decontamination. For this, one would have to know the distance from the source using gamma rays. In numerous situations, the energy of the radioactive source is attenuated by its presence in a stainless steel, concrete or lead receptacle (often with thick walls). In such situations, the number of photons detected that are coming from the source is just a small fraction of the number of photons emitted: the activity of the source is underestimated. It would be invaluable to have at one's disposal a means allowing the real activity of the source to be estimated.
In this context, it would therefore be interesting to propose Compton imaging which has fewer disadvantages than the prior art, in terms of calculation time and energy of the source to be detected.
It is therefore the object of the present invention to propose a device, system and method of Compton imaging making it possible to remedy at least some of the disadvantages of the prior art.
To this end, the invention concerns a device, system and method of using a Compton camera, characterised by the use of at least two capture centres having separate positions.
Thus, the invention makes it possible to reduce the calculation time and/or to limit the number of photons necessary to obtain reliable imaging, contrary to the prior art which uses only Compton images obtained from the same capture centre.
According to a feature, the invention is characterised in that the separate positions are separated by a distance less than or equal to twice the angular resolution of the Compton camera capture centre.
According to a feature, the invention is characterised in that the separate positions are separated by a distance less than but close to twice the angular resolution of the Compton camera capture centre.
According to a feature, the invention is characterised in that the separate positions are disposed along an arc centred on the position of the gamma-ray source.
Further characteristics, details and advantages of the invention will be apparent on reading the following description with reference to the attached drawings, which show:
In general, as introduced in its above preamble, the present application proposes to use “Compton images” from different positions of “Compton captures”. The term “Compton image” (or simply “image”) refers in the present application to the imaging obtained from vectors calculated on the basis of data collected by a single, given “capture centre” (having a given position, whatever its technical specifications). On the other hand, the term “Compton capture” (or simply “capture”) refers in the present application to the imaging obtained from vectors calculated on the basis of data collected by a plurality of given “capture centres”, whether it is a question of the same capture centre moved in space between two successive images, or two separate capture centres (CC1, CC2) which produce independent images from different positions. The present application therefore provides various embodiments for its physical implementation in terms of device, system or method. Thus, the same camera will preferably be used in two successive positions, or two identical cameras located in two separate positions, but the present application also provides the possibility of using two different cameras in two separate positions.
Numerous combinations may be envisaged without departing from the scope of the invention; the person skilled in the art will choose one or the other according to economic, ergonomic, dimensional or other constraints which he must respect.
Thus, in the prior art, as for example shown in
In general, as shown for example in
The invention relates to 2 parts:
The use of a multi-capture Compton camera and the advantages which it comprises.
Improvements to the functioning of existing cameras which may or may not be coupled to the use of a multi-capture camera.
Everything which is said for a multi-capture camera may be reproduced with a monocular camera by moving the camera by a precisely known distance and producing identical exposures, on condition of also knowing precisely the orientation of the optical axis of the camera.
However, in the case of sources which are movable or variable in time, such a system does not work. Moreover, it is impractical and less precise. The multi-capture camera is therefore the preferred embodiment of the invention.
Constitution of a Multi-Capture Compton Camera:
A multi-capture camera consists of two Compton heads C1 and C2 (each of which may consist of a pair of diffuser+absorber plates). These two heads are ideally identical in terms of optical design, their optical axes are preferably parallel, and the two heads are separated by a known distance with precision. These two cameras observe the same field of view.
The precision of the position of a radioactive point source obtained with a Compton camera is between 1° and 2°. It is this parameter which is important for the design of a multi-capture camera. The distance between the two heads must subtend an angle greater than the angular precision of location of a source within range of the camera. (For example, a separation of 10 cm between heads if the precision of position is 1° and the aforementioned range 10 m.)
If it is wished to produce a portable camera, the distance between the two heads will be relatively short, for example 20 cm, and this camera can work in the multi-capture mode up to at least 5 m.
In this modality, it is important that the distance between the heads is not too great, for example <10°, so that the field of view observed is not too different between two images.
Thus, an angular distance between the heads of >30° has a different usefulness, that of giving a three-dimensional image of a wide object.
Benefits of a Multi-Capture Camera:
Gamma triangulation of the source.
The act of measuring the angular position of a given source from two detectors C1 and C2 which are separated by a distance D, supposing an angular precision a on the measurement of position (typically 1 to 2° according to the optical configurations and the number of photons detected) makes it possible to estimate the distance of gamma emission up to a distance Z=D/tangent (α).
Reduction of Artefacts
It was seen above that Compton image reconstruction is by nature ambiguous, which causes multiple artefacts in the images, particularly when the number of photons detected is small (<300 photons/source). Surprisingly, we found that, with a slight displacement of the shot (typically 20 cm for a distance of 300 cm), the position of the artefacts was completely different between the two images acquired by cameras C1 and C2. This double imaging therefore makes it possible to improve the signal-to-noise ratio of the reconstructed image. Conventional reconstruction of the images acquired by camera C1, which are obtained by intersection of the cones arising from C1, is used. The images acquired by camera C2 are obtained by intersection of the cones arising from C2.
Detection of Weak Sources with Certainty
The multi-capture camera is particularly well adapted to the analysis of slight contamination, or even natural radioactivity. In fact, in this case the flux of photons is weak (a few photons/hour). It takes hours for the source to exceed the threshold of detection (50 photons/point source) and the natural background noise interferes with the measurement.
By contrast, if an estimate of the distance from the source is available, triangulation makes it possible to obtain detection with certainty.
In fact, the relative position of a source on the two images is known, and it was seen above that the positioning of the noise had changed between the two images.
Furthermore, once the source is located on the two images, it is possible to add up the number of photons obtained on the two images, which increases the signal-to-noise ratio.
Increase in the Angular Resolution
We saw above that, when two intense sources are moved closer together, collapse of the image occurs. This collapse occurs because a high density of intersections of Compton cones appears in an ellipse between the two sources. However, the position of the source is recognised by the density of intersections of Compton cones.
This phenomenon is amplified by the fact that the dimension of the detector in the case of a monocular camera is small compared with the distance from the source, hence the regions of intersection are wide objects (due to the uncertainties about the measurement of the different parameters) distributed about two straight lines per pair of cones.
If the plane of reconstruction is now placed at a distance from the source and only the intersections between the cones arising from C1 and C2 ((C1-C2), not C1-C1 and not C2-C2)) are used to reconstruct the image, the situation changes: due to the distance between the two optical centres, the regions of intersection are reduced to arcs of conics which are no longer degenerate at Z: the two reconstructed positions of the source change according to the plane of reconstruction chosen: the solution is Z-dependent.
We saw in claim 1 that it is relatively easy with a multi-capture camera to obtain the distance from the source and therefore the plane of reconstruction. The true angular direction of the source being the same for all photons emitted, there will be a region of high density of intersection for the true position of the source at X, Y, Z, and a diffuse halo of false interactions around this position: the problem no longer being degenerate at Z. Thus the collapse of images can be avoided.
Thus the images can be improved, and one can hope for a gain by a factor of 2 to 3 on the angular resolution of our camera and find the angular resolution which would be obtained with an optical camera.
Moreover, as the uncertainty factors on the reconstruction are greatly reduced, detection of the source will be obtained with certainty by this method with a smaller number of photons, for example 15 photons/camera.
This method is therefore particularly advantageous for the detection of very slight contamination.
Automatic Management of Imaging of Wide Objects:
We use a method of smoothing of the reconstructed images: the ML/MLEM method. This method allows a large proportion of the artefacts to be made to vanish and brings out the details, however inadequate the statistics (number of photons/image voxel).
This method proceeds by successive iterations (between 10 and 30 in most situations). By contrast, if a large number of “smoothing operations” are applied to the image of a wide object, the image is highly degraded.
It is therefore important, before choosing the number of smoothing operations carried out, to identify whether a source is a point or is wide.
Factory calibration is carried out with a point source in different angular positions in the field of the camera. Thus the point spread function (PSF) of a point source is obtained as a function of the angle in relation to the optical axis:
A region of the image to be studied is chosen
In this region, the probable contour of the main source is determined
The number of photons present in the source is estimated
The profile of the source is compared to the PSF (θ) of a point source
The maximum number of smoothing operations to be applied in this case is determined as a function of the size of the source and the number of photons present.
Precise measurement of the flux of a naked source in a noisy context
The Compton camera provides reproducibly the number of photons which have contributed to building a given image. It is therefore possible to use it to estimate a flux of gamma rays.
It often happens that the source which is being looked for is situated in the middle of other sources emitting with the same energy, which can interfere with the measurement.
In this case the Compton image must be reconstructed over the whole of the space (4π steradians). Then these sources must be identified, and from the reconstruction the cones which pass through these parasitic sources must be excluded. In this way a count rate is obtained without interference.
Measurement of the Flux of a Shielded Source
Most often, intense sources of radiation are situated in a container which may be:
A metal barrel
A concrete enclosure
A lead shield.
In a context of decommissioning, it is necessary to know the intrinsic activity of a source Fr and not its apparent activity Fa at the limits of its container. We will show that our multi-capture camera makes it possible to estimate the true activity.
First of all, if one chooses to make an image in the energy range corresponding to the peak of the source (for example 630-690 KeV for a 137 Cs source), one sees only the photons emitted by the source which have not been diffused by the container, and this allows the apparent flux Fa to be measured.
Next, the energy spectrum is observed. It must comprise diffused photons of which the energy is lower than that of the source (typically for 137 Cs 500-600 KeV) if the source is significantly shielded. Ideally, an energy range where there is no other line of apparent emission is chosen for analysis.
Next an image is produced with these diffused photons (e.g. 500-600 KeV): the source must disappear, and the photons diffused by the container of the source are seen.
Knowing the distance from the source and the solid angle subtended by the image of diffused photons, the number of photons detected can be measured and so the flux of diffused photons Fd can be estimated.
In order to estimate the flux of photons absorbed by the container, the region of energy of the diffused photons can be cut in two and one image of 500-550 KeV and one image of 550-600 KeV produced. As the photoelectric effect falls rapidly with the wavelength, comparison of the number of photons detected in these two energy ranges allows the rate of absorption of radiation Fe to be estimated.
We then have Fr=Fa+Fd+Fe.
Naturally, this type of sophisticated processing is possible only if the images are statistically valid, that is, if we have a number of photons >>50/voxel.
Precise measurement of the flux arising from a source in a context with interference.
The image is reconstructed on 47c, and all cones passing through intense artefacts or sources situated outside the region of interest are eliminated.
In most cases, an initial estimate is available, which is even precise to 10% of the distance from the source (multi-capture visible camera, physical measurement, laser telemeter, etc.).
The direction of incidence of each gamma photon having a Compton effect in the detector is positioned on a cone.
For each pair of gamma photons detected, there are one to two possible solutions which are two straight lines generating the cone.
For each plane of reconstruction there are in general two solutions, only one of which represents the position of the source.
The solution is degenerate at Z: whatever the position of the plane of reconstruction, the two solutions appear angularly in the same place.
The second solution (the one which does not contain the source) is the one which generates artefacts (false concentrations) upon reconstruction.
By contrast, the moment one changes the point of observation of a distance greater than the error of position of the source (1 to 2° for our camera), the position of artefacts which depends on statistical concentrations of more or less random points is changed (the artefacts are no longer observed in the same place), and by contrast the true source changes position foreseeably according to the displacement of positions of the cameras.
For each pair of gamma photons, if one takes into consideration only the intersections of cones arising from camera A with those arising from camera B, the possible solutions are an arc of conic (ellipse, parabola, etc.). For each plane of reconstruction there are in general two solutions, only one of which represents the position of the source. On the other hand, the solution is no longer degenerate at Z: short of a certain distance there is no longer a solution. The angular position of solutions is dependent on the plane of reconstruction. However, the true source has a fixed angular direction, therefore the true source will emerge very rapidly from noise.
In the particular case in which there are two sources close together, there are numerous “false intersections” which appear in the region between the two sources. This brings about a collapse of the image in the central region with conventional methods of reconstruction. Example: Two 137Cs sources with an intensity of 720 MBq and 230 MBq are observed from a distance of 2.5 m with a Compton camera. The angular diameter of spots is 2°. Therefore in conventional optics an angular resolution of 3° could be predicted. If the sources are 8° apart, they are perfectly resolved. If the sources are 6° apart, a collapse of the image in the central region between the two sources is observed.
In the case of a multi-capture camera, for two sources close together, the “false solutions” will be distributed in a diffuse halo at Z, while the true sources will be dense in the plane of reconstruction. The collapse should be avoided and a resolution of 3° should be recovered. This allows a camera to be obtained at reduced cost.
The Present Application Therefore Aims to Protect:
Multi-capture Compton camera consisting of two adjacent detection assemblies observing the same field of view, characterised in that the two detection assemblies are separated by an angular distance greater than 1° at the maximum range of the camera in order to make it possible to observe displacement of a point source at the maximum range between two images taken simultaneously
Camera in which the same effect is produced by a precisely known relative movement of the camera between two shots, the position of the optical axis of the camera also being known.
Use of such a camera to estimate the distance from a radioactive source by triangulation of gamma rays
Elimination of artefacts, preserving only the points which are preserved between the two images (coherent distance)
Use of such a camera to detect and produce an image of very weak sources of <0.1 microrad/h with certainty
Method of reconstruction using a multi-capture camera which consists of considering, for the reconstruction, only the intersections of cones arising from camera C1 with those arising from camera C2
Improvement to the angular resolution connected with the use of the above reconstruction
Precise measurement of the flux of photons in a given region in a noise-affected context, eliminating all cones which converge outside the region of interest
Use of a multi-capture camera to estimate the real activity of a shielded source, using measurement of the flux of direct photons and the flux of diffused photons.
