Method for forming a gamma image, by combining Compton imaging modality and coded mask.

Abstract

Method for reconstructing the position of a source using a gamma camera. The gamma camera combines two imaging modalities: a coded mask imaging modality and a Compton imaging modality. The method involves selecting at least one isotope, with reconstruction performed using a Bayesian probabilistic approach, taking into account each selected isotope.

Description

Title of the invention: Method for forming a gamma image, by combining Compton imaging modality and coded mask. Technical field

[0001] The technical field of the invention is X or gamma imaging, and more particularly the reconstruction of the position of irradiating sources using an image acquired by a gamma camera. PREVIOUS ART

[0002] Gamma cameras are devices for forming an image to establish a map of irradiating sources in a given environment. A first application is the visualization of an irradiation source in an organism, for medical diagnosis purposes. Another application is the location of an irradiating source in an installation, and in particular in a nuclear installation.

[0003] The use of gamma cameras in the medical field is relatively old. In the nuclear industry, this type of device was developed in the 1990s, and is becoming more widespread in nuclear installations, for radiological characterization purposes. The objective is to identify the main irradiating sources present in an installation. Indeed, the irradiation sources are not distributed homogeneously. They are often concentrated locally, in the form of "hot spots", a common term in the field of radiation protection. A gamma camera has the advantage of locating these hot spots remotely.

[0004] Some gamma cameras consist of a two-dimensional matrix of pixels, connected to a detector material. The detector material is generally a semiconductor material, for example CdTe or CdZnTe. Under the effect of an interaction of ionizing radiation in the detector material, one or more pixels generate an electrical pulse, the amplitude of which is correlated with the energy released by the radiation during the interaction. Each pixel is connected to an electronic circuit for processing the pulses.

[0005] Each pixel is formed by an electrode, which usually acts as an anode. When incident radiation interacts in the detector material, electrons are released in the detector material. The electrons are collected by an anode. The latter generates a pulse whose amplitude depends on the number of electrons collected by the anode, this number generally being proportional to an energy lost by the ionizing radiation in the detector material. Thus, from the amplitude of a pulse, the energy lost by the radiation during the interaction can be estimated.

[0006] Furthermore, it is possible to locate the position of an interaction according to the depth of the detector material, i.e. perpendicular to the anode matrix. The estimation of the depth of the interaction is for example described in Montemont et al. “Studying spatial resolution of CZT detectors using sub-pixel positioning for SPECT”, IEEE transactions on nuclear science, Vol. 61, No. 5, October 2014.

[0007] Some gamma cameras use a coded mask type collimator, formed from a predefined pattern. The pattern may, for example, be periodic and include an elementary mesh reproduced by rotation and / or translation. The pattern allows alternation between absorbent parts between which openings extend. The assembly forms a coded mask. The collimator allows delimitation of an observed field. From an image formed on the gamma camera, a back-projection operator makes it possible to obtain a position of the irradiating source in the observation field. With this type of gamma camera, only the interactions originating from radiation emitted by the source, and not diffused prior to the interaction in the detector material, contribute to the information useful for locating the irradiating sources in the observation field.

[0008] Another type of gamma camera takes advantage of temporally coincident interactions resulting from Compton scattering (inelastic scattering) of X-ray or gamma radiation in the detector material. Generally, the first interaction corresponds to Compton scattering, while the second interaction is photoelectric absorption. Knowledge of the coordinates of two interactions, resulting from the scattering of an ionizing photon in the detector material, as well as the energies released respectively by each interaction, makes it possible to estimate a trajectory of the incident photon. An advantage of Compton gamma cameras is the absence of collimation, which gives a field of observation of 4 ir steradians. Another advantage is the possibility of obtaining a compact device.

[0009] The invention described below allows a combination of coded mask imaging and Compton diffusion imaging modalities. Statement of the invention

[0010] A first object of the invention is a method for estimating positions of irradiating sources in an observation field, using a gamma camera, the gamma camera comprising: - a detection material; - pixels, distributed along a detection surface of the detection material, each pixel being configured to form a detection signal under the effect of detection of an interaction of an ionizing photon in the detection material; - a collimator placed facing the detection material, and defining a first observation field, included in the observation field of the gamma camera; - a localization unit, configured to assign a position to each interaction in the detection material, from the detection signal generated by said interaction; - a spectrometry unit, configured to assign an energy to each detected interaction, from the detection signal generated by said interaction; - a coincidence unit, configured to identify each interaction as a single interaction or a multiple interaction, a multiple interaction being formed from two diffusion interactions detected in temporal coincidence; - a memory, configured to store parameters of detected interactions;

[0011] the method comprising the following steps: - a) acquisition of detection signals by the pixels during an acquisition period, each detection signal being associated with a detected interaction; - b) depending on the detection signals, assignment of a position and an energy to each detected interaction; - c) identification of each detected interaction as a single interaction or a multiple interaction; - d) memorizing, by memory, the parameters of each interaction, the parameters of each interaction including the position and the energy of said interaction;

[0012] the method being characterized in that it also comprises - e) selection of an isotope from a list of isotopes; - f) depending on the energy assigned to each interaction, simple or multiple, calculation of a probability of emission by the isotope; - g) from the parameters of each interaction, calculation of a probability of emission of the photon having generated the interaction, from different points of the observation field; - h) taking into account an a priori probability of emission of a photon for the selected isotope, from the different points of the observation field; - i) based on the probabilities resulting from g) and h), calculation of a spatial distribution of emission intensity in the first observation field for the isotope selected in step e).

[0013] Steps h) and i) can be carried out iteratively, such that at each iteration: - step h) includes taking into account a spatial distribution of the emission intensity of the initial isotope or resulting from a previous iteration; - step i) includes an update of the spatial distribution of the emission intensity for the selected isotope based on: • the a priori probability of emission resulting from step h) of the same iteration; • probabilities resulting from steps f) and g).

[0014] According to one embodiment, during step e), the isotope is - selected according to the energies respectively assigned to different detected interactions; - or arbitrarily selected.

[0015] According to one embodiment, steps f) to i) are carried out successively for different selected isotopes.

[0016] According to one embodiment, step f) is implemented by calculating: - a probability that the photon having generated the interaction is a direct photon, emitted by the isotope, according to an emission energy of the isotope and having reached the detection material while conserving the emission energy, without crossing an absorbing part of the collimator; - a probability that the photon having generated the interaction is an indirect photon, emitted by the isotope, according to an emission energy of the isotope, and having reached the detection material with an energy lower than the emission energy and / or by crossing an absorbing part of the collimator.

[0017] According to one embodiment, step f) comprises taking into account, for each emission energy of the isotope: - a distribution of probabilities of detection of an interaction, in the detector material, according to different energies, assuming that the photon having generated the interaction is a direct photon; - a distribution of probabilities of detection of an interaction, in the detector material, according to different energies, assuming that the photon having generated the interaction is an indirect photon;

[0018] the detection probability distributions being previously calculated by considering a simple interaction and a multiple interaction.

[0019] According to one embodiment, step g) comprises: - when the interaction is simple, selection of a first spatial response model of the camera, the probability of emission position from different points of the first observation field being determined as a function of the first spatial response model and the position of the interaction in the detector material; - when the interaction is multiple, selection of a second spatial response model of the camera, the probability of emission position from different points of the observation field being determined as a function of the second spatial response model and the energy as well as the position of the interactions detected in temporal coincidence forming the multiple interaction in the detector material.

[0020] The first spatial response model can determine, for different two-dimensional positions of simple interactions established parallel to the detection surface, a probability of detection of a photon emitted from different points of the first observation field, each simple interaction being an interaction not considered as multiple.

[0021] The second spatial response model determines, for different three-dimensional positions of interactions detected in temporal coincidence, forming the multiple interaction, and for different energy values ​​of said interactions, a probability of detection of a photon emitted from different points of the observation field.

[0022] A second object of the invention is a gamma camera, intended to detect the presence of irradiating sources in an observation field, the gamma camera comprising: - a detection material; - a coded mask collimator, extending opposite the detection material; - pixels, distributed over a detection surface of the semiconductor material, each pixel being configured to form a detection signal under the effect of detection of an interaction of an ionizing photon in the semiconductor material; - a localization unit, configured to assign a position to each interaction in the semiconductor material, from a detection signal generated by said interaction, so as to obtain a localization according to the detection surface and in a direction perpendicular to the detection surface; - a spectrometry unit, configured to assign an energy to each detected interaction, from the detection signal generated by said interaction; - a coincidence unit, so as to identify each interaction as a single interaction or a multiple interaction, a multiple interaction being formed from two interactions detected in temporal coincidence; - a memory, configured to memorize a quantity of detected interactions; - a processing unit, configured to process the interactions stored in the memory, the processing unit being configured to implement steps d) to i) of a method according to the first subject of the invention.

[0023] The invention will be better understood upon reading the description of the exemplary embodiments presented in the remainder of the description, in conjunction with the figures listed below. FIGURES

[0024] Figures 1A and 1B show schematically the main components of a gamma camera allowing implementation of the invention.

[0025] Figures 2A and 2B represent two scattering configurations, according to which two interactions are alternately considered as scattering and absorption.

[0026] [Fig.3A] is a matrix illustrating a spectral response of gamma in the case of a simple interaction in the detector material, the interacting photon having conserved the emission energy.

[0027] [Fig.3B] is a matrix illustrating a spectral response of the gamma in the case of a simple interaction in the detector material, the interacted photon not having conserved the emission energy or having passed through an absorbing part of the collimator.

[0028] [Fig.3C] is a matrix illustrating a spectral response of the gamma in the case of a multiple interaction in the detector material, the photon having diffused in the detector material having conserved the emission energy.

[0029] [Fig.3D] is a matrix illustrating a spectral response of the gamma in the case of a multiple interaction in the detector material, the photon having diffused in the detector material not having conserved the emission energy or having passed through an absorbing part of the collimator.

[0030] [Fig.4] shows schematically the main steps of a method according to the invention.

[0031] [Fig.5A] is a graph for estimating the scattering angle as a function of the energy of two interactions resulting from scattering.

[0032] [Fig.5B] is a graph allowing estimation of a diffusion probability as a function of the energy of two interactions resulting from the diffusion.

[0033] [Fig.6] represents an estimate of a conical surface comprising the trajectory of a photon before scattering.

[0034] [Fig.7] illustrates the first field of observation and the second field of observation.

[0035] Figures 8A, 8C and 8E are examples of source positions reconstructed from a first imaging modality (coded mask imaging), a second reconstruction modality (Compton imaging), and a combination of the first modality and the second modality: in each of these figures, the reconstruction of the image was carried out with a single iteration

[0036] Figures 8B, 8D and 8F are examples of source positions reconstructed from a first imaging modality (coded mask imaging), a second reconstruction modality (Compton imaging), and a combination of the first modality and the second modality: in each of these figures, the reconstruction of the image was carried out according to several iterations.

[0037] Figures 9A and 9B illustrate a variant of a gamma camera allowing an implementation of the invention. PRESENTATION OF SPECIAL EMBODIMENTS

[0038] Figure 1A represents a gamma camera 1, or gamma camera, allowing an implementation of the invention. The gamma camera is configured to detect ionizing photons, of type X or gamma, whose energy is generally between 10 keV and 10 MeV. The objective of the gamma camera is to locate irradiating sources present in the observation field Q. The observation field Q extends around a central axis A. The gamma camera can be coupled to a visible camera 2, making it possible to form a visible image of the observation field. The term gamma camera corresponds to an imager having an observation field and configured to form an object image O allowing a location of irradiation sources in the observation field.

[0039] The gamma camera comprises a detector material 11, usually a semiconductor material allowing the creation of charge carriers (electron / hole pairs) during an interaction with X or gamma radiation. It may in particular be CdTe or CdZnTe.

[0040] The gamma camera comprises 12 pixels, distributed along a detection surface. In the example shown, the 12 pixels are coplanar and distributed according to a two-dimensional matrix, preferably regular. Other possibilities are possible and described below. The matrix may for example comprise a few tens or even hundreds of pixels. Each pixel 12 is an elementary radiation detector. In the example described, each pixel 12 is an anode, the polarization of which makes it possible to collect electrons produced during an interaction occurring in the detector material.

[0041] When an ionizing photon interacts in the detector material 11, charge carriers, for example electrons, migrate towards one or more pixels, the latter being touched pixels: each touched pixel is a pixel which collects charge carriers. An interaction can give rise to one or more touched pixels. When the pixels are small, several pixels can be touched during an interaction. When the pixels are large, the number of touched pixels is reduced and can be limited to a single touched pixel. During their migration towards a touched pixel, the charge carriers can generate a signal, usually called an induced signal, on the adjacent pixels of the touched pixel(s).

[0042] Generally speaking, each interaction gives rise to the formation of a detection signal by at least one pixel, and most often several pixels. The detection signal may be a signal resulting from the collection of charge carriers by one or more pixels 12 or a signal induced by the migration of charge carriers through the detector material 11.

[0043] The gamma camera comprises a localization unit 14, so as to define a three-dimensional position of each interaction in the detector material 11.

[0044] When the pixels have a large surface area, typically greater than 1 mm2, the localization unit can implement sub-pixelization, in order to assign a two-dimensional position (y, y) of each detected interaction, parallel to the detection surface 12, from detection signals formed by several pixels 12 following each interaction. Thus, each pixel is virtually divided into virtual pixels. The subdivision into virtual pixels makes it possible to improve the spatial resolution. Such a method is for example described in the publications Warburton WK, “An approach to sub-pixel spatial resolution in room temperature X-ray detector arrays with good energy resolution” as well as in Montemont et al. “Studying spatial resolution of CZT detectors using sub-pixel positioning for SPECT”, IEEE transactions on nuclear science, Vol. 61, No. 5, October 2014 or in patent US9322937B2.Using these methods, the size of virtual pixels can reach, for example, 0.5 mm * 0.5 mm, or 0.1 mm by 0.1 mm. The use of sub-pixelation is only justified when the pixels 12 have a surface area considered too large. Subsequently, the term pixel refers indifferently to a virtual pixel and a physical pixel.

[0045] The localization unit 14 can be configured to establish an interaction depth along an axis perpendicular to the detection surface of the detection material 11. The interaction depth can for example be determined according to of the detection signal, or detection signals, generated by the pixel(s) that collected the charge carriers. The interaction depth is determined when the detection material is sufficiently thick, for example beyond a thickness of 1 mm. When the material is considered thin, the interaction depth is fixed throughout the material. The detector can be composed of different thin and spaced apart detection layers, and preferably parallel to each other. In this case, each layer is assigned an interaction coordinate perpendicular to the detection surface.

[0046] Under the effect of the collection of charge carriers, each pixel 12 generates a pulse whose amplitude depends on the energy released, in the detector material, by an ionizing photon, during an interaction, this energy usually being designated “interaction energy”. The gamma camera 1 comprises a spectrometry unit 15. The spectrometry unit allows as precise an estimation as possible of the amplitude of the pulses resulting from the collection of charge carriers following an interaction. The spectrometry unit can comprise both electronic means (pulse shaping circuit, multichannel analyzer, analog-to-digital converter) and software means. Estimating the amplitude of a pulse makes it possible to estimate the interaction energy. This energy must be estimated as precisely as possible. The addressed energy range is generally between 10 keV and a few hundred keV, or even a few MeV.It is desirable that the energy accuracy be in the order of %, or even lower.

[0047] Thus, the spectrometry unit 15 makes it possible to obtain an energy spectrum of the interactions detected by each pixel. Each interaction is thus classified in an energy channel c. Each energy channel c extends between E ±5E. 2ÔE then corresponds to the spectral width of each energy channel. For example, 25E ​​•= 0.2 keV. Depending on the energy of the detected photons, the isotopes responsible for the irradiation can be identified. The energy of each interaction can also be processed using a spectral response from the gamma camera, as described below.

[0048] The gamma camera is bimodal. It is configured to exploit the interactions detected in the detector material 11 to reconstruct a position of irradiating sources in the observation field, and this by using a reconstruction modality dependent on each detected interaction. The gamma camera comprises a collimator 10, of the coded mask type, as described in the prior art.

[0049] According to a first modality, the detected interactions result from photons having propagated through the collimator 10 before reaching the detection material. This first modality allows localization of potentially irradiating sources present in a first observation field QL Each point of the first observation field can be projected through the entire mask, onto the detector. This means that radiation emitted by each point of the first field, projected through the entire coded mask 10, reaches the detection material. The interactions exploited by the first modality are so-called simple interactions, in the sense that they do not occur in temporal coincidence: they are detected at different times. The first observation field Ql is represented in [Fig.lB].

[0050] According to a second modality, called Compton imaging, the detected interactions result from photons emitted around the gamma camera, and interacting in the detector material 11 by Compton scattering. In a known manner, following Compton scattering, a scattered photon propagates according to a scattering angle relative to the trajectory incident to the detector material. When the scattered photon is absorbed in the detector material, it is possible to estimate the incident trajectory. The occurrence of inelastic scattering (or Compton scattering), followed by absorption of the scattered photon, generates two interactions which are detected simultaneously, i.e. temporally coincident. Knowledge of the position and energy of each interaction allows an estimation of the incident trajectory of the photon, according to the expression:

[0051] cosa = \-m<£2 [

[0052] where: - a is an angle between the trajectory of the incident photon to the detector material 11 and a straight line connecting the position of the two interactions in the detector material. This is the scattering angle. - E, and E2 are the respective energies of the first interaction and the second interaction, taken in chronological order. - me is the mass of an electron; - it is the speed of light in a vacuum.

[0053] In Figures 2A and 2B, the angle a is represented between the incident trajectory of a photon and a straight line separating two interactions generated under the effect of the inelastic scattering of the photon in the detector material. A first interaction, releasing an energy E^ corresponds to a scattering according to a scattering angle a. The energy Ei corresponds to a part of the energy of the incident photon. A second interaction, releasing an energy E2, corresponds to an absorption of the scattered radiation. The energy E2 corresponds to the energy of the scattered photon. The two interactions are offset in time but the time interval separating them is too short to be determined. Also, the two interactions are considered to be simultaneous or in temporal coincidence and form a multiple interaction of total energy E - Ey+E^ It is not possible to identify, among the two interactions, which one is prior to the other. Also, when detecting two interactions simultaneously, each interaction is successively considered to be the first interaction (scattering), then the second (absorption of the scattered photon). This results in two different surface area estimates comprising the trajectory of the incident photon. Figure 2A corresponds to a "forward" scattering, the scattering angle a being acute, while Figure 2B corresponds to a backscattering ("backward" scattering), the scattering angle a being obtuse. The front / back terms are to be considered as a function of the trajectory of the incident photon at the detector. The probability of each configuration (forward scattering or backscattering) is governed by the Klein-Nishina relation:p^£jex + cos2( (f)

[0054] where _ ^2 and a is deduced from the expression (1) e~ E

[0055] In Figures 2A and 2B, two conical surfaces are represented, each conical surface corresponding to the potential trajectories of the incident photon in each case. Thus, to each multiple interaction, one or two conical surfaces are assigned, weighted by a probability resulting from the Klein-Nishina relation. After a sufficient number of multiple interactions have been detected, the position of each irradiating source is determined by an intersection of the different conical surfaces assigned to each multiple interaction.

[0056] The second Compton imaging modality allows the detection and localization of sources located in a second field of observation, which can extend according to a solid angle of 4ir steradians around the detection material.

[0057] Thus, the first imaging modality through the coded mask collimator addresses a relatively reduced first observation field, the second modality addresses a second observation field Q2 extending beyond the first observation field Q1 and extending well beyond the latter. In the following, the first observation field Q1 is a flat surface located at a predetermined distance from the gamma camera. The second observation field Q2 is a sphere of predetermined radius around the gamma camera, and centered on the detector 11.

[0058] The field of observation of the gamma camera Q corresponds to: - either to the second observation field Q2; - either to the second observation field Q2, truncated by a flat part corresponding to the first observation field Ql, and to which is added the first observation field QL The observation field of the gamma camera Q comprises a spherical part (part of the observation field Q2 without overlapping with the observation field Ql) and a flat part, the latter corresponding to the first observation field QL Cf. [Fig.lB]. - The first field of observation and the second field of observation have a common part, in which the two imaging modalities can be combined.

[0059] The first modality can be implemented from a two-dimensional localization (i.e. parallel to the detection surface) of each single interaction. The second modality preferably requires a three-dimensional localization of each multiple interaction.

[0060] The gamma camera comprises a temporal coincidence unit 16, configured to assign a detection instant r to each detected interaction. The temporal coincidence unit makes it possible to identify simple interactions (a single interaction assigned to an instant) and multiple interactions (several interactions assigned to the same instant).

[0061] The gamma camera 1 has a memory 17, configured to memorize the simple interactions detected at each instant f. To each simple interaction, the parameters (x, y. E) are assigned - the coordinates (A ^) are coordinates of the pixels, real or virtual, having detected the interaction, determined by the localization unit 14; - energy E is the energy released by the interaction, determined by the spectrometry unit 15;

[0062] The memory is also configured to store the multiple interactions detected at each instant*. To each multiple interaction, the parameters yr zP x2, y2, z2, E', E'21 are assigned - The coordinates ( _ ) correspond to the coordinates of the x2,y2,z2) 1 two scattering interactions, forming the multiple interaction, and detected in temporal coincidence; - the energies E'} and E'2 detected at each scattering interaction forming the multiple interaction, whose total energy is E' = E\ + E'2.

[0063] The gamma camera may comprise an image forming unit 18 configured to form a gamma image G from the simple interactions. The gamma image G is defined according to coordinates (x, y), parallel to the detection surface 12, each coordinate (x, y) corresponding to a pixel 12. Each point G(x, y) of the gamma image G corresponds to a quantity of simple interactions detected by each pixel 12 of coordinates (x, y).

[0064] When implementing the spectrometry circuit 15, the memory 17 can store the energies associated with each interaction detected during an acquisition period. The duration of the acquisition period can be between a few seconds, in the case of strong irradiation, or a few minutes, possibly a few hours when the irradiation to which the detector material 11 is subjected is weak. In the case of a multiple interaction, the energy of the interaction is the total energy previously mentioned, corresponding to the sum of the energies of each diffusion interaction.

[0065] From the detected energies, an isotope selection unit 19 is configured to select one or more isotopes likely to be present in the observed field. Subsequently, the index ' designates an isotope. The selection of the isotopes is carried out according to the energy emission probabilities of known isotopes, previously stored. The decay patterns of isotopes likely to constitute the irradiating sources of the observation field are then taken into account. By decay pattern of an isotope, we mean the emission energy, or energies, as well as the branching rates (probabilities of emission of a photon for the different emission energies). The selection of the isotopes can be done automatically by the isotope selection unit 19, or manually, the selection being carried out by an operator.

[0066] The different detected energies can be presented in the form of a global detection spectrum. The global detection spectrum is a histogram of the energies of the different interactions detected, whether they are single or multiple interactions. Usually, the abscissa axis corresponds to the energies (or channel numbers) and the ordinate axis corresponds to the number of interactions (single or multiple) detected for each energy. The spectrum comprises peaks, each peak corresponding to an emission energy of an isotope. From the peaks, the isotopes likely to be present in the observation field are selected. The detection spectrum is established from all the interactions, single or multiple.The field of observation Q is the second field of observation Q2 (i.e. that of the second modality) extending beyond the first field of observation Q1 (that of the first modality), to which is added the first field of observation Ql.

[0067] According to one possibility, the isotopes located in the field of observation are known. In this case, the user selects the isotopes i to be considered in the isotope selection unit.

[0068] The gamma camera comprises a reconstruction unit 20, intended to position any irradiating sources detected in the observation field Q.

[0069] The reconstruction is carried out using a probabilistic approach. A particularity of the adopted approach is to assign a weight to each detected interaction. The weight corresponds to a probability of emission of the detected photon by an isotope selected by the isotope selection unit 19, at different points in the observation field. When implementing the first modality, only the first field is considered observation field Ql, the latter being discretized by points with coordinates X and Y. The observation field Ql is a plane parallel to the detector, located at a distance d from the latter. The distance d can be chosen arbitrarily. When implementing the second modality, we consider the observation field Q2. As previously indicated, the second observation field Q2 is for example a sphere, centered on the center of the detector material, of radius d, d being the distance between the first observation field Ql and the detector. [Fig.7] represents the observation fields Ql and Q2. The selection between the first modality and the second modality is made according to whether an interaction is considered simple or multiple by the temporal coincidence unit.

[0070] Figure 3A represents, for different emission energies (ordinate axis - unit keV), probability distributions of detection of a simple interaction, in the detector material, according to different energies (abscissa axis - channels) assuming that the photon having generated the interaction is a direct photon. In Figure 3A, the abscissa axis corresponds to an energy channel c, whose rank is between 1 and 1024, to each channel corresponding an energy band (or "energy bin"). To each channel corresponds an energy E'. The notation E' designates the energy associated with each channel, the notation E designating the energy incident on the detector. The ordinate axis corresponds to emission energies. To each emission energy corresponds a distribution of detection probabilities in the different energy channels. Figure 3A is a matrix Hsj of different probability distributions corresponding to different emission energies.The probability distribution corresponding to an emission energy is a line of the matrix, such as the one represented in dotted lines, corresponding to the energy 1300 keV. The gray level represents the associated probability level. Thus, each point Hsj(E\E) of the matrix Hsj corresponds to a probability of detection, in an energy channel c, of a photon emitted at an emission energy E. The matrix Hsj represented in [Fig.3A] thus corresponds to a spectral response matrix of the gamma camera. The spectral response matrix is ​​obtained by forming two hypotheses: . - the detected interaction is a simple interaction, which corresponds to the index j. j is a variable taking the value 1 during a simple interaction and 2 during a multiple interaction. - the detected photon is a direct photon, that is to say a photon reaching the detector material by propagating through an opening of the collimator 10, having an energy E corresponding to the emission energy by the isotope. This corresponds to the index s = +. s is a variable taking the value + when considering a direct photon and - when considering a photon indirect. An indirect photon is a photon that has interacted before reaching the detector material or that has passed through the collimator 10 without interacting, and reaches the detector material with its initial energy.

[0071] Given these assumptions, each point of the matrix Hsj is such that:

[0072] Hsj(E\E) = ^E\E,s=+, j= 1),

[0073] where p(EE, s — +. j = 1) is a probability of detection of the photon in the energy channel c.

[0074] Figure 3B represents a spectral response matrix Hsj established by considering a simple interaction (j = 1) and an indirect photon: s ~ ~. Each point of the matrix of Figure 3B is such that Hsj(E\E) = E'\E,s= -, j = 1)- An indirect photon is a photon having lost part of its energy before being detected, and / or having reached the detector, without losing energy by passing through an absorbing part of the collimator 10. The loss of energy may be due to diffusion in the collimator 10 or in the environment of the gamma camera. The matrices represented in Figures 3A and 3B can be obtained by Monte Carlo type modeling.

[0075] Each spectral response matrix can be obtained by modeling. For this, the main components of the device are modeled: chassis, collimator, detector material. The presence of irradiating sources in the field of observation of the camera is simulated and, for each photon emitted by a source, the status of the photon reaching the detector (direct or indirect photon) is estimated, as well as the energy of the photon detected in the detector. The irradiating sources are randomly arranged in the field of observation. Each spectral response matrix is ​​obtained by averaging the energies detected for the direct or indirect photons respectively.

[0076] Figures 3C and 3D respectively represent: - a spectral response matrix Hsj, as described in connection with figures 3A and 3B, considering - + (direct photon) and j = 2 (multiple interaction); - a spectral response matrix Hsj, as described in connection with figures 3A and 3B, considering5 ~ " (indirect photon) and J = 2 (multiple interaction). In the case of a multiple interaction, an indirect photon is as previously defined, to which is added a photon not releasing all its energy in the detection material: following diffusion, the diffused photon escapes from the material, without being absorbed.

[0077] It is recalled that for each multiple interaction, the energy E' corresponds to the sum of the energies simultaneously detected.

[0078] According to one possibility, each spectral response matrix can be established for each pixel. In the example described, a spectral response matrix is ​​considered defined for all pixels, for each type of interaction: j = 1 or 2 and ' = + or

[0079] The spectral response matrix can be simulated by Monte Carlo method.

[0080] [Fig.4] represents the main steps implemented by the different components of the gamma camera to obtain a reconstruction of sources in the observation field Q.

[0081] Step 100: Interaction detection. The gamma camera is placed in an environment potentially comprising irradiating sources 5. During an acquisition period: - the localization unit 14 assigns a position to each detected interaction; - the temporal coincidence unit 16 determines whether each interaction is a single or multiple interaction; - the spectrometry unit 15 assigns an energy E' to each detected interaction. In the case of a multiple interaction, the energy E' corresponds to the sum of at least two energies E'^ E'2 detected simultaneously;

[0082] Each interaction can be associated with a rank, for example chronologically. Each interaction k is assigned a set of parameters: xh yE'k for a simple interaction and X^ yz^ x^, y^, Z^ E' , E'^ for a multiple interaction.

[0083] The parameters of each interaction are stored. The number of stored interactions may be greater than several hundred or thousands, and may exceed several hundred thousand or millions.

[0084] Step 110: Selection of an isotope.

[0085] During this step, one or more isotopes 1 are selected based on the Ek energies detected during the acquisition period. The isotope, or each isotope, can be selected automatically or manually. Alternatively, the selection of each isotope is carried out on the basis of a priori assumptions regarding the isotopic composition of the irradiating sources.

[0086] Step 1 2 0: Taking into account spatial and spectral response models.

[0087] The reconstruction unit 20 performs a reconstruction of the position of the sources in the observed field from the single interactions and the multiple interactions detected. The reconstruction is carried out by successively taking into account each selected isotope i. Thus, step 120 is carried out for each isotope l.

[0088] The reconstruction of the sources assumes the taking into account of a spectral response model and a spatial response model. The spectral model D(E\ i, s) makes it possible to define, for each event case, a probability of detected energy E', for a type of photon (direct or indirect), knowing the isotope”.

[0089] So,#^#;^)

[0090] Dj(E. K s) = e)X£I>)

[0091] ^E\i) corresponds to a probability of emission of energy E by the isotope h This probability can be determined by existing databases, including, for each isotope, the emission energies and the corresponding branching rates.

[0092] In the case of a multiple interaction (j = 2), E' = E\ + E'2.

[0093] The spatial model corresponds to the probability of detecting an interaction of a photon in a coordinate (x, y) or (xy z) when the photon has been emitted in a coordinate (X, Y) of the observation field. This corresponds to a direct model, which can easily be obtained by modeling. For simple and multiple interactions, a previously established spatial model is taken into account, corresponding respectively to the first modality and the second modality.

[0094] Thus, the reconstruction uses a spectral response model and a different spatial response model depending on whether the interaction is simple or multiple.

[0095] Sub-Step 1 21: Simple interactions

[0096] In the case of simple interactions, the spectral response model is established from the Hsj matrices with j=\. The spatial response model is such that: - for direct photons, a spatial model is used that takes into account the topography of the coded mask. More precisely, for each source position (Xb Y0 in the first observation field Ql, we can estimate an interaction probability positioned according to two-dimensional coordinates (X, yj jans |c detector material. Thus, p( x,y\Xl,Yl,s~+)~M(x,y,X],Yi) (3), whereMCx^^rO is a probability of interaction according to the coordinates (x, v) for an irradiating source located in y J on the first observation field QL M ( x, y, X^ Yj ) can be determined by numerical modeling. - for indirect photons, we use a uniform position probability in the first observation field. p( x, y[Xj, = l(x, y)® r j' where 1 denotes a distribution uniform two-dimensional.

[0097] From the spectral response models D and spatial M, we can establish a response of the system R, such that R = ® is the tensor product operator.

[0098] As previously indicated, interaction depth 2 is not necessarily used to deal with simple interactions.

[0099] Sub-Step 1 2 2: Multiple interactions

[0100] In the case of multiple interactions, the spectral response model is established from the Hsj matrices with j=2. The spatial response model is such that: - for direct photons, we use a spatial model as mentioned in connection with figures 2A and 2B. Each multiple interaction corresponds to two cones C whose half-angle at the apex corresponds to the scattering angle a. P^X2, J2, z2, E\ E^E, X2, Y2, Z2, s = + j denotes a probability of Compton scattering taking place between the positions xi' zi and x2? y2' releasing the respective energies E} and E2, when a photon direct is emitted at energy E in a from a point of coordinates (X2, Z2) of the observation field Q2. The spatial model C can be established by numerical modeling, as described below. - for indirect photons, we use a uniform position probability in the second observation field Q2:

[0101] yrzvx2,y7zvE' E'2\E, X2,Y2,Z^s=- ) =€ I(xp Z)) ®l(x2. ® denotes the tensor product. S(X2, Y2,Z2) corresponds to the sphere forming the observation field Q2. It can, for example, be a sphere of unit radius.

[0102] Figures 5A and 5B illustrate the implementation of the spatial model C used for multiple interactions. Figure 5A shows the value of the scattering angle α (gray level) as a function of the detected energies of the first interaction (y-axis) and the second interaction (x-axis). On the x- and y-axes, the coordinate 100 corresponds to the energy 511 keV. Figure 5A shows an example of two interaction detections with respective energies 500 keV and 250 keV. The value of the scattering angle α, indicated by the gray level, depends on the interaction that corresponds to scattering (first interaction detected in chronological order) and the interaction that corresponds to absorption (second interaction detected in chronological order). If the interaction corresponding to scattering is that of energy 500 keV, the value of the scattering angle a is 48°.If the interaction corresponding to scattering is that of energy 250 keV, the value of the scattering angle is 97° (backward scattering). The dotted line corresponds to the sum of the energies of the first interaction and the second interaction, in this case 750 keV. The dotted line makes it possible to identify pairs E\, E'2 whose total detected energy is equal to 750 keV.

[0103] Thus, Figure 5A makes it possible to estimate the angle a from E\ and ÆA. According to the chronological order assigned to the interactions generating the energies E\ and E'2, the spatial model corresponds to two cones, extending around the same axis which corresponds to a straight line connecting the coordinates (x? zi) and (x2, j , z2) of each interaction, whose respective vertices are (xi' zi) and (x2' J2' z2), and whose respective vertex half-angles are the two scattering angles obtained by taking into account the two detection chronologies: interaction located at (xi' zi) occurring respectively before or after the interaction located at (x2, y? Z2).

[0104] Each cone is assigned a probability which depends on the chronological order of each detection.

[0105] [Fig.5B] represents the probabilities associated with each chronological order: 500 keV (scattering) then 250 keV (absorption) or 250 keV (scattering) then 500 keV (absorption). The axes of [Fig.5B] are similar to the axes of [Fig.5A]. The gray level corresponds to the probability of each chronological order: the probability associated with the configuration (500 keV; 250 keV) is 0.3; while the probability associated with the configuration (250 keV, 500 keV) is a few %. The probabilities corresponding to each configuration are determined using the Klein-Nishima formula.

[0106] Figures 5A and 5B include white areas which correspond to impossible situations.

[0107] Using the tabulated data of Figure 5A and the probabilities of Figure 5B, and knowing the positions of each scattering interaction, two coaxial cones can be determined, and corresponding half-angles determined using Figure 5A, each cone being assigned a probability determined in Figure 5B. The axis of each cone is a straight line passing through the positions (xi' zi) and / ) of J2, each interaction. The uncertainty in the scattering angle, due to the uncertainties in the respective energies E^ and E2 and in the interaction positions, can be translated into a thickness defined for each cone. In order to take into account the uncertainties, the half-angle of each cone can vary within a confidence interval a±Aa where Aa corresponds to a measurement uncertainty. The measurement uncertainty results from the uncertainty in the estimates of the energies EA and E2. Each cone is parameterized by the angles a (scattering angle) as well as 0 (elevation of the cone axis) and (azimuth of the cone axis) as represented in Figure 6. Figure 6 shows the second observation field 2, here assimilated to a spherical surface. A fixed reference frame (X, Y and Z) is associated with the second field, whose origin corresponds to the center of the detector. The X and Y axes are for example parallel to the detection surface. The Z axis is perpendicular to the detection surface. In [Fig.6], we have represented a cone of . half-angle a. When the cone passes through collimator 10, the attenuation of the latter is taken into account in the model. We can thus establish a probability of origin of the photon for each angular coordinate 0 and q>. The position of each source corresponds to an intersection between each cone and the spherical surface.

[0108] In [Fig.6], we have also represented: a reference frame (x' J' z), linked to the interactions, whose axis corresponds to the axis of the cone, that is to say to the straight axis passing through the positions (X1' zi) and (¾ J2, Z2) ' an intermediate reference frame (X',Y',Z'), which is obtained by rotating the reference frame (x' Y z) around the x axis, coincident with the X' axis, according to the angle 0; the reference frame (X,Y,Z) linked to the detector, which is obtained by rotation of the intermediate reference frame (X',Y',Z'), around the Z' axis, coincident with the Z axis, according to the angle q>.

[0109]

[0110] The coordinates of a point on the cone are: - in the frame linked to the interactions (x,y,z) ' x = sin(a')sin(s) y - cos {a) z= sin(a)cav(g)

[0111] Angle 8 corresponds to an angle of precession around the axis of the cone, uniformly distributed between 0 and 2ir. in the intermediate frame (X',Y',Z'): X' = ​​x Y' = ycos(0) -zsin(Q) Z' = ysin (3) + zcos ( 0 ) in the reference frame of observation field 2, centered on the detector:

[0113] r X2 = Z2 = Z 1

[0114] Figure 7 represents the first field of observation 1, or reduced field of observation, as well as the second field of observation 2. Certain coordinates X, Y, Z defined in the field of observation 2 can be transposed into the first field of observation 1. To do this, angular coordinates 0' and q>' are defined for each coordinate X, Y, Z in the field of observation. When the angular coordinates 0' and q>' intersect the first field of observation 1, the corresponding coordinates in the first field of observation 1 are determined. Thus, for each angular coordinate 0' and q>' included in the reduced field of observation, can apply trigonometric relations to obtain a position X^ Fj in the reduced field of observation.

[0115] Xj = tan(0')cosO')

[0116] yjManffl^sinO')

[0117] This makes it possible to assign an emission position, in the first observation field 1, of a photon having generated a multiple interaction.

[0118] Step 1 3 0: reconstruction of simple interactions. The reconstruction of the sources is carried out by implementing a MLEM (Maximum Likelihood Expectation Maximization) type reconstruction approach, which results from the application of Bayes' theorem, separating simple interactions from multiple interactions. According to Bayes' theorem,

[0119] Y{,i\Er,xy) = xp(Xj, Y^i) (4)

[0120] where: - P(Xb Fh z) is the current hypothesis on the position and composition (isotopes) of the sources: this results from the a priori knowledge of the reconstruction available relative to the position of the sources: p(XY, F], i) corresponds to the current image. Subsequently, p(Xh Fh 1) is designated - p(E' XF 1^1 0 is a direct model, resulting from the response model global R previously described, which combines the spectral response and the spatial response. - P^E'. x. y) is obtained by marginalization:

[0121] p(E\x,y)^Yx^Jj{E\x,y\Xv Y^Kp^X^ Y^i)} (5)

[0122] P^E', x, v) corresponds to what is expected on the detector given the image current Fr i).

[0123] The reconstruction process is carried out iteratively, separately for each isotope, according to the following sub-steps:

[0124] Sub-step 131: selection of an isotope from the isotopes indicated during step 110.

[0125] Sub-step 132: reconstruction of an image F{ 4^ corresponds to a spatial distribution of emission intensity of the isotope f in the first observation field, using simple interactions with: Cf\Xi Yl,i)=^kf(Xi, Y^E'^ x^y^

[0127] where k denotes each interaction. According to this embodiment, called list mode, the interactions, simple or multiple, are processed one after the other.

[0128] Using (4) and (5), we obtain:

[0129] C^(X„ Y,i)= P (X„ Y,,i)£^^

[0130] with

[0131] p( E h ^y^X^ Y v i )=^ s p(E' k \s,i)pix k ,y k \X î , Y^s)

[0132] p^E'h xy\Xi, Y},i ) = pE'^s + âj^y|X], )^+) + pE' \s-, i)pxk, y \XV Y^s-)

[0133] F^-b ) = yk, X,'yi) and y) = ye X1'F^

[0134] v , Y ) corresponds to the response function of the mask. It is obtained by classical source reconstruction algorithms for gamma cameras at coded masks.

[0135]

[0136] Substep 132 is implemented iteratively, with: (x 15 Y x ,i)= gP^x* y^oE 7—r---—'û--- Each iteration, of rank B, aims to estimate Y^ i) from a previous estimate qW ( x^ Y^ i ) • When n = 0, the estimate is made from an initial estimate O^^ÇX^ F] i) ' P31 example a uniform value at each point and for any isotope ' in the field of observation.

[0137] In expressions (7) and (8), the reconstruction is carried out according to a so-called list mode, in which the interactions of rank k stored individually are taken into account. If p^xy corresponds to the estimation of the image, formed on the detector, at iteration (n) by the simple interactions, of the photons emitted by an isotope *, we can write:

[0138] / n) (x, y, i,s) = ^XY [ y IX1' y 1' (x!' Y1' 0 ] (9)

[0139] When s ~ +' we consider that the detected photon is directly emitted by the isotope l. The image pn^xyis) corresponds to an estimate of the image formed on the camera by the isotope h When s = ', the detected photon is indirectly emitted by the isotope i. The image yis) is then uniform.

[0140]

[0141]

[0142] Expression (8) becomes: Yvi) = O{n)(X^ wj

[0143] During the first iteration, we take into account an initialized a priori reconstruction, noted Q(ll=0\x, Y,iY which is Pæ” example a uniform distribution for each isotope. In the expression ( 10). ( îv v \ corresponds to taking into account the 1' * 1' / spatial response of the camera, i.e. the probability that a photon emitted in (X^Y,) according to scenario s (direct or indirect photon) is detected in (x' 3*) during interaction k.

[0144]

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] Expression (10) can be written: Y l ,i)=O>"\x i Y^^V^X,, Yi) (11) with Uk(Xlf Y (12) Uk{X^, Yi'0 corresponds to a contribution of a detection of a simple interaction k in the update of the image / % yi)-

[0151]

[0152]

[0153]

[0154]

[0155] Step 140: Case of multiple interactions. The algorithm followed for multiple interactions is similar to that established for simple interactions. It allows an image to be formed in the previously defined observation field Q2. Sub-Step 141: Analogously to Step 131, an isotope is selected. Under Step 142: The image reconstruction process is performed analogously to simple interactions l\X2 y^z^Xy„y^z^E'E'yj= P(x^ Y^Z^i) |4j X2, Y2, Z2 belong to the observation field Q2. The process aims to reconstruct from an image Y2 Z2 ï)' ^ans 'c chamP ^2, which corresponds to a spatial distribution of emission intensity of the isotope * in the second observation field, the latter covering the first observation field and extending beyond the latter.

[0156] O^Xx* Y* Z., i) = r* z* ïu, Xu, fa.e^ 6')

[0158]

[0159]

[0160]

[0161]

[0162]

[0163]

[0164]

[0165]

[0166]

[0167]

[0168]

[0169]

[0170]

[0171] o ' Y. Zyi)= p(Xy Y : Zj«Æj—7---- : --------— 4^y ik~ And fa# z^- y& E'u} = E2k\^ / ) fa# -vwx* y^z™ E]k' E'^x^ y*Zî s ) = = ^EjkyZ^ 2s.|s' + ,ij p^xu-, zu. x^ y^ z2k E'^ E'^, À'?)',Z:.vh ) +^E w +2?2a|s"' y^ z2k E'E'^X^. V Z- s- ) with i et ale ^2' Y2, Z2I ylje Zyp X2k, Z2A, E'^ y2, Zÿ s-) = IJa-j, Jv zp x2y y2, X^ Y^ Z2j Z, Y v 7 \ corresponds to each cone representing the propagation of the t / jb ^2' * 2' ^2 / incident radiation, from coordinates X2, Y2, Z2 of the observation field, during a multiple interaction k. We recall that each multiple interaction generates two different cones, weighted by probabilities as described in connection with step 122. The object image is reconstructed iteratively, with: z., i) = ow(x2, y 2, z2. i)Yk ^iJc-y ik^Ur-.y 8' (Xt K2 Z? 0 from a Each iteration, of rank n, aims to estimate Qfa+V previous estimate Q^ÇX2 Y2 Z2 / j • When H = 0, the estimate is made from an initial estimate y^ ^y for example a value uniform or zero at each point and for any isotope z in the field of observation. During the first iteration, we take into account an initialized a priori reconstruction, noted q ( X2 Yq Z2 i ) ' clu' is P31 example a uniform distribution for each isotope. Expression (13) can be written: 6 / w+:i)(x2, y2,z2j) = o <n\x2,y2,z2,oe^(*^     (n') with U'k{XlyY2yZ2,i)=H 12' U'k(X2yY2,Z^i)corresponds to a contribution of a detection of a multiple interaction k in the update of the image Qi(n+^ ( y ). Step 150: Combined reconstruction

[0172] Step 140 can be implemented in the part of the observation field Q outside the observation field QL Step 130 can be implemented by only considering simple interactions, in the observation field QL The invention takes advantage of the fact that a part of the observation field Q is addressed by the two imaging modalities, in the overlap zone between Ql and Q2. In this example, it is considered that it is the first observation field Ql onto which a projection of a part of the second observation field Q2 can be carried out. Thus, certain emission points with coordinates X2, ^2 can be projected into the field observation Ql, their coordinates, in Q2, being linked by the equations:

[0173] X^tanWcosO') (13a)

[0174] y| = tan(0')sin( <!--) (13ô)<br-->

[0175] and

[0176] (p' = atan^ j - atan^^) (13c)

[0177] g _ afan ( ^2+y2 ) _ as-n ( ^2^2 ) (13d)

[0178] Alternatively, it is possible to project the first field of observation into the second field of observation with:

[0179] X2 = sinO cos(p (13e)

[0180] Y 2-sinO sin(p{\3f)

[0181] Z2 = co5^'(13g)

[0182] Thus, in the first observation field Ql:

[0183] O^Xx^Y^Y^U^^ (14)

[0184] where X2^j, ^2-»i correspond to the coordinates, initially determined in the second observation field Q2, and which can be projected into Ql by applying the expressions allowing the change of reference indicated in (13a) to (13g).

[0185] Outside the overlapping area, the reconstruction can be carried out according to (11'), based on the multiple interactions only. This makes it possible to obtain a reconstruction in the part of the observation field Q2 having no overlap with QL

[0186] The reconstruction can also be obtained by projection of the first field of observation onto the second field of observation: In this case, a change of variables is carried out according to expressions (13d) to (13g). This makes it possible to obtain a reconstruction in the first field of observation projected onto a part of the second field of observation, in the area in which the two fields of observation overlap.

[0187] For each isotope considered, the iterations stop when a convergence criterion is reached. This could, for example, be the quadratic difference between two successive reconstructed images.

[0188] When all the isotopes have been reconstructed, the reconstruction process is stopped: step 160.

[0189] The inventors implemented the invention by exposing the gamma camera to a 137Cs type source with an activity of 5MBq placed 1m from a gamma camera as previously described. The source was placed in the reduced observation field Q1: it was therefore visible both by the first modality (coded mask) and by the second modality (Compton imaging). Figures 8A, 8C and 8E show an image of the source obtained on the basis of a single iteration of the reconstruction algorithm, respectively - by taking into account only simple interactions: the image of the source is obtained according to the first modality, on the basis of 584 detected interactions ([Fig.8A]); - taking into account only multiple interactions: the image of the source is obtained according to the second modality, on the basis of 104 detected multiple interactions ([Fig.8C]); - taking into account both single and multiple interactions, the image of the source is obtained by combining the first and second modalities ([Fig.8E]).

[0190] [Fig.8E] leads to more accurate source detection, compared to [Fig.8C] and [Fig.8A],

[0191] Figures 8B, 8D and 8F show an image of the source obtained by implementing several tens of iterations of the previously described algorithm, respectively - by taking into account only simple interactions: the image of the source is obtained according to the first modality, on the basis of 584 detected interactions; - only multiple interactions are taken into account: the image of the source is obtained according to the second modality, on the basis of 104 detected multiple interactions; - taking into account both single and multiple interactions, the image of the source is obtained by combining the first and second modalities. According to one possibility, the camera comprises several detection materials, distributed in different planes, each detection material 11 being located facing a spherical coded mask type collimator. Such a variant is illustrated in Figures 9A and 9B. According to this variant, the first field observation field Q1 comprises different components, distributed facing each detection material 11. [Fig.9B] illustrates a hemispherical coded mask 10. The spherical coded mask is formed by assembling two hemispherical coded masks. The second observation field Q2 comprises all of the first observation fields Q1 and extends between them along a spherical surface.

Claims

1. Claims Method for estimating positions of irradiating sources in an observation field (Q, Q2), using a gamma camera (1), the gamma camera comprising: - a detection material (11); - pixels (12), distributed along a detection surface of the detection material, each pixel being configured to form a detection signal under the effect of a detection of an interaction of an ionizing photon in the detection material; - a collimator, of the coded mask type, placed facing the detection material, and defining a first observation field (Ql), included in the observation field of the gamma camera; - a localization unit (14), configured to assign a position to each interaction in the detection material, from the detection signal generated by said interaction; - a spectrometry unit (15), configured to assign an energy to each detected interaction, from the detection signal generated by said interaction; - a coincidence unit (16), configured to identify each interaction as a single interaction or a multiple interaction, a multiple interaction being formed from two interactions detected in temporal coincidence; - a memory (17), configured to store parameters of detected interactions; the process comprising the following steps: - a) acquisition of detection signals by the pixels (12) during an acquisition period, each detection signal being associated with a detected interaction; - b) depending on the detection signals, assignment of a position and an energy (x^ yE'k) to each detected interaction; - c) identification of each detected interaction as a single interaction or a multiple interaction; - d) memorization, by memory, of the parameters of each interaction, simple and multiple, the parameters of each interaction comprising the position and the energy of said interaction, the parameters of at least one simple interaction and of at least one multiple interaction being memorized; the method being characterized in that it also comprises, from the parameters of the memorized simple and multiple interactions, - e) selection of an isotope (l) from a list of isotopes; - f) depending on the energy assigned to each interaction, simple and multiple, calculation of an emission probability by - g) from the parameters of each interaction, simple and multiple, calculation of a probability of emission of the photon having generated the interaction, from different points of the first observation field ( yi-s ) > p(x^ y\e x2h y^ E\ EU|X2,Z2, s ) '

2. - h) taking into account an a priori probability of emission of a photon for the selected isotope, from the different points of the first observation field (PÏX, Y,iyp(x2,Y2,z2.i)y - i) based on the probabilities resulting from g) and h), established for single and multiple interactions, calculation of a spatial distribution of the emission intensity in the first observation field %.y„i))for the isotope selected during step e). Method according to claim 1, wherein steps h) and i) are carried out iteratively, such that at each iteration: - step h) includes taking into account a spatial distribution of the emission intensity of the initial isotope (XY i ) ) OR æsu'tan 1 a previous iteration (o (Xt, y,, / ))' - step i) includes an update of the spatial distribution of the emission intensity for the selected isotope based on: • the a priori probability of emission resulting from step h) of the same iteration; • the probabilities resulting from steps f) and g).

3. Method according to any one of the preceding claims, wherein in step e), the isotope is - selected according to the energies respectively assigned to different detected interactions; - or selected arbitrarily.

4. A method according to any preceding claim, wherein steps f) to i) are carried out successively for different selected isotopes.

5. Method according to any one of the preceding claims, in which step f) is implemented by calculating: - a probability (p ( E | i, s - + ) ) that the photon having generated the interaction is a direct photon, emitted by the isotope, according to an emission energy of the isotope and having reached the detection material while conserving the emission energy, without passing through an absorbing part of the collimator; - a probability (p{E\i, s — - )) that the photon having generated the interaction is an indirect photon, emitted by the isotope, according to an emission energy of the isotope, and having reached the detection material with an energy lower than the emission energy and / or by passing through an absorbing part of the collimator.

6. Method according to claim 5, in which step f) comprises taking into account, for each emission energy of the isotope: - a distribution of probabilities of detection of an interaction, in the detector material, according to different energies, assuming that the photon having generated the interaction is a direct photon; - a distribution of probabilities of detection of an interaction, in the detector material, according to different energies, assuming that the photon having generated the interaction is an indirect photon; the detection probability distributions being previously calculated by considering a simple interaction and a multiple interaction.

7. Method according to any one of the preceding claims, in which step g) comprises: - when the interaction is simple, selection of a first spatial response model of the camera, the probability of emission position from different points of the first observation field j|X, Y, s )) being determined as a function of the first spatial response model and the position of the interaction in the detector material; - when the interaction is multiple, selection of a second spatial response model of the camera, the probability of emission position from different points of the observation field ( P^x^, y? z? x2, yr z2, s^) being determined as a function of the second spatial response model and the energy as well as the position of the interactions detected in temporal coincidence forming the multiple interaction in the detector material.

8. Method according to claim 7, in which the first spatial response model determines, for different two-dimensional positions of simple interactions (x' ^), established parallel to the detection surface, a probability of detection of a photon emitted from different points of the first observation field (Ql), each simple interaction being an interaction not considered as multiple.

9. Method according to claim 7, in which the second spatial response model determines, for different three-dimensional positions of interactions detected in temporal coincidence / \, forming the multiple interaction, and VH yp x2' yy Zy for different energy values ​​of said interactions a probability of detection of a photon emitted from different points of the observation field (Q).

10. Gamma camera (1), intended to detect the presence of irradiating sources (5) in an observation field (Q), the gamma camera comprising: - a detection material (11); - pixels (12), distributed along a detection surface of the detection material, each pixel being configured to form a detection signal under the effect of a detection of an interaction of an ionizing photon in the detection material; - a collimator arranged facing the detection material, and defining a first observation field (Ql), included in the observation field of the gamma camera; - a localization unit (14), configured to assign a position to each interaction in the detection material, from the detection signal generated by said interaction; - a spectrometry unit (15), configured to assign an energy to each detected interaction, from the detection signal generated by said interaction; - a coincidence unit (16), configured to identify each interaction as a single interaction or a multiple interaction, a multiple interaction being formed from two interactions resulting from Compton scattering detected in temporal coincidence; - a memory (17), configured to store parameters of detected interactions; - a processing unit (18), configured to process the interactions stored in the memory, the processing unit being configured to implement steps e) to i) of a method according to any one of the preceding claims.

Images