Abstract:
Method and apparatus for determining a position of an event. The system processes signals from an assembly of N photodetectors and includes a signal generator for producing a signal representing the value of the maximum, or the energy, of a pulse delivered by the photodetector and is digitized. It also includes a signal generator for producing a threshold-exceeded signal for each photodetector when the amplitude of the signal representing the value of the maximum, or the energy, of the digitized pulse is greater than the threshold. It also includes a signal generator common to the photodetectors, for delivering a signal representing a position of an event as a function of the threshold-exceed signals. This device can be used with gamma cameras.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a device for determining the presumed position of an event inducing a signal, in photodetectors, this position being, for example, located in relation to the photodetector assembly. 
     The invention applies in particular to determination of the position of an event from signals supplied by photomultipliers equipping a gamma camera, the position being located in relation to the photomultipliers themselves. Gamma camera means a camera sensitive to gamma (γ) radiation. Such cameras are used notably for the purposes of medical imaging. 
     2. Discussion of the Background 
     At the present time, the majority of gamma cameras used in nuclear medicine are cameras operating according to the principle of Anger type cameras. This subject can be referred to U.S. Pat. No. 3,011,057. 
     Gamma cameras make it possible in particular to visualize the distribution, in an organ, of molecules labelled with a radioactive isotope previously injected into the patient. 
     The structure and operation of a known gamma camera are described and summarized below with reference to the accompanying FIGS. 1,  2 A and  2 B. 
     FIG. 1 shows a detection head  10  of a gamma camera disposed opposite an organ  12  containing molecules labelled with a radioactive isotope. 
     The detection head  10  has a collimator  20 , a scintillator crystal  22 , a light guide  24  and a plurality of photomultiplier tubes  26  juxtaposed so as to cover one face of the light guide  24 . The scintillator is, for example, a crystal of NaI (Tl). 
     The function of the collimator  20  is to select from among all the gamma radiations  30  emitted by the organ  12  those which reach the detection head substantially at normal incidence. The selective nature of the collimator makes it possible to increase the resolution and clarity of the image produced. However, the increase in resolution is made to the detriment of the sensitivity. By way of example, for around 10,000 γ photons emitted by the organ  12 , one single photon is actually detected. 
     The γ photons which have passed through the collimator reach the scintillator crystal  22  where nearly every γ photon is converted into a plurality of light photons. In the remainder of the text, event designates each interaction of a gamma photon with the crystal, causing a scintillation. 
     The photomultipliers  26  are designed to emit an electrical pulse proportional to the number of light photons received from the scintillator for each event. 
     So that a scintillation event can be localized more precisely, the photomultipliers  26  are not placed directly side by side with the scintillator crystal  22  but are separated from the latter by the light guide  24 . 
     The photomultipliers emit a signal whose amplitude is proportional to the total quantity of light produced in the scintillator by gamma radiation, that is to say proportional to its energy. However, the individual signal from each photomultiplier also depends on the distance which separates it from the point of interaction  30  of the gamma radiation with the material of the scintillator. This is because each photomultiplier delivers a current pulse proportional to the light flux it has received. In the example of FIG. 1, small graphs A, B, C show that photomultipliers  26   a ,  26   b  and  26   c  situated at different distances from a point of interaction  30  deliver signals with different amplitudes. 
     The position of the point of interaction  30  of a gamma photon is calculated in the gamma camera from signals coming from the photomultiplier assembly by performing a barycentric weighting of the contributions of each photomultiplier. 
     The principle of barycentric weighting as implemented in Anger type cameras emerges more clearly on referring to the accompanying FIGS. 2A and 2B. 
     FIG. 2A shows the electrical wiring of a detection head  10  of a gamma camera, which connects this camera to an image forming unit. The detection head has a plurality of photomultipliers  26 . 
     As shown in FIG. 2B, each photomultiplier  26  of the detection head is associated with four resistors denoted RX − , RX + , RY −  and RY + . The values of these resistors are specific to each photomultiplier and depend on the position of the photomultiplier in the detection head  10 . 
     The resistors RX − , RX + , RY −  and RY +  of each photomultiplier are connected to the output  50  of the said photomultiplier, represented in FIG. 2B by a current generator symbol. They are moreover respectively connected to common collector lines denoted LX − , LX + , LY −  and LY + , in FIG.  2 A. 
     The lines LX − , LX + , LY −  and LY +  are in turn connected respectively to analog integrators  52 X − ,  52 X + ,  52 Y −  and  52 Y + , and, by means of the latter, to analog-to-digital converters  54 X − ,  54 X + ,  54 Y −  and  54 Y + . The output of the converters  54 X − ,  54 X + ,  54 Y −  and  54 Y +  is taken to a digital operator  56 . The lines LX − , LX + , LY −  and LY +  are furthermore connected to a common path, referred to as the energy path. This path also has an integrator  57  and an analog-to-digital converter  58  and its output is also taken to the operator  56 . 
     By virtue of the device of FIG. 2, the position of the interaction is calculated according to the following equations (U.S. Pat. No. 4,672,542):        X   =           X   +     -     X   -           X   +     +     X   -                       and             Y   =         Y   +     -     Y   -           Y   +     +     Y   -                                
     in which X and Y indicate the coordinates, along two orthogonal directions, of the position of the interaction on the crystal and in which X + , X − , Y + , Y −  indicate respectively the weighted signals delivered by the integrators  52 X + ,  52 X − ,  52 Y + ,  52 Y − . 
     The values of X and Y, as well as the total energy E of the gamma ray which has interacted with the crystal, are produced by the digital operator  56 . These values are next used for constructing an image as described, for example, in the document FR-2 669 439. 
     The calculation of the position of the interaction is marred by an uncertainty related to the statistical Poisson fluctuations of the number of light photons and the number of photoelectrons produced for each event, that is to say for each gamma photon detected. The higher the number of photons or photoelectrons, the smaller is the standard deviation of the fluctuation. Because of this phenomenon, the light should be collected as carefully as possible. The intrinsic spatial resolution of the camera is characterised by the mid-height width of the distribution of the positions calculated for one and the same collimated point source placed on the scintillator crystal. 
     For gamma rays with an energy of 140 keV, the resolution is generally of the order of 3 to 4 mm. 
     The energy of a detected gamma photon is calculated by summing the contributions of all the photomultipliers which have received light. This is also marred by a statistical fluctuation. The resolution energy-wise of the camera is characterised by the ratio of the mid-height width of the distribution of the calculated energies to the mean value of the distribution, for one and the same source. 
     The resolution energy-wise is generally of the order of 9 to 11% for gamma rays with an energy of 140 keV. 
     Finally, an Anger type gamma camera has the advantage of making it possible to calculate in real time the barycentre of the signals from photomultipliers with very simple means. 
     This is because the system described previously has a limited number of components. Moreover, the resistors used to inject the signal from the photomultipliers into the collector lines are very inexpensive. 
     Such a camera has however also a major drawback, which is a reduced counting rate. Counting rate means the number of events, that is to say interactions between a γ photon and the scintillator, which the camera is capable of processing per unit of time. 
     One of the limitations of the counting rate is due notably to the fact that the camera is incapable of processing two events taking place substantially simultaneously at distinct points of the scintillator crystal. 
     This is because simultaneous but geometrically distinct events give rise to electrical signals which stack up in the collector lines LX − , LX + , LY −  and LY +  and which can no longer be distinguished. These events are also “lost” for the formation of an image. 
     The limitation of the counting rate is not too important a constraint in the traditional medical imaging techniques. This is because, as indicated above, the collimator stops a very large number of gamma rays and only a small number of events are actually detected. 
     Gamma cameras are however also used in two other medical imaging techniques where the limitation of the counting rate is a crippling constraint. 
     These techniques are the so-called “attenuation equalisation by transmission” and “coincidence PET (Positron Emission Tomography)” techniques. 
     The attenuation equalisation by transmission technique consists of taking into account, at the time of forming a medical image, the attenuation belonging to the tissue of the patient surrounding the organ examined. In order to know this attenuation, a measurement is made of the transmission of gamma radiations to a gamma camera through the body of the patient. To that end the patient is made to take a position between a highly active external source and the detection head of the gamma camera. Thus, at the time of measuring the transmitted radiation, a high number of events take place in the scintillator crystal. The high number of events per unit of time also increases the probability of having a number of substantially simultaneous events. A conventional Anger type camera then proves to be inappropriate. 
     The PET technique consists of injecting into the patient an element such as F 18  capable of emitting positrons. The annihilation of a positron and an electron releases two γ photons emitted in opposite directions and having an energy of 511 keV. This physical phenomenon is taken advantage of in the PET imaging technique. In this technique, use is made of a gamma camera with at least two detection heads disposed on either side of the patient. The detection heads used are not equipped with a collimator. This is because electronic processing of the information, referred to as coincidence processing, makes it possible to select, from among the events, those which coincide timewise, and to thus calculate the trajectory of the gamma photons. 
     The detection heads are therefore subjected to high gamma radiation fluxes. The conventional Anger type gamma cameras have a counting rate which is generally too limited for such an application. 
     For information only, an Anger type gamma camera can normally operate with a detection of 1.10 5  events per second, whereas in PET imaging at least 1.10 6  events per second are necessary for normal operation. 
     Another limitation of Anger type gamma cameras, described above, is due to the fact that the calculation of the barycentre of an event is definitively fixed by the construction of the detection head and notably by the choice of the resistors RX − , RX + , RY − , RY +  for each photomultiplier. Similarly, the calculation of the energy is fixed by the wiring of the photomultipliers on to a common path (the energy path). 
     SUMMARY OF THE INVENTION 
     The present invention proposes a device and a method for processing signals from an assembly of N photodetectors, permitting a higher counting rate than that of the known Anger type cameras. Moreover, such a device (or method) allows a rapid determination of an area of the assembly of N photodetectors concerned by an event. Taking such an area into account permits a processing of the signals which is more efficient than in the known devices. 
     An aim of the invention is also to propose a device and a method which allows a more flexible processing of the signals received by an assembly of N photomultipliers or photodetectors, permitting the implementation of various calculation algorithms, and not solely an algorithm defined by the choice of the characteristics of the photodetectors, in particular by the choice of the resistors RX + , RX − , RY + , RY −  in the case of a gamma camera head of the prior art. 
     More precisely, an object of the invention is a device for processing signals from an assembly of N photodetectors having: 
     means, associated with each photodetector, for producing a signal representing the value of the maximum, or the energy, of a pulse delivered by the photodetector and digitized; 
     means for producing a threshold-exceeded signal for each photodetector when the amplitude of the signal representing the value of the maximum, or the energy, of the digitized pulse is greater than the said threshold; 
     means, common to the photodetectors, for delivering a signal representing a position of an event as a function of the threshold-exceeded signals. 
     Such a device therefore processes previously digitized data and makes it possible to produce a position signal with the help of which the area of the field of the photodetectors concerned by the interaction, or by the event, can be located or coded. When an event produces a meaningful signal, that is to say one whose amplitude or energy exceeds a threshold in a number of photodetectors, each corresponding digitized signal produces a threshold-exceeded signal. Depending on the distribution of the threshold-exceeded signals, a position signal may be issued. 
     This position information or signal can next, for example, be transmitted to means of calculating one or more characteristics of the event. These calculation means therefore do not need to read the whole of the photodetector field in order to determine the position of the event: the said position is already “coded” by the means which make it possible to deliver a position signal. They do not need, either, to take into account the photodetector assembly in order to determine a characteristic of the event, for example its energy. The reading of all the photodetectors would be long and would lead to a pronounced degradation of the counting rate performance. 
     Such a device makes it possible to avoid fixing the calculation of the position of the event by an array of resistors and wiring, as in the Anger cameras. 
     When a number of photodetectors are concerned with one event, that is to say when threshold-exceeded signals are issued corresponding to a number of photodetectors in response to one event, it may be advantageous to obtain a simultaneous or synchronized issuing of the different exceeded signals by the different threshold-exceeded signal production means concerned. 
     To that end and according to a particular embodiment, a device according to the invention also has means, associated with each photodetector, for detecting the timewise position of the maximum of the digitized pulse and means for delaying it by a certain duration so as to put it into synchronism with the maximum of the value of the corresponding integral. 
     According to another aspect, a device according to the invention may also have: 
     means for forming a total analog signal, equal to the sum of the analog signals delivered by N 1  photodetectors (N 1 ≦N); 
     means for delivering a signal representing the timewise position of the maximum of the total analog signal; 
     means for delaying the delivery of the signal representing the position of an event for a certain duration from the timewise position of the maximum of the total analog signal. 
     Thus, the position signal is issued with a certain shift with respect to the detection of the event (represented by the total analog signal), which makes it possible to take into account the time necessary for production of the integral by the summing device. 
     It is also possible to take account of signals validating the event inside a time slot whose instant of origin is fixed on the crossing of a threshold. The width is fixed so as to take into account only the signals produced during a certain time interval after crossing of the threshold. If it is known, statistically, that an event can induce threshold-exceeded signals during such a time interval, parasitic signals occurring later, outside the said interval, and not related to the event in question, can thus be eliminated. 
     If it is wished, furthermore, to take into account the physical principle according to which an event can activate only neighbouring photodetectors, or those defining a related surface or area, the means for delivering a position signal can be provided such that a signal is issued only if all the threshold-exceeded signals produced correspond to neighbouring or related photodetectors. It can also be provided that these means do not deliver any position signal when threshold-exceeded signals are issued by non-neighbouring photodetectors. 
     An object of the invention is also a device for detecting events, having: 
     a two-dimensional array of N photodetectors, defining a map; 
     digitization means, associated with each photodetector; 
     a device according to the invention, such as already described above. 
     The signal representing a position of an event has a first and a second component, each corresponding to a coordinate of the event located in the map of the photodetector array. 
     Generally, whatever the device adopted, in accordance with the invention, the means for delivering a signal representing a position of an event can also make it possible to deliver: 
     a first signal, representing a first position, if a single threshold-exceeded signal is delivered, coming from any first photodetector among the N photodetectors; 
     a second signal, representing a second position, if two threshold-exceeded signals are delivered, coming from two neighbouring photodetectors. 
     Thus, it is possible to locate the position of an event, not only in relation to the central positions of the photodetectors, but also in relation to an intermediate position between two neighbouring photodetectors. It is thus possible to distinguish between the case where the interaction takes place, more or less precisely, at the centre of a photodetector, and the case where the interaction takes place between two or three photodetectors. The importance of being able to perform this distinction lies in the fact that an environment specific to each case can then be taken into account when it is wished to calculate characteristics of the event, for example the barycentre of the site where it occurs, or its energy, without for all that needing to consider the responses of all the photodetectors. 
     An object of the invention is also a device for processing signals from an assembly of N photodetectors, having: 
     N digital coding units; 
     digital summation means associated with each digital coding unit; 
     means of comparing an output signal from the summation means with a predetermined value; 
     two read-access memories, addressed by the outputs of the comparison means. 
     Such a device makes it possible to implement a device for processing signals from an assembly of N photodetectors, such as already described above. In particular, the digital summation means (respectively, the means of detecting a maximum) associated with each digital coding unit make it possible to produce a signal representing the value of the energy (respectively, the amplitude) of a digital pulse; the means of comparing this signal with a predetermined value make it possible to produce a threshold-exceeded signal when the value of the energy (respectively, the amplitude) of the pulse is greater than the predetermined value; the read-access memory can be programmed so as to deliver a signal representing a position of an event as a function of the various threshold-exceeded signals. 
     Such a device can also have, associated with each digital coding unit: 
     means of detecting the timewise position of the maximum of a digitized pulse, these means being controlled by the corresponding digital coding unit; 
     a shift register; and 
     an AND gate controlled by the output of the comparison means and the output of the shift register. 
     In this particular embodiment, there is associated, with each detector, means for detecting the timewise position of the maximum of a digitized pulse and for delaying the issuing of the threshold-exceeded signal until an instant to be determined. 
     The invention can also comprise: 
     analog summation means intended to receive, at the input, the outputs of the N photodetectors; 
     means of detecting a maximum of an analog signal, controlled by the analog summation means; 
     a shift register controlled by the maximum-detecting means; 
     a register controlled by the read-access memory and by the previous shift register. 
     An object of the invention is also a device for detecting events, having a two-dimensional array of N photodetectors and a device for processing signals from the assembly of N photodetectors, such as just described. 
     The invention also relates to a camera having a device, as described above, the photodetectors being photomultipliers. 
     The invention also relates to a gamma radiation imaging device, having such a camera. 
     The invention also relates to a method of processing signals produced by an assembly of N photodetectors, in response to an event, this method including: 
     for each photodetector, the production of a signal representing the value of the maximum, or the energy, of a pulse delivered by the photodetector and digitized; 
     the production of a signal of exceeding of a threshold (V r ), for each photodetector, when the amplitude of the signal representing the value of the maximum, or the energy, of the digitized pulse is greater than the said threshold; 
     the formation of a signal representing a position of an event as a function of the threshold-exceeded signals. 
     Such a method according to the invention is advantageously applicable to an attenuation equalisation by transmission imaging method or to a coincidence PET type imaging method. The principles of these methods have been described in the introduction to the present application. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In any case, the characteristics and advantages of the invention will emerge more clearly in the light of the description which follows. This description concerns the example embodiments, given by way of explanation and being non-limitative, referring to accompanying drawings in which: 
     FIG. 1, already described, is a schematic section of a detection head of a known Anger type camera; 
     FIG. 2, already described, shows schematically a device for collecting and coding signals coming from photomultipliers of the detection head of FIG. 1; 
     FIG. 3 depicts a circuit associated with a photodetector, for processing data from this photodetector; 
     FIGS. 4A and 4B depict an analog signal supplied by a photodetector (FIG.  4 A), and the digitized analog signal and a corresponding information capture timing diagram (FIG.  4 B); 
     FIG. 5 depicts an embodiment of a summing device; 
     FIG. 6 depicts an embodiment of the invention; 
     FIG. 7 depicts schematically a device for detecting the maximum of the analog signal; 
     FIG. 8 is a timing diagram depicting schematically a sum of the analog signals from a photodetector assembly, the identification of the maximum of this sum, threshold-exceeded signals, the response interval of a PROM memory and a presumed position signal issued by the PROM; 
     FIGS. 9A and 9B depict two examples of photodetector fields, with different symmetries; 
     FIG. 10 depicts positions, or an array, of columns of a photodetector field and possible positions of events in relation to this array of columns; 
     FIGS. 11A and 11B depict two examples of useful areas according to the presumed part. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a partial view of a device for processing the signal according to the invention. It depicts only a single channel of such a device, that is to say the part of the device associated with a single photodetector  60 . The photodetector  60  is, for example, a photomultiplier connected to a current-to-voltage converter  62 . In response to an event detected by the photomultiplier, there is obtained, on the output  64  of the current-to-voltage converter  62 , a signal, for example, of the type illustrated in FIG.  4 A. 
     The graph of FIG. 4A shows, as ordinates, the amplitude of the signal corresponding to the pulse and, as abscissae, the time. The amplitude of the signal and the time are shown on an arbitrary scale. In FIG. 4, t 0  indicates the starting instant of the pulse supplied by the photodetector, and t 1  indicates the instant when the pulse becomes almost zero again, after having passed through a maximum. For information only, the duration corresponding to the interval t 1 −t 0  is of the order of one microsecond, in the case of a photomultiplier of a gamma camera coupled to a crystal of NaI (Tl). 
     The analog signal present on the output terminal  64  is taken to an analog-to-digital converter  66 . The latter samples each pulse of the signal into a certain number of samples n, as illustrated in FIG.  4 B. Two consecutive samples are separated by a step, or clock interval p (the clock operating at 1/p Hz). 
     By way of example, the converter samples each pulse of the signal into n=10 samples. For a 1 microsecond signal, sampling is then performed every 100 nanoseconds. 
     The analog-to-digital converter  66  is, preferably, a high-speed converter, of “flash” type, being able to operate at a frequency of the order of 10 to 20 megahertz. 
     The digital signal originating from the analog-to-digital converter  66  is taken to a digital summing device  68 . This summing device performs a rolling sum of the samples transmitted to it by the analog-to-digital converter  66 . The rolling sum is performed on a given number of samples. This predetermined number is equal, for example, to 10. 
     At the same time, the result of the summation performed with the means  68  is stored in a register  71 . The storage function may be composed of a number of registers in order to make it possible to store a number of events which are very close timewise. 
     The value of the rolling sum is taken to comparison means  70 . The value of the rolling sum is compared therein with a fixed predetermined threshold value at an input  72  of the comparator  70 . This comparator issues, on an output  74 , a binary signal representing the result of the comparison (for example, 0 if the value of the rolling sum is less than the fixed reference value and 1 if the value of the rolling sum is greater than the reference value). 
     According to a particular embodiment, the summing device  68  may have, as illustrated in FIG. 5, a shift register  82  whose length determines the number of samples summed and an adder/accumulator  86 . The summing input of the latter is connected to the output of the coder, and its subtracting input is connected to the output of the shift register  82 , so that, at each coding step, the content of the accumulator situated in  86  (which represents the rolling sum) is corrected by the difference between the input and the output of the shift register  86 . The output of the accumulator is taken to the comparator  70  which then supplies a binary threshold-exceeded signal. So as to limit the duration of this exceeding, the latter is preferably valid only during a time slot centred on the maximum of the rolling sum. This makes it possible to separate events which are close in time but geographically distinct over the field of the detector. 
     This slot can be positioned by taking as a reference the instant of passage T 1  of the coded signal through a maximum. This detection is carried out in the entity  88  by comparing the current value of the coder output with the previous value. When the current value is less than the previous value, the comparator  88  issues a pulse referred to as MAX (FIG.  8 ). This pulse is sent to a shift register  90  whose delay n 1  is controlled in order to generate a time slot DS 1  centred on the maximum of the rolling sum IS 1 . In order to take into account the inaccuracy (plus or minus one sampling step) of the determination of the position of the maximum of the coded signal, the time slot is preferably activated for no sampling steps (FIG. 4B) with n 0 &gt;3 (for example n 0 =3), this choice of a minimum of three guaranteeing a minimum of simultaneity of threshold-exceeded signals between the photomultipliers activated by one and the same event. 
     An AND gate  92 , whose inputs are the signal obtained at the output of the comparator  70 , and the output signal of the shift register  90 , make it possible to obtain, on its output  94 , a threshold-exceeded signal (FLAG) at the desired instant with respect to the passage through the maximum of the digital sum. 
     In short, each photomultiplier, equipped with the device described previously, supplies two information items. A first information item is the output of the summing device  68  which represents the current integral, and a second information item, referred to as FLAG, at the output  94  of the AND  92 , synchronous with the maximum of the current integral, which gives substance to the fact that the current integral has exceeded, or has not exceeded, a threshold. 
     FIG. 6 depicts a device, in accordance with the invention, for processing signals originating from a number of photodetectors  60 ,  60 - 1 ,  60 - 2 . In this figure, identical references to those of FIG. 3 designate similar or corresponding elements therein. 
     In this figure, it may be seen that it is possible to take, at the output of the photodetector  60 , an analog signal  100 , of the type described above in connection with FIG.  4 A. In FIG. 6, the reference  102  designates as a whole all the analog signals taken from the other photodetectors  60 - 1 ,  60 - 2 , etc. All these signals go into an analog summing device  98  which delivers a signal S (FIG.  8 ), the sum of all the analog signals supplied by a certain number of photodetectors, for example by all the photodetectors. A device  104  makes it possible to deliver a pulse I at the time at which the signal S passes through its maximum. This device  104  has, for example, as illustrated in FIG. 7, a differentiator  110  (a capacitor  112 , an amplifier  114  and a resistor  116  between the input and the output of the amplifier  114 ); the output of this differentiator  110  feeds a comparator  118  which makes it possible to detect the output of the differentiator going to 0. The pulse I feeds the input of a shift register  106  whose step p is controlled by the clock H. The output  107  of this register is referred to as the storage pulse and makes it possible, in particular, to activate the storage register  71  corresponding to the photodetector  60  and the register  122  corresponding to the presumed position. It also activates each storage register associated with each photodetector. The delay of the shift register  106  is controlled so that the rising edge of the storage signal  107  is synchronous with the instant at which the sums must be stored in the registers  71 . 
     The assembly of photodetectors  60 ,  60 - 1 ,  60 - 2 , etc. is distributed for example in a two-dimensional array, of the type described later in connection with FIG. 9A or  9 B. 
     The photodetectors can therefore be located by their position along rows and columns of the two-dimensional array. 
     In order to locate the position of an event in relation to this two-dimensional field, or array, of photodetectors, there is advantageously associated a read-access memory with a first direction of location in the photodetector field and a read-access memory with a second direction of location in the photodetector field. If this field is located by rows and columns, there can therefore in this way be associated a read-access memory for locating a “row” coordinate and a read-access memory for locating a “column” coordinate. 
     More precisely, in a device according to the invention, of the type illustrated in FIG. 6, the outputs  94 , which represent, when active, the photodetectors of the centre of the interaction, are used in the following manner: 
     an OR circuit  202  gathers together the outputs of type  94  of the photodetectors of one and the same column and generates a signal  222  which is active when at least one of its inputs is active. There are as many circuits of type  222  as there are columns, 
     an OR circuit  212  gathers together the outputs of type  94  of the photodetectors of one and the same row and generates a signal  232  which is active when at least one of its inputs is active. There are as many circuits of type  212  as there are rows. 
     The signals of type  222  are the addresses of a PROM  76  which is programmed so as to supply the coordinate  80  of the presumed position in relation to the columns. Similarly, the signals of type  232  are the addresses of a second PROM  77  which is programmed so as to supply the coordinate  81  of the presumed position in relation to the rows. The presumed position, represented by the pair of values  80 ,  81 , is stored in a register  122 , at the same time as the contributions of all the photodetectors are stored in their respective registers  71 . This storing is activated by the signal  107  generated by the register  106 . 
     FIG. 8 depicts chronologically the different signals: 
     the signal S represents the analog sum of all the photodetectors at the time of an isolated event, 
     the signal I represents the pulse related to the passage of S through a maximum. This pulse occurs at the instant T 0 , 
     the signal S 1  represents the output of a photodetector which is among those covering the interaction site of the event, and the signal CS 1  represents the signal S 1  after coding (at the output of the analog-to-digital coder  66 ), 
     the signal MAX represents the pulse related to the passage of CS 1  through a maximum when CS 1  is greater than a threshold. This threshold is used to validate only meaningful maxima, that is to say those above the noise. The signal MAX occurs at the instant T 1 . The duration T 1 −T 0  is related to the coding time. The instant T 1  is defined to within ±1 coding step. This is due to the noise of the coder on the one hand and to the noise of the signal on the other hand, 
     the signal IS 1  represents the signal at the output of the summing device  68 . The instant T 2  represents the instant at which the integral passes through a maximum, and therefore the theoretical instant at which the value of the integrals and the corresponding presumed position must be stored. The duration T 2 −T 1  is related to the integration time, that is to say to the number of samples taken into account by the summing device (this number being itself related to the coding step and to the mean duration of the pulses), 
     the signal DS 1  is generated by the shift register  90 ; it takes into account the inaccuracy of the determination of T 1 . It lasts for three coding steps, and is delayed with respect to T 1  so as to be centred on T 2 , 
     the signal COMP is supplied by the comparator  70 . The comparison threshold is controlled so as to validate only the photodetectors close to the site of the interaction, 
     the signals  94 ( 1 ),  94 ( 2 ) and  94 ( 3 ) are the outputs  94  of three other photodetectors close to the site of the interaction for which the energy is greater than the threshold. These outputs take into account all the inaccuracy of the determination of T 1 , and therefore of T 2 , and this is why the storage signal  107  is generated by a separate analog chain, 
     the storage signal  107  is generated from the signal I, delayed by the circuit  106 . This shift is controlled so that the integrals and the corresponding presumed position are stored at the instant which represents the mean value of the different instants T 2 . 
     Examples of two-dimensional arrays of photodetectors are illustrated in FIGS. 9A and 9B. More precisely, these figures depict, in each case, a position of an array of photodetectors in a top view (or, in other words, if reference is made to FIG. 1, seen from the scintillator  22 ). The cross-section of a photodetector can have various forms, for example, square, hexagonal, or circular. 
     FIG. 9A corresponds to a photodetector field, in a top view, each photodetector having a square section. In this case, each photodetector can issue a threshold-exceeded signal which can locate it along one row and along one column. Thus, in FIG. 9A, the photodetector  60 - 1  can, in response to an event, issue a signal which, if it exceeds a certain threshold, will cause the issuing of a threshold-exceeded signal locating the row L 1  and the column C 1 . Similarly, the photodetector  60 - 2  can be located by the row L 1  and the column C 2 , threshold-exceeded signals being sent to the memories  76 ,  77 , which correspond to the row L 1  and the column C 2 . 
     FIG. 9B depicts, in a top view, a mixed field of hexagonal and round photodetectors. The photodetectors  60 - a ,  60 - b  have a circular cross-section, while the other photodetectors have a hexagonal cross-section. In such a case, the photodetector field clearly defines juxtaposed columns  60 - i ,  60 - e ,  60 - a  (C 4 );  60 - k ,  60 - g ,  60 - c  (C 3 ),  60 - j ,  60 - f ,  60 - b  (C 2 );  60 - l ,  60 - h ,  60 - d  (C 1 ). Each photodetector corresponds to one column and one only. On the other hand, the assignment of one row to one photodetector is made unambiguously solely for the case where the said row passes through the middle of the photodetector: in FIG. 9B, this is the case for the photodetectors  60 - k ,  60 - l  (row L 3 );  60 - g ,  60 - h  (row L 2 );  60 - c ,  60 - d  (row L 1 ). For the other photodetectors, situated between two rows, it is preferable that their threshold-exceeded signal results, to the corresponding read-access memory, in an issuing of addressing signals corresponding at the same time to the two rows between which they are situated. Thus, if the photodetector  60 - f  delivers a signal whose energy exceeds the fixed threshold, a threshold-exceeded signal is issued for the column C 2  and for the rows L 1  and L 2 . Similarly, when an event causing the threshold to be exceeded for the photodetector  60 - j , the result of this is a signal for addressing column C 2  and for addressing rows L 2  and L 3 . 
     A method will now be described which makes it possible to define, or code, the signal representing a position of an event, as a function of the threshold-exceeded signals. In the case of a read-access memory, it is a matter of carrying out the coding of the row and/or column combinations for which a threshold-exceeded signal appears at the time of an event. 
     As illustrated in FIG. 10, and still keeping the example of an assembly of four columns of photodetectors C 1 -C 4 , it is possible to locate, not only the position of the columns along an axis, but also the position of the points P 1 , P 2 , P 3  situated halfway between the columns (points to which odd figures are assigned on the scale at the bottom of FIG.  10 ). These intermediate points correspond to the limits between two neighbouring photodetectors of two neighbouring columns. 
     The photodetector or photomultiplier fields used at present, in particular in the case of a gamma camera, typically have at maximum between 50 and 100 photodetectors. Consequently, a dozen columns at maximum (and a dozen rows) is sufficient to describe these photodetector fields. It is therefore possible to use a read-access memory (PROM) of reasonable size (of the order of 4 kbytes) for coding the groups of columns, the successive columns constituting the addresses of the PROM. For larger fields, or arrays, of photodetectors, the size of the PROM will be chosen accordingly. 
     Furthermore, if the fact is taken into account that an event can in general activate only contiguous or related columns (for example, 1, 2, 3 or 4 depending on the energy of the event and the threshold used), only a restricted number of “valid” combinations is to be retained and coded. The expression of the position of the event on the X axis of location of the columns does not require eight bits; it is possible to use the most significant bit (Q 7 ) for coding the validity of the position obtained. Table I below gives the signals sent to three addressing inputs A 0 , A 1 , A 2  of a PROM, as a function of the combination of the threshold-exceeded signals for the columns C1, C2, C3 (0=no threshold-exceeded signal, 1=a threshold-exceeded signal). The decimal content of the PROM, resulting from these combinations, is given in the fourth column (A 0 =1, A 1 =2, A 2 =2 2 ). The fifth column gives the validity bit: when two columns, separated by a third, issue a threshold-exceeded signal, while the third does not issue one, the combination is not valid and is not retained (validity bit=0). The programming of the PROM next makes it possible to assign, as a function of the decimal content, an event position: 
     when only the first column issues a threshold-exceeded signal (address A 0 , the decimal content=1), the event is identified in position  2  on the X scale of FIG. 10; 
     when a threshold-exceeded signal corresponding to the single column C2 is issued, the event is localized in position  4  on the X scale of FIG. 10; 
     when threshold-exceeded signals corresponding to the columns C 1  and C 2  are issued, the event is positioned, not on one or other of the columns, but in the intermediate position  3  on the X scale (FIG.  10 ); 
     when a single threshold-exceeded signal is issued, corresponding to the column C 3 , the event is localized in position  6  on the X scale; 
     when two threshold-exceeded signals are issued, corresponding to the columns C 2  and C 3 , the event is localized in the intermediate position  5 , on the X scale, between the columns. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Address 
                 Address 
                 Address 
                   
                   
                   
               
               
                 A 0   
                 A 1   
                 A 2   
                 Decimal 
                 Validity 
                 Position of 
               
               
                 C 1   
                 C 2   
                 C 3   
                 content 
                 bit 
                 event (X) 
               
               
                   
               
             
             
               
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 1 
                 0 
                 0 
                 1 
                 1 
                 2 
               
               
                 0 
                 1 
                 0 
                 2 
                 1 
                 4 
               
               
                 1 
                 1 
                 0 
                 3 
                 1 
                 3 
               
               
                 0 
                 0 
                 1 
                 4 
                 1 
                 6 
               
               
                 1 
                 0 
                 i 
                 5 
                 0 
                 0 
               
               
                 0 
                 1 
                 1 
                 6 
                 1 
                 5 
               
               
                 1 
                 1 
                 1 
                 7 
                 1 
                 4 
               
               
                   
               
             
          
         
       
     
     This coding can be generalized to four columns and over. Once the coding of the PROM has been carried out, a defined combination of columns for which threshold-exceeded signals are issued corresponds to a unique position of the event in relation to all the columns. This position is coded on the output bits of the PROM, except for the most significant bit which makes it possible to code the validity of the combination. 
     The same reasonings and the same type of coding can be carried out for determining the position of the event in “Y”, along the rows. The threshold-exceeded signals ordered along the rows are coded with the help of a second (PROM  77  of FIG. 6, for example), which can be identical to the previous one. For the case of a field of detectors with hexagonal and/or circular section (the case of FIG.  9 B), the coding of the positions of photodetectors situated between two consecutive rows does not pose any more problems: thus, the identification of an event along the column C 2  and along the rows L 1  and L 2  corresponds to an event localized above the photodetector  60 - f.    
     The position of an event can consequently be represented by a pair (X, Y) which locates this event in the two-dimensional assembly or field of photodetectors or photomultipliers. There are retained, as the only valid positions, the only pairs (X, Y) of which both elements satisfy the validity criteria. 
     A device and a method according to the invention make it possible to localize an event, not only above one or other of the photodetectors of a field of photodetectors, but also at the limit of two photodetectors, perhaps even of three photodetectors. 
     In FIG. 6, the signal  126  issued contains this localization information. This signal can be transmitted to a microcomputer specifically programmed to select a specific environment of the group of photodetectors concerned by the interaction (that is to say of the group of photodetectors having issued a threshold-exceeded signal), which next makes it possible to calculate the characteristics of the event from the environment adopted. 
     Thus, FIGS. 11A and 11B each depict a field of photodetectors with hexagonal section, supplemented on the edges by photodetectors with circular section. 
     In FIG. 11A, two photodetectors  60 - 1 ,  60 - 2 , have issued threshold-exceeded signals. This results in identification of the position of the event at the limit (point A) between the two photodetectors  60 - 1  and  60 - 2 . The computer to which this information is transmitted can then select, for example from a memory in which the data of intensities emitted by the photodetectors are stored, the first ring  160  of photodetectors and the second ring  260  of photodetectors, which surround the photodetectors  60 - 1  and  60 - 2  directly concerned by the interaction. The total energy of the event can, for example, be calculated, by summing all the signals delivered by the photodetectors directly concerned by the interaction ( 60 - 1 ,  60 - 2  in FIG. 11A) and all the signals issued by the photodetectors of the first ring and the second ring ( 160  and  260 ). 
     FIG. 11B depicts the case of an event which has induced a threshold-exceeded signal for three photodetectors  60 - 3 ,  60 - 4 ,  60 - 5  disposed as illustrated in FIG.  11 B. The event is therefore localized to the point B, the intersection between the three photodetectors. The computer must therefore take into account the information concerning the photodetectors of the first ring  360  and the photodetectors of the second ring  460  for calculating characteristics, for example, the energy of the event. 
     The device according to the invention has been described using a threshold-exceeded criterion for the energy of the digitized signal. It is also possible to adopt other criteria, for example, a threshold-exceeded criterion for the maximum of the digitized signal. In this case, the comparator  70  of FIG. 6 compares the digitized signal originating from the digitizer  66  with a predetermined threshold value. 
     The method of coding by PROM which has been described makes it possible to obtain, with very great speed, a positioning signal for the event. An algorithmic method would be slower. 
     The PROM memories can be replaced by any type of device or memory in which a positioning coding can be entered or stored. Thus, the description produced with a PROM can be generalized to any programmable read-access coding medium or means. 
     It can be worthwhile to carry out the coding with the help of memories which can be reprogrammed. The device is then in fact reusable for other photodetector configurations.