Patent Application
20050139777
The present invention is connected to a radiation detection device and concerns, in a general manner, the field of the detection and the measurement of exposure to any type of radiation of a corpuscular or wave nature, such as particles and photons, particularly radiations of protons, neutrons, electrons, positrons, α,β,γ radiation, X-ray photons, visible light photons and photons outside the visible spectrum.
The invention applies, among other things, to matrix architecture detection devices used in imaging, particularly in the field of radiology, where X-ray matrix imagers are used, commonly known as “sensors”.
Radiation detection devices of the prior art are known in which the structure is formed of one or several detector elements combined with an electronic device for processing and counting pulses generated by the detector element during a capture event of a photon or a particle.
In particular, radiation detection devices arranged in a matrix architecture that comprise a detection entity combined with a processing/acquisition entity are known.
The matrix assembly formed by the detection entity (commonly known as “detection circuit”) combined with the electronic acquisition and counting entity (commonly known as “reading circuit”) constitutes a “sensor”.
The detection entity may be formed by a gas, a scintillator associated with a semi-conductor, or by layers of detector material, which absorb the photons or the particles and finally transform them into electric pulses (electronic charge packets).
The expression “charge packet” herein designates, in a general manner, positive or negative charge packets, particularly holes or electrons or positron-electron pairs.
The electronic entity is formed of a matrix arrangement of electronic processing and acquisition circuits, the function of which is to carry out a counting of detection events of photons or particles.
Matrix electronic circuits may be made using silicon technology, for example using Bipolar, CMOS or BICMOS technology.
The imager comprises a plurality of detector elements 1 that may consist of discrete elements such as gas scintillator tubes or a continuum of elementary detectors formed in one or several layers of detection layers, for example a semi-conductor.
Each detector element 1 is combined with an acquisition and counting chain 1,2,3,4, the function of which is to process the electric pulses coming from the detector and to count the detection events normally corresponding to the capture of a photon or a particle.
For each particle detected in the semi-conductor element 1, the counter 4 of the particles detector must be incremented by one unit. Not incrementing the counter amounts to not using the detected particle and, consequently, degrading the statistic and the quality of the image formed.
When the particle or the photon P creates charges in the same place of the detection layer in the median zone separating two detector elements DET1/DET2, in other words the normal zone at the boundary between two adjoining pixels, the “cloud” of charges CC risks being divided into two charge packets CC′ and CC″ on approaching the anodes AN1/AN2 of the two detectors. Consequently, the charge packets CC′ and CC″, which do not necessarily have equal charges, reach at the same instant the two anodes AN1 and AN2, which are connected to two acquisition chains AMP1-CMP1 and AMP2-CMP2. There is then triggering of two comparators CPM1 and CMP2 in the two pixels PIX1 and PIX2, which leads to a double counting, even though there has only been a single interaction: there is then a generation of a false event, which is at least as serious as not counting a real event.
Thus, when a particle is absorbed in a place close to the boundary between two neighboring detector elements, the delivered charges may be spread out in the absorption layers of the two adjoining pixels, principally due to diffusion phenomena. Two particles are counted whereas only one should actually be counted. This problem becomes even more acute the smaller the dimensions of the elementary detectors, which is the case, for example, in mammography. Since the evolution of technologies is leading to a miniaturization of circuits, this problem is likely to be encountered with increasing frequency in other applications.
One known solution, which works well with synchrotron radiation sources and with detection layers in silicon or in gallium arsenide, consists in adjusting the threshold of the comparators CMP1 and CMP2 as near as possible to an amplitude equal to half of the amplitude of the electric pulse that the detected particle generates. In this case, only the pixel detector that has collected more than half of the charges counts the particle. This solution resolves numerous cases. However, there remains the problem of particles detected very close to the boundary between two adjoining pixels and the problem of adjustment dispersions.
A further disadvantage of this solution consists in that it does not apply to the field of conventional radiology, for two reasons. Firstly, X-ray generator tubes emit a continuous spectrum of energy. The ratio between the maximum energy emitted and the minimum energy emitted is typically from 2 to 3. Consequently, there is no sense in defining a half amplitude of a “standard” photon. Secondly, the detector materials may be semi-conductors of lower quality than silicon or gallium arsenide, such as, for example, selenium (Se), cadmium telluride (CdTe), lead oxide (PbO), lead iodide (PbI2), mercury iodide (HgI2), thallium bromide (TlBr). In these materials, the electronic charge transport properties are mediocre and the charge finally read depends on the depth of absorption of the photon X into the layer. Said depth can vary in an important and random manner from one photon absorbed to another. Again, in this case, there is no sense in defining an amplitude half of the amplitude of a detected photon.
Another possible retort to double counting is to ensure that the first pulse CC′ that appears in time is counted and that the second pulse CC′ which then appears is not taken into account.
A single pulse is counted, which conforms to the “one single event” hypothesis.
However, this solution excludes, by principle, any event in the adjoining pixel PIX2 after the detection of the first pulse CC′ in the pixel PIX1, which therefore dismisses the interactions due to other photons, which are real events.
The higher the radiation flux, the higher this problem of loss of information.
The aim of the invention is to provide a radiation detection device that does not have these disadvantages.
One aim of the invention is to provide a detection system that makes it possible to identify real events compared to false events.
The aim of the invention is to obtain a system that makes possible the counting of two real events caused by two interactions while avoiding the superfluous counting of a false event during a single interaction.
A further objective of the invention is to provide a detection system that makes it possible to distinguish real events from false events, at a high working frequency (thus under a high flux of particles or photons) however brief the time interval separating two pulses.
The final aim of the invention is to differentiate the case where a single photon or a unique particle arrives on two adjoining pixels, and the case where two photons or two particles arrive on two adjoining pixels.
In a surprising manner, the invention provides for differentiating the nature of events by analyzing the chronological correlation of the electric signal pulses corresponding to these events, said analysis making it possible to distinguish the following cases:
The invention is realized with a radiation detection device comprising a plurality of detector elements, each detector element being associated with a circuit for acquiring pulse signals and counting detection events, the device comprising at least one correlator, or means of correlation, of detection events corresponding to at least two detector elements.
Advantageously, said at least one correlator receives at least two pulse signals coming from respective detector elements and controls the counters or means of counting corresponding respectively to each detector element, in such a way as to count a single detection event when two pulse signals are correlated, in other words concord, converge or coincide.
According to the invention, the events correlator is capable of comparing the temporal overlap of a pulse signal from one detector element compared to another pulse signal from another detector element.
Advantageously, the correlator makes it possible to distinguish:
It is provided, according to the invention, that the correlator is capable of:
According to an embodiment of the invention, the correlator has an asynchronous logic capable of comparing the occurrence chronologies of transition fronts of at least two pulse signals coming from respective detector elements.
It is provided that the correlator detects:
Other aims, characteristics and advantages of the invention will become clear on reading the following description of embodiments, given uniquely by way of example and in nowise limitative, and by referring to the appended drawings, in which:
The principle of the invention is based on the distinction of real events, in other words electrical signals created by different interactions, compared to false events, in other words electrical signals coming from a single and same interaction.
Thus, in a surprising manner according to the invention, one observes that one can identify the nature of events by their correlation:
The terminology used in the present document uses several terms to differentiate these very distinct situations:
On the other hand, when two pulses are produced in a concomitant manner in close time intervals, one of the pulses overlapping at least partially the other pulse, one distinguishes, according to the invention, two other situation(s):
In order to distinguish real events from false events, the invention provides for associating the electronic acquisition circuits of at least two pixels through the intermediary of an events correlation system that makes it possible to compare the pulse signals from different detector elements corresponding to said pixels.
Thus, the correlation system COR is arranged to receive a first pulse signal from a first detector DET1 and a second signal from a second detector DET2, in such a way as to be able to compare their respective chronologies and in such a way as to deliver, in a coordinated manner, a first increment signal C1 intended for the first counter CNT1 and a second increment signal C2 intended for the second counter CNT2.
Thus, as represented in
According to the invention, the events correlator is capable of comparing the temporal overlap of a pulse signal from one detector element compared to another pulse signal from another detector element.
The invention is realized with a logic system that determines the nature of the event—real or false—by analyzing the incident sequence of pulse signals, and provides at the output one or several count signals adapted to each of the pixels in order to count the real events and not to count false events.
The anti-coincidence logic system that supplies, to the pixel, an appropriate signal that makes it possible to count the real events and not to count the false events is advantageously realized in asynchronous logic.
Asynchronous systems function with the single knowledge of the occurrence of events: in other words that each transition on an input or output contains an information, independently of the temporal variations between these transitions. The asynchronous systems may evolve in a locally synchronized manner, and the triggering of actions uniquely depends on the presence of an event to be processed.
This type of circuit is suited to the realization of a correlator according to the invention, which makes it possible to distinguish sequences of transition fronts of pulses and to trigger the above mentioned counting of events.
In an advantageous manner, the asynchronous circuit is tuned to signals transmitted in input (events).
Advantageously, the asynchronous circuit is capable of triggering the processing locally when all of the information is available to distinguish a real event from a false event.
According to the invention, the asynchronous circuit must produce at the output values that make it possible to count or not count events.
In order to distinguish a false event from real events and to trigger a single counting or a double counting of events, the correlation system identifies the two sequences of pulse level transitions indicated in the following table.
It appears on examining the previous table that it is at the third transition, in other words at the first final front of the first pulse H1 or the second pulse H2, following the initial fronts of pulses H1 and H2, that the state logic system can determine and discriminate if it is faced with a case of correlation or non-correlation.
Thus, as shown in
However, the logic system can only decide to increment or not the second counter C2 after the third transition C/D or C/B of pulse signals present in the input.
It is uniquely in the case where the final front D of the first pulse H1 appears 8 first (before the final front of the second pulse) that the state logic system “decides” to increment the second counter C2.
The state, diagrams of the asynchronous logic system, illustrated in
In this embodiment and in the following description of the operation of the correlation system, the event counting is active on the low to high transitions. Alternatively, in other embodiments, an inverse logic may be used.
The initial state A corresponds to an idle state in which the system emits two count signals C1 and C2 at an idle level, here the logic level 1. This idle state is maintained while the pulse signals H1 and H2 remain at an idle level, here the low logic level 0.
When the first input signal H1 receives the initial low to high transition H1+ of a pulse from the detector DET1, the logic system enters a cycle of states B and C. In the first state B of this cycle, the first count signal C1 leaves the idle level, here 1, and takes an active level, here the level 0, the second count signal C2 remaining at the idle level 1. The idle level is, in principle, immaterial, it may be 0 or 1.
If a final front H1− appears in the state B, for example if the H1 immediately returns to the idle level 0 (case of an isolated pulse H1), the logic system returns to the initial idle state A and the first count signal C1 returns to idle level 1. During this transition cycle A/B/A, the counter CNT1 is therefore incremented by one unit corresponding to the isolated event H1 for example.
On the other hand, as shown in
From this third state C, the logic system can either make a return to the second state B of the cycle A/B/C, or a transition to a fourth state D belonging to another cycle of states A/D/E, depending on whether one is in the case of
In the case of correlation, shown in
Thus, in the case of correlation, as shown in
Now, if, as shown in
During this state transition C/D, the logic system modifies the output C1, which goes to level 1. The low to high transition C1+ increments the counter CNT1.
In this case of non-correlation, as shown in
Thus, in the case of non-correlation shown in
At the end of the two pulses H1 and H2 of the two input signals, the logic system according to the invention has therefore incremented by one unit each of the two counters CNT1 and CNT2 corresponding to each of the detectors DET1 and DET2, thus counting two events in accordance with the objective of processing two real non-correlated events.
The state diagram of
The transition from the state D to the state E takes place in the case of detection of a low to high transition H1+ of the first input signal H1 while the second input signal H2 is already in the active state D. In this case, during the state transition D/E the logic system generates a high to low transition C1− on the output of the first count signal C1.
When the logic system is in the fifth state E and detects the final front H1− of the pulse of the first input signal H1, in other words if the first pulse of signal H1 ends first (before any high to low transition of the second input signal H2), the logic system returns to the fourth state D, the count signals C1 and C2 remaining at the same levels.
The states cycle A/D/E/D/A corresponds to the incrementing operations INC2 of the second counter C2 and to the processing of another case of correlation in which a pulse of signal H2 has a time base that completely overlaps that of a pulse of signal H1 (example not illustrated) and in which only the counter CNT2 is incremented during the state changes D/A.
On the other hand, when the logic system is in the fifth state E and the final front H2− of the pulse of the second input signal H2 appears first, the logic system makes a transition to the second state B during which the output signal C2 changes level, here taking the level 1. This low to high transition C2+ of the second count signal increments the second counter CNT2.
During the succession of states A/D/E/B/A, the logic system then increments by one unit the second counter CNT2 (transition C2+ between the state E and B) then the first counter CNT1 (transition C1+ between the state B and A). This succession of states A/D/E/B/A corresponds to the processing of another case of non-correlation (not shown) in which the two inputs H1 and H2 receive two pulses H1 and H2 of comparable durations, the pulse H1 this time having a late time shift compared to the pulse H2.
At the end of the two pulses H1 and H2 of two input signals, the logic system according to the invention has therefore incremented by one unit each of the two counters CNT1 and CNT2 corresponding to each of the detectors DET1 and DET2, thus counting two events in accordance with the objective of processing two real non-correlated events.
It may be observed that the conditions of transition H2+, H2− and H1+, H1− as well as the output instructions C2−, C2+ and C1− of the fourth and fifth states D and E are transpositions of the conditions of transition H1+,H1− and H2+,H2− as well as the respective output instructions C1−,C1+ and C2− of the second and third states (by permutation of H1 and H2 as well as C1 and C2), the state diagram of the correlator illustrated in
Thus, according to the embodiment of
Or instead, according to the embodiment of
Thus, as shown in
In this embodiment of the asynchronous logic states correlation system, it appears that the system has five states and, consecutively, five state memories.
The above description of embodiments of the invention deals with the case of the correlation of the pulse signals H1 and H2 of two acquisition chains AMP1/CMP1 and AMP2/CMP2 coming from two neighboring detector elements DET1 and DET2 (two adjoining pixels).
Other embodiments of state diagrams and variations of input and output state transitions may be envisaged from the example of this embodiment without going beyond the scope of the present invention.
In order to conceive a device according to the invention, or a state machine according to the invention, or a state device or machine using a method according to the invention, several solutions may be envisaged: one may either use synthesis tools specific to the synthesis of asynchronous state machines, or use an appropriate description language (such as CHP or CSP) and then synthesize this circuit with asynchronous circuit synthesis tools. One may refer to the document of M. Renaudin et al. “A design framework for asynchronous/synchronous circuits based on CHP to HDL translation”, IEEE, 199 M, p. 135-144.
In a more general manner, the correlator provided according to the invention may be used not just for rejecting concordances of events between two adjoining pixels but also for accepting concordances of events between any pixels, whether adjoining or not.
The invention advantageously applies to radiation detection devices arranged in a matrix architecture, in which the detection layer(s) constitute a network of closely related elementary detectors combined with an acquisition chain matrix, particularly those used in imaging.
Within the scope of such an application, each acquisition chain can be linked to several adjoining acquisition chains, each link comprising a correlation system capable of comparing the temporal overlap of the electric signals emitted by the chain with the signals concerned by the link.
Thus, it is provided that the correlators of-the acquisition and event counting circuits are laid out in a matrix arrangement, each correlator being connected to a respective detector element and to at least one adjoining detector element, the correlator being capable of comparing the temporal overlap of linked pulse signals.
In an advantageous manner, the invention improves the quality of images obtained with matrix sensors by avoiding losses of statistical information represented by the counting of false events and improves the performance and thee quality of the imager, such as the contrast and the sharpness of the image.
The invention applies particularly advantageously to the field of medical imaging, especially X-ray imaging, a field in which any loss of information or deterioration in the performance would be prejudicial to the quality of the diagnosis.
As another specific application example, the invention is suitable for digital tomography devices, known as scanners, and in which it is advantageous to be able to reject the correlations or coincidences between adjoining pixels.
The invention further applies to the case of electron-positron pair emission tomography (PET). In this case, each recombination of an electron with a positron leads to the simultaneous emission of two y radiations in opposite directions at an angle of 1800.
In said tomography, in order to recognize that a γ radiation captured by a first detector and another γ radiation captured by a second detector, situated in the opposite zone to the first, originate S from a true recombination and not non-correlated false events (disintegrations, emissions of any γ radiation, etc.) and to localize the place and more precisely the point of origin of the emission of the two γ radiations, one detects the capture coincidences of two photons γ by two detectors situated in opposite zones. Advantageously, according to the invention, the correlators are used to detect such coincidences, with the exception that this time one accepts the coincidences, in other words that in the case of correlation of detection of events by two detectors situated in opposite zones, one accepts the correlation instead of rejecting this case, and one increments a corresponding counter. Then, in the case of non correlation, in other words in the case of detection of detection events with a time gap that do not correspond therefore to a real recombination, one considers that it involves false events and the counter associated with the two detectors is not incremented.
The invention may be further realized with a radiation detection device comprising a plurality of detector elements, each detector element being associated with a circuit for acquiring pulse signals and counting detection events, the device comprising correlators of detection events corresponding to two detector elements and receiving the pulse signals coming from the respective detector elements, particularly situated in opposite zones, each correlator controlling the means of counting corresponding to the two paired detector elements and being used in such a way as to count a detection event when two pulse signals are correlated (or concordant) and not to count an event when two pulse signals diverge or are not correlated.
Apart from the fields of radiology and X-ray radiography, the invention also applies to the fields of measuring the exposure to other types of radiation, such as imaging based on the emission of electron-positron pairs (positron camera), the remote detection of α,β,γ, radiation, visible light or beyond visible light imaging (photons of ultraviolet radiation and wavelengths on this side, photons of infrared radiation and wavelengths on the other side).
Other applications, variations and embodiments could be implemented by those skilled in the art, without going beyond the scope of the present invention.
The realization of a device operating according to the invention may be achieved in accordance with the indications contained in the thesis of Pascal VIVET, 21 Jun. 2001, entitled “Méthodologie de conception de circuits intégrés quasi-insensibles aux délais: application à l'étude et la réalisation d'un processeur RISC 16 bit asynchrone” Methodology for designing integrated circuits that are virtually insensitive to time lags: application to the study and realization of an asynchronous RISC 16 bit processor), Institut National Polytechnique de Grenoble.
| Number | Date | Country | Kind |
|---|---|---|---|
| 03 51233 | Dec 2003 | FR | national |
