Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
  • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
  • G01T1/16—Measuring radiation intensity
  • G01T1/17—Circuit arrangements not adapted to a particular type of detector
  • G01T1/171—Compensation of dead-time counting losses

Definitions

• the present invention relates to nuclear medical imaging systems, and more specifically to the correction of the stacking effects of the electronic pulse signals delivered by the gamma radiation detectors.
• the present invention relates to a pulse signal detector occurring randomly over time.
• the invention can be used for the detection of nuclear radiation using a scintillation detector, for example a gamma camera.
• a gamma camera includes means for locating scintillations occurring in the scintillator crystal under the effect of incident gamma radiation.
• U.S. Patent No. 3,011,057 describes a gamma camera in accordance with the preamble.
• This includes a collimator for selecting the direction of incident gamma photons, a scintillator crystal with which incident photons interact to give rise to light pulses or scintillations, and a network of photomultiplier tubes which in turn transform the received scintillations into pulses. electric.
• This network of photomultiplier tubes is part of the localization means which, from the electrical pulses provided by the tubes, deliver, in a known manner, X and Y coordinate signals from the place where the scintillation has occurred, as well as a signal E proportional to the energy of the photon that produced the scintillation.
• Signal E is used to differentiate the different types of incident gamma photons and to identify noise, direct radiation, and scattered radiation.
• the scintillation produces a sheaf of photons emitted in all directions isotropically within the scintillator crystal, it is simultaneously seen by several tubes photomultipliers.
• the determination of the location of this scintillation on the crystal, itself representative of the emission site of the incident gamma photon, is obtained by calculating the location of the center of gravity, that is to say a weighted sum of electrical pulses. delivered by all the photomultiplier tubes excited by the scintillation considered.
• the energy of the incident photon is obtained by unweighted summation of the electric pulses delivered by all the photomultiplier tubes.
• These electrical pulses generally have a known and constant average shape for a given scintillator material and a given light detection and signal processing electronics. This shape, or amplitude of the signal as a function of time, represents the average flux of scintillation photons emitted over time. For most scintillator materials, this pulse can be with a good approximation described by the following bi-exponential model:
• ⁇ 2 is the pulse rise time constant
• ⁇ is typically of the order of 250 nanoseconds and ⁇ 2 typically of the order of 80 nanoseconds.
• ⁇ is typically of the order of 250 nanoseconds and ⁇ 2 typically of the order of 80 nanoseconds.
• X, Y the calculated values of the coordinates of the scintillation site and the calculated E value of the total energy of the scintillation, which determine the spatial resolution and the energy resolution. of the instrument.
• these pulses are integrated over a certain time before being used for the calculation of X, Y and E.
• This Integration operation physically corresponds to counting the total number of scintillation photons emitted throughout the duration of the pulse.
• the integration time is typically of the order of several times the decay time of the pulse, so that the amount of residual scintillation photons after integration is negligible compared to the amount of photons that have been integrated.
• this integration time is typically of the order of 1 microsecond.
• the disadvantage of integrating the signals is that the quality of the measurement of the values of X, Y and E (and therefore the performance of the detector) becomes very dependent on the counting rate to which the detector is exposed. that is, the number of scintillations that occur per second. Indeed, scintillations occurring randomly in time according to the statistical law of Poisson, it happens that pulses occur very closely in time and that as a result the electronic signals overlap partially, causing a known phenomenon under the name of stacking. Uncorrected, stacking effects produce severe distortions when calculating (X, Y) and E and are likely to lead to unacceptable degradation of image quality.
• the stacking phenomenon can be of two types: the upstream stack (or prepulse), and the downstream stack (or postpulse).
• Upstream or pre-impulse stacking is due to events occurring before the beginning of the integration of the current event.
• the pulse signal of the current event is thus superimposed on the tail of the previous event.
• the downstream or post-pulse stack is due to events occurring after the beginning of the integration of the current event, that is, while the current event is being integrated. Different methods of correcting these two types of stacking have been described in the literature.
• Some gamma cameras of the prior art use a method based on simple rejection of events. Indeed, as the stacking phenomenon has the effect of artificially increasing the integral of the signals, the selection of events is done on the basis of the comparison of their energy with a given acceptance window. Events whose energy is outside this acceptance window are rejected. This method avoids taking into account events whose values (X, Y, E) are distorted or distorted, but suffers from severe losses of events induced by high rejection rates occurring at high counting rates.
• US Pat. No. 5,276,615 describes a method for shortening pulse duration by deconvolution or signal filtering, so as to reduce the integration time and thus reduce both losses due to dead time and the probability of stacking. .
• the signal after deconvolution can theoretically be shortened at will without loss of information, a practical limit is imposed by the noise component in the signal.
• the shortening of the signal is obtained by amplification of the high frequencies (in the Fourier space), which carry more and more noise and less and less signal as the frequency increases.
• the noise level can reach such values as the losses in precision, that is to say the losses in spatial resolution and in energy, can become unacceptable.
• filtering is a stationary operator, these impairments are introduced independently of the count rate, which makes the system optimized for only one count rate at a time.
• this method has the disadvantage of requiring that the two events, current and next, are necessarily detected and integrated, fully or partially, even in the case where one of these events is unnecessary and could have been eliminated at one time.
• early stage of treatment for example by selection of the amplitude of events or another method.
• Such superfluous treatment of unnecessary events ultimately requires additional computational capacity which inevitably has an impact on both the downtime of the system and the cost of the equipment.
• this method does not properly process bursts of three or more events close enough to be stacked on top of each other. Indeed, as downstream and upstream stacking corrections are performed independently, it is not taken into account a possible
• the second event is corrected from the upstream stack by the tail of the previous event as if this event queue were integrally integrated, i.e. integrated over a time that may be longer than the actual integration time of the second event, which overestimates the upstream stacking correction of the second event.
• This non-correlation of upstream and downstream stacking corrections is the reason why such a system performs a false correction, the average value of which can be partially compensated by any phenomenological model (for example as a function of the average counting rate or other ), but can not correct exact stacks for a given sequence of multiple events.
• a third disadvantage of this method is that the fact of estimating the tail of a pulse from the instantaneous value of the signal assumes that the pace of the pulse is constant, for example purely exponential or linear, as well as described in the aforementioned patent. This approximation becomes invalid for more complex pulse patterns such as the bi-exponential model described in the preamble, and it would be necessary to take into account not only the instantaneous amplitude of the signal, but also the time at which this amplitude is measured. relative to the beginning of the impulse.
• US Patent No. 7,439,515 only bypasses the second problem mentioned above, by executing the upstream and downstream stacking corrections in a correlated manner, in order to compensate for the upstream stacking correction of the downstream stack or the shortening of the stack. signal integration time.
• the efficiency of the method relies on the effective detection by the trigger of all the events independently of their characteristics, in order to detect the appearance of stacking conditions and to be able to process the events
• the stacking correction fails at a high counting rate because it is highly dependent on the effectiveness of the trigger, but it also requires significant calculation means so as to be able to continuously deal with a high flow of data. much of which is unnecessary, which in any case increases the loss of events associated with the system timeout.
• the present invention describes a new and improved method and apparatus that do not have these disadvantages as well as others of the prior art.
• the object of the present invention is to provide, in particular for radiation detection devices delivering electronic pulse signals in response to events arising from the interaction of quanta radiation incident with the detector, a method for performing a correction of upstream and downstream stacking effects, the method of detecting the occurrence of an incident event, integrating a portion of the signal preceding the impulse of the event over a predetermined time interval, integrating the pulse of the event on a predetermined time interval or until a new event is detected, applying an upstream or prepulse stack correction based on the integral of the signal portion preceding the pulse of the event and integration time of the event pulse, application of a downstream or post-pulse stack correction based on the time of the event integration of the impulse of the event.
• the method may further comprise a step of measuring the time that has elapsed between the current event (being integrated) and the previous event, so that the upstream stacking correction is based not only on the integral of the signal portion preceding the event pulse and the integration time of the event pulse, but also on the time that has elapsed between the current event and the previous event. This makes it possible to take into account the shape factor of the pulse and to accurately calculate the recovery integrals corresponding to complex pulse shapes.
• the invention also relates to devices that make it possible in particular to implement the method described above.
• the subject of the invention is a signal processing device for a radiation detector delivering electronic pulse signals in response to events arising from the interaction of incident radiation quanta with the detector, which device comprises means (a) for detecting the occurrence of an incident event, means (b) for integrating a signal portion preceding the pulse of the incident event over a predetermined time interval, means (c) for integrating the pulse of the incident event incident on a predetermined time interval or until a new event is detected, means (d) for performing an upstream stack correction based on the integral of the signal portion preceding the pulse of the event and the time
• event pulse integrating means (e) for performing a downstream stack correction based on the event pulse integration time.
• the means (d) for performing the upstream stacking correction furthermore include a counter intended to measure the time which has elapsed between the current event (being integrated) and the preceding event, so as to the upstream stacking correction is based on the integral of the signal portion preceding the pulse of the current event, the integration time of the pulse of the current event, and the time that 'is passed between the current event and the previous event.
• the integration (b) of the signal portion preceding the pulse of the event and the integration (c) of the event pulse are performed by two separate integrators operating in parallel, the signal signals input applied to the integrators being delayed by two different delays before being applied to the integrators.
• This architecture makes it possible to perform both integration operations simultaneously, then to execute the upstream and downstream stacking corrections in the same treatment cycle independently of the arrival time of the successive events.
• the means for performing the upstream stacking correction (d) is the means for performing the upstream stacking correction
• the means for performing the upstream stack correction further comprise a counter measuring the time elapsed between the current event (being integrated) and the previous event, the address of the memory below. above mentioned is obtained by a combination of the integration time of the pulse of the current event and the value of said counter.
• the means for performing the downstream stacking correction comprise a multiplier and a memory, the said correction being performed by multiplying with the aid of the said multiplier the integral of the pulse of the corrected event of the stack upstream by the output of said memory addressed by the integration time of the pulse of the event.
• the present invention has the advantage of making accurate and reliable upstream and downstream stacking corrections, which improve the spatial and energy resolutions of the instruments, particularly at high count rates.
• Another advantage of the present invention is that the improvement of the spatial and energy resolutions depends to a lesser extent on the efficiency of the triggering system.
• the present invention may be advantageously used in nuclear medical imaging systems such as gamma cameras or positron emission tomographs.
• FIG. 1 shows by way of example the signal associated with a burst of three events approximated in time, the time intervals used to measure the different integrals of the pulse signals of the events, and the integrals themselves without stacking correction.
• FIG. 2 shows the signal associated with a burst of three time-close events and the integral of the pulse tails of the previous events to be subtracted from the raw integrals shown in FIG. 1 so as to perform upstream stacking correction.
• FIG. Figure 3 shows the signal associated with a burst of three time-lapse events and the time intervals used to measure the different integrals of the pulse signals of the events that will be used to calculate the integral of the pulse tails of the previous events indicated in FIG. . 2.
• FIG. 4 shows a preferential way of producing an integrator comprising upstream and downstream stacking corrections.
• the invention is not limited to gamma cameras and can be broadly applicable to any type of particle detector delivering electronic pulse signals in response to incident events.
• it can be applied to the individual photomultiplier tubes as well as to any signal obtained by summing the output signals of the photomultipliers.
• analog signals delivered by the detector are converted into digital signals using Analog-Digital Converters (ADCs) which convert the signal continuously and at a constant rate. frequency high enough to satisfy the Nyquist criterion. As a result, all operations described in the following method are performed on digital signals.
• ADCs Analog-Digital Converters
• the event trigger referred to below is a device delivering a logic signal when an event is detected and is used to trigger the sequence of integration and stack correction of the pulse signal of an event.
• FIG. 1 illustrates the stacking phenomena occurring when a burst of three events close together in time is detected.
• the beginning of the signal corresponds to a moment when one does not have any information on the presence or the absence of a possible previous event. This corresponds to situations where the event trigger did not detect any event in a significantly long time interval before the first burst event.
• This significantly long time interval corresponds to the value of the time from which the energy contained in the remaining portion of a pulse falls below the energy of an individual scintillation photon, i.e. it corresponds to the extinction time or time at which the scintillation actually ends.
• this value is of the order of several microseconds, that is to say that in practice it can be significantly greater than the time typical integration of an event.
• the trigger may have detected no events during this time for various reasons, such as: trigger makes a selection of events with peak amplitude values that must be included in a predefined acceptance window, or an event has occurred while the trigger is idle because the dead time following the detection of a previous event has not yet elapsed, or the counting rate is so high that the stacking phenomena make events difficult to discern, or the signal undergoes statistical fluctuations such that the trigger is unable to identify valid pulses of events.
• the signal portion corresponding to the first event is integrated from to to ti, giving a value integral Ii.
• the integration is stopped by the detection by the trigger of a second event.
• the signal portion corresponding to the second event is integrated from t 1 to t 2 , giving an integral of value I 2 .
• the integration is stopped due to the detection by the trigger of a third event.
• the signal portion corresponding to the third event is integrated from t 2 to t 3 , giving an integral of value I 3 .
• the integration is stopped after the nominal integration time because no event is detected before t 3 , and therefore (t 3 -t 2 ) is equal to the nominal integration time.
• FIG. 2 indicates the portions of the pulse tails of the previous events which are integrated with the integrals I 1 , I 2 and I 3 and which must be subtracted from them in order to carry out the upstream stacking correction.
• the integral contains a portion of a possible residual tail of a previous event that has not been detected. This residual tail integral is denoted ⁇ .
• the integral I 2 also contains a portion of the residual tail of the previous event, noted ⁇ and whose value also depends on fo-ti), which shows how the upstream stacking correction is in close correlation with the integration time and therefore can not be performed independently of the correction
• the integral I 3 also contains a portion of the residual tails of the previous events, denoted ⁇ 2 , which itself includes a portion of the residual tail of the first event, denoted
• FIG. 3 describes, on the same salvo of three close events, the different operations to be performed in order to make precise corrections of upstream and downstream stacking.
• Co is of particular importance because it corresponds to the measurement of a possible pulse tail of an event that would have occurred before the first burst event, and is used to extrapolate an estimate of I'o, which is used later to correct the value of Ii.
• the signal portion corresponding to the first event is integrated from t0 to t5 giving a value integral Ii. At the time the integration is stopped due to the detection by the trigger of a second event.
• the signal portion preceding the second event i.e., a signal portion corresponding to the first event
• This value is used to calculate by extrapolation an estimate of ⁇ , which is used later to correct the value of I 2 .
• the signal portion corresponding to the second event is integrated from t 1 to t 2 , giving an integral of value I 2 .
• the integration is stopped due to the detection by the trigger of a third event.
• the signal portion preceding the third event i.e., a signal portion corresponding to the second event
• This value is used to extrapolate an estimate of I ' 2 , which is used later to correct the value of I 3 .
• the signal portion corresponding to the third event is integrated from t 2 to t 3 , giving an integral of value I 3 .
• the measurement and correction method performs in parallel the measurements of the integrals of the event impulse signals, namely L, I 2 and I 3 , and the measurements of the integral of the events. signals preceding events Co, Ci and C 2 . Then it corrects L, I 2 and I 3 upstream stacking corrections obtained from Co, Ci and C 2 to obtain corrected signals of the upstream stack Ji, and J 3 , and finally it corrects the values of the signals. corrected the upstream stack Ji, J 2 and J 3 to obtain final values K 1 , K 2 and K 3 corrected for both upstream and downstream stacking effects.
• the entire correction process is described in the following sequence of operations:
• K 2 J 2 ⁇ g ⁇ t 2 - t x )
• the function ⁇ Ats, ti, PE represents the correction fraction due to upstream stacking, ie the proportion of the tail of the previous event which is integrated in same time as (i.e., stacked on) the pulse signal of the current event, where:
• ti is the integration time of the pulse of the current event
• tpE is the time that elapsed between the current event and the previous event.
• the integrals of the signal over two successive and distinct time intervals are linked by a simple linear relation that depends only on the respective positions of the bounds of the integrals.
• the time separating the current event from the previous event plays no role, and function / only depends on Ats and ti, removing the need to have t PE in the input argument list of function /
• the integrals of the signal over two successive and distinct time intervals depend not only on the respective positions of the bounds of the integrals, but also the time at which these limits are positioned with respect to the temporal distribution of the event, that is to say with respect to the time at which the event occurred.
• the signal integrals are positioned far from the top of the pulse, the purely exponentially decreasing model can be considered a good approximation, whereas when the integrals are near the vertex or even include the peak of the pulse, this approximation ceases to be valid.
• the accuracy of the correction is also independent of the integration time of the current event, ie it always takes into account the exact contribution of the remaining queues of the impulses of the previous events (detected or not) to integration, regardless of the value of the integration time.
• the value of ÎPE time separating current and previous events
• the value of the associated upstream stacking correction Co will be equal to zero and the correction, made in all cases, will have no effect.
• the values of the Atsi integration times used to measure the Co, Ci and C 2 values, namely Atso, Atsi and Ats 2 can be selected on the fly for each event to optimize the final accuracy on the Ko calculation. , Ki and K 2 , depending on different relevant parameters, such as the time between the current event of the previous event and the average count rate.
• the low limit for the Atsi integration time values is to consider only one sample, but as previously stated, this introduces a high error due to statistical fluctuations. Integrating several samples decreases the error due to statistical fluctuations and the upper limit is to take for the Atsi values equal to the integration time of the previous events, that is, the upstream stacking correction uses the value of the previous impulse integrated since the beginning of the impulse.
• the values of the integration times Atsi must be carefully optimized so that the signal-to-noise ratio in the measurement of C is maximal, which usually causes the Ats, to be relatively constant (whatever the counting rate) and of the same order of magnitude as the material's decay time constant scintillator, that is to say typically three. or four times less than the nominal signal integration time of an event. This also represents a significant improvement in terms of accuracy and final position and energy resolutions compared to the state of the prior art.
• the function g (ti) represents the correction fraction due to the downstream stack, that is to say the correction for the missing part of the tail of the pulse, where:
• ti is the integration time of the pulse.
• This correction for the missing part of the tail of the pulse is obtained by multiplying the value J by a tabulated coefficient which depends on the integration time 3 ⁇ 4. Indeed, considering a constant shape for all pulses regardless of their amplitude, which is the case for most scintillation detectors, one skilled in the art can determine the remaining part of the signal knowing the value of its integral. and the integration time, and the multiplicative coefficient is thus inversely proportional to the collection efficiencies associated with the different integration times.
• FIG. 4 is a block diagram of a preferred embodiment of the invention. It is a device comprising two integrators performing the integration and correction functions of the upstream and downstream stacks, and having a sliced structure so as to perform the sequence of operations described above regardless of the number of events in the salvo and their temporal distribution.
• the digital samples of the pulse signal are simultaneously received by the FIFO delay 400 having a delay time of value At + and the FIFO delay 410 having a value delay time At-
• the output of the FIFO delay 400 is applied to the integrator 401, and the output of the FIFO delay 410 is applied to the integrator 411.
• the value ⁇ + of the FIFO delay 400 is adjusted so that when an event is detected by the trigger, the integrator 401 starts the integration.
• the multiplier 420 receives the output of the integrator 411 and the output of a memory 403 which contains the tabulated values of the function /
• the input address of the memory 403 is obtained by concatenating the time ti of the integrator 401 (Current event pulse integration time), integrator time 411, and EPI time elapsed between the current event and the previous event.
• the PEI value is provided by a clock counting the time between events and is reset at each event.
• a time limiting device makes it possible to avoid an overflow of the counter when the time separating two successive events becomes too high, so that tpE is always in the interval [1, tMAx], where ÎMAX is a fixed value dependent on the capacity of the memory 403.
• the arithmetic unit 430 subtracts the output of the multiplier 420 from the output of the integrator 401.
• the multiplier 440 receives the output of the arithmetic unit 430 and the output of a memory 402 which contains the tabulated values of function g.
• the input address of the memory 402 is obtained by the integration time 3 ⁇ 4 of the integrator 401.
• the integration time Ats signal before. the pulse of the current event is taken as constant, it disappears from the input address of the memory 403.
• the input address is obtained by concatenation of the only integration times t i. pulse of the current event and the time PEI elapsed between the current event and the previous event.
• the chosen pulse shape model is a purely exponential decreasing function, it becomes unnecessary to take into account the form factor and the function / cease to depend on the elapsed PEI time. between current and previous events.
• tpE disappears from the input address of the memory 403 and this input address is obtained by the integration time 3 ⁇ 4 of the integrator 401.
• integrator 411 or end of integration of Ci and sending of Ci downstream event 3 detected by the trigger

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