FR2754656A1 - PHOTODETECTOR SIGNAL CODING UNIT AND METHOD, WITH INPUT LEVEL CORRECTION, AND USE OF SUCH A UNIT FOR A GAMMA-CAMERA - Google Patents

Abstract

The invention concerns a unit encoding the analog electric signal of a photodetector (110) comprising an analog-to-digital-converter (112), connected to the photodetector (110), the analog-to digital converter (112) being capable for receiving the analog electric signal of the photodetector and transmitting an encoded corresponding digital signal. The invention is characterised in that the unit further comprises means (130) for automatically correcting the level of the signal received by the analog-to-digital converter (112) on the basis of an encoded signal transmitted by the analog-to-digital converter in the absence of a pulse from the photodetector. The invention is applicable to gamma-cameras.

Description

PHOTODETECTOR SIGNAL CODING UNIT AND METHOD,
WITH ENTRY LEVEL CORRECTION, AND USE OF A
SUCH A UNIT FOR A GAMMA-CAMERA.

DESCRIPTION
Technical area
The present invention relates to an input level correction signal coding unit.

 The invention can find applications in any device in which an analog signal must be converted into a digital signal. However, the invention is more particularly intended for the processing of an electrical detection signal marred by a statistical fluctuation, such as the detection signal emitted by a photomultiplier, for example.

 The coding unit of the invention can therefore be used in particular for the detection heads of gamma-cameras equipped with photomultipliers. It can also be used for y or x radiation detectors using other types of photodetectors such as photodiodes or semiconductor detectors, for example.

State of the art
A detection head of a gamma-camera and in particular a detection head of a gamma-camera of the Anger type comprises a scintillator crystal and a plurality of photomultipliers optically coupled to this crystal.

 When gamma radiation reaches the scintillator crystal, it is converted into light photons. The photons produced in the scintillating crystal of the gamma camera are detected by photomultipliers. For each event, that is to say for each interaction between a gamma radiation (gamma photon) and the scintillator, the photomultipliers receiving light photons produced by the interaction, deliver an electrical pulse. These electrical pulses each have an amplitude proportional to the quantity of light received by the corresponding photomultiplier.

Reference may be made to this subject in documents (1) and (2), the reference of which is indicated at the end of this description.

 It should be noted that the signals delivered by the photomultipliers undergo fluctuations linked to Poisson fluctuations in the number of light photons produced during each interaction and in the number of photoelectrons generated in the photomultiplier.

 In the absence of an event, the signal from a photomultiplier does not present an electrical pulse but a continuous background, also affected by fluctuations. The level of the continuous background can vary from one photomultiplier to another from the same detection head. Most of it is noise.

 The signals from all the photomultipliers of the detection head of the gamma-camera are collected and directed to a calculation unit capable of calculating, for example, the position of each event detected on the crystal. For precise calculation of the position of events on the crystal, as well as for rapid processing of signals during detection with high counting rates, i.e. with a large number of events per unit of At the present time, it is currently envisaged to associate an analog-digital converter with each photomultiplier of a gamma-camera. The analog-to-digital converter converts the analog signal from the photomultiplier into a coded digital signal, which is directed to the computing unit.

 By way of illustration, FIG. 1 shows, in the form of a functional diagram, a signal processing channel corresponding to a single photomultiplier of a gamma-camera.

 The photomultiplier, with the reference 10, is connected to an analog-digital converter 12 by means of a current-voltage converter 14. The signal emitted by the photomultiplier is, in fact, a current signal which should be converted into a voltage signal before applying it to the input 16 of the analog-digital converter 12.

The output 18 of the analog-digital converter 12 is connected to a digital summator 20. The summator 20 is provided for integrating the digital signal formed by the analog-digital converter in response to the analog signal applied to its input. The integrated signal is then directed to a calculation unit 22 capable, for example, of calculating the energy of a gamma photon, at the origin of a detected event.

 The calculation unit 22 can receive signals from a plurality of processing channels comparable to that shown in FIG. 1, corresponding to the set of photomultipliers of the detection head, to calculate, for example, the position of an event detected on the scintillator crystal.

 A detection device in which an analog-digital converter is associated with each photomultiplier, in accordance with FIG. 1, however poses a certain number of problems.

 In fact, as mentioned previously, in the absence of an event-related pulse, each photomultiplier emits a signal with a continuous background, the amplitude of which is specific to this photomultiplier.

In addition, the gain of photomultipliers is also very variable from one copy to another. Finally, the amplitude of the continuous background of the signal may be modified over time, in particular by the play of statistical fluctuations.

 Thus, in the absence of a pulse, the signal present on the input terminals of the various analog-digital converters has very variable levels from one channel to another.

 This phenomenon has a negative influence on the quality of signal processing.

 In fact, if the level of the continuous background of the signal is high in the absence of a pulse, a large portion of the dynamic range of the analog-digital converter is used by this background. The analog-to-digital converter is then not able to correctly convert large amplitude pulses.

 The analog-digital converter is effectively characterized by its dynamic range which can be understood as the voltage difference existing between a minimum voltage converted and corresponding to a minimum value of the digital signal supplied by the analog-digital converter, and a maximum voltage converted , corresponding to the maximum value of the digital signal supplied by the analog-digital converter.

 To compensate for the dispersions of characteristics between the photomultipliers and to apply an appropriate signal input level to the analog-digital converters, a solution consists a priori of providing between each photomultiplier and the analog-digital converter which is associated with it, an adjustable gain amplifier. .

 This solution is however insufficient for several reasons.

 First of all, it appears that the level of the continuous background of the signal emitted by a photomultiplier, in the absence of a pulse corresponding to a detected event, can vary over time. Thus, when this level at the input of the analog-to-digital converter is initially adjusted to a low value in order to preserve a large dynamic range, the signal applied to the analog-digital converter risks taking values of polarity opposite to that of the pulse ( negative if the pulse is positive) if the level of the continuous background decreases over time. One consequence of this phenomenon is an inaccurate consideration of the analog signal.

 Furthermore, when a detection head of a gamma-camera comprises of the order of a hundred photomultipliers, the individual adjustment and adjustment of the input level of each analog-digital converter is a long and delicate operation .

Statement of the invention
To remedy the difficulties set out above, the object of the invention is to propose a coding unit for a photodetector making it possible to make the best use of the dynamic range of the coder, that is to say of the analog-digital converter which it comprises.

 Another object is to propose a coding unit making it possible to respect the positive polarity of the signal applied to the input of the analog-to-digital converter and to avoid in particular the appearance of negative components of the signal.

 An object of the invention is also to avoid a long and tedious adjustment of the input level of the analog-digital converter.

 To achieve these aims, the invention more specifically relates to a unit for coding the analog electrical signal of a photodetector capable of emitting pulses, comprising an analog-digital converter, connected to the photodetector, the analog-digital converter being able to receiving the analog electrical signal from the photodetector and transmitting a coded digital signal corresponding to the analog signal and comprising a succession of samples. According to the invention, the coding unit further comprises means for automatically correcting the level of the photodetector signal received by the analog-to-digital converter, as a function of the coded signal emitted by the analog-to-digital converter in the absence of a pulse. of the photodetector.

 For the purposes of the present invention, photodetector is understood to mean both a photomultiplier and a photodiode or a semiconductor detector, of the CdTe type for example.

 Thanks to the correction means, taking into account the coded signal output by the analog-digital converter, the individual adjustment of the signal level emitted by the photodetectors is done automatically and does not require intervention on the device. This aspect is particularly advantageous for gamma-cameras with photomultipliers comprising the coding unit of the invention.

 In addition, the correction means can permanently adjust the input level and thus correct changes in the characteristics of the photodetectors over time. The correction means also make it possible to take account of the characteristics of the measurements carried out and of the fluctuations which they generate.

 Finally, the automatic correction means can be designed to make the best use of the dynamic range of the analog-digital converter while preventing the signal at the input of the analog-digital converter from becoming negative.

 The correction means are connected, for example, between the photodetector and the analog-digital converter of the coding unit and are connected to an output for transmitting the digital signal from the analog-digital converter.

According to a particular embodiment of the coding unit of the invention, the correction means can comprise - a detector of absence of pulse in the signal
digital to analog-to-digital converter - means for comparing the digital signal to a
first threshold says low threshold and at a second threshold
said high threshold - a correction counter capable of transmitting a signal
level down correction when signal
numeric is greater than the second threshold and
issue an increase correction signal
level when the digital signal is lower than
first threshold, the correction signal being emitted
upon detection of no pulse; and - means for adding to the photodetector signal
a continuous correction voltage corresponding to the
correction signal.

 Thanks to the absence of pulse detector, it is possible to determine the necessary correction of the signal level of the photodetector in the absence of a pulse, that is to say while the photodetector transmits a continuous background signal.

 The comparison means make it possible to compare the digital signal emitted by the analog-digital converter at the low threshold and at the high threshold.

Thus, when an absence of pulse is detected, the correction made takes account of the fact that the level of the continuous background emitted by the photodetector, and converted into a signal coded by the analog-to-digital converter, is higher or lower than the high and low.

 When the level of the continuous background is such that the corresponding digital signal is above the high threshold, it is considered that the latter occupies too large a part of the dynamic range of the analog-digital converter. The input signal from the analog-digital converter is then compensated by adding to it a correction voltage tending to reduce its amplitude (level).

 Conversely, when the level of the continuous background is such that the corresponding digital signal is below the low threshold, it is considered that the latter is too low. This means that the analog signal applied to the input of the analog-to-digital converter may take negative values.

 The input signal from the analog-digital converter is then compensated by adding to it a correction voltage tending to increase its amplitude.

 When the level of the continuous background is such that the corresponding digital signal is between the high threshold and the low threshold, no correction is made.

 The choice of the value of the high threshold and the low threshold depends in particular on the characteristics of the analog-digital converters used and their resolution. It also depends on the dispersion of the characteristics of the photodetectors.

 In particular, in a use of the coding unit for a gamma camera with photomultipliers, the difference between the high threshold and the low threshold is chosen sufficient to be able to adapt the correction means equally to any photomultiplier in the range of photomultipliers used for the gamma-camera detection head.

 According to a particular aspect of the embodiment of the coding unit, the means for adding a correction voltage to the signal from the photodetector may comprise a digital-analog converter for converting the correction signal from the correction counter into an analog correction voltage, and an operational amplifier for adding the analog correction voltage to the photodetector signal.

According to another particular aspect of the embodiment of a coding unit according to the invention, the absence of pulse detector can comprise - a shift register, comprising a number
determined n of positions and capable of emitting a signal
detection of an absence of pulse when all
the positions are in a validation state - a control clock for the analog converter
digital, also connected to the shift register
to successively put in a validation state
a position of the shift register at each
digital signal sample - a comparator able to compare a digital value
of each sample to the numerical value of
previous sample in succession
digital signal samples capable of transmitting
a reset register reset signal
when the value of a sample differs from that
from the previous sample by more than a quantity
predetermined; and - a system for resetting the shift register to zero
when at least one bit among a predetermined number
of most significant bits of a sample is non-zero.

 Within the meaning of the invention, it is considered that the coder, or analog-digital converter, delivers a coded, or digital signal, formed of a succession of samples whose value reflects the value of the analog signal applied to the analog to digital converter, given moments. These times are determined by the clock.

 The present invention also relates in particular to the use of the coding unit for the processing of photodetector signals of a gammacamera, in particular of a gamma-camera with photomultipliers.

 Furthermore, the invention relates to a gamma camera comprising a plurality of photodetectors and a signal calculation and processing unit. According to the invention, a coding unit as described above is connected respectively between each photodetector and the signal calculation and processing unit.

 Finally, the invention relates to a method for coding the analog electrical signal of a photodetector capable of emitting pulses. According to the method, the analog signal from the photodetector is converted into a corresponding digital signal and the level of the analog signal is automatically corrected as a function of the digital signal obtained in the absence of a pulse from the photodetector.

 According to a particular implementation of the method, the digital signal is compared to a low threshold and to a high threshold and a correction voltage is added to the analog signal tending to lower the level of the analog signal when the digital signal produced in the absence pulse exceeds the high threshold and tends to increase the level of the analog signal when the digital signal produced in the absence of pulse is below the low threshold.

 Other characteristics and advantages of the invention will emerge from the description which follows, with reference to the figures of the appended drawings, given purely by way of non-limiting illustration.

Brief description of the figures
- Figure 1, already described, is a simplified diagram of a signal processing channel of a photomultiplier of a gamma-camera.

 - Figure 2 is a simplified diagram of a signal processing channel of a photomultiplier including a coding unit according to the present.

invention.

 - Figure 3 is a detailed diagram of a coding unit according to the present invention.

Detailed description of embodiments
The following description refers more precisely to the processing of the signal from a photomultiplier of a gamma camera. It should however be noted that the invention can also be used for processing the signal from a photomultiplier or another type of photodetector in other applications such as measurement applications in the field of nuclear physics, for example. example.

 FIG. 2 shows a channel 100 for processing the signal from the photomultiplier of a gamma-camera. The gamma camera includes a plurality of photomultipliers and a plurality of channels operating according to the same scheme.

 For reasons of simplification, the parts of FIG. 2, identical or similar to parts of FIG. 1, have the same references to which 100 have been added.

 The photomultiplier 110 is connected to an analog-digital converter 112 by means of a current-voltage converter 114 and an operational amplifier 115.

 The current-voltage converter 114 is for example an operational amplifier with total feedback to convert the signal into current produced by the photomultiplier into a signal into voltage.

 The output 118 of the analog-digital converter 112 is connected to a digital adder 120 associated with a digital computing unit 122. The adder and the computing unit are not in themselves part of the coding unit in the sense of the invention, but are part of the gamma camera using the coding unit.

 The digital output 118 is also connected to means 130 for automatic correction of the level of the signal received by the analog-to-digital converter. The output of the correction means 130 is connected to the input of the analog-digital converter 112 via the operational amplifier 115.

 More specifically, the operational amplifier 115 makes it possible to add to the signal coming from the current-voltage converter 114 a correction voltage delivered by the correction means 130.

In the diagram of FIG. 2, the output of the current-voltage converter 114 is connected to an inverting input of the operational amplifier 115 and the output of the correction means 130 is connected to a non-inverting input of the operational amplifier 115.

 The operation of the correction means is described in more detail with reference to FIG. 3.

 As shown in FIG. 3, the input of the correction means 130, connected to the output 118 of the analog-digital converter, comprises a switch 132 capable of directing the most significant bits of the samples coming from the analog-digital converter to a first channel 134 and to direct the least significant bits of the samples coming from the analog-digital converter to a second channel 136.

 The separation between most significant and least significant bits can be established according to criteria adapted to the envisaged application and according to the quality of the analog-digital converter used.

 By way of example for an analog-digital converter able to deliver samples coded on 8 bits, it can be considered that the bits denoted bO, bl, b2, b3 and b4, of lower significance (20, 21,22,23, 24) are the least significant bits and that the bits denoted b5, b6 and b7 (25,26,27) are the most significant bits.

 The correction means 130 also comprise a shift register 140. This register comprises a determined number n of positions which may be in a validation state, for example the logic state 1, or in a non-validation state, for example logic state 0.

 The shift register is part of an absence of pulse detector described below. Within the meaning of the present invention, it is considered that there is absence of pulse when a digital signal corresponding to the continuous background emitted by the photomultiplier is detected in the absence of an event. This signal is referred to as "baseline". The baseline is defined as a succession of n sample values, positive weak, and equal to each other, except for coding noise, that is to say to a bit of least significant weight. The number n of values determining the presence of a baseline is equal to the number n of positions of the shift register 140.

 As an indication n can be chosen in the order of 10, for a sampling frequency of 10 MHz.

 The shift register 140 is used as a counter.

 When the criteria defining the baseline are met, the positions of the shift register are successively put into the validation state, one after the other, respectively in response to a synchronization signal from a clock. On the other hand, the positions of the shift register are reinitialized in a non-validation state when at least one of the criteria defining the baseline is not fulfilled. In FIG. 3, the clock and the synchronization signal are simply identified by a letter H or H. The same clock is also used to clock the operation of the analog-digital converter.

 The first criterion defining the baseline is checked from the first channel 134 which receives the most significant bits.

 The first criterion defining the baseline is that of the succession of samples of low positive values.

 This criterion is satisfied when all the most significant bits, directed in the channel 134 are zero.

 Channel 134 is connected to a reset terminal 142 of the shift register 140 via a NON or NOR gate 144 and an AND (AND) gate 146. Thus, when one of the most significant bits of a sample is in logic state "1", the shift register is reset to put all of its positions in a non-validation state (logic state "O").

 The verification of the second criterion defining the baseline is carried out from channel 136 which receives the least significant bits.

 The second criterion defining the baseline is the fact that the positive values of the samples forming the baseline are equal to each other to the nearest coding noise (+ 1/2 LSB).

 To verify the equality of the values of the successive samples, the correction means 130 have a comparator 150.

 The comparator has a first input 152 to which the channel 136 is directly connected.

The input 152 thus receives the least significant bits of the samples supplied by the analog-to-digital converter.

 Channel 136 is also connected to a second input 154 of the comparator via a delay unit 156. The delay unit makes it possible to delay the signal by a clock time and to keep in memory the value of the previous sample. To this end, a clock signal H is applied to the delay unit 156.

 Thus, the comparator 150 compares, at each clock signal, the digital sample currently present on channel 136 with the sample which was present there at the previous clock time.

 When the digital samples do not differ by more than the value of a least significant bit, then the comparator does not emit a reset signal. Its output 158 is then in logic state "1".

On the other hand, when the value of the samples differs by more than the value of a least significant bit, the output 158 goes to the logic state
O which corresponds to a reset signal. As the output 158 is connected to the shift register 140 via the AND gate 146, the register is then reset.

 As shown in FIG. 3, the signal comprising the least significant bits of channel 136 is taken from input 154 of comparator 150 and is directed to comparison means 160. This signal can also be taken from input 152 of the comparator.

 The comparison means 160 comprise two threshold comparators 162 and 164.

 The signal from channel 136 is applied to a first input 162a, 164a respectively of each threshold comparator.

 The threshold comparators 162, 164 also each have a second input 162b, 164b. A high threshold value and a low threshold value of the signal are permanently applied to these inputs, these values are denoted SH and SB in FIG. 3.

 Comparators with threshold 162 and 164, by comparing the digital signal with the values of high and low thresholds, make it possible to verify that the level of the baseline is sufficiently high to avoid the risk of negative values of the analog signal applied to the input of the analog-to-digital converter, and sufficiently small, however, not to cut off too much of the dynamic range of the analog-to-digital converter.

 As an indication, the level of the analog signal applied to the input of the analog-digital converter is considered to be part of the baseline when it is less than one eighth of the dynamic range of the analog-digital converter. The values of the high threshold and the low threshold can be fixed for example at 7 and 5, for a converter with a dynamic output range of 256 channels (8 bits).

 The outputs of the threshold comparators 162 and 164 bear the references 162c and 164c in FIG. 3. They are respectively applied to the input of a NOR OR (NOR) gate 166 and an AND gate (-AND) 168 It is noted that the output signal of the threshold comparator 164 is inverted at the input of the AND gate 168.

 The outputs of the logic gates 166 and 168 respectively deliver a correction validation signal and a correction direction signal.

 The outputs 166 and 168, as well as an output 148 of the shift register 140, are connected to a correction counter 170.

 The correction counter 170 delivers on its output 172, a digital initial correction value when the system is powered up.

 The output 172 is connected to a digital-analog converter capable of converting the digital signal present at the output 172 of the corrector counter 170 into an analog voltage called correction voltage.

 The output 176 of the analog digital converter is finally connected to the operational amplifier 115 also visible in FIG. 2.

The operational amplifier 115 adds the correction voltage to the voltage signal from the photomultiplier to apply the sum of these voltages to the input 116 of the analog-to-digital converter.

 When a baseline detection signal, that is to say no pulse signal, is supplied at output 148 of the shift register and that, moreover, a correction signal is supplied by logic gate 166 then the correction voltage added to the photomultiplier signal is either too high or insufficient. The presence of the baseline detection signal and the correction signal causes a modification of the correction signal emitted by the counter-corrector 170 on its output 172. The modification of the correction signal also depends on the signal coming from the logic gate 168 which indicates a direction of correction. When the baseline is at too low a level, the correction signal delivered by the counter 170 is modified in a direction tending to increase the voltage applied to the input of the analog-digital converter. Conversely, when the baseline is at too high a level, the correction signal is modified in a direction tending to decrease the voltage applied to the input of the analog-digital converter.

 According to a particular implementation, the counter-corrector 170 can be adjusted so as to increase or decrease the digital value of the correction signal respectively by a unit of lesser weight at each correction step.

 Resistors 180, 182 connected to voltage sources (+ v, -v) allow the gain of the correction to be adjusted.

 The analog correction voltage produced by a modification of the digital correction signal by a value corresponding to the least significant bit is adjusted to a sufficiently low value. It is more precisely chosen so as to cause a correction of the level of the baseline less than half the difference between the high threshold and the low threshold.

Documents cited - (1) US-A-3 011 057
(2) FR-A-2 669 439

Claims (11)

 1. Unit for coding the analog electrical signal of a photodetector (110) capable of emitting pulses, comprising an analog-to-digital converter (112) connected to the photodetector, the analog-digital converter (112) being able to receive the signal electric analog of the photodetector and emitting a coded digital signal, corresponding to the analog signal and comprising a succession of digital samples, characterized in that the coding unit further comprises means (130) for automatic correction of the level of the received signal by the analog-digital converter (112) according to a coded signal emitted by the analog-digital converter in the absence of pulse from the photodetector.  2. Unit according to claim 1, characterized in that the correction means (130) are connected between the photodetector (110) and the analog-digital converter (112) and are connected to a transmission output (118) of the signal digital to analog-to-digital converter (112).  3. Unit according to claim 1 or 2, characterized in that the correction means (130) comprise -a pulse absence detector (140,144,146,150,  156) in the digital signal of the converter  analog-digital; - means for comparing (160) the digital signal to  a first threshold (SB) called the low threshold and a second  threshold (SH) known as high threshold - a correction counter (170) capable of transmitting a  level down correction signal when  the digital signal is above the second threshold  (SH) and to issue a correction signal  level increase when the digital signal  is lower than the first threshold (SB), the signal  correction being issued upon absence detection  impulse; and - means (115,174) for adding to the signal of the  photodetector continuous correction voltage  corresponding to the correction signal.  4. Unit according to claim 3, characterized in that the means for adding a correction voltage to the signal from the photodetector comprise a digital-analog converter (174) for converting the correction signal from the correction counter (170) into an analog voltage correction and an operational amplifier (115) for adding the analog correction voltage to the photodetector signal.  5. Unit according to claim 3 characterized in that the absence of pulse detector comprises - a shift register (140) comprising a number  determined n of positions and capable of emitting a signal  detection of an absence of pulse when all  the positions are in a validation state; - a control clock (H, I) of the converter  analog-digital, also connected to the register  offset (140) to successively put in a  validation status a position of the register of  offset to each sample of the digital signal - a comparator (150) capable of comparing a value  numeric of each sample to numeric value  from the previous sample in the estate  digital signal samples capable of transmitting  a reset register reset signal  when the value of a sample differs from that  from the previous sample by more than a quantity  predetermined; and - a reset system (144,146) to zero of the register to  offset when at least one bit among a number  predetermined high order bits of a sample  is non-zero.  6. Unit according to claim 3, characterized in that the comparison means' comprise - a first threshold comparator (164) for comparing the  value of each sample at the low threshold (SB); and - a second threshold comparator (162) for comparing  the value of each sample at the high threshold (SH).  7. Unit according to claim 3, characterized in that the comparison means comprise a first output for transmitting a correction validation signal to the correction counter and a second output for transmitting a correction direction signal to the correction counter .  8. Use of a coding unit according to any one of claims 1 to 7 for the processing of photomultiplier signals of a gamma-camera.  9. Gamma camera comprising a plurality of photodetectors and a signal calculation and processing unit, characterized in that it further comprises a coding unit in accordance with at least one of claims 1 to 7, connected between each photodetector and signal calculation and processing unit.  10. Method for coding the analog electrical signal of a photodetector capable of emitting pulses, characterized in that - the analog signal of the photodetector is converted  into a corresponding digital signal, and - the signal level is automatically corrected  analog according to the digital signal obtained in  absence of pulse from the photodetector.  11. Coding method according to claim 10, characterized in that - the digital signal is compared to a low threshold and to a  high threshold, and - a voltage of  correction to lower the signal level  analog when the digital signal, produced in  the absence of pulse, exceeds the high threshold and  tending to increase the level of the analog signal  when the digital signal produced in the absence  pulse is below the low threshold.