WO1998017002A1 - Unit and method for encoding photodetector signal, with correction of input level, and use of such a unit for a gamma-camera - Google Patents
Unit and method for encoding photodetector signal, with correction of input level, and use of such a unit for a gamma-camera Download PDFInfo
- Publication number
- WO1998017002A1 WO1998017002A1 PCT/FR1997/001809 FR9701809W WO9817002A1 WO 1998017002 A1 WO1998017002 A1 WO 1998017002A1 FR 9701809 W FR9701809 W FR 9701809W WO 9817002 A1 WO9817002 A1 WO 9817002A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- signal
- analog
- correction
- digital
- photodetector
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/06—Continuously compensating for, or preventing, undesired influence of physical parameters
- H03M1/0602—Continuously compensating for, or preventing, undesired influence of physical parameters of deviations from the desired transfer characteristic
- H03M1/0604—Continuously compensating for, or preventing, undesired influence of physical parameters of deviations from the desired transfer characteristic at one point, i.e. by adjusting a single reference value, e.g. bias or gain error
- H03M1/0607—Offset or drift compensation
-
- 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
- G01T1/161—Applications in the field of nuclear medicine, e.g. in vivo counting
- G01T1/164—Scintigraphy
- G01T1/1641—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras
- G01T1/1642—Static instruments for imaging the distribution of radioactivity in one or two dimensions using one or several scintillating elements; Radio-isotope cameras using a scintillation crystal and position sensing photodetector arrays, e.g. ANGER cameras
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
- H03M1/00—Analogue/digital conversion; Digital/analogue conversion
- H03M1/12—Analogue/digital converters
Definitions
- 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.
- 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 ⁇ or x-ray detectors using other types of photodetectors such as photodiodes or semiconductor detectors, for example.
- 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.
- 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.
- 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.
- a calculation unit capable of calculating, for example, the position of each event detected on the crystal.
- the analog-to-digital converter converts the analog signal from the photomultiplier into a coded digital signal, which is directed to the computing unit.
- 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 is connected to an analog-digital converter 12 via a current-voltage converter 14.
- the signal emitted by the photomultiplier is, in fact, a current signal which should be convert it 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 designed to integrate the digital signal formed by the analog-to-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.
- each photomultiplier 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.
- the gain of photomultipliers is also very variable from one copy to another.
- the amplitude of the continuous background of the signal may be modified over time, in particular by the play of statistical fluctuations.
- the signal present on the input terminals of the various analog-digital converters has very variable levels from one channel to another.
- the analog-digital converter is effectively characterized by its dynamic range which can be understood as the difference in voltage 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.
- a solution consists a priori of providing between each photomultiplier and the analog-digital converter which is associated with it, an amplifier with adjustable gain.
- 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.
- this level at the input of the analog-digital converter is initially adjusted to a low value to preserve a large dynamic range, the signal applied to the analog-digital converter risks taking values of polarity opposite to that of the impulse (negative if the impulse is positive) if the level of the continuous background decreases over time.
- This phenomenon is an inaccurate consideration of the analog signal.
- 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-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.
- 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.
- the coding unit further comprises means for automatically correcting the level of the photodetector signal received by the analog-digital converter, as a function of the coded signal transmitted by the analog-digital converter in the absence of pulse from the photodetector.
- photodetector is understood to mean both a photomultiplier than a photodiode or a semiconductor detector, of the CdTe type for example.
- 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.
- 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.
- 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.
- the correction means can include:
- a correction counter capable of transmitting a level reduction correction signal when the digital signal is greater than the second threshold and of transmitting a level increase correction signal when the digital signal is less than the first threshold, the signal correction being emitted upon detection of absence of pulse;
- 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.
- the correction made takes into account the fact that the level of the continuous background emitted by the photodetector, and converted into a signal coded by the analog-digital converter, is higher or lower than the high thresholds. and low.
- 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 analog-digital converter is then compensated by adding a correction voltage tending to reduce its amplitude (level).
- 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.
- the analog signal applied to the input of the analog-digital converter risks taking 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.
- 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.
- 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.
- the means for adding to the photodetector signal a voltage of correction may include a digital-to-analog converter to convert the correction signal from the correction counter to an analog correction voltage, and an operational amplifier to add the analog correction voltage to the photodetector signal.
- the absence of pulse detector can include:
- - a shift register comprising a determined number n of positions and capable of emitting a signal for detecting an absence of pulse when all the positions are in a validation state; - a control clock of the analog-digital converter, also connected to the shift register to successively put in a validation state a position of the shift register at each sample of the digital signal; a comparator able to compare a digital value of each sample with the digital value of the previous sample in the succession of samples of the digital signal and able to send a reset register reset signal when the value of a sample differs that of the previous sample by more than a predetermined quantity; and
- the encoder or analog-digital converter, delivers a coded or digital signal formed by a succession of samples whose value reflects the value of the analog signal applied to the analog-digital converter, at given times. 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 gamma-camera, in particular of a gamma-camera with photomultipliers.
- the invention relates to a gamma camera comprising a plurality of photodetectors and a signal calculation and processing unit.
- a coding unit as described above is connected respectively between each photodetector and the signal calculation and processing unit.
- the invention relates to a method for coding the analog electrical signal of a photodetector capable of emitting pulses.
- 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.
- 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 a pulse is below the low threshold.
- FIG. 1 is a simplified diagram of a signal processing channel of a photomultiplier of a gamma-camera.
- FIG. 2 is a simplified diagram of a signal processing channel of a photomultiplier including a coding unit according to the present invention.
- FIG. 3 is a detailed diagram of a coding unit according to the present invention.
- 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.
- the photomultiplier 110 is connected to an analog-digital converter 112 via 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-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.
- 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.
- the output of the current-voltage converter 114 is connected to one 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, as shown in FIG.
- 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.
- the bits denoted bO, bl, b2, b3 and b4, of lower significance (2 °, 2 1 , 2 2 , 2 3 , 2 4 ) are the least significant bits and that the bits denoted b5, b6 and b7 (2 5 , 2 6 , 2 7 ) 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 can 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.
- 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.
- 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.
- 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.
- 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.
- 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 except for coding noise ( ⁇ + _ 1/2 LSB).
- 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-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 one 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.
- 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 logic state 0 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.
- 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. On these inputs are permanently applied a high threshold value and a low threshold value of the signal, these values are denoted SH and SB in FIG. 3. Threshold comparators 162 and 164, by comparing the digital signal with the values of the thresholds high and low, make it possible to check that the baseline level is high enough to avoid the risk of negative values of the analog signal applied to the input of the analog-digital converter, and low enough however not to amputate too much significant range of the analog-to-digital converter.
- 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 logic gate 166 and an AND gate
- 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 digital-analog 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 at input 116 of the analog-digital converter.
- a baseline detection signal that is to say no pulse signal
- a correction signal is supplied by logic gate 166
- 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.
- the baseline is at a level too low, 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.
- 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.
- 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.
Abstract
Description
Claims
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP51805998A JP2001505729A (en) | 1996-10-14 | 1997-10-10 | Apparatus and process for encoding a photodetector signal with input level correction and use of this apparatus for a gamma camera |
EP97909383A EP0931381A1 (en) | 1996-10-14 | 1997-10-10 | Unit and method for encoding photodetector signal, with correction of input level, and use of such a unit for a gamma-camera |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR96/12490 | 1996-10-14 | ||
FR9612490A FR2754656B1 (en) | 1996-10-14 | 1996-10-14 | PHOTODETECTOR SIGNAL CODING UNIT AND METHOD, WITH INPUT LEVEL CORRECTION, AND USE OF SUCH A UNIT FOR A GAMMA-CAMERA |
Publications (1)
Publication Number | Publication Date |
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WO1998017002A1 true WO1998017002A1 (en) | 1998-04-23 |
Family
ID=9496620
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/FR1997/001809 WO1998017002A1 (en) | 1996-10-14 | 1997-10-10 | Unit and method for encoding photodetector signal, with correction of input level, and use of such a unit for a gamma-camera |
Country Status (4)
Country | Link |
---|---|
EP (1) | EP0931381A1 (en) |
JP (1) | JP2001505729A (en) |
FR (1) | FR2754656B1 (en) |
WO (1) | WO1998017002A1 (en) |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN110518909A (en) * | 2019-08-15 | 2019-11-29 | 中国科学院新疆天文台 | A kind of calibration method of multicore analog-digital converter |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2592257A1 (en) * | 1985-12-19 | 1987-06-26 | Sgs Microelettronica Spa | METHOD AND APPARATUS FOR COMBINATION PELLET FOR IMPULSE CODE MODULATION HAVING AN IMPROVED AUTOMATIC RESET CIRCUIT |
FR2669439A1 (en) * | 1990-11-21 | 1992-05-22 | Commissariat Energie Atomique | NUCLEAR DETECTION METHOD WITH BASIC POTENTIAL CORRECTION AND APPARATUS (IN PARTICULAR GAMMA-CAMERA) CORRESPONDING. |
-
1996
- 1996-10-14 FR FR9612490A patent/FR2754656B1/en not_active Expired - Fee Related
-
1997
- 1997-10-10 WO PCT/FR1997/001809 patent/WO1998017002A1/en not_active Application Discontinuation
- 1997-10-10 JP JP51805998A patent/JP2001505729A/en active Pending
- 1997-10-10 EP EP97909383A patent/EP0931381A1/en not_active Withdrawn
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
FR2592257A1 (en) * | 1985-12-19 | 1987-06-26 | Sgs Microelettronica Spa | METHOD AND APPARATUS FOR COMBINATION PELLET FOR IMPULSE CODE MODULATION HAVING AN IMPROVED AUTOMATIC RESET CIRCUIT |
FR2669439A1 (en) * | 1990-11-21 | 1992-05-22 | Commissariat Energie Atomique | NUCLEAR DETECTION METHOD WITH BASIC POTENTIAL CORRECTION AND APPARATUS (IN PARTICULAR GAMMA-CAMERA) CORRESPONDING. |
Cited By (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
CN110518909A (en) * | 2019-08-15 | 2019-11-29 | 中国科学院新疆天文台 | A kind of calibration method of multicore analog-digital converter |
Also Published As
Publication number | Publication date |
---|---|
FR2754656A1 (en) | 1998-04-17 |
EP0931381A1 (en) | 1999-07-28 |
JP2001505729A (en) | 2001-04-24 |
FR2754656B1 (en) | 1998-12-18 |
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