ADJUSTMENT OF PROPAGATION TIME AND GAIN IN PHOTOMULTIPLIER TUBES

Abstract: A method of providing a photomultiplier tube (PMT) having a desired gain and transit time comprising: a) adjusting the gain of the PMT such that it has a desired gain; and b) adjusting the transit time of the PMT such that it has a desired transit time.

Description: ADJUSTMENT OF PROPAGATION TIME AND GAIN IN PHOTOMULTIPLIER TTTBES FIELD OF THE INVENTION The present invention is concerned with adjustments in the propagation time of Photo- Multiplier Tubes (PMT) and in particular to the adjustment of gain and propagation time of PMTs as used in gamma cameras. BACKGROUND OF THE INVENTION Multi-headed gamma cameras comprise at least two large detectors. Each of the detectors detects events (interactions of gamma rays with a crystal which forms the face of the detector) and determines the position of the events. In many cases the detectors are Anger type gamma detectors in which a plurality of light detectors, such as PMTs view the crystal and determine the position of events based on the amount of light detected by the PMTs. Since the events generate light at a point at which the interaction takes place, the magnitude of the signals detected by the various PMTs can be used to determine the position of the event on the detector. In order to accurately determine the position of the event, the gain of the PMTs must be the same. In the normal course of calibration of PMTs the gain of the PMTs is adjusted by adjusting the voltage on the PMTs or the voltage distribution between the electrodes of the PMTs. This adjustment of the voltage allows for good gain tracking of the PMTs and for accurate determination of the position of events. When such multi headed cameras are used as detectors for Positron Emission Tomography (PET), an additional problem arises. In PET, an event is characterized not only by the detection of an interaction on one of the detectors but also by the simultaneous detection of two interactions on both gamma detectors. If more than two detectors are used, events may be determined by coincident interactions on any two detectors. However, it is well known that the propagation time for signals varies from PMT to PMT. Thus, while the propagation time for PMTs is of the order of 65 nanoseconds, this time can vary by up to about ±10 nanoseconds between PMTs. This spread in propagation times results in coincident events registering at slightly different times on different positions on the same or different detectors. Since each interaction is viewed by different PMTs, this difference is not constant and varies from position to position on the surface of the detectors. In general, a coincidence (time) window is utilized in the determination of coincidence between two detected interactions. Any two interactions are considered to be coincident if they occur within the same time window. In view of the variable propagation time of the detection of the interactions (as well as other variations in the time of flight of the gamma rays whose interaction is measured) a relatively large coincidence window is necessary in order to assure 1 that all truly coincident events are acquired. However, use of such a large window also reduces the discrimination of the system against non-coincident events, such as those caused by unrelated gamma rays and those associated with scattered events. Unfortunately, while it is possible to adjust the propagation time of PMTs and to adjust their gain, no method of independently adjusting the gains and propagation times has been published. Methods which have been used to reduce the size of the coincidence window include sorting PMTs by characteristics such that the PMTs used in a particular set of detectors have the same characteristics. However, in addition to being inexact (since there is still a sizable spread between PMTs), this solution does not allow for changes in gain and propagation time with aging of the PMTs and for the easy replacement of PMTs which fail. Another possible way to reduce the spread between signals generated by PMTs is to provide a variable delay line for each PMT. This solution allows for the independent variation of gain (by changing the voltage on the PMT) and propagation time (utilizing the delay line). However, this solution is relatively expensive. SUMMARY OF THE INVENTION The present invention is concerned, in one aspect thereof, with the independent adjustment of gain and propagation time of individual PMTs. A second aspect of the invention is concerned with the matching of the gain and propagation time of a plurality of PMTs. A third aspect of the invention is concerned with the production and calibration of Anger cameras in which the PMTs have matching propagation times and gains. Since the outputs of the PMTs have the same timing, resulting sum and weighted sum pulses will be shorter and more consistent with respect to their amplitude and timing. In accordance with this aspect of the invention, unintegrated (or less integrated) signals may be used for determining whether events meet an energy criteria. In general, in the prior art, signals must be at least partially integrated, at least in part, before any energy determination is made. A fourth aspect of the invention is concerned with coincidence cameras with reduced coincidence (time) window width. As indicated above, the gain and propagation time of PMTs do not track. Thus, if the gains of PMTs in an Anger type detector are matched for gain (as they must be in order to provide accurate position information for detected interactions) the propagation times of the PMTs will be sufficiently different such that the coincidence window must be increased to 15 to 20 nanoseconds. In accordance with preferred embodiments of the invention the coincidence window may be reduced to as little as 5-10 nanoseconds. 2 The present invention is based on the understanding mat, while both the gain and propagation delay of a PMT depend on the voltage applied to the PMT and to the individual electrodes of the PMT, the dependence on voltage is different for the two characteristics. In a multi-dynode PMT having n dynodes, the gain of the PMT is proportional to [Nι*V2*—*V n ]0-7 5 where V ) is the voltage difference between the first dynode and the cathode of the tube and the other indexed Vs are the voltage between the numbered dynode and the previous dynode, i.e., the gain is not a function of the grid voltage, but only a function of the voltage on the individual dynodes. It is thus clear that the gain of a PMT is dependent on the overall voltage applied to the PMT and the distribution of the voltages among the electrodes on the PMT. The propagation time of the PMT is substantially equal to the sum of the flight times of the electrons between the electrodes. This is made up of the time of flight between the cathode and the fist dynode, the time of flight between the individual dynodes and the time of flight from the last dynode to the anode. In general, the time of flight between the dynodes is dependent on the distance between the dynodes (a constant for a particular PMT) and the voltage between the dynodes. The distance between the cathode and the first dynode is made up of two portions, namely, the portion between the cathode and the grid and the portion between the grid and the first dynode. The effect of the grid on the transit time can be understood simply in the following manner. For the first portion of the flight, the time is dependent on the voltage between the grid and the cathode. Since this distance is relatively large, this voltage has a substantial effect on the flight time. Between the grid and the first dynode, the velocity of the electrons further increases due to the voltage difference between the grid and the dynode. However, since this distance is relatively small and the electrons already have a substantial velocity the effect of variations in this voltage drop are much smaller than the effect of the photo-cathode to grid voltage. In general, however, the time of flight is decreased as the grid to cathode voltage is increased, with the cathode-first dynode remaining constant. However, since the photocathode is current limited, the change in grid to cathode voltage does not have a great effect on the gain of the PMT. While equations for the gain and time of flight can be formulated and solved, at least numerically, this is seldom necessary. In practice, the gain and propagation time can be sufficiently varied by changing the cathode (overall) voltage and the voltage on the grid or, less preferably, one or more of the early dynodes. Such variation is preferably performed empirically, by measuring the gain and propagation delay for the PMTs and adjusting the 3 aforesaid voltages such that all of the PMTs have the same gain and the same propagation delay. Alternatively, the gain and delay are adjusted for each tube such that the tube has a standard gain and transit time. In particular it is possible to adjust the overall gain by changing a single voltage, namely the overall voltage (which also changes, proportionally, the voltage on each dynode) to adjust d e gain and then change the grid voltage to adjust the propagation time. In a preferred embodiment of the invention, a source of gamma rays is placed adjacent to a detector crystal in front of each of a plurality of PMTs. More preferably, a pulsed light source such as a LED is used to illuminate the PMT directly with light to which it is sensitive. The overall voltage (photocathode to anode) and then the cathode grid voltage of each PMT are adjusted such that the signals produced by the PMTs are of the same amplitude and are coincident. After all of the PMTs in a detector have been adjusted such that their gains and propagation times coincide, the PMTs are mounted in a detector, and the procedure is repeated for the other detector or detectors, preferably utilizing the same standard gain and propagation time for the PMTs in the two detectors. After both detectors have been made self consistent as to gain and propagation time, a positron annihilation source is placed between the two detectors, preferably, at a point equidistant from the two detectors and at the center of the detectors. Ideally, truly coincident events will result in signals generated in the two detectors having slightly different timing (on the average) and somewhat different amplitude. In one preferred embodiment of the invention, coincidence between two interactions is defined based on this timing (or on a difference from some fixed timing). Any differences in amplitude are corrected for in the settings of the electronics associated with the two detectors. It should be understood that the above procedure makes it especially simple to change PMTs when they become defective with age. Since all the PMTs are set at the same standard (or at most to a limited number of standards), only minor adjustments, if any, will be necessary to match the gain and propagation time of the new PMT to the old PMTs. This can be done by placing a source mid way between the new PMT and an adjacent PMT and adjusting the new PMT until the amplitude and propagation time of the new PMT matches that of the old one. It should be understood that an additional benefit of the calibration process of the invention is that when the outputs of the PMTs are summed for energy and position determination, they are coincident such that the shape (and thus the maximum value) of the sum is more uniform from pulse to pulse. This advantage is also available if the gains and propagation times of the PMTs of a single Anger detector are calibrated and aligned as in the above procedure. In a further preferred embodiment of the invention, the gain is varied by varying the overall voltage of the PMT without changing the propagation time of the PMT, per se. In order to compensate for variations in propagation time, a delay line is inserted after at least some of the PMTs to compensate for the variations in propagation times. These delay lines may be fixed delay lines, where each delay line is chosen from delay lines having different delays to best match the delay required. Alternatively or additionally, they may be tapped delay lines where the tap is chosen to provide the required delay. Alternatively or additionally a continuously variable or electronically tunable delay lines may be used and the delay may then be varied in a similar manner to the variation in delay with grid voltage described above. When more than one delay line is used for each PMT, gross propagation time correction is achieved using a fixed delay line or a tapped delay line and fine propagation time correction is achieved using either a tapped, continuously tunable or electronically tunable delay line. Alternatively a tapped delay line may be used for gross corrections and a continuously tunable delay line used for fine correction. The use of more than one delay line may result in lower cost and/or finer tuning capability than using a single delay line. There is thus provided, in accordance with a preferred embodiment of the invention, a method of providing a photomultiplier tube (PMT) having a desired gain and transit time comprising: (a) adjusting the gain of the PMT such that it has a desired gain; and (b) adjusting the transit time of the PMT such that it has a desired transit time. Preferably, adjusting the gain comprises adjusting the overall voltage on the PMT such that the tube has the desired gain. Preferably, adjusting the transit time comprises adjusting the voltage on a grid or an early dynode of the PMT such that the PMT has the desired transit time. Preferably, the voltage of the grid or early dynode is adjusted without substantially varying the voltage on the other electrodes. Alternatively, adjusting the voltage on the grid changes the overall gain of the PMT and including repeating (a) and (b) at least once. There is further provided, a method of producing a gamma detector comprising providing a scintillator crystal; providing a plurality of PMTs which view a surface of the crystal; and 5 adjusting the gain and transit time of the PMTs so that they are substantially identical for the plurality of PMTs. Preferably, the gains and transit times are adjusted according to the method of the invention. There is further provided, in accordance with a preferred embodiment of the invention, a multi-detector PET camera comprising: a plurality of Anger gamma camera detectors each of which produces a timing signal when an event is detected, the gamma camera detectors having been produced in accordance with the method of the invention; and a coincidence circuit which determines the presence of a valid event based on a time difference between the timing signals produced by the gamma cameras. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly and fully understood from the following detailed description of preferred embodiments thereof, read in conjunction with the following drawings in which: Fig. 1 is a schematic diagram of a single PMT; Figs. 2A-2D shows a number of configurations suitable for independently adjusting the gain and propagation time of the PMT, in accordance with preferred embodiments of the invention; Fig. 2E is a schematic circuit drawing of circuitry for providing electronically variable gain to a PMT; Fig. 3 is a diagram of a portion of an Anger detector, in accordance with a preferred embodiment of the invention; Fig. 4 is a block diagram of a coincidence Anger Camera utilizing two of the Anger detectors of Fig. 3; Fig. 5 is a block diagram of a coincidence unit in accordance with a preferred embodiment of the invention; Figs. 6A and 6B illustrate a method of calibrating and adjusting PMTs in accordance with a preferred embodiment of the invention; and Fig. 7 schematically shows a system utilizing external delay lines for compensating for variations in propagation time of PMTs. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 very schematically illustrates a PMT 10. PMT 10 comprises a cathode 12, a grid 14 and a series of dynodes 16 to 23. PMT 10 further includes a final collector electrode (anode) 6 26 at which a signal is produced. It should be understood that Fig. 1 is a schematic representation and that the actual construction of the PMT may vary among the many configurations which are available. In operation, each of the cathode 12, grid 14 and dynodes 16 to 23 are supplied with a voltage, with the cathode having a lowest (most negative) voltage and each successive electrode having a more positive voltage. Preferably, cathode 12 is a photocathode, i.e., it emits electrons in response to light which impinges on it, the number of electrons being generally proportional, for a give wavelength of light, to the total flux of the impinging light. Assuming that light of a given intensity impinges on cathode 12, a number of electrons, having substantially zero momentum, is generated at the cathode. Under the influence of the field generated by the voltage difference between cathode 12 and grid 14, the electrons are accelerated toward the grid and then past the grid to the first dynode 16. In general, to preserve proportionality between the light and the number of electrons which pass the grid, the cathode and grid voltages are chosen such that, for the light impinging on the photocathode, all of the electrons are swept away from the cathode and no space charge develops. Since the light intensity is very low, this is not a substantive limitation. The electrons, which have been accelerated by the field between the cathode and the grid and further by the field between the grid and the first dynode, impinge on the first dynode 16, whose surface is formed with a material having a high secondary emission ratio. In general, the ratio of the number of electrons emitted by the dynode and the number of electrons impinging on the dynode is proportional to the voltage through which the electrons have fallen (the cathode-first dynode voltage) to the 0.7 power. Thus, the ratio of electrons for the first dynode, i.e., its electronic gain, is (Vι-VjN^-7. Similarly the gain for the other dynodes is (V j - Vj_ι)0- '. Thus, the overall gain for the PMT is proportional to: \0.7 **πfa -r(ι-i) where i is the number of the dynode and the cathode voltage is defined as VQ. This gain is not a function of the grid voltage, but only a function of the voltage on the individual dynodes. The propagation time of the PMT is substantially equal to the sum of the flight times of the electrons between the electrodes. This is made up of the time of flight between the cathode and the fist dynode and the time of flight between the individual dynodes. In general, the time of flight between the dynodes is dependent on the distance between the dynodes (a constant for a particular PMT) and the voltage between the dynodes. In general, the time of flight between two successive dynodes i-1 and i can be expressed as: tj=ki*(Vj) " 0-5 ; where k j is a constant dependent on the distance between the two dynodes i and i-1 and Vj is the voltage between the dynode i and dynode i-1. The distance between the cathode and the first dynode is made up of two portions, namely, the portion between the cathode and the grid and the portion between the grid and the first dynode. The effect of the grid on the transit time can be understood simply in the following manner. For the first portion of the flight, the time is dependent on the voltage between the grid and the cathode, with the time decreasing with the voltage to the 0.5 power. Between the grid and the first dynode the velocity of the electrons further increases due to the voltage difference between the grid and the dynode. In general, this leads to a complex formulation for the time of flight which is reduced as the grid voltage increases. Note that while the grid voltage effects the time of flight it does not effect the gain of the PMT, unless the voltage on the first dynode 16 is also changed. In sum, the transit time can be expressed as f(Vi, Vg) where the VJS are the voltages on the dynodes with respect to the cathode (for the first dynode) or the previous dynodes (for the other dynodes) and Vg is the voltage difference between the grid and the cathode. While these equations can be solved, at least numerically, in preferred embodiments of the invention, this is not necessary. In practice, the gain and propagation time can be varied sufficiently by changing the cathode (overall) voltage and the voltage on the grid (or one of the early dynodes) respectively. Such variation is preferably performed empirically, by measuring the gain and propagation delay for the PMTs and adjusting the aforesaid voltages such that all of the PMTs have the same gain and the same propagation delay. In particular it is possible to adjust the overall gain by changing a single voltage, namely the overall voltage (which also changes, proportionally, the voltage on each dynode) to adjust the gain and then changing the grid voltage to adjust the propagation time. In general, the electrode voltages are supplied to the individual dynodes utilizing a series of voltage dropping resistors. Fig. 2A shows one configuration of resistors, variable resistors and potentiometers used to supply voltages to the dynodes, for independent adjustment of the gain and propagation time of the PMT. A high voltage power supply 30 supplies a high voltage to a potentiometer 40 in series with a fixed resistors 32 to 38 and an electronic gain control circuit 42, which is shown schematically on Fig. 2A. Details of the construction of such a gain control is described below. Preferably, the variable high voltage is supplied via a variable resistor 44 from a high voltage buss which supplies all of the PMTs in the gamma camera. In a preferred embodiment of the invention, grid 14 is connected to the wiper of potentiometer 40. One side of potentiometer 40 is connected to the high voltage (and to the cathode) and the other end is connected to the first dynode. The other dynodes are connected serially to the junctions of fixed resistors 32-38. Typically, anode 26 which collects the charge generated by the last dynode 23, is at virtual ground at the input of a current to voltage converter 46. In practice, all of the electronics shown in Fig. 2A is preferably placed on a PC board which is attached to the PMT. After adjustment of potentiometer 40 and variable resistor 44 to set the gain and transit time of the PMT, as described below, the PMT is ready for installation in a gamma camera. In order to calibrate the PMT and its associated circuitry, electronic gain 42 is set to the middle of its range and high voltage supply 30 is set to the standard high voltage to be used in the system. Cathode 12 is illuminated with a short burst of light having a standard intensity, from a LED or the like. A pulse length of 1 nanosecond or the like is appropriate. The amplitude of the output signal is adjusted by varying the resistance of variable resistance 44 until the output signal is in accordance with the standard output to which all the PMTs are to conform. The time delay between the burst of light and the appearance of a signal at an output 48 of converter 46 is measured and adjusted by varying the grid 14 voltage (by adjusting the position of the wiper of potentiometer 40) such that the time delay conforms to the desired standard time delay. While the adjustment of potentiometer 44 should have little or no effect on the gain of the PMT, additional iterations may be performed, if necessary. Fig. 2B shows an alternative method of adjusting the gain and time delay of a PMT. In this method fixed resistors 51 and 52 replace potentiometer 40 of Fig. 2A. The gain is adjusted by varying the resistance of resistor 44 as in Fig. 2A. The time delay is varied by varying the resistance of variable resistance 50. In general, resistance 50 is made substantially smaller than the sum of the resistances of resistors 51, 52 and 33 to avoid an untoward effect on the gain of the tube when the time delay is adjusted. However, with this method there is an interaction between the gain and the time delay and several iterations of adjustment may be required to achieve both the desired gain and time delay. • Fig. 2C shows a configuration of resistors in which the overall gain is less sensitive to the transmission time adjustment. In this configuration the cathode-grid resistance is a variable resistor 53 and a fixed resistor 55 connects grid 14 to first dynode 16. A resistor 54, having a resistance that is much smaller than the sum of the resistances of resistors 53 and 55 connects cathode 12 and first dynode 16. The presence of resistor 54 fixes, to great measure, the voltage 9 at the first dynode (and hence the overall gain) irrespective of the value of variable resistance 53 which is used to set the time delay. Fig. 2D shows yet another way of substantially independently controlling the gain and transit time of the PMT, in accordance with a preferred embodiment of the invention. In this embodiment of the invention, the transit time is substantially controlled by varying the relative voltage on the grid and the gain is varied by varying the overall voltage on the tube, by setting the resistance 39 and varying the resistance of variable resistor 41. It has been found that reasonable variations in resistor 41 do not have a pronounced effect on the transit time of the PMT but do give enough variation in the gain of the tube for the purposes of the present invention. While the configurations shown in Figs. 2A-2D are preferred, it will be clear to a person of skill in the art that many different combinations of variable resistors, potentiometers and methods of varying the various voltages may be employed in calibrating and standardizing PMTs in accordance with the invention. Furthermore, while it is usually more convenient to provide variable resistors an/or potentiometers as shown in Figs. 2A-2D, the same effect may be achieved by replacing the variable elements by fixed resistors after the values are determined or by determining the resistance values by replacement and trial and error. This method is not usually as convenient as the other methods and are often less precise. In addition while in some methods of adjustment, in accordance with a preferred embodiment of the invention, a gridded PMT is required, other methods may employ non-gridded tubes. Fig. 2E shows a circuit 56 useful for achieving variable electronic gain in a PMT, in accordance with a preferred embodiment of the invention. In operation the gain is varied by supplying a varying voltage at terminal 58. While this circuit varies the gain by adjusting the effective resistance between dynodes 21-23, its effect on the overall time delay is small, at least in part because the proportion of the time delay due to electron transit between these dynodes is relatively small. However, as discussed below, this electronic adjustment allows for the adjustment of gain in situ as tubes age with little degradation of the coincidence of the PMTs in a detector. Fig. 3 shows an Anger camera head 70 useful in a coincidence detection type gamma camera. Portion 70 includes a scintillation crystal 72, such as an Nal crystal. Crystal 72 emits a faint flash of light whenever a gamma ray interacts with and is absorbed by it. An array of PMTs 74, preferably arranged in a hexagonal configuration, are situated behind and view the scintillation crystal such that each of the PMTs detects a portion of the light generated by the interaction and produces a signal which is proportional to the brightness of the light which it 10 views. High voltage is supplied to each PMT from a common high voltage source 30 and preferably, each PMT includes circuitry shown in one or more of Figs. 2A-2D and has been adjusted and calibrated as described above. As is conventional in such cameras the outputs of the PMTs are summed (after amplification and optionally partial integration) in a summer 76. The output of summer 76 contains information on the time of occurrence of an interaction of a gamma ray with crystal 72 and on the energy of the gamma ray. This information is used to trigger an event detector 80 which sends a timing signal to Anger electronics 78 and to a coincidence unit 90 as described below. Many variants are known in the art for Anger electronics 78 and the present invention may be adapted for use with all such systems. In general Anger electronics 78 produces, responsive to the output of the PMTs, a position signal and an energy signal as well as other signal as is well known in the art. Fig. 4 is a block diagram of a coincidence camera 88, in accordance with a preferred embodiment of the invention. In particular, camera 88 incorporates a plurality of detectors 70, as for example described in conjunction with Fig. 3, and a coincidence unit 90 which receives event detection trigger signals from event detector 80 shown in Fig. 3. When the triggers are received within a small, generally preset, range of time delays coincidence unit 90 produces a valid coincidence output signal at an output 92 thereof. The valid coincidence output signal controls a pair of switches 94 and 96 which receive information from Anger electronics 78 of the detectors. If the coincidence detector does not determine that coincident events were produced by the detectors, generally no information is transferred from the detectors to the rest of the coincidence camera. If a valid coincidence signal is generated, the information from the detectors is fed to an Anger Processor 98. Such Anger processors are well known in the art and this unit as well as a host computer, display, user interface and storage may be any design and construction as known in the art. Fig. 5 shows a more detailed block diagram of coincident unit 90, in accordance with a preferred embodiment of the invention. Coincidence unit 90 receives trigger signals from two detectors and provides a variable delay to each by delay circuits 100 and 102. The two delayed signals cause variable pulse generators 104 and 106 to generate pulses of different pulse width such that if the triggers arrive at the coincidence unit within a given delay from each other, an AND gate 108 which receives the pulses will receive a signal simultaneously from both of them. For example, if a valid coincidence output is desired whenever the triggers are less than 10 nsec apart, one of the signals is delayed by 10 nsec from the other. The pulse width of the 11 earlier channel is made 10 nsec longer than that for the later channel. Preferably, AND gate 108 triggers a one-shot 110 to generate a valid coincidence output of constant length. However, if the width of the shorter pulse is sufficient for switching switches 94 and 96 for a desired time period, the one-shot can be omitted. Alternatively, the trigger signals may be digitized and a digital coincidence unit may be employed. Figs. 6A and 6B illustrate a method of adjusting the gain and transit time of a PMT assembly, in accordance with a preferred embodiment of the invention. As shown in Fig. 6A the calibration setup comprises a pulse generator 124 ad LED 126 which supply a very short light pulse to PMT 10. Associated with PMT 10 is resistor electronics 121, preferably as shown in Figs. 2A-2E. Electronics 121 is electrified utilizing two voltage sources, high voltage source 30 and a power supply 128 for amplifier 46 (and element 42 when it is present), which is part of electronics 121. Electronics 10 preferably includes two knobs 120 and 122 which are used to vary the resistances, potentiometers or voltages as appropriate in the various embodiments of Figs. 2A-2D. Alternatively, an external driver may be used to drive these variable elements. The light pulse causes PMT 10 and electronics 121 to produce a short pulse which is viewed utilizing a high speed scope 126, which is preferably a digital sampling scope. The trigger for the scope is supplied from the same pulse which drives LED 126. A signal appears on the scope which is similar to that shown in Fig. 6B. Fig. 6B shows two traces. One of these is a reference trace which represents the desired pulse amplitude and timing for the PMT. The other pulse is the actual output of the PMT. The operator varies the positions of knobs 120 and 122 until the two traces match to the extent possible. After assembling the PMTs in the detectors, fine tuning of the gain of the PMTs in the detector may be achieved by providing a coincident light signal to adjacent PMTs and adjusting the gains so that they each provide a signal of the same amplitude. Such a coincident light signal may be provided by irradiating a region between (and equidistant from) the PMTs by a collimated source of radiation which interacts with crystal 72. Light generated in the crystal by the interaction should result in equal signals being generated by the adjacent PMTs. Of course the signals should also be coincident. The electronic fine gain control is then adjusted to provide the required equal gain. This adjustment may be performed automatically by the gamma camera. Additionally or alternatively, any overall differences in time delay of the two detectors and the time delay of cables, electronics and other components may be compensated for by adjusting the delay of delay circuit 100. 12 While the invention has been described with respect to manual adjustment of the gain and transit time of the PMTs, the potentiometers and variable resistors utilized in the preferred embodiments of the invention may be of a type which can be controlled externally. If such devices are used, the adjustments may be performed automatically. In a further preferred embodiment of the invention, the gain is varied by varying the overall voltage of the PMT without changing the propagation time of the PMT, per se. In order to compensate for variations in propagation time, a delay line is inserted after at least some of the PMTs to compensate for the variations in propagation times. These delay lines may be fixed delay lines, where each delay line is chosen from delay lines having different delays to best match the delay required. Alternatively or additionally, they may be tapped delay lines where the tap is chosen to provide the required delay. Fig. 7 shows three tapped delay lines 130, 132, and 134 used to compensate for the variations in propagation time of three PMTs 136, 138 and 140. Preferably, the outputs of the PMTs are amplified by one of three amplifiers 142 that are preferably charge to voltage converters. Optionally the gain of the amplifiers may be made variable to provide at least a portion (in particular an electronically controllable portion) of the compensation for the gain variations between the PMTs. Alternatively, the amplifiers are simple charge converters and the gain of the PMTs is equalized by the methods described above. Alternatively or additionally a continuously variable or electronically tunable delay lines may be used and the delay may then be varied in a similar manner to the variation in delay with grid voltage described above. When more than one delay line is used for each PMT, gross propagation time correction is achieved using a fixed delay line or a tapped delay line and fine propagation time correction is achieved using either a tapped, continuously tunable or electronically tunable delay line. Alternatively a tapped delay line may be used for gross corrections and a continuously tunable delay line used for fine correction. The use of more than one delay line may result in lower cost and/or finer tuning capability than using a single delay line. The present invention has been described by reference to a number of preferred embodiments thereof, having various features. It should be clear that these features may be combined in various ways and that some of the features may be dispensed with in some preferred embodiments of the invention as defined by the claims. 13

Claims: CLAIMS 1. A method of providing a photomultiplier tube (PMT) having a desired gain and transit time comprising: (a) adjusting the gain of the PMT such that it has a desired gain; and (b) adjusting the transit time of the PMT such that it has a desired transit time. 2. A method according to claim 1 wherein adjusting the gain comprises: adjusting the overall voltage on the PMT such that the tube has the desired gain. 3. A method according to claim 1 or claim 2 wherein adjusting the transit time comprises: adjusting the voltage on a grid or an early dynode of the PMT such that the PMT has the desired transit time. 4. A method according to claim 3 wherein the voltage of the grid or early dynode is adjusted without substantially varying the voltage on the other electrodes. 5. A method according to claim 3 wherein adjusting the voltage on the grid changes the overall gain of the PMT and including the step of: repeating steps (a) and (b) at least once. 6. A method of providing a photomultiplier tube system (PMT) having a desired gain and transit time comprising: (a) adjusting the gain of the PMT system such that it has a desired gain; and (b) adjusting the transit time of the PMT system such that it has a desired transit time. 7. A method according to claim 6 wherein the PMT system comprises a PMT and wherein adjusting the gain comprises: adjusting the overall voltage on the PMT such that the tube has the desired gain. 8. A method according to claim 7 wherein the PMT system comprises an amplifier that amplifies an output of the PMT and including: adjusting the overall gain of the amplifier such that the PMT system has the desired gain. 14 9. A method according to claim 6 wherein the PMT system includes a PMT and wherein adjusting the transit time comprises: delaying an output of the PMT such that the PMT has the desired transit time. 10. A method according to any of claims 1-4 or 7-8 and comprising: delaying an output of a PMT such that the PMT has the desired transit time. 11. A method according to claim 9 or 10 wherein delaying includes delaying said output with a delay line. 12. A method of producing a gamma detector comprising: providing a scintillator crystal; providing a plurality of PMTs which view a surface of the crystal; and adjusting the gain and transit time of the PMTs so that they are substantially identical for the plurality of PMTs. 13. A method according to claim 6 wherein the gains and transit times have been adjusted according to the methods of any of claims 1-11. 14. A photomultiplier tube (PMT) system comprising: a plurality of photomultiplier tubes; and a plurality of delay lines which delay the outputs of individual ones of the PMTs such that the delays are substantially the same. 15. A PMT system according to claim 14 wherein the gain of the photomultiplier tubes is substantially the same. 16. A PMT system according to claim 15 and including means for varying a voltage on the PMT to produce said substantially equal gains. 17. A method of producing a gamma detector comprising: providing a scintillator crystal; and 15 providing a PMT system according to any of claims 14-16 that views a surface of the crystal. 18. A multi-detector PET camera comprising: a plurality of Anger gamma camera detectors each of which produces a timing signal when an event is detected, the gamma camera detectors having been produced in accordance with the method of claim 12, 13; or 17 and a coincidence circuit which determines the presence of a valid event based on a time difference between the timing signals produced by the gamma cameras. 16

Inventors: HEFETZ YARON, HEFETZ, YARON
Applicants: ELGEMS LTD, HEFETZ YARON, ELGEMS LTD., HEFETZ, YARON
Application year: 1998
Main classification: H01J43
IPC codes: G01T1, H01J43