WO1993010472A1 - Detecting gamma rays - Google Patents

Detecting gamma rays Download PDF

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Publication number
WO1993010472A1
WO1993010472A1 PCT/AU1992/000620 AU9200620W WO9310472A1 WO 1993010472 A1 WO1993010472 A1 WO 1993010472A1 AU 9200620 W AU9200620 W AU 9200620W WO 9310472 A1 WO9310472 A1 WO 9310472A1
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WO
WIPO (PCT)
Prior art keywords
scintillation
rays
crystals
signals
array
Prior art date
Application number
PCT/AU1992/000620
Other languages
French (fr)
Inventor
Geoffrey Norman Taylor
Gary Francis Egan
Original Assignee
The University Of Melbourne
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The University Of Melbourne filed Critical The University Of Melbourne
Publication of WO1993010472A1 publication Critical patent/WO1993010472A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
    • G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
    • G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
    • G01T1/2914—Measurement of spatial distribution of radiation
    • G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)
    • 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
    • G01T1/20—Measuring radiation intensity with scintillation detectors
    • G01T1/2018—Scintillation-photodiode combinations
    • G01T1/20182—Modular detectors, e.g. tiled scintillators or tiled photodiodes
    • 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
    • G01T1/20—Measuring radiation intensity with scintillation detectors
    • G01T1/2018—Scintillation-photodiode combinations
    • G01T1/20184—Detector read-out circuitry, e.g. for clearing of traps, compensating for traps or compensating for direct hits
    • 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
    • G01T1/20—Measuring radiation intensity with scintillation detectors
    • G01T1/2018—Scintillation-photodiode combinations
    • G01T1/20185—Coupling means between the photodiode and the scintillator, e.g. optical couplings using adhesives with wavelength-shifting fibres

Definitions

  • the light emitted from at least one end of a scintillation crystal is detected and measured by the light detection means comprising an array of photodiodes.
  • X-ray or gamma ray detection apparatus comprising an array of elongate scintillation crystals, first light detection and measuring means at one end of each scintillation crystal, second light detection and measuring means at the other end of each scintillation crystal, and comparator means to receive and compare signals from the first and second light detection and measuring means whereby when an X-ray or gamma ray is absorbed by one of the scintillation crystals the signals from the first and second light detection means at both ends of that crystal are compared to determine the scintillation point within the crystal.
  • a preferred method of determining the longitudinal location of the scintillation point within an elongate crystal is to calculate the ratio of the number of photons detected at one end of the crystal to the sum of the number of photons measured by both detection means.
  • the signal output of each detector is proportional to the number of photons detected by that detector and so the ratio of the pulse height from one end to the total pulse heights can be used as a measurement of the required ratio of photons. This method is insensitive to the actual number of photons produced as only ratios are used.
  • the array of scintillation crystals are preferably arranged around the X-ray or gamma ray source in an annular or cylindrical arrangement so that a pair of gamma rays emitted from the source in opposite directions will be absorbed in generally opposed crystals of the array.
  • the light detectors at the inner ends of the crystals it is necessary for the light detectors at the inner ends of the crystals to be of a very thin construction, and a photodiode array satisfies this requirement so that the amount of material in front of the inner ends of the crystals does not distort the imaging capacity of the array to any significant extent.
  • the present invention therefore enables considerably smaller annular or cylindrical arrays to be used, and this substantially reduces: the manufacturing cost and complexity; the space needed to install an imaging camera; and the complexity and cost of maintenance.
  • the scintillation crystals may comprise crystals of any suitable scintillation material, for instance bismuth germanate (BGO).
  • BGO bismuth germanate
  • the light detection means at the ends of the crystals remote from the photodiode array may comprise any convenient form of detector, for instance, a photomultiplier tube, because in an annular arrangement of crystals it is not necessary for the detectors at the other ends of the crystals to be of a thin construction.
  • a method of determining the coincidence of X-rays or gamma rays in opposed or nearly opposed scintillation crystals of X-ray or gamma ray detection apparatus comprising a plurality of blocks of scintillation crystals and associated light detectors disposed around a source of X-rays or gamma rays, wherein the method comprises the steps of: supplying detection signals from each of the plurality of blocks of light detectors to logic gates of coincidence logic circuitry including a programmable memory; the logic gates being arranged to produce output trigger signals when detection signals from at least two detectors are received by a logic gate within a desired time interval; programming the memory to provide identification signals to respective logic gates of the coincidence logic circuitry; and using the identification signals to determine when trigger signals are produced by a coincidence of detection of X-rays or gamma rays absorbed in opposed or nearly opposed crystals.
  • apparatus for detecting X-rays or gamma rays comprising a plurality of blocks of scintillation crystals and associated light detectors disposed in an array around a source of X-rays or gamma rays, and coincidence logic circuitry connected to the light detectors associated with the blocks of crystals, wherein the coincidence logic circuitry includes a plurality of AND gates, each connected to at least two detectors, memory means programmed to provide respective identification signals to each of the AND gates, clock means to provide clock signals to the AND gates, and an OR gate connected to the outputs of the AND gates, wherein each AND gate provides an output signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals, and the OR gate provides a trigger signal in response to the output signals from the AND gates to indicate a coincidence between X-rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals
  • Figure 2 is a side view of part of the scintillator crystal and photodiode array of the apparatus of Figure 1 ;
  • Figure 4 is a graph of the number of photons detected at one end of a scintillation crystal plotted against distance along the crystal;
  • Figure 5 is a graph of the sum of the photons detected at both ends of the crystal plotted against distance along the crystal;
  • Figure 6 is a graph showing the ratio of the number of photons measured at one end to the sum of the number of photons measured at both ends plotted against distance along the crystal
  • Figure 7 is a schematic diagram of an analogue electronic circuit suitable for use with the diode array of the gamma imaging apparatus
  • Figure 8 is a schematic diagram of a crystal and detector arrangement with detectors on four sides of the gamma source under study
  • Figure 9 is a diagram of the coincidence logic circuitry associated with the crystal and detector arrangement of Figure 8.
  • Figure 10 is a schematic timing diagram for the electronic circuit of Figure 7.
  • FIG. 1 of the drawings there is shown a ring 10 of elongate scintillation crystals 12 forming an annular array of crystals surrounding an object 14 under study. It will, however, be appreciated that a plurality of rings of crystals may be provided to form a cylindrical array. If, for instance, positron annihilation takes place at point X in the object 14 two high energy X-rays or gamma rays ⁇ , ⁇ will be propagated in opposite directions from point X along flight path F.
  • the gamma rays ⁇ , ⁇ thus enter respective scintillation crystals 12a, 12b on opposite sides of the annular array 10 by passing through thin light detectors 16 on the inner ends of the crystals 12a and 12b at absorption points Z to generate photons of light which travel longitudinally in opposite directions within the crystals 12a and 12b.
  • Further light detectors 18 are provided on the outer ends of the crystals 12 so that light generated by absorption of the gamma rays in a crystal 12a or 12b is detected at each end of the crystal to enable the longitudinal position of the scintillation point Z within the crystal to be determined. Information on the transverse position of the scintillation points Z is obtained from the opposed or nearly opposed crystals in which scintillation takes place.
  • the region R 1( of possible trajectory of the gamma rays ⁇ , ⁇ is wider than the region R 2 of possible trajectory if information on both the longitudinal and transverse position of the scintillation points Z is determined.
  • Each of the graphs refers to data obtained from an elongate scintillation crystal of bismuth germanate (BGO) having a length of 20mm and a square cross-section of 3x3mm 2 .
  • BGO bismuth germanate
  • a diffuse reflector coating of 98% reflectance is applied to the side surfaces of the crystal and the index of refraction of the detectors is about 1.5.
  • the dip in the middle of Figure 5 is due to losses in reflections from the walls, where the distance from both ends is largest.
  • the number of reflections off the surface of a crystal is large and hence losses become important where the point of interaction Z is at a maximum from the detectors at the ends of the crystals.
  • the method for obtaining information on the longitudinal position of interaction Z is to take the ratio of photons measured at one detector to the sum of the number of photons detected at both ends N A /(N A + N B ) as shown in Figure 6. This determination is insensitive to the number of photons produced because only ratios are used.
  • the signal output from a detector is proportional to the number of photons detected by that detector and so the ratios of pulse heights can be used as a measurement of the ratio of photons plotted in Figure 6.
  • the plot in that graph can then be used to convert the ratio to a measured position along the crystal.
  • each scintillation crystal 12 comprises an array of photodiodes 16 whereas the light detector 18 on the front or outer end of each crystal may comprise a photomultiplier tube (PMT) or a light guide in combination with a PMT.
  • PMT photomultiplier tube
  • Figure 4 a set of four quadrants each containing a sub-array of 4x4 crystals preferably of dimensions 3x3x20 mm 3 although it will be appreciated that different crystal configurations and dimensions of crystals may be used.
  • the diode arrays 16 and four 16-channel multiplexed preamplifier chips 20 are mounted on a substrate 22 (e.g. of machined ceramic).
  • the connections to the diode array active areas is made via metallised tracks on the silicon out to one edge where a wire bond connection 23 is made to the preamplifier chips 20.
  • the control signals, power and signal connections to the preamplifier 20 are made via flexible circuit 24 board which contains ancillary electronics, such a output driver circuits, before a connection to the data acquisition circuitry via a cable connector.
  • the electronics are attached in such a way that modules can be ganged together into the required annular formation for the purpose of PET imaging.
  • the entire module may be contained in a sheet metal case thin enough not to affect the required close packaging of modules into an annulus.
  • This metal case provides support to hold the crystals in their correct orientation and provides the necessary shielding against electromagnetic radiation noise.
  • FIG. 7 to 10 there is shown the associated electronic circuitry for the detectors of the gamma ray imaging system of the invention.
  • the design of the electronic circuitry to trigger the data acquisition system upon a coincidence between two annihilation X-rays or gamma rays interacting in two opposing or nearly opposing scintillation crystals is an important feature in the system of the invention.
  • An aim of the invention is to use a coincidence array capable of being remotely programmed for selection of specific combinations of inputs coincidentally within a predetermined time window, and to achieve this without the encumbrance and inflexibility of "hard wiring" specific combinations.
  • Figures 8 and 9 illustrate schematically how this is achieved in a simplified application with only four blocks of crystals 12 and associated detectors A,B,C and D around the object 14 under study.
  • the coincidence trigger circuitry comprises a respective amplifier 30 associated with each detector A, B, C and D, a matrix of six AND gates 32 a programmable RAM 34, and an OR gate 36.
  • Each amplifier 30 is connected to inputs of three of the AND gates 32 and the RAM 34 has six outputs 38, each of which is connected to an input of a respective AND gate 32.
  • the selection of the specific combinations that will register a trigger is made by loading the RAM, the set bits of which provide one of the inputs to the selected AND gates in the matrix, for a trigger to be flagged.
  • the corresponding element in the RAM must be set, providing one of the inputs to the AND gate with channels A and C as inputs.
  • the fourth input to all gates is a clock signal CK to provide the desired time window for the coincidence.
  • the outputs of all the AND gates 32 are logically summed by OR gate 36 to provide a trigger signal on any coincidence.
  • the identifiers of the elements that provided the trigger are clocked out for use by the data acquisition system.
  • This relatively simple "self-coincidence” idea is a far more flexible and economical logic circuit than any trigger electronics presently used in PET cameras. It is also ideally suited to implementation in very large scale integration (VLSI), electronics. An efficient trigger is desirable for the use of the high resolution gamma detector. The number of channels possibly containing data is high so for fast event rate capability is mandatory.
  • FIG. 7 Further details of the electronics circuitry are shown schematically in Figure 7 wherein similar "rear end" circuits are connected to each array of photodiodes 16.
  • Each circuit (only one of which is illustrated in Figure 7) comprises a pre-ampl ' ifier 42, a shaping amplifier 44, a buffer amplifier 46 and a multiplexer 48, all of which may be incorporated in a single integrated CMOS, low-noise amplifier chip 40, the output of which is connected to a flash analogue-digital converter 50 of data acquisition circuitry, with a clock input 52 from a real time clock 54.
  • the "front end” circuitry connected to the photomultiplier tube comprises amplifier 30 the output of which is connected both to the analogue-digital converter 50 via a sample and hold circuit 60 and, via a discriminator 62, to the coincidence logic circuitry illustrated in more detail in Figure 9.
  • the digital output from the discriminator 62 is put into coincidence with a not BUSY signal. If the BUSY signal is off, a signal from the photomultiplier tube is passed to the coincidence electronics. No signal is passed from the discriminator to the coincidence electronics when the BLOCK BUSY signal for the block is set.
  • a valid coincidence is required to trigger digitization of the photomultiplier tube signals and to commence the multiplex cycle for digitizing the diode array signals of the specific blocks involved in the coincidence.
  • Digitized signals are immediately clocked through a digital pedestal (noise) subtraction and zero suppression circuit into a local buffer memory. This process reduces the number of channels of data to be stored.
  • a readout request for the two blocks involved in the coincidence is posted to the data acquisition circuitry.
  • the queued requests to read the local buffer memories corresponding to the blocks in the trigger are handled in turn by the data acquisition circuitry.
  • the BLOCK BUSY signal is issued to the blocks in the coincidence and held in that state until after the data acquisition control circuitry has serviced the trigger and cleared the local memories of the blocks involved.
  • a RESET signal is sent to the blocks involved and BLOCK BUSY signal is dropped, enabling triggers from those blocks once more.
  • Clock speed for loading the digitized data into local memory is high (50- 100Mhz) as all diode channels must be processed.
  • Clock speed for loading data from local memory into the data acquisition control circuitry is slower f 10Mhz) as zero suppression has reduced the number of channels to data to be loaded.
  • the present invention provides not only an effective method and apparatus for detecting gamma rays in which a high degree of spacial resolution of the scintillation point within a crystal is achieved, but also reliable electronic circuitry for triggering a data acquisition system upon a coincidence between two gamma rays interacting in opposed or nearly opposed scintillation crystals so that the location of the source of the gamma rays can be determined more accurately. It will also be appreciated that various modifications or alterations may be made to the apparatus described above without departing from the scope of the invention.

Abstract

A method of, and apparatus for, detecting X-rays and gamma rays is provided in which scintillation crystals (12) are disposed around a source of X-rays or gamma rays in an annular or cylindrical array with light detectors (16, 18) to detect and measure the light emitted from both ends of each crystal (12) when it absorbs an X-ray or gamma ray, whereby the position (Z) of a scintillation point within the crystal (12) can be determined from a comparison of the measurement of the light emitted from each end of the crystal. An array of thin photodiodes (16) disposed on the inner end of each crystal (12) is used to measure the light emitted from the inner end of the crystal (12) to enable the apparatus to have a small ring diameter thereby reducing cost and complexity. The scintillation crystals of the array may be arranged in blocks with the apparatus including coincidence logic circuitry arranged to determine when a coincidence between X-rays or gamma rays is from opposed or nearly opposed blocks of the array.

Description

DETECTING GAMMA RAYS
This invention relates to a method of, and apparatus for detecting electromagnetic radiation, particularly high energy X-rays and gamma rays. The invention has particular application in gamma ray imaging systems for use in positron emission tomography (PET) and other industrial and scientific applications such as non-destructive testing systems.
It is known in gamma ray imaging systems to use scintillation crystals with means for detecting scintillation light emitted within the crystals when gamma rays are absorbed therein. However, in previously proposed systems, the precise point of scintillation is difficult to determine and data provided by the gamma ray detection apparatus may be flawed if the scintillation point is not accurately determined.
It is therefore desirable to provide a method of detecting gamma rays using scintillation crystals in which a high degree of spacial resolution of the scintillation point within the crystals is achieved.
It is desirable to provide gamma ray detection apparatus with an improved means for measuring the light from a scintillation crystal which is able to determine accurately the scintillation point within the crystal.
It is also desirable to provide gamma ray detection apparatus which is less expensive to manufacture and maintain and in which the space needed to install an imaging camera is reduced.
It is further desirable to provide a gamma ray imaging system with electronic means for triggering a data acquisition system upon a coincidence between two annihilation X-rays or gamma rays interacting in opposed or nearly opposed scintillation crystals.
According to one aspect of the invention there is provided a method of detecting X-rays or gamma rays comprising the steps of: providing an array of elongate scintillation crystals around a source of X- rays or gamma rays; providing light detection means at both ends of each scintillation crystal for detecting light emitted from a scintillation point within the crystal when an X- ray or gamma ray is absorbed therein; measuring the light emitted from both ends of the scintillation crystal; and comparing the measurement to provide an indication of the position of the scintillation point within the crystal.
Preferably, the light emitted from at least one end of a scintillation crystal is detected and measured by the light detection means comprising an array of photodiodes.
According to another aspect of the invention there is provided X-ray or gamma ray detection apparatus comprising an array of elongate scintillation crystals, first light detection and measuring means at one end of each scintillation crystal, second light detection and measuring means at the other end of each scintillation crystal, and comparator means to receive and compare signals from the first and second light detection and measuring means whereby when an X-ray or gamma ray is absorbed by one of the scintillation crystals the signals from the first and second light detection means at both ends of that crystal are compared to determine the scintillation point within the crystal.
A preferred method of determining the longitudinal location of the scintillation point within an elongate crystal is to calculate the ratio of the number of photons detected at one end of the crystal to the sum of the number of photons measured by both detection means. The signal output of each detector is proportional to the number of photons detected by that detector and so the ratio of the pulse height from one end to the total pulse heights can be used as a measurement of the required ratio of photons. This method is insensitive to the actual number of photons produced as only ratios are used.
The array of scintillation crystals are preferably arranged around the X-ray or gamma ray source in an annular or cylindrical arrangement so that a pair of gamma rays emitted from the source in opposite directions will be absorbed in generally opposed crystals of the array. In such an arrangement, it is necessary for the light detectors at the inner ends of the crystals to be of a very thin construction, and a photodiode array satisfies this requirement so that the amount of material in front of the inner ends of the crystals does not distort the imaging capacity of the array to any significant extent. The determination of the longitudinal location of a scintillation point, ie. "the depth of interaction" within a crystal, has the advantage that gamma ray pairs emanating from positions off the axis of the array do not suffer from degraded position resolution. It is therefore not necessary to have an annular or cylindrical array much larger than the object being imaged to avoid rays emanating from radii close to the inner radius of the detecting cylinder.
The present invention therefore enables considerably smaller annular or cylindrical arrays to be used, and this substantially reduces: the manufacturing cost and complexity; the space needed to install an imaging camera; and the complexity and cost of maintenance.
The scintillation crystals may comprise crystals of any suitable scintillation material, for instance bismuth germanate (BGO). The light detection means at the ends of the crystals remote from the photodiode array may comprise any convenient form of detector, for instance, a photomultiplier tube, because in an annular arrangement of crystals it is not necessary for the detectors at the other ends of the crystals to be of a thin construction.
According to another aspect of the invention, there is provided a method of determining the coincidence of X-rays or gamma rays in opposed or nearly opposed scintillation crystals of X-ray or gamma ray detection apparatus comprising a plurality of blocks of scintillation crystals and associated light detectors disposed around a source of X-rays or gamma rays, wherein the method comprises the steps of: supplying detection signals from each of the plurality of blocks of light detectors to logic gates of coincidence logic circuitry including a programmable memory; the logic gates being arranged to produce output trigger signals when detection signals from at least two detectors are received by a logic gate within a desired time interval; programming the memory to provide identification signals to respective logic gates of the coincidence logic circuitry; and using the identification signals to determine when trigger signals are produced by a coincidence of detection of X-rays or gamma rays absorbed in opposed or nearly opposed crystals.
According to a further preferred aspect of the invention there is provided apparatus for detecting X-rays or gamma rays comprising a plurality of blocks of scintillation crystals and associated light detectors disposed in an array around a source of X-rays or gamma rays, and coincidence logic circuitry connected to the light detectors associated with the blocks of crystals, wherein the coincidence logic circuitry includes a plurality of AND gates, each connected to at least two detectors, memory means programmed to provide respective identification signals to each of the AND gates, clock means to provide clock signals to the AND gates, and an OR gate connected to the outputs of the AND gates, wherein each AND gate provides an output signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals, and the OR gate provides a trigger signal in response to the output signals from the AND gates to indicate a coincidence between X-rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals from the memory means enabling determination of whether the coincidence is from opposed or nearly opposed blocks of crystals.
The coincidence trigger circuitry used in the method and apparatus described above is vastly simpler and more flexible than any existing system, reducing complexity whilst providing the essential triggering of the system.
The various aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 is a schematic part plan view of an annular array of scintillation crystals of gamma ray imaging apparatus in accordance with the invention;
Figure 2 is a side view of part of the scintillator crystal and photodiode array of the apparatus of Figure 1 ;
Figure 3 is a plan view of the layout of the photodiode array and associated amplifier layout;
Figure 4 is a graph of the number of photons detected at one end of a scintillation crystal plotted against distance along the crystal;
Figure 5 is a graph of the sum of the photons detected at both ends of the crystal plotted against distance along the crystal;
Figure 6 is a graph showing the ratio of the number of photons measured at one end to the sum of the number of photons measured at both ends plotted against distance along the crystal,
Figure 7 is a schematic diagram of an analogue electronic circuit suitable for use with the diode array of the gamma imaging apparatus,
Figure 8 is a schematic diagram of a crystal and detector arrangement with detectors on four sides of the gamma source under study;
Figure 9 is a diagram of the coincidence logic circuitry associated with the crystal and detector arrangement of Figure 8; and
Figure 10 is a schematic timing diagram for the electronic circuit of Figure 7.
Referring to Figure 1 of the drawings there is shown a ring 10 of elongate scintillation crystals 12 forming an annular array of crystals surrounding an object 14 under study. It will, however, be appreciated that a plurality of rings of crystals may be provided to form a cylindrical array. If, for instance, positron annihilation takes place at point X in the object 14 two high energy X-rays or gamma rays γ, γ will be propagated in opposite directions from point X along flight path F. The gamma rays γ, γ thus enter respective scintillation crystals 12a, 12b on opposite sides of the annular array 10 by passing through thin light detectors 16 on the inner ends of the crystals 12a and 12b at absorption points Z to generate photons of light which travel longitudinally in opposite directions within the crystals 12a and 12b.
Further light detectors 18 are provided on the outer ends of the crystals 12 so that light generated by absorption of the gamma rays in a crystal 12a or 12b is detected at each end of the crystal to enable the longitudinal position of the scintillation point Z within the crystal to be determined. Information on the transverse position of the scintillation points Z is obtained from the opposed or nearly opposed crystals in which scintillation takes place.
As illustrated in Figure 1 , if no determination of the longitudinal position of the scintillation points Z within the crystals 12a to 12b is made, the region R1( of possible trajectory of the gamma rays γ, γ is wider than the region R2 of possible trajectory if information on both the longitudinal and transverse position of the scintillation points Z is determined.
The method by which determination of the longitudinal position of the scintillation point is made will be described with particular reference to the graphs of Figures 4 to 6.
Each of the graphs refers to data obtained from an elongate scintillation crystal of bismuth germanate (BGO) having a length of 20mm and a square cross-section of 3x3mm2. A diffuse reflector coating of 98% reflectance is applied to the side surfaces of the crystal and the index of refraction of the detectors is about 1.5.
On the graph of Figure 4, there is shown the relative number of photons NA reaching one end (End A) of the scintillation crystal against the longitudinal location of the scintillation point Z within the scintillation crystal. The relative number of photons NA is normalized to the maximum number detected (which occurs when gamma absorption takes place very close to end A). By itself, this position dependence could not be differentiated from what would be observed with interactions producing different numbers of photons at the same position in the crystal. It is therefore necessary, in accordance with one aspect of the invention, to detect the number of photons detected at both ends of the crystal (NA + NB) so that the sum of the photons detected by both detectors can be determined as illustrated in the graph of Figure 5. The dip in the middle of Figure 5 is due to losses in reflections from the walls, where the distance from both ends is largest. For long thin crystals, the number of reflections off the surface of a crystal is large and hence losses become important where the point of interaction Z is at a maximum from the detectors at the ends of the crystals. The method for obtaining information on the longitudinal position of interaction Z is to take the ratio of photons measured at one detector to the sum of the number of photons detected at both ends NA /(NA + NB) as shown in Figure 6. This determination is insensitive to the number of photons produced because only ratios are used. It will be appreciated that the signal output from a detector is proportional to the number of photons detected by that detector and so the ratios of pulse heights can be used as a measurement of the ratio of photons plotted in Figure 6. The plot in that graph can then be used to convert the ratio to a measured position along the crystal.
A preferred gamma ray detection apparatus of the invention will be more particularly described with reference to Figures 2 and 3 in which the thin light detectors 16 on the rear or inner ends of each scintillation crystal 12 comprises an array of photodiodes 16 whereas the light detector 18 on the front or outer end of each crystal may comprise a photomultiplier tube (PMT) or a light guide in combination with a PMT. There is shown in Figure 4 a set of four quadrants each containing a sub-array of 4x4 crystals preferably of dimensions 3x3x20 mm3 although it will be appreciated that different crystal configurations and dimensions of crystals may be used.
The diode arrays 16 and four 16-channel multiplexed preamplifier chips 20 are mounted on a substrate 22 (e.g. of machined ceramic). The connections to the diode array active areas is made via metallised tracks on the silicon out to one edge where a wire bond connection 23 is made to the preamplifier chips 20. The control signals, power and signal connections to the preamplifier 20 are made via flexible circuit 24 board which contains ancillary electronics, such a output driver circuits, before a connection to the data acquisition circuitry via a cable connector. The electronics are attached in such a way that modules can be ganged together into the required annular formation for the purpose of PET imaging.
The crystal array is kept spaced and oriented correctly via a machined PTFE support matrix 26 which also provides the necessary reflectance coating around the individual crystals. The crystals 12 could be glued to the diode array 16 with optical epoxy resin or similar.
At the end opposite to the diode array the crystals could be glued to a light guide or directly to the photomultiplier 18.
The entire module may be contained in a sheet metal case thin enough not to affect the required close packaging of modules into an annulus. This metal case provides support to hold the crystals in their correct orientation and provides the necessary shielding against electromagnetic radiation noise.
Referring now to Figures 7 to 10 there is shown the associated electronic circuitry for the detectors of the gamma ray imaging system of the invention. The design of the electronic circuitry to trigger the data acquisition system upon a coincidence between two annihilation X-rays or gamma rays interacting in two opposing or nearly opposing scintillation crystals is an important feature in the system of the invention.
An aim of the invention is to use a coincidence array capable of being remotely programmed for selection of specific combinations of inputs coincidentally within a predetermined time window, and to achieve this without the encumbrance and inflexibility of "hard wiring" specific combinations. Figures 8 and 9 illustrate schematically how this is achieved in a simplified application with only four blocks of crystals 12 and associated detectors A,B,C and D around the object 14 under study.
The coincidence trigger circuitry comprises a respective amplifier 30 associated with each detector A, B, C and D, a matrix of six AND gates 32 a programmable RAM 34, and an OR gate 36. Each amplifier 30 is connected to inputs of three of the AND gates 32 and the RAM 34 has six outputs 38, each of which is connected to an input of a respective AND gate 32.
The selection of the specific combinations that will register a trigger is made by loading the RAM, the set bits of which provide one of the inputs to the selected AND gates in the matrix, for a trigger to be flagged.
For example, to program a trigger on a coincidence between detector element A and C, the corresponding element in the RAM must be set, providing one of the inputs to the AND gate with channels A and C as inputs. The fourth input to all gates is a clock signal CK to provide the desired time window for the coincidence.
The outputs of all the AND gates 32 are logically summed by OR gate 36 to provide a trigger signal on any coincidence. The identifiers of the elements that provided the trigger are clocked out for use by the data acquisition system.
This relatively simple "self-coincidence" idea is a far more flexible and economical logic circuit than any trigger electronics presently used in PET cameras. It is also ideally suited to implementation in very large scale integration (VLSI), electronics. An efficient trigger is desirable for the use of the high resolution gamma detector. The number of channels possibly containing data is high so for fast event rate capability is mandatory.
Further details of the electronics circuitry are shown schematically in Figure 7 wherein similar "rear end" circuits are connected to each array of photodiodes 16. Each circuit (only one of which is illustrated in Figure 7) comprises a pre-ampl'ifier 42, a shaping amplifier 44, a buffer amplifier 46 and a multiplexer 48, all of which may be incorporated in a single integrated CMOS, low-noise amplifier chip 40, the output of which is connected to a flash analogue-digital converter 50 of data acquisition circuitry, with a clock input 52 from a real time clock 54.
The "front end" circuitry connected to the photomultiplier tube comprises amplifier 30 the output of which is connected both to the analogue-digital converter 50 via a sample and hold circuit 60 and, via a discriminator 62, to the coincidence logic circuitry illustrated in more detail in Figure 9.
The operation of the "front end" circuitry and coincidence logic circuitry will be described with particular reference to the timing diagrams in Figure 10.
As described above, coincidences between photomultiplier tube signals from appropriate pairs of block detectors are sought. Signals from the photomultiplier tube 18 are passed through the discriminator 62, set to a minimum signal threshold.
The digital output from the discriminator 62 is put into coincidence with a not BUSY signal. If the BUSY signal is off, a signal from the photomultiplier tube is passed to the coincidence electronics. No signal is passed from the discriminator to the coincidence electronics when the BLOCK BUSY signal for the block is set.
A valid coincidence is required to trigger digitization of the photomultiplier tube signals and to commence the multiplex cycle for digitizing the diode array signals of the specific blocks involved in the coincidence. Digitized signals are immediately clocked through a digital pedestal (noise) subtraction and zero suppression circuit into a local buffer memory. This process reduces the number of channels of data to be stored. Upon completion of the loading of digitized data to local memory, a readout request for the two blocks involved in the coincidence is posted to the data acquisition circuitry.
The queued requests to read the local buffer memories corresponding to the blocks in the trigger are handled in turn by the data acquisition circuitry.
The BLOCK BUSY signal is issued to the blocks in the coincidence and held in that state until after the data acquisition control circuitry has serviced the trigger and cleared the local memories of the blocks involved. At completion of acquisition of data for a given trigger a RESET signal is sent to the blocks involved and BLOCK BUSY signal is dropped, enabling triggers from those blocks once more.
Clock speed for loading the digitized data into local memory is high (50- 100Mhz) as all diode channels must be processed. Clock speed for loading data from local memory into the data acquisition control circuitry is slower f 10Mhz) as zero suppression has reduced the number of channels to data to be loaded.
From the description above, it will be appreciated that the present invention provides not only an effective method and apparatus for detecting gamma rays in which a high degree of spacial resolution of the scintillation point within a crystal is achieved, but also reliable electronic circuitry for triggering a data acquisition system upon a coincidence between two gamma rays interacting in opposed or nearly opposed scintillation crystals so that the location of the source of the gamma rays can be determined more accurately. It will also be appreciated that various modifications or alterations may be made to the apparatus described above without departing from the scope of the invention.

Claims

1. A method of detecting X-rays or gamma rays characterized by the steps of: providing an array of elongate scintillation crystals around a source of X- rays or gamma rays; providing light detection means at both ends of each scintillation crystal to detect light emitted from a scintillation point within the crystal when an X-ray or gamma ray is absorbed therein; measuring the light emitted from both ends of the scintillation crystal; and comparing the measurement to provide an indication of the position of the scintillation point within the crystal.
2. A method according to claim 1 wherein the light emitted from at least one end of each scintillation crystal is measured by an array of photodiodes.
3. A method according to claim 1 wherein the scintillation crystals are arranged in an annular or cylindrical array around the source of X-rays or gamma rays so that X-rays or gamma rays propagated in opposite directions from the source produce scintillations in crystals on substantially opposite sides of the array.
4. A method according to claim 3 wherein the light emitted from the inner end of a scintillation crystal is measured by an array of photodiodes disposed on the inner end of said crystal.
5. A method according to claim 4 wherein the light emitted from the outer ends of the scintillation crystals is measured by a photomultiplier tube.
6. A method according to any one of the preceding claims wherein the longitudinal position of a scintillation point within a crystal is determined by calculating the ratio NA/(NA + NB) where NA represents the number of light photons detected at one end of the crystal and NB represents the number of light photons detected by the light detection means at the opposite ends of the crystal.
7. A method according to claim 6 wherein the ratio of the pulse height of the signal output from the light detection means at one end of the crystal to the total pulse height of the signal outputs of the light detection means at both ends of the crystal is used to calculate the ratio NA/(NA + NB).
8. A method according to any one of claims 1 to 5 further comprising the steps of: arranging the scintillation crystals and associated light detection means in a plurality of blocks around the source of X-rays or gamma rays; supplying detection signals from each of the plurality of blocks of light detectors to logic gates of coincidence logic circuitry including a programmable memory; the logic gates being arranged to produce output trigger signals when detection signals from at least two detectors are received by a logic gate within a desired time interval; programming the memory to provide identification signals to respective logic gates of the coincidence logic circuitry; and using the identification signals to determine when trigger signals are produced by a coincidence of detection of X-rays or gamma rays absorbed in opposed or nearly opposed crystals.
9. Apparatus for detecting X-rays or gamma rays comprising an array of elongate scintillation crystals, first light detection and measuring means at one end of each scintillation crystal, second light detection and measuring means at the other end of each scintillation crystal and comparator means to receive and compare signals from the first and second light detection and measuring means whereby when an X-ray or gamma ray is absorbed by one of the scintillation crystals the signals from the first and second light detection means at both ends of that crystal are compared to determine the scintillation point within the crystal. 10. Apparatus according to claim 9 wherein the array of scintillation crystals is in the form of an annular or cylindrical array with the source of X-rays or gamma rays located substantially at the centre of the annular array and with the longitudinal axis of each scintillation crystal extending radially outwards from the centre of the array so that each crystal has a radially inner end facing towards the source and a radially outer end.
11. Apparatus according to claim 10 wherein the light detection means on at least the inner end of each scintillation crystal comprises an array of photodiodes.
12. Apparatus according to claim 11 wherein at least one photomultiplier tube is used as the light detection means on the outer ends of the scintillation crystals.
13. Apparatus according to any one of claims 9 to 12 wherein the scintillation crystals are formed of bismuth germanate.
14. Apparatus according to claim 9 or claim 10 wherein the scintillation crystals of the array are arranged in blocks and the apparatus also includes coincidence logic circuitry including a plurality of logic gates connected to light detectors associated with the blocks of crystals, memory means programmed to provide respective identification signals to respective logic gates of the coincidence logic circuitry, and clock means arranged to provide clock signals to the logic gates, wherein the coincidence logic circuitry is arranged to provide a trigger signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals to indicate a coincidence between X-rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals from the memory means enabling determination of whether the coincidence is from opposed or nearly opposed blocks of crystals. 15. A method of determining the coincidence of X-rays or gamma rays in opposed or nearly opposed scintillation crystals of X-ray or gamma ray detection apparatus comprising a plurality of blocks of scintillation crystals and associated light detectors disposed around a source of X-rays or gamma rays, wherein the method comprises the steps of: supplying detection signals from each of the plurality of blocks of light detectors to logic gates of coincidence logic circuitry including a programmable memory; the logic gates being arranged to produce output trigger signals when detection signals from at least two detectors are received by a logic gate within a desired time interval; programming the memory to provide identification signals to respective logic gates of the coincidence logic circuitry; and using the identification signals to determine when trigger signals are produced by a coincidence of detection of X-rays or gamma rays absorbed in opposed or nearly opposed crystals.
16. Apparatus for detecting X-rays or gamma rays comprising a plurality of blocks of scintillation crystals and associated light detectors disposed in an array around a source of X-rays or gamma rays, coincidence logic circuitry including a plurality of logic gates connected to light detectors associated with the blocks of crystals, memory means programmed to provide respective identification signals to respective logic gates of the coincidence logic circuitry, and clock means arranged to provide clock signals to the logic gates, wherein the coincidence logic circuitry is arranged to provide a trigger signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals to indicate a coincidence between X- rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals from the memory means enabling determination of whether the coincidence is from opposed or nearly opposed blocks of crystals. 17. Apparatus according to claim 14 or claim 16 wherein the coincidence logic circuitry includes a plurality of AND gates, each connected to at least two detectors, the memory means being programmed to provide respective identification signals to each of the AND gates, and an OR gate connected to the outputs of the AND gates, wherein each AND gate provides an output signal when detection signals are received from at least two detectors within the predetermined time window determined by the clock signals, and said trigger signal is provided by the OR gate in response to the output signals from the AND gates.
93 1
AMENDED CLAIMS
[received fay the International Bureau on 19 April 1993 (19.04.93) ; original claims 1-13 replaced by amended claims 1 -16 ; original claims 14-17 renumbered as claims 17-20 (5 pages)]
1. A method of detecting X-rays or gamma rays characterized by the steps of: providing an array of elongate scintillation crystals pointing towards a source of X-rays or gamma rays, providing first scintillation light detection means substantially transparent to X-rays or gamma rays at a proximal end of each scintillation crystal which faces the source to detect reverse scattered scintillation light emitted when an X-ray or gamma ray passing through the first light detection means is absorbed within the crystal; providing second scintillation light detection means at the opposite end of the array of scintillation crystals remote from the source to detect forward scattered scintillation light when an X-ray or gamma ray is absorbed within the crystal; measuring the reverse scattered scintillation light detected by the first light detection means; measuring the forward scattered scintillation light detected by the second light detection means; and analysing measurements of the reverse scattered and the forward scattered scintillation light to construct an image of the X-ray or gamma ray trajectory.
2. A method according to claim 1 wherein the X-rays or gamma rays pass directly through the first scintillation light detection means substantially without deviation and are subsequently absorbed within the scintillation crystals.
3. A method according to claim 1 wherein the scintillation crystals are arranged in a cylindrical or spherical array around the source of X-rays or gamma rays.
4. A method according to claim 3 wherein the X-ray or gamma ray source is disposed substantially at the centre of the cylindrical or spherical array and with the longitudinal axis of each scintillation crystal extending radially outwards from the centre of the array so that each crystal has a radially inner proximal end and a radially outer distal end.
5. A method according to claim 4 wherein the first scintillation light detection means is of thin construction, whereby the scintillation crystals can be cylindricaily or spherically close packed without hindrance from the first light detection means.
6. A method according to claim 1 wherein the measured reverse scattered scintillation light detected by the first scintillation light detection means transparent to the X-rays or gamma rays is used to determine the scintillation crystals which absorbed the X-ray or gamma ray.
7. A method according to any one of the preceding claims wherein the first scintillation light detection means comprises an array of photodiodes on the proximal ends of the scintillation crystals.
8. A method according to claim 1 wherein the second scintillation light detection means is substantially opaque to X-rays or gamma rays.
9. A method according to claim 1 wherein the measured forward scattered light detected by the second scintillation light detection means is used to determine the time and energy characteristics of the X-ray or gamma ray.
10. A method according to claim 8 or claim 9 wherein the second scintillation light detection means comprises at least one photomultiplier tube.
11. Apparatus for detecting X-rays or gamma rays comprising an array of elongate scintillation crystals pointing towards a source of X-rays or gamma rays, first light detection and measuring means substantially transparent to X- rays or gamma rays at a proximal end of each scintillation crystal facing the source, second light detection and measuring means at the opposite end of each scintillation crystal remote from the source, and analysing means to receive and analyse signals from the first and second light detection and measuring means whereby when an X-ray or gamma ray is absorbed by one of the scintillation crystals the signals from the first and second light detection and measuring means at both ends of that crystal are used to construct an image of the X-ray or gamma ray trajectory.
12. Apparatus according to claim 11 wherein the array of scintillation crystals is in the form of a cylindrical or spherical array with the source of X-rays or gamma rays located substantially at the centre of the annular array and with the longitudinal axis of each scintillation crystal extending radially outwards from the centre of the array so that each crystal has a radially inner proximal end facing towards the source and a radially outer distal end.
13. Apparatus according to claim 12 wherein the first scintillation light detection means is of thin construction, whereby the scintillation crystals can be cylindricaily or spherically close packed without hindrance from the first light detection means.
14. Apparatus according to any one of claims 11 to 13 wherein the first scintillation light detection means comprises an array of photodiodes on the proximal ends of the scintillation crystals.
15. Apparatus according to claim 11 wherein the second scintillation light detection means is substantially opaque to X-rays or gamma rays.
16. Apparatus according to claim 15 wherein the second scintillation light detection means comprises at least one photomultiplier tube.
17. Apparatus according to claim 10 or claim 11 wherein the scintillation crystals of the array are arranged in blocks and the apparatus also includes coincidence logic circuitry including a plurality of logic gates connected to light detectors associated with the blocks of crystals, memory means programmed to provide respective identification signals to respective logic gates of the coincidence logic circuitry, and clock means arranged to provide clock signals to the logic gates, wherein the coincidence logic circuitry is arranged to provide a trigger signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals to indicate a coincidence between X-rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals from the memory means enabling determination of whether the coincidence is from opposed or nearly opposed blocks of crystals.
18. A method of determining the coincidence of X-rays or gamma rays in opposed or nearly opposed scintillation crystals of X-ray or gamma ray detection apparatus comprising a plurality of blocks of scintillation crystals and associated light detectors disposed around a source of X-rays or gamma rays, wherein the method comprises the steps of: supplying detection signals from each of the plurality of blocks of light detectors to logic gates of coincidence logic circuitry including a programmable memory; the logic gates being arranged to produce output trigger signals when detection signals from at least two detectors are received by a logic gate within a desired time interval; programming the memory to provide identification signals to respective logic gates of the coincidence logic circuitry; and using the identification signals to determine when trigger signals are produced by a coincidence of detection of X-rays or gamma rays absorbed in opposed or nearly opposed crystals.
19. Apparatus for detecting X-rays or gamma rays comprising a plurality of blocks of scintillation crystals and associated light detectors disposed in an array around a source of X-rays or gamma rays, coincidence logic circuitry including a plurality of logic gates connected to light detectors associated with the blocks of crystals, memory means programmed to provide respective identification signals to respective logic gates of the coincidence logic circuitry, and clock means arranged to provide clock signals to the logic gates, wherein the coincidence logic circuitry is arranged to provide a trigger signal when detection signals are received from at least two detectors within a predetermined time window determined by the clock signals to indicate a coincidence between X- rays or gamma rays detected by absorption in two or more blocks of scintillation crystals of the array, the identification signals from the memory means enabling determination of whether the coincidence is from opposed or nearly opposed blocks of crystals.
20. Apparatus according to claim 17 or claim 19 wherein the coincidence logic circuitry includes a plurality of AND gates, each connected to at least two detectors, the memory means being programmed to provide respective identification signals to each of the AND gates, and an OR gate connected to the outputs of the AND gates, wherein each AND gate provides an output signal when detection signals are received from at least two detectors within the predetermined time window determined by the clock signals, and said trigger signal is provided by the OR gate in response to the output signals from the AND gates.
PCT/AU1992/000620 1991-11-20 1992-11-20 Detecting gamma rays WO1993010472A1 (en)

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WO2000025149A1 (en) * 1998-10-28 2000-05-04 Koninklijke Philips Electronics N.V. Computer tomograph detector
CN100563575C (en) * 2007-03-21 2009-12-02 中国科学院高能物理研究所 Meet system in a kind of position emissron tomography and meet method
CN110520760A (en) * 2017-03-13 2019-11-29 通用电气公司 Pixel for radiation detector designs

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US5665971A (en) * 1993-04-12 1997-09-09 Massachusetts Institute Of Technology Radiation detection and tomography
WO2000025149A1 (en) * 1998-10-28 2000-05-04 Koninklijke Philips Electronics N.V. Computer tomograph detector
CN100563575C (en) * 2007-03-21 2009-12-02 中国科学院高能物理研究所 Meet system in a kind of position emissron tomography and meet method
CN110520760A (en) * 2017-03-13 2019-11-29 通用电气公司 Pixel for radiation detector designs

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