Fast timing and depth of interaction simplified pet detector with projective-style anode-cathode readout

Abstract

A Positron Emission Tomography (PET) detector includes a sensor. The sensor includes a photodetector array, an anode summing circuit, a cathode summing circuit, and a processor. The photodetector array has a plurality of photodetectors arranged in a two-dimensional grid having rows and columns, each photodetector configured to generate an anode signal and a cathode signal in response to an event at the photodetector. The anode summing circuit is configured to determine a sum of anode currents generated by photodetectors within a column of the photodetector array. The cathode summing circuit is configured to determine a sum of cathode currents generated by photodetectors within a row of the photodetector array. The processor is configured to determine parameters of the event based on at least one of the sum of anode currents and the sum of cathode currents.

Description

FAST TIMING AND DEPTH OF INTERACTION SIMPLIFIED PET DETECTOR WITH PROJECTIVE-STYLE ANODE-CATHODE READOUTBACKGROUND OF THE INVENTIONFIELD OF THE INVENTION[1] The present invention relates generally to Positron Emission Tomography and in particular to a sensor circuit and associated scintillation crystals topology used in Positron Emission Tomography.DESCRIPTION OF THE RELATED ART[2] Positron Emission Tomography (PET) typically employs a PET scanner having multiple gamma-ray detectors that detect positron annihilation photons generated within an object, usually a living organism. A radiotracer is placed within the organism. The radiotracer undergoes positron emission that produces pairs of annihilation photons which propagate in diametrically opposite directions from each other. The annihilation photons are received at opposed detector pairs of the PET scanner. Each annihilation photon interacts with the detector to register an event. The events are correlated with each other and used to mathematically reconstruct a three-dimensional distribution of radiotracers within the organism, thereby providing biologically relevant information about the organism.[3] A detector of the PET scanner includes a plurality of scintillation crystals and light sensors such as silicon photomultipliers (SiPMs). An event occurs when a gamma ray impinges on a scintillation crystal and deposits some or all of its energy therein, causing scintillation photons to be emitted. These scintillation photons are detected by one or more light sensors. Each light sensor generates an electrical signal in response to absorption of the scintillation photons. The magnitude of the electrical signal is proportional to the number of scintillation photons detected at the sensor. These electrical signals can be processed using an analog signal processor and the resulting outputs are then digitized. A processor reads the digitized signals to determine variousparameters of the event, including an event position, an energy of the event and a relative timing of the event.[4] The accuracy of determination of parameters of the event (e.g., event timing) is important for image quality. However, current detector topologies limit this accuracy. It is therefore advantageous to design a detector with a topology that enhances the accuracy of the event information.BRIEF SUMMARY OF THE INVENTION[5] Disclosed herein is a sensor for a Positron Emission Tomography detector. The sensor includes a photodetector array, an anode summing circuit, a cathode summing circuit, and a processor. The photodetector array has a plurality of photodetectors arranged in a two-dimensional grid having rows and columns, each photodetector configured to generate an anode signal and a cathode signal in response to an event at the photodetector. The anode summing circuit is configured to determine a sum of anode currents generated by photodetectors within a column of the photodetector array. The cathode summing circuit is configured to determine a sum of cathode currents generated by photodetectors within a row of the photodetector array. The processor is configured to determine parameters of the event based on at least one of the sum of anode currents and the sum of cathode currents.[6] Disclosed herein also is a Positron Emission Tomography (PET) detector. The PET detector includes a photodetector array, an anode summing circuit a cathode summing circuit, and a processor. The photodetector array has a plurality of photodetectors arranged in a two-dimensional grid having rows and columns, each photodetector configured to generate an anode signal and a cathode signal in response to an event at the photodetector. The anode summing circuit is configured to determine a sum of anode currents generated by photodetectors within a column of the photodetector array. The cathode summing circuit configured to determine a sum of cathode currents generated by photodetectors within a row of the photodetector array. The processor is configured to determine parameters of the event based on at least one of the sum of anode currents and the sum of cathode currents.BRIEF DESCRIPTION OF THE DRAWINGS[7] These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, and accompanying drawings where:[8] FIG. 1 illustrates one embodiment of a Time-Of-Flight Positron Emission Tomography (TOF PET) system;[9] FIG. 2 shows a scintillation device suitable for use as a detector of the PET scanner of the TOF PET system;

[0010] FIG. 3 shows a circuit that includes the photodetector array of FIG. 2, in an embodiment;

[0011] FIGS. 4 and 4A are diagrams showing various components of the one or more processing units of the TOF PET system, in embodiments;

[0012] FIG. 5 shows a circuit that includes the photodetector array of FIG. 2 in an alternative embodiment; and

[0013] FIG. 6 shows a circuit that includes the photodetector array of FIG. 2, in another alternative embodiment.

[0014] It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings.DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention as well as to the examples included therein. All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may include numbers that are rounded to the nearest significant figure.

[0016] FIG. 1 illustrates one embodiment of a Time-Of-Flight Positron Emission Tomography (TOF PET) system 100. The TOF PET system 100 includes a first gantry 102a and a control system 104. The first gantry 102a includes a PET scanner 106 having a plurality of detectors 108 arranged in a ring 110. A patient 112 lies on a patient bed114 that can be moved into and out of the PET scanner 112. The plurality of detectors 108 of the PET scanner 106 are configured to detect annihilation photons, gamma rays, and / or other nuclear imaging events occurring in the patient 112 due to radiotracers within the patient 112. In some embodiments, the TOF PET system 100 includes a second gantry 102b including a second imaging module 116 such as, for example, a computerized tomography (CT) module, a magnetic resonance imaging (MRI) module, and / or any other suitable imaging module. The patient bed 114 can be moved between the first gantry 102a and the second gantry 102b.

[0017] The control unit 104 includes various circuity which performs TOF PET using data from the PET scanner 106. The control unit 104 includes one or more databases 120 and a processer 122. An intermediate processor 118 is disposed between the plurality of detectors 108 and the control unit 104. In various embodiments, the intermediate processor 118 is a field-programmable gate array (FPGA). The FPGA can receive outputs from the plurality of detectors 108 and create event characterization parameters (e.g., event energy, event position, event timing, etc.). The event characterization parameter can be stored at the one or more computer databases 140 and / or can be processed by the processor 122. The processor 122 may include one or more separate computing components, as discussed herein with respect to FIG. 4.

[0018] FIG. 2 shows a scintillation device 200 suitable for use as a detector 108 of the PET scanner 106 of the TOF PET system 100. The scintillation device 200 includes a photodetector array 202 and a crystal cluster 204. The crystal cluster 204 generally includes a plurality of crystals, each in the shape of a cuboid and arranged in a box shaped cluster. The photodetector array 202 is located at a face of the crystal cluster 204 (i.e. a face of the box). Each crystal has a face that is optically coupled to one or more photodiodes of the photodetector array 202. The photodetector array 202 includes a plurality of photodetectors arranged within a two-dimensional grid having (or defined by) a first dimension and a second dimension. In an embodiment, the first dimension isreferred to as a row of the grid and the second dimension is referred to as a column of the grid. A photodetector can be a photodiode with a gain applied. In an exemplary embodiment, the photodiode can be a silicon photomultiplier (SiPM). The photodetector array 202 can be formed on a substrate 220. In various embodiments, individual photodiodes can be formed on separate substrates. Processing circuitry 222 for the photodetector array 202 can be formed within or embedded within the substrate 220.

[0019] For illustrative purposes only, the photodetector array 202 is shown in FIG. 2 as a 3x5 array of photodiodes, with 3 rows (R0, Rl, R2) and 5 columns (CO, Cl, C2, C3, C4). It is understood, however, that the photodetector array 202 can have any number of rows and columns. The crystal cluster 204 includes a first row of crystals 206, a second row of crystals 208, and a third row of crystals 210. Each row can include a scintillation crystal for each diode in the row, such that each diode is registered to an associated scintillation crystal. However, it is also possible for the number of crystals in the row to be less than or greater than the number of photodiodes within the row. Each crystal has a face (the “adjacent face”) that is disposed adjacent the diode or diodes with which it is registered. Each photodiode is optically coupled to the adjacent face or faces via an optical medium between the photodiode and the adjacent faces. The area of a diode can be different than the area of its adjacent face. For example, the photodiode can cover a smaller area, a larger area, etc., than the adjacent face.

[0020] An event occurs when a gamma ray interacts with the crystal cluster 204 generating scintillation photons. These scintillation photons are detected when one or more of the diodes of the photodetector array 202 generate electrons in response to the scintillation photons. The electron generation results in a current at an anode of the diode (i.e., an anode current) and / or a current at a cathode of the diode (i.e., a cathode current). Different aspects of the anode current and the cathode current, such as magnitudes, pulse shapes and timing relative to a reference clock signal, can be measured to determine various parameters that characterize the event. The parameters include, but are not limited to, a timing of the event, an energy of the event, a position of the event (or a location in the photodetector array at which the event occurs), a depth of interaction (DOI) of the event, etc.

[0021] The rows of crystals can be optically isolated from each other. In one embodiment, optical reflectors can be placed between adjacent rows to perform the optical isolation. In various embodiments, the optical reflector can be a layered polymer reflector such as a 3 M™ Enhanced Specular Reflector fdm. As shown in FIG. 2, optical reflectors are placed between rows. A first optical reflector 212 is disposed between the first row of crystals 206 and the second row of crystals 208. A second optical reflector 214 is disposed between the second row of crystals 208 and the third row of crystals 210. Each row of crystals can be registered one-to-one with its associated rows of diodes.

[0022] Light propagation between neighboring crystals within a selected row (i.e., crystals within the same row but in neighboring columns), and thus the distribution of light to the photodiodes within a row, may be modified by various means. Affecting the distribution of light to photosensors within the row can improve the accuracy of event characterization, particularly the accuracy of event positioning within the row and the accuracy of DOI determination. Control of light propagation or light distribution between neighboring crystals within the row can be achieved by placing an air gap between neighboring crystals (as shown in FIG. 2). Alternatively, faces of the neighboring crystals can be polished or chemically etched to have a smoother or rougher surface, thereby modifying light transfer. Reflecting, isolating or partially reflecting or isolating barriers, or optical compounds of different refractive indices, can be placed between some or all of the crystals in a row to modify the light distribution. The crystals can also be coupled by means of various optical compounds of certain refraction index. Alternatively, a single crystal can extend along the row and an internal structure can be created within the crystal at locations between the columns or at other locations. The crystal can be internally modified using sub-surface laser engraving or other methods.

[0023] FIG. 3 shows a circuit 300 that includes the photodetector array of FIG. 2, in an embodiment. The photodetector array 202 is shown having 25 photodiodes arranged within 5 rows and 5 columns of the array. A top left photodiode is labelled as doo and the remaining photodiodes are correspondingly labelled. Each photodiode includes a cathode and an anode. A plurality of common-cathode wires (K0, KI, K2, K3, K4) are provided, with each common-cathode wire associated with a respective row (R0, Rl, R2, R3, R4). Each common-cathode wire (e.g., common-cathode wire K0) is hard-wired to each of the cathodes of the photodiodes (e g., cathodes of doo, doi, do2, do3, do4) in its associated row (e.g., row RO). The common-cathode wires have an associated capacitor and are individually connected to a high voltage (+HV) bus via a load. The connection to the +HV bus provides a cathode bias. A plurality of common-anode wires (AO, Al, A2, A3, A4) are provided, with each common-anode wire associated with a respective column (CO, Cl, C2, C3, C4). Each common-anode wire (e.g., commonanode wire AO) is hard-wired to each of the anodes of the photodiodes (e.g., anodes of doo, dio, d2o, dso, d4o) in its associated column (e g., column CO).

[0024] The common cathode wires (KO, KI, K2, K3, K4) provide cathode signals from respective rows (RO, Rl, R2, R3, R4) to a cathode processing circuit 302. A cathode signal is a sum of the cathode currents for the photodiodes connected by the common cathode wire (e.g., a wire-summed cathode current). For example, for the row RO, the cathode signal SKO is shown by Eq. (1):The cathode processing circuit 302 includes circuitry for measuring the timing of cathode signals relative to a system clock to generate a time stamp. The time stamp can be a digital output of a time to digital converter (TDC). The cathode processing circuit 302 provides this time stamp to the intermediate processor 118. The cathode processing circuit 302 can also include circuitry for determining a magnitude of a cathode signal and provide the magnitude to the intermediate processor 118. The cathode processing circuit 302 can also generate multiplexed signals that can be provided to the intermediate processor 118.

[0025] The common anode wires (A0, Al, A2, A3, A4) provide anode signals from respective columns (CO, Cl, C2, C3, C4) to an anode processing circuit 304. An anode signal is a sum of the currents at the anodes connected by the common anode wire (i.e., a wire-summed anode signal). For example, for the column CO, the common anode current SAO is shown by Eq. (2):■^40=jt^cmode currents of di0) Eq. (2)The anode processing circuit 304 therefore measures summed anode currents and provides this information to the intermediate processor 118. The common anode current SAJ can also represent column energies for columns Aj . The anode processing circuit 304 can include preamplifiers, current integrators and analog to digital converters suitable for determining magnitudes of the anode signals. Signals can be individually digitized or alternatively combined (or multiplexed) to generate fewer signals, which are then digitized. Multiplexing can include amplification, buffering, sums and / or differences, comparators with fixed or event dependent thresholds, timing discriminators, etc. The anode processing circuit 304 can provide these signals to the intermediate processor 118.

[0026] FIG. 4 is diagram 400 showing the processing circuitry of the TOF PET system, in an embodiment. The processing circuitry includes the cathode processing circuits 302a-302n of the various detectors 108, the anode processing circuits 304a-304n of the various detectors 108, the intermediate processor 118 and the processor 112. The intermediate processor 118 includes a field-programmable gate array (FPGA) that receives the signals from each of the cathode processing circuits 302a-302n and the anode processing circuits 304a-304n and generates event characterization parameters. The processor 122 includes at least a coincidence processor 404 and a reconstruction processor 406. The output of the intermediate processor 118 is provided to the coincidence module 402. The coincidence module 402 and the reconstruction module 404 perform various calculations to form an image, which is sent to display 406.

[0027] FIG. 4A is a diagram 410 showing the processing circuitry of the TOF PET system, in another embodiment. The processing circuitry includes the cathode processing circuits 302a-302n of the various detectors 108, the anode processing circuits 304a-304n of the various detectors 108, the intermediate processor 118 and the processor 112. The intermediate processor 118 includes a plurality of FPGAs 412a-412n. Each FPGA 412a-412n is dedicated to a selected detector 108. Thus, a selected FPGA (e.g., FPGA 412a) receives signals from the cathode processing circuit (e.g., cathode processing circuit 302a) and the anode processing circuit (e.g., anode processing circuit 304a) of the associated detector 108. Each FPGA 412a-412n provides its output to the coincidence module 402 of the processor 122.

[0028] The calculations disclosed herein can be performed at one or more of the cathode processing circuit 302. Analog discriminators can be applied to analog signals from a common cathode (row) to produce digital signals. These digital signals can be sent to a time to digital converter (TDC), which generates an event time stamp. A time stamp for the event can be determined based on a first crossing of a cathode signal (e.g., SKO, SKI, SK2, SK3, SK4) across a threshold. For example, when more than one row provides a cathode signal, the first cathode signal to cross the threshold can be used to mark the time stamp. The time stamp can alternatively be determined by using a logical OR circuit. Alternatively, a time stamp can be determined by determining a first crossing of a sum of the cathode signals (i.e., across all cathode rows) across the threshold. In various scenarios, scattering can occur in crystals in adjacent rows, thereby causing current in photodiodes of adjacent rows. In this case, a time stamp can be obtained by calculating a weighted average of time stamps derived from the crossings of multiple cathode signals, using the magnitudes of the cathode signals of adjacent rows.

[0029] An energy of an event can be determined by summing a signal strength from all of the anode signals, as shown in Eq. (3):E event ^n ^An Eq. (J)

[0030] A position of the event can be determined by determining the indices of the row and column at which maximum signals are detected, as shown in Eq. (4):In another embodiment, the position of an event within a row can be determined by a weighted average of the magnitudes of anode signals or an Anger function (i.e., center of gravity of signals) derived from all or some of the magnitudes of the anode signals. In other embodiments, the position of the event can be determined from the magnitudes of the anode signals using machine learning techniques. If row signal magnitude information is not directly measured, the row position of an event can be determined as the row index of the first cathode signal to cross a threshold. When an event occurs in adjacent rows, a position of the event can be best estimated by selecting the row index ofthe row for which the recorded magnitude for the event is the lowest, due to the highest likelihood of Compton forward scattering.

[0031] A depth of interaction (DOI) of the event can be determined as a function of a ratio of the maximum anode signal strength (i.e., a maximum column energy represents the energy of the column having the maximum energy) to the total energy (i.e., the sum of anode signals) and a position of the event, as shown in Eq. (5):The value of Eq. (5) can be a number between 0 and 1, which can be input to a lookup table. The lookup table can bin the value and output a bin value for depth of interaction.The accuracy of a DOI determination can be improved by altering the calculation based on the event column position, or by using machine learning techniques.

[0032] Optically isolating rows of the detector and obtaining timing readouts along common cathodes of these rows reduces each common cathode timing domain’s area, and thus reduces dark noise (i.e., random photoelectrons due to thermal processes or other processes within the photodiode) and increases the signal magnitude, thereby improving the timing resolution for the event. The smaller scintillator volume of each segment (as compared to that of the entire array) can also reduce jitter in the arrival time of the optical scintillation photon, thus improving time resolution. A projective style connection of cathodes and anodes may reduce the number of channels that require processing, while maintaining enough information to determine position energy and DOI. For example, in this way the number of channels can be reduced from all channels (e.g., n x m channels) to (m + n) channels.

[0033] FIG. 5 shows a circuit 500 that includes the photodetector array 200 of FIG. 2 in an alternative embodiment. A buffer 502 is associated with the anode of each photodiode. The anode currents are buffered before being summed. In circuit 400, the buffered signals are summed via a common-anode wire (i.e., A0, Al, A2, A3, A4).

[0034] FIG. 6 shows a circuit 600 that includes the photodetector array 200 of FIG. 2, in another alternative embodiment. For each diode, a buffer 502 is associated with the anode. For each photodetector array of a column, the anode currents arebuffered and then provided to a summing circuit (e.g., summing circuits 602, 604, 606, 608, 610) associated with the associated column (e.g., columns CO, Cl, C2, C3, C4). A summing circuit can be a circuit external to the photodiode array, (such as the FPGA 402 (FIG. 4) or an internal circuit that is embedded with the substrate 220 (FIG. 2).

[0035] It is to be understood that the terms “row” and column” can be interchanged without affecting operation of the PET scanner. Also, the roles of cathodes and anodes in determining event timing and event position data can be reversed.

[0036] Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.

[0037] The reader’s attention is directed to all papers and documents which are filed concurrently with this specification, and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0038] All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[0039] Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C §112, sixth paragraph. In particular, the use of “step of’ in the claims herein is not intended to invoke the provisions of 35 U.S.C §112, sixth paragraph.

Claims

What Is Claimed Is:

1. A sensor for a Positron Emission Tomography detector, comprising: a photodetector array having a plurality of photodetectors arranged in a two- dimensional grid having rows and columns, each photodetector configured to generate an anode signal and a cathode signal in response to an event at the photodetector; an anode summing circuit configured to determine a sum of anode currents generated by photodetectors within a column of the photodetector array; a cathode summing circuit configured to determine a sum of cathode currents generated by photodetectors within a row of the photodetector array; and a processor configured to determine parameters of the event based on at least one of the sum of anode currents and the sum of cathode currents.

2. The sensor of claim 1, wherein a photodetector includes at least one of: (i) a diode; (ii) a solid state detector; (iii) a vacuum detector; and (iv) a hybrid detector including a vacuum detector and a solid state detector.

3. The sensor of claim 1 where the photodetector array includes a silicon substrate and at least one of the anode summing circuit and / or the cathode summing circuit is embedded within the silicon substrate.

4. The sensor of claim 1, wherein each photodetector has an anode and a cathode and wherein the cathodes within a row of the photodetector array are hardwired to the cathode summing circuit via a common cathode wire and at least one of: (i) the anodes within a column of the photodetector array are hardwired to the anode summing circuit via a common anode wire; (ii) each anode within the column of the photodetector array has an associated buffer between the anode and the common anode wire; and (iii) buffered anode current from each anode within the column of the photodetector array is provided to a summing circuit associated with the column.

5. The sensor of claim 4, wherein the processor is further configured to determine anenergy of the event from the sum of anode currents related to the event for a plurality of columns.

6. The sensor of claim 4, wherein the processor is further configured to determine at least one of: (i) a column energy from each of the anode signals within a column, (ii) a total energy of the event; (iii) a position of interaction of the photon of the event; and (iv) a depth of interaction of the photon as a function of the column energy.

7. The sensor of claim 1, wherein the cathode processing circuit is configured to determine a time stamp for the event from the sum of cathode currents.

8. The sensor of claim 7, wherein the time stamp is one of (i) the time of a first crossing of a cathode signal over a threshold; (ii) the time of a first crossing of a sum of the cathode signals associated with the event over the threshold; and (iii) a weighted average of the time stamps of first crossings of cathode signals associated with the event over the threshold.

9. The sensor of claim 1, further comprising a crystal cluster associated with the photodetector array, the crystal cluster having a plurality of crystals arranged in rows, wherein the rows of the crystals are optically isolated from each other.

10. The sensor of claim 9, wherein light distribution between neighboring crystals within a selected row is controlled by at least one of (i) an air gap; (ii) a roughness of a face of the crystal; (iii) an internal structure within a crystal of the row; and (iv) a reflecting barrier; and (v) an optical compound of different refractive index.

11. A Positron Emission Tomography (PET) detector, comprising: a photodetector array having a plurality of photodetectors arranged in a two- dimensional grid having rows and columns, each photodetector configured to generate an anode signal and a cathode signal in response to an event at the photodetector;an anode summing circuit configured to determine a sum of anode currents generated by photodetectors within a column of the photodetector array; a cathode summing circuit configured to determine a sum of cathode currents generated by photodetectors within a row of the photodetector array; and a processor configured to determine parameters of the event based on at least one of the sum of anode currents and the sum of cathode currents.

12. The PET detector of claim 11, wherein a photodetector includes at least one of: (i) a diode; (ii) a solid state detector; (iii) a vacuum detector; and (iv) a hybrid detector including a vacuum detector and a solid state detector.

13. The PET detector of claim 11, wherein the photodetector array includes a silicon substrate and at least one of the anode summing circuit and the cathode summing circuit is embedded within the silicon substrate.

14. The PET detector of claim 11, wherein each photodetector has an anode and a cathode and wherein the cathodes within a row of the photodetector array are hardwired to the cathode summing circuit via a common cathode wire and at least one of: (i) the anodes within a column of the photodetector array are hardwired to the anode summing circuit via a common anode wire; (ii) each anode within the column of the photodetector array has an associated buffer between the anode and the common anode wire; and (iii) buffered anode current from each anode within the column of the photodetector array is provided to a summing circuit associated with the column.

15. The PET detector of claim 14, wherein the processor is further configured to determine an energy of the event from the sum of anode currents related to the event for a plurality of columns.

16. The PET detector of claim 14, wherein the processor is further configured to determine at least one of: (i) a column energy from each of the anode signals within a column, (ii) a total energy of the event; (iii) a position of interaction of the photon of theevent; and (iv) a depth of interaction of the photon as a function of the column energy.

17. The PET detector of claim 11, wherein the cathode processing circuit is configured to determine a time stamp for the event from the sum of cathode currents.

18. The PET detector of claim 17, wherein the time stamp is one of: (i) the time of a first crossing of a cathode signal over a threshold; (ii) the time of a first crossing of a sum of the cathode signals associated with the event over the threshold; and (iii) a weighted average of the time stamps of first crossings of cathode signals associated with the event over the threshold.

19. The PET detector of claim 11, further comprising a crystal cluster associated with the photodetector array, the crystal cluster having a plurality of crystals arranged in rows, wherein the rows of the crystals are optically isolated from each other.

20. The PET detector of claim 19, wherein light distribution between neighboring crystals within a selected row is controlled by at least one of: (i) an air gap; (ii) a roughness of a face of the crystal; (iii) an internal structure within a crystal of the row; (iv) a reflecting barrier; and (v) optical compounds of different refractive indices.

Images