Ultra low silicon photomultiplier terminal capacitance design for time-of-flight positron emission tomography block detectors

Abstract

A Positron Emission Tomography system includes a circuit for a detector of the system. The detector includes an array of silicon photomultipliers. The circuit includes the array of silicon photomultipliers, an amplifier, and a transformer between the amplifier and the array of silicon photomultipliers. The array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer. The array includes at least two silicon photomultipliers connected in series.

Description

ULTRA LOW SILICON PHOTOMULTIPLIER TERMINAL CAPACITANCE DESIGN FOR TIME-OF-FLIGHT POSITRON EMISSION TOMOGRAPHY BLOCK DETECTORSSUMMARY

[0001] According to an embodiment of the present invention, a detector for a Positron Emission Tomography system is disclosed.

[0002] A detector for Positron Emission Tomography (PET) typically includes a plurality of scintillation crystals and photosensors including silicon photomultipliers. An array of silicon photomultipliers detects a gamma ray and sends an electrical signal to event energy, position, and timing pickoff circuits. The timing pickoff circuit generates a timing pulse in response to the timing information for gamma events. Often silicon photomultipliers of the array are connected in parallel. However, the parallel connection increases a terminal capacitance of the array, thereby reducing a coincidence timing resolution of the PET system. It is therefore desirable to provide an array that increase a resolution of the timing pulse.BRIEF SUMMARY OF the INVENTION

[0003] Disclosed herein is a detector for a Positron Emission Tomography system. The detector includes an array of silicon photomultipliers, an amplifier, and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.

[0004] Disclosed herein also is a Positron Emission Tomography system. The system includes an array of silicon photomultipliers, an amplifier, and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of siliconphotomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.

[0005] Disclosed herein also is a circuit for a detector of a Positron Emission Tomography device. The circuit includes an array of silicon photomultipliers, an amplifier, and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0006] FIG. 1 illustrates one embodiment of a Time-Of-Flight Positron Emission Tomography (TOF PET) system;

[0007] FIG. 2 illustrates a circuit model of an Analog Silicon Photomultiplier (aSiPM) microcell, in accordance with some embodiments;

[0008] FIG. 3 shows a scintillation device suitable for use as a detector of the TOF PET system;

[0009] FIG. 4 shows a parallel wiring configuration of the diodes of the scintillation device;

[0010] FIG. 5 shows a high-level circuit diagram for a detector of the TOF PET system, in an illustrative embodiment; and

[0011] FIG. 6 a detailed circuit diagram for the detector, in an illustrative embodiment.DETAILED DESCRIPTION

[0012] This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description.

[0013] FIG. 1 illustrates one embodiment of a Time-Of-Flight Positron Emission Tomography (TOF PET) system 100. The TOF PET system 100 includes a scanner for at least a PET module 112 provided in a first gantry 116a. The PET module 112 includes a plurality of detectors 50 configured to detect an annihilation photon, gamma ray, and / or other nuclear imaging event. In various embodiments, the PET module 112 performs TOF PET. A patient 117 lies on a movable patient bed 118 that may be movable between a gantry. In some embodiments, the TOF PET system 100 includes a scanner for a second imaging module 114 provided in a second gantry 116b. The second imaging module 114 can be any suitable imaging module, such as, for example, computerized tomography (CT), magnetic resonance imaging (MRI) and / or any other suitable imaging module.

[0014] Scan data from the PET module 112 is stored at one or more computer databases 140 and processed by one or more computer processors 150 of a computer 130. The graphical depiction of computer 130 in FIG. 1 is provided by way of illustration only, and computer 130 may include one or more separate computing devices. The imaging data sets can be provided by the PET module 112 and / or may be provided as a separate data set, such as, for example, from a memory coupled to the computer 130.

[0015] FIG. 2 illustrates a circuit model of an Analog Silicon Photomultiplier (aSiPM) microcell 2, in accordance with some embodiments. The aSiPM microcell 2 can be configured as part of a detector 50 for one or more nuclear imaging modules 112, 114, such as, for example, a PET module 112. The aSiPM microcell 2 includes one or more firing cells 4 and one or more passive cells 6. The firing cell 4 is configured to respond to an annihilation photon event, for example, by generating a signal when receiving a gamma ray during a PET scan. The firing cell 4 generates a scintillation pulse output at cathode 58 and / or anode 60. The scintillation pulse is provided to analog frontend electronics (AFE) coupled to the aSiPM microcell 2. The scintillation response in a PET system 100can be considered a homogeneous single-input, single-output (SISO) system, with a single input of the photo-electron current pulse from the aSiPM microcell 2 and a single output (voltage or current signal) from the AFE.

[0016] The firing cell 4 includes a diode portion 8 configured to respond to an annihilation photon event by initiating a cascade and a quenching portion 10 configured to quench the cascade. The diode portion 8 is configured to generate a predetermined voltage in response to an annihilation photon event. In some embodiments, the diode portion 8 is modeled as a voltage source 12 in series with a diode resistor 14 (which is representative of the diode resistance value Rd) and in parallel with a diode capacitor 16a (which is representative of the diode capacitance value Cd). In some embodiments, the voltage source 12 and the diode resistor 14 are replaced with a current source in parallel with the diode resistor 14. A switch 18 controls a breakdown response of the firing cell 4 to an annihilation photon event. A breakdown event occurs when the aSiPM microcell 2 receives a gamma ray from an annihilation photon event. The diode portion 8 is coupled in series to the quenching portion 10. The quenching portion 10 includes a quenching resistor 22a (having a quenching resistance value Rq) in parallel with a quenching capacitor 24a (having a quenching capacitance value Cq). The quenching portion 10 limits current through the aSiPM microcell 2 and facilitates transition of the diode portion 8 from an active (or avalanche) state to a pre-charge (or ready) state to allow detection of additional annihilation photon events.

[0017] In aSiPM devices, the firing cell 4 is coupled in parallel to one or more passive cells 6. Each of the passive cells 6 includes a quenching resistor 22b and a quenching capacitor 24 > coupled in parallel. The quenching portion 10 of the passive cell 6 is coupled in series to the diode capacitor 16b.

[0018] In operation, when the aSiPM microcell 2 encounters an annihilation photon event (such as receiving a gamma ray during a PET scan), the firing cell 4 generates an output signal. The avalanche event produces a predetermined scintillation output signal between the anode 58 and the cathode 60 of the aSiPM microcell 2. The avalanche eventis quenched by the respective quenching portions 10 of each of the firing cell 4. When the firing cell 4 initiates an avalanche event, a large charge is generated by the aSiPM microcell 2. The quenching portion 10 spatially distributes the charge, allowing the aSiPM to recover (e.g., recharge) to detect additional annihilation photon events.

[0019] FIG. 3 shows a scintillation device 300 suitable for use as a detector 50 of the PET system 100. The scintillation device 300 includes an array 302 of silicon photomultipliers at an end of a crystal cluster 304. The silicon photomultipliers can be modelled as diodes and therefore may also be referred to herein as diodes. For illustrative purposes, the array 302 includes four didoes, labelled Di, Dz, Ds and D4. Also for the illustrative purposes, the crystal cluster 304 includes a first crystal 306, second crystal 308, third crystal 310 and fourth crystal 312. The dimensions and the number of the crystals in the array can be configurated based on the specific detector design. An electron enters the crystal cluster 304 and interacts with the crystals, thereby generating photons that can be detected by one or more of the diodes of the array 302. The diodes generate signals that are sent to circuits for determining a timing of the detection as well as an energy and event position of the detection.

[0020] FIG. 4 shows a parallel wiring configuration of the diodes. Each diode has an associated capacitance CD„ (where n = 1, 2, 3, 4). The capacitance of each diode is the same or substantially the same. As a result of the parallel configuration, the terminal capacitance across the terminal is increased, as shown in Eq. (1):The increase in terminal capacitance reduces the timing resolution of a gamma event in the parallel circuit.

[0021] FIG. 5 shows a high-level circuit diagram 500 for the detector 50, in an illustrative embodiment. The high-level circuit diagram 500 includes a diode section 502, a timing pickoff circuit 504, and an energy and position detection circuit 506. The diodesection 502 includes the diodes DI, D2, D3 and D4 of the array 302. The timing pickoff circuit 504 is a high frequency circuit and the energy and position detection circuit 506 is a low frequency circuit. The diodes are wired to the timing pickoff circuit 504 via a first wiring configuration. In particular, the diodes are wired to the timing pickoff circuit 504 in a 2x2 series wiring configuration. The diodes are wired to the energy and position detection circuit 506 via a second wiring configuration.

[0022] In a non-limiting embodiment, a high-frequency circuit operates at radio frequencies (RF) or higher frequencies, such as in a range from a few Megahertz (MHz) to a few Gigahertz (GHz), a low-frequency circuit operates operate at frequencies ranging from a few Hertz (Hz) to a few Kilohertz (kHz), and an energy and position detection circuit measures an energy and position of a particle or of a photon.

[0023] The timing pickoff circuit 504 includes circuit elements for determining a timing for a signal from the diode section 502. The circuit elements include a transformer 508 and a high frequency amplifier 516. A primary side 514 of the transformer 508 is electrically coupled to a high frequency amplifier 516 and a secondary side 512 of the transformer 508 is electrically coupled to the diodes. The transformer includes a center tap 510 on its secondary side 512.

[0024] The diodes of the diode section 502 are wired in series with the secondary side 512 of the transformer 508. The center tap 510 separates the diodes into a first group 518 including two diodes (i.e., first diode DI and second diode D2) and a second group 520 including two diodes (i.e., third diode D3 and fourth diode D4).

[0025] Within the first group 518 of diodes, a cathode end of the first diode DI is connected to a positive terminal at the secondary side 512 of the transformer 508 by a first wire trace 524. An anode end of the first diode DI is connected to the cathode end of the second diode D2. An anode end of the second diode D2 is connected to the center tap 510 on the secondary side 512 of the transformer 508 by a second wire trace 526.

[0026] Within the second group 520 of diodes, an anode end of the third diode D3 is connected to a negative terminal of the secondary side 512 of the transformer 508 by a third wire trace 528. A cathode end of the third diode D3 is connected to the anode end of the fourth diode D4. A cathode end of the fourth diode D4 is connected to the center tap 510 on the secondary side 512 of the transformer 508 by a fourth wire trace 530.

[0027] The aSiPM bootstrapping hybrid readout circuit disclosed herein includes a high- frequency capacitive-coupling readout for PET timing pickoff that includes the elements of the diode section 502 and the timing pickoff circuit 504 (as shown by circuit loop 540) and a low-frequency inductive-coupling readout for PET energy and event positioning that includes the diode section 502 and the energy and position detection circuit 506.

[0028] FIG. 6 a detailed circuit diagram 600 for the detector 50, in an illustrative embodiment. The detailed circuit diagram 600 depicts the diode section 502, the timing pickoff circuit 504, and the energy and position detection circuit 506.

[0029] The transformer 508 performs a bootstrapping and impedance conversion of the signals from the diodes. The amount of bootstrapping and impedance conversion is a based on the turn ratio between the primary side 514 and the secondary side 512, which yields an impedance ratio. For example, a turn ratio of 1 : sqrt(N) (secondary : primary) leads to an impedance ratio of 1 / N. A terminal capacitance of the high frequency circuit loop 540 is shown in Eq. (2):where A can be any suitable value, such as N= 8 for example. The capacitance for the 2x2 series configuration is reduced with respect to that of the parallel configuration (FIG. 4), as shown in Eq. (3):Eq. (3)

[0030] The overall aSiPM terminal capacitance of the circuit disclosed in FIGS. 5 and 6 therefore is greatly reduced with respect to a single aSiPM device as well as with respect to the parallel configuration of FIG. 4. The reduced capacitance results in a faster timing pulse (i.e., a faster signal slope to the timing pulse) and an improved “equivalent noise charge (ENC)” (i.e., lower noise) with respect to the single and parallel circuits, thereby improving the “noise-to-slope ratio (NSR)” of the timing signal. The first wire trace 524 connecting the first diode DI to the positive terminal of the secondary side 512 of the transformer 508 has a trace length Iti. The second wire trace 526 connecting the second diode D2 to the center tap 510 has a trace length It?. The third wire trace 528 connecting the third diode D3 to a negative terminal of the secondary side 512 of the transformer 508 by a third wire trace 528 has a trace length Its. The fourth wire trace 530 connecting the fourth diode D4 to the center tap 510 has a trace length / .

[0031] To increase a resolution of a timing signal at the amplifier, signals from each of the diodes reach the transformer at the same time. In other words, a signal transit time between each diode and the transformer is the same for each diode. To achieve this, the trace length of each wire trace is the same or substantially the same, such as shown in Eq. (4): ltQ= lt = lt2= lt3Eq. (4)By having each trace length be the same, the transit time added by each trace travelled by a signal is the same. This allows for synchronized signals, thereby improving the timing pickoff accuracy. In other words, the mismatch in transit timing that occurs in serial and hybrid readout circuits is effectively eliminated.

[0032] Although the SiPM array shown herein is in a 2x2 configuration, the methods disclosed herein are also suitable for other array configurations (e.g., 3x3, 4x4, etc.).

[0033] The circuit disclosed herein reduces overall SiPM terminal capacitance, as well as transit time mismatching from each SiPM device. As a result, the circuit disclosed hereinachieves an improved PET detector timing pickoff accuracy and realizes an improved PET coincidence timing resolution (CTR), thereby improving PET TOF performance.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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

CLAIMSWhat is claimed is:

1. A detector for a Positron Emission Tomography system, comprising: an array of silicon photomultipliers; an amplifier; and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.

2. The detector of claim 1, wherein each silicon photomultiplier has an associated transit time for which a signal travels between the silicon photomultiplier and the transformer, wherein the transit time is the same for each silicon photomultiplier.

3. The detector of claim 1, wherein each silicon photomultiplier is connected to the transformer via a wire trace, each wire trace having a same length.

4. The detector of claim 1, wherein the array of silicon photomultipliers includes four silicon photomultipliers separated into a first group having two silicon photomultipliers and a second group having two silicon photomultipliers, wherein the first group is connected between a positive terminal of the transformer and a center tap of the transformer and the second group is connected between the center tap and a negative terminal of the transformer.

5. The detector of claim 1, wherein the transformer and the amplifier form a timing pickoff circuit with the array of silicon photomultipliers through a first wiring configuration and the array of silicon photomultipliers is connected to an energy and position detection circuit through a second wiring configuration.

6. The detector of claim 5, wherein the timing pickoff circuit is a high frequency circuit and the energy and position detection circuit is a low frequency circuit.

7. The detector of claim 5, wherein a turn ratio of the transformer reduces a capacitance of the timing pickoff circuit.

8. A Positron Emission Tomography system, comprising: an array of silicon photomultipliers; an amplifier; and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.

9. The Positron Emission Tomography system of claim 8, wherein each silicon photomultiplier has an associated transit time for which a signal travels between the silicon photomultiplier and the transformer, wherein the transit time is the same for each photomultiplier.

10. The Positron Emission Tomography system of claim 8, wherein each silicon photomultiplier is connected to the transformer via an associated wire trace, each wire trace having a same length.

11. The Positron Emission Tomography system of claim 8, wherein the array of silicon photomultipliers includes four silicon photomultipliers separated into a first group having two silicon photomultipliers and a second group having two silicon photomultipliers, wherein the first group is connected between a positive terminal of the transformer and a center tap of the transformer and the second group is connected between the center tap and a negative terminal of the transformer.

12. The Positron Emission Tomography system of claim 8, wherein the transformer and the amplifier form a timing pickoff circuit with the array of silicon photomultipliers through a first wiring configuration and the array of silicon photomultipliers is connected to an energy and position detection circuit through a second wiring configuration.

13. The Positron Emission Tomography system of claim 12, wherein the timing pickoff circuit is a high frequency circuit and the energy and position detection circuit is a low frequency circuit.

14. The Positron Emission Tomography system of claim 12, wherein a turn ratio of the transformer reduces a terminal capacitance of the silicon photomultiplier array.

15. A circuit for a detector of a Positron Emission Tomography device, comprising: an array of silicon photomultipliers; an amplifier; and a transformer between the amplifier and the array of silicon photomultipliers, wherein the array of silicon photomultipliers is connected between a first end of the transformer and a second end of the transformer, the array including at least two silicon photomultipliers connected in series.

16. The circuit of claim 15, wherein each photomultiplier has an associated transit time for which a signal travels between the photomultiplier and the transformer, wherein the transit time is the same for each photomultiplier.

17. The circuit of claim 15, wherein each photomultiplier is connected to the transformer via an associated wire trace, each wire trace having a same length.

18. The circuit of claim 15, wherein the array of silicon photomultipliers includes four silicon photomultipliers separated into a first group having two silicon photomultipliers and a second group having two silicon photomultipliers, wherein thefirst group is connected between a positive terminal of the transformer and a center tap of the transformer and the second group is connected between the center tap and a negative terminal of the transformer.

19. The circuit of claim 18, wherein the transformer and the amplifier form a timing pickoff circuit with the array of silicon photomultipliers through a first wiring configuration and the array of silicon photomultipliers is connected to an energy and position detection circuit through a second wiring configuration.

20. The circuit of claim 18, wherein the timing pickoff circuit is a high frequency circuit and the energy and position detection circuit is a low frequency circuit.

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