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 photomultiplier tubes. The circuit includes the array of silicon photomultiplier tubes, an amplifier, and a transformer between the amplifier and the array of silicon photomultiplier tubes. The array of silicon photomultiplier tubes is connected between a first end of the transformer and a second end of the transformer. The array includes at least two silicon photomultiplier tubes connected in series.

Description

Technical Field

[0001] According to an embodiment of the present invention, a detector for a positron emission tomography system is disclosed. Background Technology

[0002] Detectors used in positron emission tomography (PET) typically include multiple scintillation crystals and photodetectors, including silicon photomultiplier tubes (SPTs). The SPT array detects gamma rays and sends electrical signals to event energy, position, and timing pick-off circuitry. The timing pick-off circuitry generates timing pulses in response to timing information for the gamma event. The SPTs in the array are typically connected in parallel. However, parallel connections increase the terminal capacitance of the array, thereby reducing the timing resolution of the PET system. Therefore, it is desirable to provide an array that increases the resolution of the timing pulses. Summary of the Invention

[0003] This paper discloses a detector for a positron emission tomography (PET) system. The detector includes a silicon photomultiplier tube array, an amplifier, and a transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first terminal and a second terminal of the transformer, and the array includes at least two silicon photomultiplier tubes connected in series.

[0004] This paper also discloses a positron emission tomography (PET) system. The system includes a silicon photomultiplier tube array, an amplifier, and a transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first terminal and a second terminal of the transformer, and the array includes at least two silicon photomultiplier tubes connected in series.

[0005] This paper also discloses a circuit for a detector in a positron emission tomography (PET) apparatus. The circuit includes a silicon photomultiplier tube array, an amplifier, and a transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first terminal and a second terminal of the transformer, and the array includes at least two silicon photomultiplier tubes connected in series. Attached Figure Description

[0006] Figure 1 An embodiment of a time-of-flight positron emission tomography (TOF PET) system is shown; Figure 2 A circuit model of an analog silicon photomultiplier tube (aSiPM) microcell according to some embodiments is shown; Figure 3 A scintillation device suitable for use as a detector in a TOF PET system is shown; Figure 4 The parallel wiring configuration of the diodes in the scintillation device is shown; Figure 5 A high-level circuit diagram of a detector for a TOF PET system is shown in an illustrative embodiment; and Figure 6 This is a detailed circuit diagram of the detector in the illustrative embodiment. Detailed Implementation

[0007] The description of exemplary embodiments is intended to be read in conjunction with the accompanying drawings, which will be considered part of the entire written description.

[0008] Figure 1 An embodiment of a time-of-flight positron emission tomography (TOF PET) system 100 is illustrated. The TOF PET system 100 includes a scanner, at least for a PET module 112, provided in a first gantry 116a. The PET module 112 includes a plurality of detectors 50 configured to detect annihilation photons, gamma rays, and / or other nuclear imaging events. In various embodiments, the PET module 112 performs TOF PET. A patient 117 lies on a movable bed 118, which is movable between gantry units. 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, computed tomography (CT), magnetic resonance imaging (MRI), and / or any other suitable imaging module.

[0009] The scan data from the PET module 112 is stored in one or more computer databases 140 and processed by one or more computer processors 150 of the computer 130. Figure 1 The graphical representation of computer 130 is provided only by illustration, and computer 130 may include one or more separate computing devices. Imaging datasets may be provided by PET module 112 and / or may be provided as separate datasets (such as from memory coupled to computer 130, for example).

[0010] Figure 2A circuit model of an analog silicon photomultiplier tube (aSiPM) microcell 2 according to some embodiments is shown. 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, PET module 112). The aSiPM microcell 2 includes one or more firing units 4 and one or more passive units 6. The firing units 4 are configured to respond to annihilation photon events, for example, by generating a signal when receiving gamma rays during a PET scan. The firing units 4 generate scintillation pulse outputs at a cathode 58 and / or an anode 60. The scintillation pulses are provided to an analog front-end electronics (AFE) coupled to the aSiPM microcell 2. The scintillation response in the PET system 100 can be considered a uniform single-input single-output (SISO) system with a single input of a photoelectron current pulse from the aSiPM microcell 2 and a single output (voltage or current signal) from the AFE.

[0011] The ignition unit 4 includes a diode portion 8 and a quenching portion 10. The diode portion 8 is configured to respond to an annihilation photon event by initiating a cascade, and the quenching portion 10 is configured to quench a 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 in conjunction with a diode resistor 14 (which represents the diode resistance value R). d ) in series with a diode capacitor 16a (which represents the diode capacitance value C) d A voltage source 12 is connected in parallel with the diode resistor 14. In some embodiments, the voltage source 12 and the diode resistor 14 are replaced by a current source connected in parallel with the diode resistor 14. Switch 18 controls the breakdown response of the ignition unit 4 to an annihilation photon event. A breakdown event occurs when the aSiPM microcell 2 receives gamma rays from an annihilation photon event. Diode portion 8 is coupled in series to quenching portion 10. Quenching portion 10 includes a quenching capacitor 24a (which has a quenching capacitance value C). q A quenching resistor 22a connected in parallel (which has a quenching resistance value R) q The quenching section 10 limits the current through the aSiPM microcell 2 and helps the diode section 8 transition from the active (or avalanche) state to the precharge (or ready) state to allow the detection of additional annihilation photon events.

[0012] In the aSiPM device, the ignition unit 4 is connected in parallel with one or more passive units 6. Each of the passive units 6 includes a quenching resistor 22b and a quenching capacitor 24b coupled in parallel. The quenching portion 10 of the passive unit 6 is coupled in series to a diode capacitor 16b.

[0013] In operation, when the aSiPM microcell 2 encounters an annihilation photon event (e.g., receiving gamma rays during a PET scan), the ignition unit 4 generates an output signal. An avalanche event generates a predetermined scintillation output signal between the anode 58 and cathode 60 of the aSiPM microcell 2. The avalanche event is quenched by a corresponding quenching portion 10 of each ignition unit 4. When the ignition unit 4 initiates an avalanche event, the aSiPM microcell 2 generates a large amount of charge. The quenching portion 10 spatially distributes the charge, thereby allowing the aSiPM to recover (e.g., recharge) to detect additional annihilation photon events.

[0014] Figure 3 A scintillation device 300 suitable for use as a detector 50 in a PET system 100 is shown. The scintillation device 300 includes a silicon photomultiplier tube array 302 at one end of a crystal cluster 304. A silicon photomultiplier tube can be modeled as a diode and is therefore also referred to herein as a diode. For illustrative purposes, the array 302 includes four diodes, labeled D1, D2, D3, and D4. Also for illustrative purposes, the crystal cluster 304 includes a first crystal 306, a second crystal 308, a third crystal 310, and a fourth crystal 312. The size and number of crystals in the array can be configured based on a specific detector design. Electrons enter the crystal cluster 304 and interact with the crystals, thereby generating photons that can be detected by one or more diodes in the array 302. The diodes generate signals that are sent to a circuit to determine the timing of the detection, as well as the energy and location of the event.

[0015] Figure 4 The diagram shows a parallel wiring configuration of diodes. Each diode has an associated capacitance C. Dn (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 terminals increases, as shown in equation (1): Equation (1) Increasing the terminal capacitance reduces the timing resolution of gamma events in parallel circuits.

[0016] Figure 5 A high-level circuit diagram 500 for detector 50 is shown in an illustrative embodiment. The high-level circuit diagram 500 includes a diode section 502, a timing pickup circuit 504, and an energy and position detection circuit 506. The diode section 502 includes diodes D1, D2, D3, and D4 in an array 302. The timing pickup circuit 504 is a high-frequency circuit, and the energy and position detection circuit 506 is a low-frequency circuit. The diodes are connected to the timing pickup circuit 504 via a first wiring configuration. Specifically, the diodes are connected to the timing pickup circuit 504 in a 2×2 series wiring configuration. The diodes are connected to the energy and position detection circuit 506 via a second wiring configuration.

[0017] In a non-limiting embodiment, the high-frequency circuit operates at radio frequency (RF) or higher (such as in the range of several megahertz (MHz) to several gigahertz (GHz), the low-frequency circuit operates in the range of several hertz (Hz) to several gigahertz (kHz), and the energy and position detection circuit measures the energy and position of the particle or photon.

[0018] The timing pickup circuit 504 includes circuit elements for determining the timing of the signal from the diode section 502. The circuit elements include a transformer 508 and a high-frequency amplifier 516. The primary side 514 of the transformer 508 is electrically coupled to the high-frequency amplifier 516, and the secondary side 512 of the transformer 508 is electrically coupled to the diode. The transformer includes a center tap 510 on its secondary side 512.

[0019] The diodes in diode section 502 are connected in series with the secondary side 512 of transformer 508. Center tap 510 divides the diodes into a first group 518 comprising two diodes (i.e., first diode D1 and second diode D2) and a second group 520 comprising two diodes (i.e., third diode D3 and fourth diode D4).

[0020] In the first group of 518 diodes, the cathode of the first diode D1 is connected to the positive terminal of the secondary side 512 of the transformer 508 via the first trace 524. The anode of the first diode D1 is connected to the cathode of the second diode D2. The anode of the second diode D2 is connected to the center tap 510 on the secondary side 512 of the transformer 508 via the second trace 526.

[0021] Within the second group of 520 diodes, the anode of the third diode D3 is connected to the negative terminal of the secondary side 512 of the transformer 508 via the third trace 528. The cathode of the third diode D3 is connected to the anode of the fourth diode D4. The cathode of the fourth diode D4 is connected to the center tap 510 on the secondary side 512 of the transformer 508 via the fourth trace 530.

[0022] The aSiPM bootstrapping hybrid readout circuit disclosed herein includes: a high-frequency capacitively coupled readout for PET timing pickup, comprising diode portion 502 and elements of timing pickup circuit 504 (as shown by circuit loop 540); and a low-frequency inductively coupled readout for PET energy and event localization, comprising diode portion 502 and energy and position detection circuit 506.

[0023] Figure 6This is a detailed circuit diagram 600 for the detector 50 in an illustrative embodiment. Detailed circuit diagram 600 depicts a diode section 502, a timing pickup circuit 504, and an energy and position detection circuit 506.

[0024] Transformer 508 performs bootstrapping and impedance switching on the signal from the diode. The amount of bootstrapping and impedance switching is based on the turns ratio between the primary side 514 and the secondary side 512, which produces an impedance ratio. For example, a turns ratio of 1:sqrt(N) (secondary:primary) results in an impedance ratio of 1 / N. The terminal capacitance of the high-frequency circuit loop 540 is shown in equation (2): Equation (2) Where N can be any suitable value, such as, for example, N = 8. The capacitance of a 2×2 series configuration is smaller than that of a parallel configuration. Figure 4 As shown in equation (3): Equation (3).

[0025] Therefore, relative to a single aSiPM device and relative to Figure 4 Parallel configuration, in Figure 5 and Figure 6 The total aSiPM terminal capacitance of the disclosed circuit is significantly reduced. Compared to single circuits and parallel circuits, the reduced capacitance results in faster timing pulses (i.e., a faster signal slope for the timing pulses) and improved "equivalent noise charge (ENC)" (i.e., lower noise), thereby improving the "noise slope ratio (NSR)" of the timing signal. A first trace 524, connecting the first diode D1 to the positive terminal of the secondary side 512 of transformer 508, has a trace length lt1. A second trace 526, connecting the second diode D2 to the center tap 510, has a trace length lt2. A third trace 528, connecting the third diode D3 to the negative terminal of the secondary side 512 of transformer 508 via a third trace 528, has a trace length lt3. A fourth trace 530, connecting the fourth diode D4 to the center tap 510, has a trace length lt4.

[0026] To improve the resolution of the timing signal at the amplifier, the signal from each diode arrives at the transformer simultaneously. In other words, the signal transit time between each diode and the transformer is the same for each diode. To achieve this, the trace lengths of each trace are the same or substantially the same, as shown in equation (4): Equation (4) By making each trace the same length, the transit time added by each trace travelled by a signal is the same. This allows for synchronized signals, thus improving timing pickup accuracy. In other words, it effectively eliminates the transit timing mismatch that occurs in serial and mixed readout circuits.

[0027] Although the SiPM array shown in this paper is a 2×2 configuration, the methods disclosed herein are also applicable to other array configurations (e.g., 3×3, 4×4, etc.).

[0028] The circuit disclosed herein reduces the total SiPM terminal capacitance and the transit time mismatch from each SiPM device. As a result, the circuit disclosed herein achieves improved PET detector timing pickup accuracy and improved PET coincidence timing resolution (CTR), thereby improving PET TOF performance.

[0029] Although the invention has been described in considerable detail with reference to certain preferred variations, other variations are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred variations contained herein.

[0030] The reader’s attention is drawn to all papers and documents submitted concurrently with this specification and made publicly available for examination, and the contents of all such papers and documents are incorporated herein by reference.

[0031] All features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features for the same, equivalent, or similar purposes, unless expressly stated otherwise. Therefore, unless expressly stated otherwise, each disclosed feature is merely one example of a series of equivalent or similar features.

[0032] Any element of a “means” or “step” for performing the specified function that is not expressly stated in the claims shall not be construed as a “means” or “step” as defined in paragraph 6 of 35 USC §112. In particular, the use of “step” in the claims is not intended to reference the provisions of paragraph 6 of 35 USC §112.

Claims

1. A detector for a positron emission tomography (PET) system, comprising: Silicon photomultiplier tube array; Amplifier; and A transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first end and a second end of the transformer, the array comprising at least two silicon photomultiplier tubes connected in series.

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

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

4. The detector according to claim 1, wherein the silicon photomultiplier tube array comprises four silicon photomultiplier tubes, the four silicon photomultiplier tubes being divided into a first group having two silicon photomultiplier tubes and a second group having two silicon photomultiplier tubes, wherein the first group is connected between the positive terminal of the transformer and the center tap of the transformer, and the second group is connected between the center tap of the transformer and the negative terminal.

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

6. The detector according to claim 5, wherein the timing pickup circuit is a high-frequency circuit and the energy and position detection circuit is a low-frequency circuit.

7. The detector according to claim 5, wherein the turns ratio of the transformer reduces the capacitance of the timing pickup circuit.

8. A positron emission tomography (PET) system, comprising: Silicon photomultiplier tube array; Amplifier; and A transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first end and a second end of the transformer, the array comprising at least two silicon photomultiplier tubes connected in series.

9. The positron emission tomography system of claim 8, wherein each silicon photomultiplier tube has an associated transit time for signal propagation between the silicon photomultiplier tube and the transformer, wherein the transit time is the same for each photomultiplier tube.

10. The positron emission tomography system of claim 8, wherein each silicon photomultiplier tube is connected to the transformer via an associated trace, each trace having the same length.

11. The positron emission tomography system of claim 8, wherein the silicon photomultiplier array comprises four silicon photomultiplier tubes, the four silicon photomultiplier tubes being divided into a first group having two silicon photomultiplier tubes and a second group having two silicon photomultiplier tubes, wherein the first group is connected between the positive terminal of the transformer and the center tap of the transformer, and the second group is connected between the center tap of the transformer and the negative terminal of the transformer.

12. The positron emission tomography system of claim 8, wherein the transformer and the amplifier form a timing pickup circuit with the silicon photomultiplier tube array via a first wiring configuration, and the silicon photomultiplier tube array is connected to an energy and position detection circuit via a second wiring configuration.

13. The positron emission tomography system of claim 12, wherein the timing pickup 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 the turns ratio of the transformer reduces the terminal capacitance of the silicon photomultiplier array.

15. A circuit for a detector in a positron emission tomography (PET) apparatus, comprising: Silicon photomultiplier tube array; Amplifier; and A transformer between the amplifier and the silicon photomultiplier tube array, wherein the silicon photomultiplier tube array is connected between a first end and a second end of the transformer, the array comprising at least two silicon photomultiplier tubes connected in series.

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

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

18. The circuit of claim 15, wherein the silicon photomultiplier array comprises four silicon photomultiplier tubes, the four silicon photomultiplier tubes being divided into a first group having two silicon photomultiplier tubes and a second group having two silicon photomultiplier tubes, wherein the first group is connected between the positive terminal of the transformer and the center tap of the transformer, and the second group is connected between the center tap of the transformer and the negative terminal of the transformer.

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

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

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