Methods and systems for pet detectors

Methods and systems are provided for signal processing in a detector assembly of an imaging system. In one embodiment, an imaging system may include a plurality of detector blocks, each detector block including an array of silicon photomultiplier (SiPM) devices divided into at least two regions, with the SiPM devices in the two or more regions transmitting signals to two or more distinct timing pick-off circuits. In this way, a SiPM array may be subdivided into regions with a signal summed from SiPMs of a single region being transmitted to a separate timing pick-off circuit.

FIELD

Embodiments of the subject matter disclosed herein relate to non-invasive diagnostic imaging, and more particularly, to signal processing in positron emission tomography (PET) detectors.

BACKGROUND

A positron emission tomography (PET) scanner generates images that represent the distribution of positron-emitting nuclides within the body of a patient. When a positron interacts with an electron by annihilation, the entire mass of a positron-electron pair is converted into two photons (also referred to as events). The photons are emitted in opposite directions along a line of response (LOR). The annihilation photons are detected by detectors that are placed on both sides of the LOR, in a configuration such as a detector ring. In a time of flight (TOF) PET, in addition to measurement of the distance and attenuation of photons, an actual time difference between the detection of photons released during coincident events is measured to more accurately identify the distance from the annihilation event to the detector. The detectors convert the incident photons into useful electrical signals that can be used for image formation. An image thus generated based on the acquired image data includes the annihilation photon detection information.

Modern PET scanners include detectors such as silicon photomultiplier (SiPM) devices, wherein the detectors are typically tuned such that data collection is normalized for the energy event that is being detected. A TOF PET detector including analog application specific integrated chips (ASICs) is desired to demonstrate a coincidence timing resolution (CTR) of lower than 200 ps. In one example, the ASICs may include one to one coupling between a single crystal and a SiPM device (also referred to herein as a SiPM). In order to account for Compton scattering between crystals in the detector, input signals from an array of SiPMs may be consolidated into one timing channel. The response of photonic detectors in the absence of light is termed dark count. In a SiPM, thermionic emission of electrons is a major source of dark counts. During consolidation of input signals from an array of SiPMs, the dark count from the SiPMs s are also summed. In order to maintain the CTR within a range, it is desirable to reduce the effect of dark count on the CTR.

BRIEF DESCRIPTION

In one embodiment, an imaging system comprises one or more detector blocks, each detector block including an array of silicon photomultiplier (SiPM) devices divided into two or more regions, with the SiPM devices in the two or more regions transmitting signals to two or more distinct timing pick-off circuits. In this way, by dividing a SiPM array into regions with each region corresponding to a separate timing pick-off circuit, the detrimental effect of dark counts on a single timing pick-off circuit may be reduced.

DETAILED DESCRIPTION

The following description relates to various embodiments of medical imaging systems. In particular, methods and systems are provided for processing signal from a silicon photomultiplier (SiPM) array of a detector. An example of a positron emission tomography (PET) imaging system including a detector that may be used to detect energy events in accordance with the present techniques is provided inFIGS. 1-2. An example SiPM array constituting a detector block is shown inFIG. 3. The SiPM array may be divided into multiple regions, with each region corresponding to a separate timing pick-off circuit.FIGS. 4-6show examples of a SiPM array divided into regions. An example timing pick-off circuit corresponding to a single region of a SiPM array is shown inFIG. 7.FIG. 8shows an example method for processing signal from a SiPM array.

Though a PET imaging system is described by way of example, it should be understood that the present techniques may also be useful when applied to images acquired using other imaging modalities, such as CT, tomosynthesis, MM, C-arm angiography, and so forth. The present discussion of a PET imaging modality is provided merely as an example of one suitable imaging modality.

In one example, analog application specific integrated chips (ASICs) may include one-to-one coupling between a single detector crystal and a SiPM causing each SiPM to readout individually. However, in order to account for Compton scattering amongst crystals and to reduce the cost of the associated electronics (Time-to-Digital converter (TDC), Analog-to-Digital converter (ADC), FPGAs, etc.), input signals from an array of SiPMs may be consolidated into one timing channel. The response of photonic detectors in the absence of light is known as dark count. In a SiPM, thermionic emission of electrons is a major source of dark counts. During consolidation of input signals from an array of SiPMs, along with the actual signal, the dark counts from the SiPMs are also summed. In order to maintain the CTR within a range, it is desirable to reduce the effect of dark count on the CTR.

In case of a one-to-one coupling between a single crystal and a SiPM, dark count (such as caused by thermionic emission of electrons) of a single SiPM may be added to the gamma ray signal regenerated from the SiPM. However, due to Compton scattering, certain gamma rays may interact with a plurality of detector crystals. As an example, when a gamma ray is scattered, the energy of the gamma ray may be split over two or more crystals which reduces the size of the signal going into the comparator and degrades the coincidence timing resolution (CTR) of Compton scattered gamma rays. In order to overcome the effect of Compton scattering, instead of channeling signal from a single SiPM to a single timing pick-off circuit, signal from an array of SiPMs may be summed and channeled to the single timing pick-off circuit. With an increase in the number of SiPMs in a detector block, the CTR may be improved while the number of detector blocks may be reduced for the system, thereby reducing the cost of the associated electronic components.

However, the dark counts from each SiPM in an array may be summed up in the input signal to the timing pick-off circuit which may increase the noise associated with the signal. The increased dark counts may degrade the CTR. The dark count effect on the CTR increases as a number of SiPMs in a detector block increase. In order to limit the influence of dark counts on the CTR, a system is proposed to limit the number of SiPM inputs to one timing pick-off circuit by adding additional timing pick-off circuits without a change in the size of the detector block and the cost of the associated electronics low

As an example, a SiPM array may be divided into two or more regions with signal from each region used as an input signal for a single timing pick-off circuit. Dark counts from SiPMs in a single region may be summed in the input signal. By summing dark counts from a lower number of SiPMs, the detrimental effect of dark counts on the input signal may be reduced. In this way, the detrimental effects of Compton scattering may be reduced by incorporating an array of SiPMs and the detrimental effects of dark count summation may be lowered by dividing the array of SiPMs into multiple regions with each region corresponding to a separate timing pick-off circuit.

If each divided region in a SiPM array is small, a non-negligible amount of Compton events may be present which may degrade the CTR. To improve the CTR caused by inter-region Compton scatter, an overlap region may be created by duplicating the SiPM signal in the overlap region, which may capture the energy of inter-region Compton scatter fully, and adding one or more timing pick-off circuits. Signals from the plurality of timing pick-off circuits may be directed to an OR gate and the timing signal which reaches the OR gate first may be transmitted to a time-to-digital converter. By keeping the signal delay lines same from SiPMs to a timing pick-off circuit and then to an OR gate, the largest signal among the signals of the regions reaches the OR gate first

Various embodiments of the invention provide a multi-modality imaging system10as shown inFIGS. 1 and 2. Multi-modality imaging system10may be any type of imaging system, for example, different types of medical imaging systems, such as a Positron Emission Tomography (PET), a Single Photon Emission Computed Tomography (SPECT), a Computed Tomography (CT, an ultrasound system, Magnetic Resonance Imaging (MM), or any other system capable of generating tomographic images. The various embodiments are not limited to multi-modality medical imaging systems, but may be used on a single modality medical imaging system such as a stand-alone PET imaging system or a stand-alone SPECT imaging system, for example. Moreover, the various embodiments are not limited to medical imaging systems for imaging human subjects, but may include veterinary or non-medical systems for imaging non-human objects.

Referring toFIG. 1, the multi-modality imaging system10includes a first modality unit11and a second modality unit12. The two modality units enable the multi-modality imaging system10to scan an object or patient in a second modality using the second modality unit12. The multi-modality imaging system10allows for multiple scans in different modalities to facilitate an increased diagnostic capability over single modality systems. In one embodiment, multi-modality imaging system10is a Computed Tomography/Positron Emission Tomography (CT/PET) imaging system10, e.g., the first modality11is a CT imaging system11and the second modality12is a PET imaging system12. The CT/PET system10is shown as including a gantry13representative of a CT imaging system and a gantry14that is associated with a PET imaging system. As discussed above, modalities other than CT and PET may be employed with the multi-modality imaging system10.

The gantry13includes an x-ray source15that projects a beam of x-rays toward a detector array18on the opposite side of the gantry13. Detector array18is formed by a plurality of detector rows (not shown) including a plurality of detector elements which together sense the projected x-rays that pass through a medical patient22. Each detector element produces an electrical signal that represents the intensity of an impinging x-ray beam and hence allows estimation of the attenuation of the beam as it passes through the patient22. During a scan to acquire x-ray projection data, gantry13and the components mounted thereon rotate about a center of rotation.

FIG. 2is a block schematic diagram200of the PET imaging system12illustrated inFIG. 1in accordance with an embodiment of the present invention. The PET imaging system12includes a detector ring assembly40including a plurality of detector blocks. The PET imaging system12also includes a controller or processor44, to control normalization, image reconstruction processes and perform calibration. Controller44is coupled to an operator workstation46. Controller44includes a data acquisition processor48and an image reconstruction processor50, which are interconnected via a communication link52. PET imaging system12acquires scan data and transmits the data to data acquisition processor48. The scanning operation is controlled from the operator workstation46. The data acquired by the data acquisition processor48is reconstructed using the image reconstruction processor50.

The detector ring assembly40includes a central opening, in which an object or patient, such as patient22may be positioned using, for example, a motorized table24(shown inFIG. 1). The motorized table24is aligned with the central axis of detector ring assembly40. This motorized table24moves the patient22into the central opening of detector ring assembly40in response to one or more commands received from the operator workstation46. A PET scanner controller54, also referred to as the PET gantry controller, is provided (e.g., mounted) within PET system12. The PET scanner controller54responds to the commands received from the operator workstation46through the communication link52. Therefore, the scanning operation is controlled from the operator workstation46through PET scanner controller54.

During operation, when a photon collides with a crystal in a detector block62on a detector ring40, it produces a scintillation event on the crystal. Each photomultiplier tube or photosensor produces an analog signal that is transmitted on communication line64when a scintillation event occurs. A set of acquisition circuits66is provided to receive these analog signals. Acquisition circuits66produce digital signals indicating the three-dimensional (3D) location and total energy of the event. The acquisition circuits66also produce an event detection pulse, which indicates the time or moment the scintillation event occurred. These digital signals are transmitted through a communication link, for example, a cable, to an event locator circuit68in the data acquisition processor48.

The data acquisition processor48includes the event locator circuit68, an acquisition CPU70, and a coincidence detector72. The data acquisition processor48periodically samples the signals produced by the acquisition circuits66. The acquisition CPU70controls communications on a back-plane bus74and on the communication link52. The event locator circuit68processes the information regarding each valid event and provides a set of digital numbers or values indicative of the detected event. For example, this information indicates when the event took place and the position of the scintillation crystal in a detector block62that detected the event. An event data packet is communicated to the coincidence detector72through the back-plane bus74. The coincidence detector72receives the event data packets from the event locator circuit68and determines if any two of the detected events are in coincidence. Coincidence is determined by a number of factors. First, the time markers in each event data packet must be within a predetermined time period, for example, 4.5 nanoseconds, of each other. Second, the line-of-response (LOR) formed by a straight line joining the two detectors that detect the coincidence event should pass through the field of view in the PET imaging system12. Events that cannot be paired are discarded. Coincident event pairs are located and recorded as a coincidence data packet that is communicated through a physical communication link78to a sorter/histogrammer80in the image reconstruction processor50.

The image reconstruction processor50includes the sorter/histogrammer80. During operation, sorter/histogrammer80generates a data structure known as a histogram. A histogram includes a large number of cells, where each cell corresponds to a unique pair of detector crystals in the PET scanner. Because a PET scanner typically includes thousands of detector crystals, the histogram typically includes millions of cells. Each cell of the histogram also stores a count value representing the number of coincidence events detected by the pair of detector crystals for that cell during the scan. At the end of the scan, the data in the histogram is used to reconstruct an image of the patient. The completed histogram containing all the data from the scan is commonly referred to as a “result histogram.” The term “histogrammer” generally refers to the components of the scanner, e.g., processor and memory, which carry out the function of creating the histogram.

The image reconstruction processor50also includes a memory module82, an image CPU84, an array processor86, and a communication bus88. During operation, the sorter/histogrammer80counts all events occurring along each projection ray and organizes the events into 3D data. This 3D data, or sinogram, is organized in one exemplary embodiment as a data array90. Data array90is stored in the memory module82. The communication bus88is linked to the communication link52through the image CPU84. The image CPU84controls communication through communication bus88. The array processor86is also connected to the communication bus88. The array processor86receives data array90as an input and reconstructs images in the form of image array92. Resulting image arrays92are then stored in memory module82.

The images stored in the image array92are communicated by the image CPU84to the operator workstation46. The operator workstation46includes a CPU94, a display96, and an input device98. The CPU94connects to communication link52and receives inputs, e.g., user commands, from the input device98. The input device98may be, for example, a keyboard, mouse, a touch-screen panel, and/or a voice recognition system, and so on. Through input device98and associated control panel switches, the operator can control the operation of the PET imaging system12and the positioning of the patient22for a scan. Similarly, the operator can control the display of the resulting image on the display96and can perform image-enhancement functions using programs executed by the workstation CPU94.

The detector ring assembly40includes a plurality of detector units. The detector unit may include a plurality of detectors, light guides, scintillation crystals and analog application specific integrated chips (ASICs). For example, the detector unit may include twelve silicon photomultipliers (SiPM) devices, four light guides, 144 scintillation crystals, and two analog ASICs.

As an example, a detector block62may include an array of scintillation crystals and an array of SiPM devices. The SiPM devices may be divided into two or more regions with the SiPMs devices in the two or more regions transmitting signals to two or more distinct timing pick-off circuits. Each of the two or more regions in the array of SiPM devices may include at least one SiPM device. In one example, the at least one SiPM device may be included in one of the two or more regions, each of the two or more regions mutually exclusive. In another example, the at least one SiPM device may be included in two of the two or more regions, the two of the two or more regions overlapping each other. Each region of the two or more regions may transmit signal to one of the two or more distinct timing pick-off circuits via an output array. The output array may include a number of output channels with the number of output channels in the output array being equal to a number of SiPM devices in a region coupled to the output array with each SiPM device mapped to an output channel in the output array. An output channel may receive an analog signal from a SiPM device and each analog signal from each output channel in the output array may be summed to generate an input signal for a timing pick-off circuit coupled to the output array, the analog signal generated by collection of scintillation photons with the SiPM device.

Each timing pick-off circuit of the two or more distinct timing pick-off circuits may include a summing amplifier, a capacitor, and a comparator connected in series, each timing pick-off circuit comparing the input signal to a threshold signal and in response to the input signal being higher than the threshold signal, generating a square pulse signal. Each timing pick-off circuit output may be coupled to a logic circuit such as an OR gate. The logic circuit may receive square pulse signals from the two or more timing pick-off circuits and transmit a square pulse signal reaching the logic circuit first to a time to digital converter (TDC). The TDC may be part of acquisition circuits66of the PET imaging system.

An example detector block with a plurality of SiPMs is shown inFIG. 3arranges in an array. As such, each SiPM may further include a plurality of pixels (6, for example). An example where the detector unit300includes sixteen SiPMs is shown inFIG. 3. In such an example, each SiPM302includes plurality of pixels. For example, the SiPM302may include 6 pixels. As such the detector unit300then includes a total of 96 pixels (determined by 6×16). The signal output from each of the SiPMs may be summed and the summed signal may be used as an input signal for a timing pick-off circuit. However, with summing the signal outputs from all SiPMs in an array, the collective dark counts may be amplified, thereby affecting the signal quality. As shown inFIGS. 4-7, a SiPM array may be divided into multiple regions with each region corresponding to a timing pick-off circuit.

FIG. 4shows a first example400of a detector block402including sixteen individual SiPMs404forming an array. The SiPMs404may be arranged in a 4×4 matrix. The sixteen SiPMs may be divided into two separate regions with the top eight SiPMs forming a first region406and the lower eight SiPMs forming a second region408. The division of regions as shown here is an example configuration and any number of SiPMs may be configured as a region. In one example, the first region may include 7 SiPMs while the second region may include the remaining 9 SiPMs.

The SiPMs from the first region406may correspond to a first output array410. The first output array410may include eight individual output channels411(also referred herein as front-end amplifier) with each output channel411directly receiving signal from one SiPM. Each output channel411may be mapped to a single SiPM in the first region406. Similarly, the SiPMs from the second region408may correspond to a second output array412. The second output array412may include eight individual output channels413with each output channel413directly receiving signal from one SiPM. Each output channel413may be mapped to a single SiPM in the second region408.

The first output array410may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a first timing pick-off circuit414. The second output array412may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a second timing pick-off circuit416. By channeling signal from a plurality of SiPMs to a single timing pick-off circuit, the detrimental effect of Compton scattering on signal quality may be reduced. By dividing the SiPMs into two groups, the accumulated dark count in the input signals for the first timing pick-off circuit414and the second timing pick-off circuit416may each be reduced relative to an accumulated dark count in an input signal obtained from summing all sixteen SiPMs in the detector unit. In this way, by including an array of SiPMs in a detector block and separately processing signals from regions in the array, detrimental effects of Compton scattering and dark count accumulation may be reduced.

An example timing pick-off circuit700is shown inFIG. 7. The timing pick-off circuit700may be the first timing pick-off circuit414or second timing pick-off circuit416inFIG. 4. The timing pick-off circuit includes a summing amplifier702, a capacitor710, and a comparator704connected in series.

An output array may sum the signals received from individual SiPMs and generate an input signal706for the summing amplifier702. At the summing amplifier702, all input signals are summed and may be amplified with a low noise and high bandwidth amplifier. The output signal708from the summing amplifier702may pass through the capacitor710wherein only the high frequency component of the signal passes through to improve CTR while a low frequency electronic noise and a low frequency component of the summed signal708gets filtered out. After passing through the capacitor710, the signal712passes through the comparator704. At the comparator, the signal712is compared to a threshold signal714and if the signal712is higher than the threshold signal714, a square pulse may be generated. The square pulse may be termed as a hit signal generated from the timing pick-off circuit700.

Returning toFIG. 4, the respective hit signals generated from the first timing pick-off circuit414and the second timing pick-off circuit416may be transmitted to an OR gate418. At the OR gate, the first arriving hit signal between two binary signals (hit signals) generates its output that gets transmitted to TDC420. As such, when the signal trace lengths are kept same, the largest signal, among the signals of706, reaching the timing pick-off comparator704may generate the hit signal first and reach the OR gate418first.

The signal from the OR gate418may then be transmitted to a time to digital converter (TDC)420. The TDC420may recognize events in the form of signals reaching the TDC from the OR gate418and provide a digital representation of the time they occurred. For example, a TDC might output the time of arrival for each incoming signal. In one example, each of the first timing pick-off circuit414, the second timing pick-off circuit416, the OR gate418, and the TDC420may be part of acquisition circuits (such as acquisition circuits66inFIG. 2) of the PET imaging system.

Compton scattering may take place between the two regions in the SiPM array in the detector block402. In order to further reduce the effect of inter-region Compton scattering and improve the CTR, an overlap region may be included by duplicating the SiPM signal in the overlap region, which may capture the energy of inter-region Compton scatters fully, and adding one or more timing pick-off circuits.

FIG. 5shows a second example500of a detector unit502including sixteen individual SiPMs504divided into overlapping regions corresponding to separate timing pick-off circuits. The SiPMs may be arranged in a 4×4 matrix. The sixteen SiPMs may be divided into three regions with the top eight SiPMs forming a first region506, the lower eight SiPMs forming a second region508, and the middle eight SiPMs forming a third region510. Four SiPMs in the third region510is shared with the first region506while the four other SiPMs in the third region510is shared with the second region508. In this way, the third region510overlaps with each of the first region506and the second region508. The division of regions as shown here is an example configuration and any number of SiPMs may be configured as a region.

The SiPMs from the first region606may correspond to a first output array512. The first output array512may include eight individual output channels511with each output channel511directly receiving signal from one SiPM. Each output channel511may be mapped to a single SiPM in the first region506. Similarly, the SiPMs from the second region508may correspond to a second output array514. The second output array514may include eight individual output channels513with each output channel513directly receiving signal from one SiPM. Each output channel513may be mapped to a single SiPM in the second region508. The third output array516may include eight individual output channels515with each output channel515directly receiving signal from one SiPM. Each output channel515may be mapped to a single SiPM in the third region510. In this way, each SiPM in the third region510provides output to two output channels.

In one example, a SiPM in the third region510provides outputs to each of an output channel511in the first output array512and an output channel515in the third output array516. In another example, a SiPM in the third region510provide outputs to each of an output channel513in the second output array514and an output channel515in the third output array516. In this way, by duplicating signals from a plurality of SiPMs, the detrimental effect of inter-region Compton scattering may be reduced and signal to noise ratio may be improved. The duplication of a signal from a SiPM may be accomplished at the output channels.

The first output array512may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a first timing pick-off circuit518. The second output array514may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a second timing pick-off circuit522. The third output array516may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a third timing pick-off circuit520.

The respective hit signals generated from the first timing pick-off circuit518, the second timing pick-off circuit522, and the third timing pick-off circuit520may be transmitted to an OR gate524. The signal trace lengths from the SiPMs504to the OR gate524may be matched among the sub groups (such as the SiPMs in the first, second, and third regions). At the OR gate, the first arriving hit signal between three signals (hit signals) may generate an output pulse to be transmitted to a TDC. Whichever hit signal (from one of the first timing pick-off circuit518, the second timing pick-off circuit522, and the third timing pick-off circuit520) reaches the OR gate first will be transmitted to a time to digital converter (TDC)526. The TDC526may recognize events in the form of signals reaching the TDC from the OR gate526and provide a digital representation of the time they occurred.

FIG. 6shows a third example600of a detector block602including 36 individual SiPMs604divided into overlapping regions corresponding to separate timing pick-off circuits. The SiPMs may be arranged in a 6×6 matrix. As the size of a detector block gets larger with more SiPMs, the cost of associated electronics may be reduced. However, the total dark count may increase with more SiPMs. The 36 SiPMs may be divided into four regions to reduce the effect of dark counts. Twelve SiPMs in a top left corner of the 6×6 array may form a first region606, twelve SiPMs in a top right corner of the 6×6 array may form a second region608, twelve SiPMs in a lower left corner of the 6×6 array may form a third region610, and twelve SiPMs in a lower right corner of the 6×6 array may form a fourth region612.

The first region606overlaps with each of the second region608and the third region610with three shared SiPMs between the first and the second regions and another three shared SiPMs between the first and the third regions. The second region608overlaps with each of the first region606and the fourth region612with three shared SiPMs between the first and the second regions and another three shared SiPMs between the second and the fourth regions. The third region610overlaps with each of the first region606and the fourth region612with three shared SiPMs between the third and the first regions and another three shared SiPMs between the third and the fourth regions. The fourth region612overlaps with each of the second region608and the third region610with three shared SiPMs between the fourth and the second regions and another three shared SiPMs between the fourth and the third regions.

All SiPMs in the third region510inFIG. 5are overlapped with the neighboring regions. InFIG. 6, only a fraction of SiPMs in a region are overlapped with the neighboring regions to minimize the number of output channels614,616,618,620, and timing pick-off circuits622,624,626,628. The minimization of the necessary electronics may reduce both the power consumption of the ASIC and the electronic noise sources. The regions inFIG. 6is an example embodiment and the size of regions may be adjusted based on the amount of dark count from each SiPM, the size of SiPMs, the targeted size of a block detector, and the desired of the CTR level.

The division of regions as shown here is an example configuration and any number of SiPMs may be configured as overlapping regions.

The SiPMs from the first region606may correspond to a first output array614. The first output array614may include eight individual output channels613with each output channel613directly receiving signal from one SiPM. Each output channel613may be mapped to a single SiPM in the first region606. Similarly, the SiPMs from the second region608may correspond to a second output array616. The second output array616may include eight individual output channels615with each output channel615directly receiving signal from one SiPM. Each output channel615may be mapped to a single SiPM in the second region608. The third output array620may include eight individual output channels619with each output channel619directly receiving signal from one SiPM. Each output channel619may be mapped to a single SiPM in the third region610. The fourth output array618may include eight individual output channels617with each output channel617directly receiving signal from one SiPM. Each output channel617may be mapped to a single SiPM in the fourth region612.

In one example, a SiPM in the first region606may provide outputs to each of an output channel613in the first output array614and an output channel615in the second output array616. In another example, another SiPM in the first region606may provide outputs to each of an output channel613in the first output array614and an output channel619in the third output array620. In this way, by duplicating signals from a plurality of SiPMs, the detrimental effect of inter-region Compton scattering may be reduced and signal to noise ratio may be improved.

The first output array614may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a first timing pick-off circuit622. The second output array616may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a second timing pick-off circuit624. The third output array620may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a third timing pick-off circuit628. The fourth output array618may receive individual signals from eight SiPMs and sum the signal to form a single input signal for a fourth timing pick-off circuit626.

The respective hit signals generated from each of the first timing pick-off circuit622, the second timing pick-off circuit624, the third timing pick-off circuit628, and the fourth timing time-off circuit626may be transmitted to an OR gate630. At the OR gate, the first arriving hit signal amongst four hit signals may generate the output of the OR gate. Whichever hit signal (from one of the first timing pick-off circuit622, the second timing pick-off circuit624, the third timing pick-off circuit628, and the fourth timing pick-off circuit626) reaches the OR gate first may initiate the timing digitization in a time to digital converter (TDC)632. By design, when the signal trace lengths are kept same, the largest signal among the outputs of the output arrays of614,616,616,620, may generate the hit signal first at the timing pick-off circuits622,624,626,628and reach the OR gate630first. The largest signal may contain more collected SiPM signals that minimizes the effect of Compton scatter. The TDC632may recognize events in the form of signals reaching the TDC from the OR gate630and provide a digital representation of the time they occurred.

In this way, the system shown inFIGS. 1-7provides for a detector ring including a plurality of detector blocks, each detector block including an array of scintillation crystals and an array of silicon photomultiplier (SiPM) devices, the array of SiPM devices divided into two or more regions with each region including at least one SiPM device, and two or more timing pick-off circuits with each of the two or more timing pick-off circuits coupled to one of the two or more regions via a distinct output array. The two or more timing pick-off circuits may transmit individual signals from the two or more regions of the array to a single logic circuit. The two or more regions may include at least one common SiPM device, the at least one common SiPM coupled to at least two timing pick-off circuits.

FIG. 8shows a high-level flow chart illustrating an example method800for processing signal from a silicon photomultiplier (SiPM) array. Method800may be carried out using the systems and components described herein above with regard toFIGS. 1-7. For example, it may be accomplished by a combination of an analog ASIC and a TDC or a hybrid ASIC. However, it should be understood that the method may be carried out using different systems and components without departing from the scope of the disclosure.

At802, an analog signal is generated at each SiPM (such as SiPM404inFIG. 4) in a detector block corresponding to a scintillation event. A detector block may include a plurality of SiPMs arranged in an array. The SiPMs within an array may be divided into two or more regions. When a photon collides with a detector crystal on a detector ring, it produces a scintillation event on the crystal. Each SiPM constituting the crystal produces an analog signal when a scintillation event occurs.

At804, signals from each SiPM in a region (such as first region406inFIG. 4) may be transmitted to a dedicated output channel (such as output channel411inFIG. 4). The output channel may be part of an output array (such as output array410inFIG. 4). The output array may include a plurality of output channels with each output channel corresponding to a specific SiPM within a region. Each region within a SiPM array may correspond to an output array. As an example, a region with eight SiPMs may correspond to an output array with eight output channels.

At806, signals from each output channel within an output array may be summed. In this way, signals from all SiPMs within a region may be summed in an output array. At808, the summed signal from an output array may be transmitted to a timing pick-off circuit. Each output array may correspond to a specific timing pick-off circuit. In this way, signal from each region in the SiPM array is summed and transmitted to a corresponding timing pick-off circuit.

At810, at the timing pick-off circuit, the signal is transmitted through an amplifier, a capacitor, and a comparator. At the timing pick-off circuit, the signal is compared to a threshold signal and if the signal is higher than the threshold signal, a square pulse may be generated. The square pulse may be termed as a hit signal generated from the timing pick-off circuit. Hit signals may be generated at each timing pick-off circuit.

At812, hit signals from each timing pick-off circuit may be transmitted to an OR gate (such as OR gate418inFIG. 4). At814, whichever hit signal reaches the OR gate first may be transmitted to a time to digital converter (such as TDC420inFIG. 4). At816, the TDC may provide a digital representation of the time of occurrence of the scintillation event. For example, a TDC might output the time of arrival for each incoming signal.

At818, the digital signal generated at the TDC may be transmitted to a data acquisition processor (such as data acquisition processor48inFIG. 2). The data acquired by the data acquisition processor is reconstructed using an image reconstruction processor.

In this way, first signals generated at silicon photomultiplier (SiPM) devices included in a first region of a SiPM device array may be transmitted to a first timing pick-off circuit via a first output array, a first hit signal corresponding to the first region of the SiPM device array may be generated at the first timing pick-off circuit, second signals generated at SiPM devices included in a second region of a SiPM device array may be transmitted to a second timing pick-off circuit via a second output array, a second hit signal corresponding to the second region of the SiPM device array may be generated at the second timing pick-off circuit, and one of the first hit signal and the second hit signal may be transmitted via a logic circuit coupled to each of the first timing pick-off circuit and the second timing pick-off circuit to reduce dark counts.

In this way, with even an increased number of SiPMs in a detector block, the CTR may be improved. The technical effect of dividing a SiPM array into two or more regions with signal from each region used as an input signal for a single timing pick-off circuit is that the detrimental effects of dark counts may be reduced. By summing dark counts from a fewer number of SiPMs, signal quality may be improved. By dividing the SiPM array into overlapping regions and adding one or more further timing pick-off circuits, SiPM signal may be duplicated in the overlap region and the CTR of the inter-region Compton scatter signals may be improved.