1. Development Process of PET Digital Technology
In the initial PET systems, energy, position, and time information were converted into digital signals using photomultiplier tubes (PMT), front-end analog circuits, and subsequent analog-to-digital converters (ADC/TDC) for event matching and image reconstruction. Due to the relatively backward chip technology at that time, the analog-to-digital conversion unit could not be integrated within the detector module and was instead placed outside, with some designs requiring long cables to transmit signals to the ADC. The extensive analog processing and long transmission paths resulted in poor signal quality and limited system scalability and performance.
In the 1990s, SiPM photoconversion devices emerged. These devices utilize integrated technology to form arrays of avalanche photodiodes (APD), making them chip-based devices with short transmission paths and high integration levels. With the development of electronic technology, the analog-to-digital conversion and signal processing circuits at the backend of SiPM have also gradually become chip-based, implemented through application-specific integrated circuits (ASIC) or field-programmable gate arrays (FPGA). The integration of PET detectors has further improved, with ADCs designed based on ASIC or FPGA gradually moving closer, reducing analog signal processing while increasing digital signal processing. Positioning the analog-to-digital conversion unit as close to the photoconversion device as possible can further enhance performance.
ASICs feature low power consumption, small size, and high performance, which can further simplify the structure of PET detectors and make them easier to scale. The combination of SiPM and ASIC allows for the reading and processing of complex signals, fully leveraging the fast transmission and low attenuation characteristics of digital signals, greatly enhancing overall system performance. Compared to traditional digitalization schemes that rely heavily on analog processing components, the reduction in signal transmission paths leads to gradual improvements in overall system performance.
2. Differences Among Various Photoconversion Devices
Since PMTs require high voltage power supplies (around 1000V) and the signal can easily degrade due to interference during the amplification process (around 10cm), the emergence of silicon photomultiplier tubes (SiPM) has gradually attracted attention in the PET field, leading to the use of SiPM instead of PMT as the photoconversion device in PET detectors. SiPMs only require low voltage power supplies (around 30V), have short signal transmission paths (<1mm), and are easy to mass-produce chip devices. The use of SiPM devices results in better signal quality, modularity, and scalability, enhancing key performance metrics of PET.
APD photoconversion devices operate in avalanche mode, with sizes around 3mm, while the tiny APD units in SiPMs operate in Geiger mode and are typically only around 35um. The integration level of SiPMs has significantly increased, with SiPM arrays often integrating thousands or even tens of thousands of diodes, whereas APD arrays can typically only integrate a few diodes. Due to their magnetic resonance compatibility, APD photoconversion devices were used in early PET/MR equipment. However, with the advancement of integration technology, they are being replaced by SiPM photoconversion devices.
3. Integration Level of Detectors
Analog detectors often use discrete components for circuit design, or their digital circuit parts utilize small-scale ASIC or FPGA designs. Constraints such as the size limitations of photoconversion devices, microelectronics technology levels, or cost considerations result in analog detector modules that include scintillation crystals, photoconversion devices, and analog signal processing components, physically packaged together. This design is common in detector modules based on PMT and APD photoconversion devices, and some SiPM-based detectors also adopt this structure. The detector outputs an analog signal, which is then converted to digital.
Digital detectors predominantly utilize integrated circuits in their design, employing large-scale ASIC or FPGA technology. With the emergence of SiPM devices and advancements in microelectronics technology, PET detector modules represented by SiPM increasingly adopt large-scale integrated circuits. The size of SiPMs is significantly smaller than that of PMTs, and data processing units such as ADCs also use integrated chip designs (based on ASIC or FPGA). The physical entity of the detector module can package scintillation crystals, photoconversion devices, and digital processing units, even including individual event processing stages. Detectors based on SiPM often adopt this structure, with the detector module directly outputting digital signals.
4. Impact on Overall System Performance
The choice of digitalization approach is one of the important factors affecting the performance of PET systems. Additionally, factors such as the type of crystal material, crystal size, detector structure and arrangement, optimization details of the digitalization scheme, image reconstruction, and calibration algorithms also significantly influence PET performance. Typically, it is not possible to enhance overall system performance through a single factor; only by collaboratively optimizing various factors across the imaging chain can the system performance be maximized to achieve the best image quality.
5. Explanation of Terms Related to PET Digitalization
Due to the significant differences in digitalization technology routes among different products, various terminology has emerged. To facilitate understanding of different manufacturers and technologies, the following common terms are explained.
Multi-Pixel Photon Counter (MPPC): A term for SiPM based on its principle, with SiPM being the general term.
Micro Cell: A single SiPM unit that can correspond to multiple APD anodes.
Unit: The smallest unit in the processing or design of detectors, consisting of scintillation crystals, photoconversion devices (PMT or SiPM micro units), and subsequent circuits.
SiPM Crystal Array Coverage: The ratio of the sensitive area of all crystal cross-sections in the crystal array to the actual area of the light-emitting surface of the crystal array.
Module: The smallest detector module that can be independently assembled or replaced.
Number of Detector Rings: The number of detector units arranged along the FOV axis in a PET detection system (it is recommended that applicants specify the number of crystal rings in registration materials).
Block Ring Count: The number of block detector units arranged along the FOV axis in a PET detection system (it is recommended that applicants specify the number of crystal rings in registration materials).
ASIC (Application-Specific Integrated Circuit): Commonly used in the PET digitalization process to process multiple signals and extract digital signal amplitude and timing information.
FPGA (Field-Programmable Gate Array): Commonly used in the PET digitalization process for analog-to-digital conversion or event matching processing.
Raw Pulse Waveform: The light pulse waveform generated by gamma rays in the scintillation detector.
Multi-Voltage Threshold Sampling (MVT): A method that presets multiple voltage thresholds and records the time for pulses to pass each threshold, using mathematical models to restore the original waveform of the pulse.
Digital Photon Counter (DPC): A photoconversion device capable of digitally recording the number of arriving photons.
Free-Sample ADC: A high-frequency clock-driven device that continuously samples the voltage amplitude of the signal path, accurately recording the shape of the signal pulse, baseline fluctuations, pulse tails, and the analog-to-digital conversion of signal accumulation.