Scientists at Philips have developed a new fully digital silicon photomultipliers (SiMP’s) that may replace detectors within PET scanners as well as open up possibilities for other ultra-sensitive detectors for DNA sequencing and protein/DNA microarrays.
By integrating low-power CMOS electronics into the silicon photomultiplier chip, the team at Philips has developed a digital silicon photomultiplier in which each photon detection is converted directly into an ultra high speed digital pulse that can be directly counted by on-chip counter circuitry. In contrast to conventional silicon photomultipliers, the Philips digital silicon photomultiplier is therefore an all-digital (digital-in/digital-out) device. As a result, it produces faster and more accurate photon counts with extremely well defined timing of the first photon detection, both of which are important factors in applications such as medical imaging scanners and high-energy nuclear particle detectors.
The PET system detects pairs of gamma rays (high energy electromagnetic radiation) originating from a radioactive tracer, a small amount of which is injected into the patient prior to the scan. To image metabolic activity, PET typically uses a radioactive derivative of glucose called fluorodeoxyglucose (FDG). This compound mimics the behavior of glucose in the body and can be detected by the PET system.
For so-called ‘time-of-flight’ PET scanners, accurately determining the time at which the first photon arrives at the detector is extremely important. Philips’ digital silicon photomultiplier prototypes achieve a timing accuracy for the detection of the first photon of around 190 ps (full-width, half-maximum using a standard scintillator crystal (LYSO) at 511 keV for two detectors in coincidence).
Conventional silicon photomultipliers (SiPMs) consist of a two-dimensional array of avalanche photodiodes (APDs) each of which is connected in series with its own polysilicon ‘quenching’ resistor. All of these diode/resistor ‘microcells’ are then connected in parallel and the entire microcell array is reverse-biased to a voltage above the diodes’ normal breakdown voltage – typically in the range 30V to 70V. Operating in this so-called ‘Geiger mode’, the diodes are ultra-sensitive to single electron-hole pairs that result in individual diodes experiencing avalanche breakdown. These electron-hole pairs can be generated either by the absorption of a photon (the desired signal), or by thermal energy or electron tunneling (unwanted background noise). The unwanted background noise produced by thermally generated electron-hole pairs and/or electron tunneling, together with false counts due to defective microcells, are collectively referred to as the SiPM’s ‘dark count’.
To eliminate a conventional SiPM’s need for an external digitizing ASIC, the digital silicon photomultiplier developed by Philips equips each individual avalanche photodiode with its own 1-bit on-chip ADC (Analog to Digital Converter) in the form of a CMOS inverter. Each microcell that experiences avalanche breakdown therefore produces its own digital output that is captured, along with the digital outputs from all other triggered microcells, by an on-chip counter. The Philips digital SiPM therefore converts digital events (photon detections) directly into a digital photon count. As a result, it is capable of achieving significantly better resolution than conventional SiPMs.
To overcome the ‘dark count’ problem associated with conventional SiPMs, each microcell in the Philips digital SiPM is also equipped with an addressable static memory cell that can be used to disable or enable the microcell. Microcells that show high dark count levels can therefore be prevented from contributing false counts to the SiPM’s output. This facility allows the Philips’ digital SiPM to achieve better signal-to-noise ratios than conventional devices. Because defective microcells in the array can be disabled, it also helps to improve production yield.
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