All-optical Temporal Correlation for PET

PET requires temporal correlation of the two 511 keV “annihilation photons” simultaneously emitted for every PET tracer radioactive decay. Time-of-flight (TOF)-PET uses photon arrival time measurements that provide information on the two-photon emission location within the body to substantially increase image signal-to-noise ratio, enabling significant improvements in disease visualization and quantification, or large reduction in injected dose and/or scan time. Precise photon arrival time information requires very high temporal resolution, however. Our lab has developed a method of directly modulating vertical cavity surface emitting lasers (VCSELs) using the output of very fast photodetectors known as solid-state photomultipliers (SSPMs) coupled to fast scintillation crystals.

This technique allows conversion of the electrical pulses from PET scintillation detectors into optical laser pulses. We hypothesize that purely optical temporal correlation of these light pulses from coincident 511 keV photons based on nanophotonic crystals rather than conventional electronic coincidence processing will significantly improve photon arrival time resolution (<100ps), advancing TOF-PET performance. We have devised a method of encoding PET ring detector position by introducing characteristic pulse delay times in each channel using nanophotonic crystals, then multiplexing all channels into a single system output (extremely fast photodetector). This will allow us to construct a PET scanner with fast time resolution for TOF, while reducing electronics cost and complexity.