Performance evaluation of a 64-pixel positron-sensitive surgical probe in simulated sentinel lymph node surgical environment

The prototype 64-pixel positron-sensitive surgical probe has been repackaged in preparation for moving the probe into clinical surgical environment The probe detector front opening is covered with a 25 μm thick aluminum foil, which makes the probe a both light-tight and sterilizable system. As previously reported, the probe consists of a dual-layer detector and a multi-anode PMT (Hamamatsu H7546). The first layer consists of an 8×8 array of thin plastic scintillators, which is 1:1 optically coupled to an 8×8 array of 2×2×10 mm³ GSO crystals. Each pixel element is optically isolated using CaCO₃ sheet reflectors. With this dual-layer design and customized 64 channels of electronics (with FPGA), a valid event is a signal in the plastic layer characterized with an overshoot by the pulse-shaping circuit, in coincidence with an annihilation 511 keV gamma in the GSO layer, selected by an energy threshold on the PMT summed dynode signal. Operating this repackaged probe system outside of the black box, the new measurements show that the 64-pixel probe has a sensitivity of ∼36 cps/pixel per μCi/cc using optimal signal selection criteria, which is 80% higher than the sensitivity of the previous system which did not have a foil covering. The repackaged probe system is also confirmed to have similar spatial resolution and good lesion identification ability as previous system. For example, it is able to identify in 10 seconds a ∼2.8 mm lesion with a contrast of ∼2.6 at the presence of both 511 keV gamma (tumor to background ratio: 7:1) and 140 keV gamma background (tumor to background ratio: 1:16).