Performance evaluation of a 64-pixel surgical probe for FDG imaging

We report on the performance of a completed 64-pixel positron-sensitive surgical probe using wall-less sphere sources in preparation for clinical testing. The method of making volumetric positron sources by uniformly mixing ¹⁸F with beeswax through a chemical procedure enables us to evaluate the performance of the probe in a more realistic environment. As previously reported, the probe consists of a dual-layer detector and a multianode 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 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. Now using wall-less sphere sources we more completely characterize the probe´s performance based on a more accurate simulation of the sentinel lymph node surgical environment. The new measurements show that the 64-pixel probe has a sensitivity of ∼20 cps/pixel per μCi/cc using optimal signal selection criteria. The probe is able to identify in 10 seconds a ∼4 mm lesion with a true-to-background ratio of ∼3 at a tumor uptake ratio of ∼8:1.