Performance of a dual-layer positron-sensitive surgical probe

A positron-sensitive surgical probe is being built based on a multi-anode PMT and a dual-layer detector, which consists of an 8×8 array of thin plastic scintillators and a matched GSO crystal array. Our probe uses three selection criteria to identify positrons and suppress background gammas, including annihilation 511 keV gammas. First an energy threshold was applied on the plastic signals; next a second energy threshold was applied on the PMT sum signal; finally, a coincidence technique between the positrons and the annihilation 511 keV gammas was applied. These selection criteria were individually tested and optimized, and have been implemented with 9 channels of electronics. Experiments were conducted using phantoms with ¹⁸F-FDG and ^99mTc, commonly used in sentinel lymph node (SLN) surgery. Measurements based on the 9-channel electronics indicate that the sensitivity of the 9-channel probe to positrons from ¹⁸F-FDG is ∼69-152 cps/kBq (2.5-5.6 kcps/μLCi) at different signal selection criteria The final 64-channel probe is expected to have ∼40% higher positron sensitivity. The pixel separation is ∼3.2 in terms of the peak to valley ratio. The second layer of the detector gives superior rejection power for 140 keV gammas. The true to false positron count ratio in the presence of 511 keV and 140 keV background gammas is expected to be high (>10) at a tumor to background ratio of 10:1.