Relevance of a multi-hole collimator for peroperative gamma detection

Aim: Peroperative detection is usually performed with a gamma probe equipped with a single-hole collimator (SHC). The aim of this study was to compare SHC with a new highly resolutive multi-hole collimator (MHC) in situations simulating the in vivo detection of two radioactive sources. Materials and Methods: The probe was composed of a SHC or a MHC, a CsI(Tl) crystal, a photomultiplier tube and a count rate unit. Sensitivity response curves were experimentally obtained with a 57Co source. From these values, the response curve in front of two small sources was simulated with an Excel software. A 30 MBq source (A) was placed at 1 cm depth and a 300 kBq source (B) was placed, successively, at 1, 3 and 5 cm. B was then moved towards A by 1 mm steps. A threshold background/signal of 50% was used to determine the minimal distance between the sources (Dmin) to remain separately detectable. Results: At 1, 3 and 5 cm depth, Dmin was, respectively, 35, 45 and 53 mm for SHC and 14, 17 and 20 mm for MHC and the count rate in front of B was, respectively, 3060, 480 and 210 cps with the SHC and 900, 360 and 150 cps with the MHC. Conclusion: When compared with SHC, the loss in sensitivity with the MHC is only moderate for long source–probe distances while the gain in resolution remains important at any distance. MHC is an interesting option for peroperative detection.