Optimization of a gamma imaging probe for axillary sentinel lymph mapping

Sentinel lymph node (SLN) mapping is a technique for assessing whether early-stage invasive breast cancer has metastasized, thus determining prognosis and treatment options. SLN identification is achieved using the blue-dye and radioactive colloids techniques, which are sometimes combined with lymphoscintigraphy. Furthermore, intra-operative gamma acoustic probes, as well as gamma imaging probes are used during surgery. The purpose of this study is the construction of a gamma probe for sentinel lymph node imaging and its optimization in terms of sensitivity with respect to spatial resolution. The reference probe has small field of view (2.5 x 2.5 cm(2)) and is based on a position sensitive photomultiplier tube (PSPMT) coupled to a pixellated CsI(Tl) scintillator. Following experimental validation, we simulated the system using the GATE Monte Carlo toolkit (GATE v6.1) and modeled various collimator geometries, in order to evaluate their performance and propose the optimal configuration. The constraints of the proposed gamma imaging probe are i) sensitivity close to 2 cps/kBq and ii) spatial resolution equal to 6 mm at 2 cm source-to-collimator distance and similar to 10mm at 5 cm. An integrated structure that achieves those requirements is a tungsten collimator with 2 x 2 mm(2)square holes, 16 mm thickness, 0.15 mm septa, where each CsI(Tl) 2 x 2 x 5 mm(3) crystal pixel is placed inside the collimator.