Monte Carlo evaluation of a novel hybrid-shielded gamma probe with integrated counting and emission direction-finding capabilities

OBJECTIVE: Gamma probes for sentinel lymph node localization usually work in counting mode only, with no information on the angular origin of detected gammas. Active-shielded designs that add emission direction finding reach about 95-97% shielding effectiveness, below the >99% level of passive-shielded commercial probes. We propose and evaluate a hybrid-shielded gamma probe combining both capabilities. Approach. The module integrates a central GAGG(Ce) scintillator for counting, an asymmetric lead or sintered tungsten shielding layer, and an outer ring of GAGG(Ce) scintillators that provide passive attenuation and direction finding functionality. The scintillators are read out by a 4 x 4 silicon photomultiplier array. We performed GEANT4 Monte Carlo simulations for Tc-99m source. Sensitivity, spatial and angular resolution, and shielding effectiveness were characterized according to the NEMA NU3-2004 protocol in air and water, and direction-finding accuracy was assessed over a 360° azimuthal scan with a centre-of-gravity estimator and a von Mises maximum-likelihood fit. Main results. Shielding effectiveness reached 99.09 ± 0.10% with the lead collimator and 99.26 ± 0.12% with the sintered tungsten one, both within the range of passive-shielded commercial probes. Sensitivity at 10 mm in air was around 4560 cps/MBq, with spatial and angular resolutions comparable to commercial devices. Over the 360° scan, the mean absolute deviation of the estimated direction was 4.05° (centre-of-gravity) and 4.55° (maximum-likelihood), with a maximum below 7.2° in both cases. Significance. Keeping shielding and directional sensing in separate elements allows each to be optimized independently. To our knowledge, this is the first gamma probe concept combining emission direction finding with shielding above 99%, which has been the main limitation of active-shielded designs. GAGG(Ce), not used in any commercial probe yet, is non-hygroscopic and well matched to silicon photomultipliers. These results motivate experimental prototyping and clinical validation.