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 (Formula presented) (Formula presented) silicon photomultiplier array. We performed GEANT4 Monte Carlo simulations for ^{99 m}Tc 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 (Formula presented) (Formula presented) azimuthal scan with a centre-of-gravity (COG) estimator and a von Mises maximum-likelihood fit. Main results. Shielding effectiveness reached (Formula presented) (Formula presented) with the lead collimator and (Formula presented) (Formula presented) 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 (Formula presented) (Formula presented), with spatial and angular resolutions comparable to commercial devices. Over the 360 (Formula presented) (Formula presented) scan, the mean absolute deviation of the estimated direction was 4.05 (Formula presented) (Formula presented) COG and 4.55 (Formula presented) (Formula presented) (maximum-likelihood), with a maximum below 7.2 (Formula presented) (Formula presented) 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.