Characterization of a handheld gamma camera for intraoperative use for sentinel lymph node biopsy

Gamma counting probes play an important role in radioguided sentinel lymph node biopsy surgical procedures. The miniaturization of gamma camera components and associated electronics has allowed the introduction of small handheld gamma cameras to be used in place of simple gamma counting probes in these procedures. We report here on the design of a compact handheld gamma camera and first results from the characterization of this device. A handheld gamma camera with field of view 13.2×13.2 mm² was constructed based on the SensL SPMArray4SL 4×4 pixel SiPM detector coupled to a 4×4 element CsI:Tl scintillator array with crystal size 3.3×3.3×5 mm³. The detector is mounted in a 114×32×26 mm³ ABS plastic enclosure together with a low energy high resolution (LEHR) parallel hole collimator and front end electronics. The total package has a weight of only 320 g. Each SiPM pixel is individually read out using a 2 channel ADC and a custom 8:1 analog multiplexor. The camera is controlled through a custom GUI program written in National Instruments LabView. A preliminary characterization of the detector was performed using a ⁵⁷Co point source (1 mm diameter). The spatial resolution, measured as the full-width at half-maximum (FWHM) of the crystal response profiles measured by stepping the point source across the detector face in 0.25 mm increments, was 3.46, 3.92, 4.56 and 6.24 mm for source-detector distances of 1, 10, 25 and 50 mm, respectively. The FWHM energy resolution at 122 keV was 40.2% and the detection efficiency was 162.9 and 149.7 cps/MBq for source-detector distances of 1 and 50 mm, respectively. The system count rate vs. incident gamma-ray fluence was linear up to 10⁴ cps.