Performance Evaluation of a Position-Sensitive SiPM-based Gamma Camera for Intraoperative Imaging

The POSiCS camera is a handheld, small field-of-view gamma camera developed for multipurpose use in radio-guided surgery (RGS), with sentinel lymph node biopsy (SLNB) as its benchmark application. This compact and lightweight detector (weighing approximately 350 g) can map tissues labeled with Tc-99m nanocolloids and guide surgeons to the location of target lesions. By enabling intraoperative visualization in close proximity to the surgical field, its primary objective is to minimize surgical interventional invasiveness and operative time, thereby enhancing localization accuracy and reducing the incidence of post-operative complications. The design and components of the POSiCS camera emphasize ergonomic handling and compactness, providing, at the same time, rapid image formation and a spatial resolution of a few millimeters. These features are compatible with routine operating-room workflow, including wireless communication with the computer and a real-time display to support surgeon decision-making. The spatial resolution measured at a source-detector distance of 0 cm was 1.9 +/- 0.1 mm for the high-sensitivity mode and 1.4 +/- 0.1 mm for the high-resolution mode. The system sensitivity at 2 cm was evaluated as 481 +/- 14 cps/MBq (high sensitivity) and 134 +/- 8 cps/MBq (high resolution). For both working modes, we report an energy resolution of approximately 20 percent, even though the high-resolution collimator exhibits an increased scattered component due to the larger amount of tungsten.

💡 Research Summary

The manuscript presents a comprehensive performance evaluation of the POSiCS (Position‑sensitive SiPM Compact and Scalable) handheld gamma camera, a wireless, lightweight (≈ 350 g) device designed for intra‑operative radio‑guided surgery (RGS), with sentinel lymph node biopsy (SLNB) as the benchmark application. The system integrates a thin, pixelated LYSO:Ce scintillator (30 × 30 mm², 30 × 30 pixel matrix) with a 3 × 3 array of linearly graded silicon photomultipliers (LG‑SiPMs) produced by FBK. Each LG‑SiPM features a 25 µm microcell pitch and an active area of ~10 × 10 mm²; a resistive network enables Anger‑type center‑of‑gravity (CoG) reconstruction using only eight readout channels (four for x, four for y). This architecture yields sub‑millimeter intrinsic positional precision while keeping the electronics compact and power‑efficient, allowing wireless data transmission via an ESP32 module.

Two interchangeable parallel‑hole collimators are provided: a low‑energy high‑sensitivity (LEHS) collimator with short septa and larger holes for rapid scanning, and a low‑energy high‑resolution (LEHR) collimator with longer septa and smaller holes for precise lesion delineation. Both are fabricated from tungsten, with septal penetration engineered to stay below 5 % and external shielding walls to suppress side‑incident photons.

Performance testing followed a subset of the 2023 NEMA standards, adapted for a small‑field device. Spatial resolution was measured using a 57Co point source (122 keV) placed at the detector centre; 100 k events were acquired without post‑processing smoothing. Gaussian fits to the x‑ and y‑profiles yielded full‑width‑at‑half‑maximum (FWHM) values of 1.9 ± 0.1 mm for the LEHS mode and 1.4 ± 0.1 mm for the LEHR mode, representing the extrinsic resolution limited primarily by the scintillator pixel size and SiPM readout precision. Sensitivity was assessed with a uniform 99mTc liquid phantom at a 2 cm source‑detector distance, resulting in 481 ± 14 cps/MBq (LEHS) and 134 ± 8 cps/MBq (LEHR). These values surpass many previously reported small‑field intra‑operative cameras by a factor of two to three. Energy resolution at the 140.5 keV photopeak was approximately 20 % for both configurations; the LEHR collimator exhibited a modestly higher scatter component due to its greater tungsten mass. Additional measurements with 177Lu (multiple photon energies up to >300 keV) demonstrated that the system can handle multi‑energy emissions, with acceptable degradation in sensitivity and resolution.

The authors discuss several practical considerations. The intrinsic radioactivity of LYSO (from 176Lu) contributes a background count rate of ~420 cps, which is mitigated by energy windowing and can be exploited for self‑calibration. Collimator side‑leakage, while reduced by the external shielding, remains a source of scattered photons that may affect image uniformity. The wireless architecture introduces a modest (~1 s) latency, acceptable for real‑time surgical guidance. Limitations include the trade‑off between sensitivity and resolution inherent to the two collimators, and the need for further optimization of septa thickness and material to minimize scatter without sacrificing mechanical robustness.

In conclusion, the POSiCS camera delivers millimeter‑scale spatial resolution, high sensitivity, and real‑time wireless imaging in a ergonomically favorable package. Its dual‑mode collimator system allows surgeons to select the optimal balance between speed and precision intra‑operatively. The demonstrated performance with both 99mTc and 177Lu suggests suitability for a broad range of theranostic applications. Future work should focus on larger clinical trials, advanced image reconstruction (e.g., AI‑based de‑blurring), and refined collimator designs to further suppress scatter and improve uniformity, positioning POSiCS as a potential new standard for intra‑operative nuclear imaging.