A high-sensitivity hand-held medical gamma camera with mosaic-patterned scintillation detector

Introduction: Portable medical gamma cameras are widely used in today’s clinics. Their applications include tumor localization, sentinel lymph node detection, intraoperative imaging guidance and radiation monitoring. However, current gamma cameras suffer from low detection efficiency due to the mechanical collimator, which brings the challenges for real-time low-dose gamma imaging. To overcome this limitation, we propose a collimator-less medical gamma camera with high detection efficiency for fast and accurate low-dose gamma imaging. With a mosaic-patterned 3D position-sensitive scintillator detector, we build the correlation between detected photon distribution and incoming gamma-rays, which enables high-sensitivity gamma imaging without metal collimators. In this work, we build a hand-held gamma camera prototype and evaluate its performance through experiments.

Methods: In this work, we use bar-shaped GAGG scintillators with polyvinyl chloride (PVC) element (as the mechanical supporting frame) to form a mosaic-patterned 3D position-sensitive scintillation detector. With this design, the absorption of photons on one scintillator naturally forms collimation for other scintillators. Therefore, the measured photon events distribution in the whole detector is sensitive to incoming gamma-rays, which enable high-sensitivity gamma imaging without mechanical collimation.

The detector is composed of 32 GAGG scintillators and 32 PVC elements, forming an 8 × 8 mosaic pattern. The size of each scintillator and PVC elements is 3.36 mm × 3.36 mm × 20.00 mm and the whole detector block is 26.88 mm × 26.88 mm × 20.00 mm elements in size. We use the dual-end-readout technique to measure the 3D photon interaction positions with SiPMs coupled at both ends. We integrate the detector, readout electronics, a power supply module, a wide-field optical camera and a tablet computer in the portable hand-held gamma camera with an overall size of 332 mm × 213 mm × 75 mm and a total weight of 2.7 kg.

We use a ^99mTc point source with a diameter of 5 mm to mimic a target imaging object. We build an experimental platform that can accurately control the relative position between the gamma camera and point source. The distance between the gamma camera and point source is 250 mm. We use a 15.8 mCi ^99mTc point source to measure the detection sensitivity in a 300 mm × 300 mm field of view at 250 mm source-to-camera distance. and to calibrate the system matrix for image reconstructions. We measure a series of testing projection data with different activity and measurement time. Projection data at different positions are mixed to mimic multiple sources imaging cases. We use ML-EM algorithm for reconstruction. We fuse the reconstructed gamma images with optical images to test the feasibility of realistic applications.

Results: The average detection sensitivity in the whole FOV is 300 cps/MBq. The point source imaging study shows that the proposed gamma camera can accurately locate a point source with 100 μCi ^99mTc total activity and 1 s measurement time. The average positioning accuracy of a 300 μCi ^99mTc point source is 4.8 ± 2.0 mm, 6.2 ± 2.8 mm and 7.4 ± 4.2 mm with 10 s, 3 s and 1 s measurement, respectively. With 1 s measurement, four 300 μCi point sources are clearly resolved. Fusion of gamma activity distribution and optical image shows that the gamma camera is suitable for realistic medical applications such as surgical guidance.

Conclusions: Our study shows that the proposed high-sensitivity medical gamma camera design has the potential to achieve fast and accurate low-dose gamma imaging. Further investigations are ongoing to fully characterize the system performance and perform real pre-clinical and clinical tests.

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