High-Resolution Photon Counting Using a Lens-Coupled EMCCD Gamma Camera

A lens-coupled electron multiplying charge-coupled device (EMCCD)-based gamma camera capable of performing photon counting for both Tc-99m and I-125 sources has been constructed. This system differs from previous EMCCD-based gamma cameras by using lens-coupling rather than fiber-optic coupling to transfer the light from the scintillating crystal to the EMCCD. The gamma camera described herein uses a micro-columnar CsI(Tl) crystal, two f/0.95 lenses, and a commercial camera containing the e2v CCD97 EMCCD that was cooled to -70 degrees C. Acquisition of the video-rate frames from the CCD97 was performed using LabVIEW software. Real-time photon counting analysis of the individual scintillation flashes within the CCD97 frames was performed by using the LabVIEW IMAQ software module. An intrinsic resolution of 56 mu m FWHM was measured by using a 25 mu m slit collimator and I-125 source. A single 0.5 mm diameter pinhole collimator was used for SPECT reconstruction of a mouse thyroid gland containing 100 mu Ci (3.7 MBq) of I-125 uptake. We found that although the photopeak for Tc-99m (140 keV) could be resolved, the photopeak for I-125 (approximate to 27 keV) could not be fully resolved due to the low optical transfer efficiency of dual lens coupling (< 5%). Nonetheless, energy windowing for I-125 sources was used to eliminate most of the background events, proving that high-resolution photon counting for low-energy sources can be achieved by using simple lens-coupling.