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

摘要

A lens-coupled electron multiplying charge-coupled device (EMCCD)-based gamma camera capable of performing photon counting for both $^{99m}{rm Tc}$ and $^{125}{rm I}$ 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 $-70,^{circ}$ 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 $^{125}{rm I}$ source. A single 0.5 mm diameter pinhole collimator was used for SPECT reconstruction of a mouse thyroid gland containing $100~mu{rm Ci}$ (3.7 MBq) of $^{125}{rm I}$ uptake. We found that although the photopeak for $^{99m}{rm Tc}$ (140 keV) could be resolved, the photopeak for $^{125}{rm I}$ $(approx 27~{rm keV})$ could not be fully resolved due to the low optical transfer efficiency of dual lens coupling $(u0003C; 5%)$. Nonetheless, energy windowing for $^{125}{rm I}$ 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.