On-Chip Pixel Binning in Photon-Counting EMCCD-Based Gamma Camera: A Powerful Tool for Noise Reduction

Charge Coupled Devices (CCDs) optically coupled to scintillation crystals can be used to construct high resolution gamma cameras. Previously, several groups have reported intrinsic detector spatial resolutions below 100 microns. When the CCD images can be read out fast enough, the energy and interaction position of individual gamma quanta can be estimated by a real-time image analysis of the scintillation light flashes. The Electron-Multiplying CCD (EMCCD) is well-suited for fast read out, since even at high frame rates it has extremely low readout noise. However, due to the often very low light levels present in scintillation gamma cameras, further reduction of noise is desirable. Here, the EMCCD is optically coupled to a 1000-mum-thick columnar CsI(Tl) crystal for Tc-99m and 1-125 imaging. We investigate noise reduction through summing of signals in pixels before the gain register and readout circuit of the EMCCD ("pixel binning"). We compare the energy and spatial resolution, photopeak efficiency (PE) and signal-to-noise ratio (SNR) of an EMCCD-based gamma camera for the case of binning vs. non-binning. When pixels were read out simultaneously in groups of 4 the spatial resolution is slightly worse in the direction of binning, but the number of false-positive counts resulting from background noise for 1-125 was reduced by 74% compared to the no binning case. We conclude that the use of charge binning when reading out EMCCDs may significantly improve the energy spectra and noise properties of CCD-based high-resolution gamma cameras.