Development and Evaluation of an EMCCD Based Gamma Camera for Preclinical SPECT Imaging
The electron-multiplying charge-coupled device (EMCCD) offers improved quantum efficiencies (40 to 95%) over a broader range of wavelengths (400 to 900 nm) and a higher intrinsic resolution (<100 mum using photon counting) when compared to photomultiplier tubes. The electron gain achieved in the multiplication register of an EMCCD effectively reduces the readout noise to less than 1 electron/pixel, making them sensitive to single photoelectrons. Our prototype camera uses the Texas Instruments Impactrontrade EMCCD model TC253SPD-B0 (7.4 mum square pixels) which is cooled under vacuum to -50degC using a four stage Peltier and liquid heat exchanger. Shuttered lens-coupling is used to image the optical light from a 3 mm thick monolithic CsI(Tl) crystal. Precise clocking for the EMCCD is provided by a National Instruments FPGA controller (PCI-7811R) and LabVIEW FPGA module (version 8.0). A custom built electronics box contains the clock driver circuitry and 16-bit video board for digital conversion of the video signal. Our optical coupling method differs from other EMCCD SPECT systems by using lenses rather than fiber optic bundles for transfer. TC253 characterization tests measured a maximum gain just over 1000times, dark current rate of 0.14 e/p/s, read noise of 18.2 e/p, and spurious charge generation of 4 e/p. A light integration intrinsic resolution of 110 mum FWHM was measured. Light integration images of a line phantom using a single pinhole collimator were used for SPECT reconstruction. We found the relative high spurious charge generation and low quantum efficiency of the TC253 made it incapable of photon counting for low energy sources using lens coupling.