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A lens-coupled electron multiplying charge-coupled device (EMCCD)-based gamma camera capable of performing photon counting for both 99mTc and 125I 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°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 µm FWHM was measured by using a 25 µm slit collimator and 125I source. A single 0.5 mm diameter pinhole collimator was used for SPECT reconstruction of a mouse thyroid gland containing 100 µCi (3.7 MBq) of 125I uptake. We found that although the photopeak for 99mTc (140 keV) could be resolved, the photopeak for 125I (≈ 27 KeV) could not be fully resolved due to the low optical transfer efficiency of dual lens coupling (< 5%). Nonetheless, energy windowing for 125I 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. [ABSTRACT FROM AUTHOR]

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 μm 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 Impactron™ EMCCD model TC2S3SPD-B0 (7.4 μm square pixels) which is cooled under vacuum to -50°C 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(TI) crystal. Precise clocking for the EMCCD is provided by a National Instruments FPGA controller (PCI-7811R) and Lab VIEW 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 1000 ×, 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 μm 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. [ABSTRACT FROM AUTHOR]