Recent advances have been made in a new class of CCD-based, single-photon-counting gamma-ray detectors
which offer sub-100 μm intrinsic resolutions.1-7 These detectors show great promise in small-animal SPECT
and molecular imaging and exist in a variety of cofigurations. Typically, a columnar CsI(Tl) scintillator or a
radiography screen (Gd2O2S:Tb) is imaged onto the CCD. Gamma-ray interactions are seen as clusters of signal
spread over multiple pixels. When the detector is operated in a charge-integration mode, signal spread across
pixels results in spatial-resolution degradation. However, if the detector is operated in photon-counting mode, the
gamma-ray interaction position can be estimated using either Anger (centroid) estimation or maximum-likelihood
position estimation resulting in a substantial improvement in spatial resolution.2 Due to the low-light-level nature
of the scintillation process, CCD-based gamma cameras implement an amplfication stage in the CCD via electron
multiplying (EMCCDs)8-10 or via an image intensfier prior to the optical path.1
We have applied ideas and techniques from previous systems to our high-resolution LumiSPECT detector.11, 12
LumiSPECT is a dual-modality optical/SPECT small-animal imaging system which was originally designed to
operate in charge-integration mode. It employs a cryogenically cooled, high-quantum-efficiency, back-illuminated
large-format CCD and operates in single-photon-counting mode without any intermediate amplfication process.
Operating in photon-counting mode, the detector has an intrinsic spatial resolution of 64 μm compared to 134
μm in integrating mode.