Detectors working in photon counting mode offer an interesting alternative to the common charge integrating detectors for computed tomography (CT), because they can potentially measure the energy of every detected X-ray photons and achieve better image contrast sensitivity for a given dose. Unfortunately, most current X-ray detectors suffer from limited count rate capability, due either to a long charge migration time in semiconductor and gas detectors, or to a slow decay time in ceramic scintillators. To overcome these difficulties, we propose to use pixel detectors based on fast light emitting inorganic scintillators individually coupled to avalanche photodiodes with parallel, low-noise, fast digital processing electronics to provide real time single event detection and recording. The proposed detector was investigated with 2 × 2 × 10 mm<sup>3</sup> Lu<sub>1.9</sub>Y<sub>0.1</sub>SiO<sub>5</sub> (LYSO), a fast decay time (40 ns), heavy (7.19 g/cc) scintillator that is also suitable for coincidence detection of annihilation radiation (511 keV) in positron emission tomography (PET). Therefore, the detector characteristics make it a good candidate for implementation in a combined PET/CT dual-modality scanner. Although only coarse spectral analysis is possible in the X-ray energy range, it is demonstrated that appropriate CT images for anatomical localization can be obtained at very low dose in counting mode using a PET/CT simulator set up for small animal imaging. Data are reported on CT image resolution, noise, contrast and dose.
Positron Emission Tomography (PET) scanners dedicated to small animal studies have seen a swift development in recent years. Higher spatial resolution, greater sensitivity and faster scanning procedures are the leading factors driving further improvements. The new LabPET<sup>TM</sup> system is a second-generation APD-based animal PET scanner that combines avalanche photodiode (APD) technology with a highly integrated, fully digital, parallel electronic architecture. This work reports on the performance characteristics of the LabPET quad detector module, which consists of LYSO/LGSO phoswich assemblies individually coupled to reach-through APDs. Individual crystals 2×2×~10 mm<sup>3</sup> in size are optically coupled in pair along one long side to form the phoswich detectors. Although the LYSO and LGSO photopeaks partially overlap, the good energy resolution and decay time difference allow for efficient crystal identification by pulse-shape discrimination. Conventional analog discrimination techniques result in significant misidentification, but advanced digital signal processing methods make it possible to circumvent this limitation, achieving virtually error-free decoding. Timing resolution results of 3.4 ns and 4.5 ns FWHM have been obtained for LYSO and LGSO, respectively, using analog CFD techniques. However, test bench measurements with digital techniques have shown that resolutions in the range of 2 to 4 ns FWHM can be achieved.