Digital Identification of Fast Scintillators in Phoswich APD-Based Detectors

Significant progress has been made in the last 15 years to improve the spatial resolution of small animal PET scanners, mainly by reducing the size of detector pixels. Spatial resolution can be further improved by using phoswich scintillator assemblies, either to increase pixellization or to measure the depth-of-interaction. A number of high-density, fast and high light output Ce-activated Lu-based scintillating materials with a range of decay times are now available, which has widened the list of potential candidates for applications in medical imaging. Most of them have suitable emission wavelength (above 400 nm) for readout with APDs, but their decay times are often too similar for accurate identification in phoswich detectors using standard analog pulse shape discrimination techniques. This study investigates the spectroscopic characteristics of these fast Lu-based scintillators and their diverse combinations into phoswich assemblies for high resolution PET detectors. Crystal identification was assessed using advanced numerical methods derived from signal recognition theory to fit a mathematical model to the digitized APD output signals and then to discriminate the crystal of interaction based on model parameters. Identification errors were evaluated as the overlapping peak area in the model parameter spectra. Identification is virtually error-free for decay time differences (Δτ) larger than 20 ns, while the measured error is generally less than 5% for Δτ > 10 ns. Whereas the Δτ between crystals is the major factor influencing identification performance, others factors such as the initial photon emission rate and the decay time also affect the identification accuracy. The phoswich pair consisting of LSO:Ce, Ca (τ = 32 ns) and LGSO (10%Gd-0.75%Ce) (τ = 45 ns) achieves the best overall performance for the PET application.