LabPET II, an APD-based Detector Module with PET and Counting CT Imaging Capabilities

Abstract

Computed tomography (CT) is currently the standard modality to provide anatomical reference for positron emission tomography (PET) in molecular imaging applications. Since both PET and CT rely on detecting radiation to generate images, using the same detection system for data acquisition is a compelling idea even though merging PET and CT hardware imposes stringent requirements on detectors. These requirements include large signal dynamic range with high signal-to-noise ratio for good energy resolution in PET and energy-resolved photon-counting CT, high pixelization for suitable spatial resolution in CT, and high count rate capability for reasonable CT acquisition time. To meet these criteria, the avalanche photodiode (APD)-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two monolithic $ 4\times 8$ APD pixel arrays mounted side-by-side on a custom ceramic holder. Individual APD pixels have an active area of $ 1.1\times 1.1~\hbox{mm}^{2}$ at a 1.2 mm pitch. The APD arrays are coupled to a 12-mm high, $8 \times 8$ LYSO scintillator array made of $1.12 \times 1.12~\hbox{mm}^{2}$ pixels also at a pitch of 1.2 mm to ensure direct one-to-one coupling to individual APD pixels. The scintillator array was designed with unbound specular reflective material between pixels to maximize the difference between refractive indices and enhance total internal reflection at the crystal side surfaces for better light collection, and the APD quantum efficiency was improved to $ \sim60\%$ at 420 nm to optimize intrinsic detector performance. Mean energy resolution was $20 \pm 1\%$ at 511 keV and $ 41\pm 4\%$ at 60 keV. The measured intrinsic spatial and time resolution for PET were respectively $0.81 \pm 0.04~\hbox{mm}~{\rm FWHM}/1.57 \pm 0.04~\hbox{mm}$ FWTM and $ 3.6\pm 0.3~\hbox{ns}$ FWHM with an energy threshold of 400 keV. Initial phantom images obtained using a CT test bench demonstrated excellent contrast linearity as a function of material density. With a magnification factor of 2, a CT spatial resolution of 0.66 mm FWHM/1.2 mm FWTM, corresponding to 1.18 lp/mm at ${\rm MTF}_{10\%}/0.67~\hbox{lp/mm}$ at ${\rm MTF}_{50\%}$ , was measured, allowing 0.75 mm air holes in an Ultra-Micro Hot Spot resolution phantom to be clearly distinguished.