Singles list mode data processing for MADPET-II

The MADPET-II small animal PET scanner consists of a ring of 18 dual-layer detector modules (O 71 mm), each containing a 4×8 array of 2×2×6 mm (front) and 2×2×8 mm (back) LSO crystals coupled one-to-one to a 4×8 APD array. To take advantage of the detector geometry, each of the 1152 crystals in the system are read out individually without any light sharing or data reduction. Thus, the system must be able to handle high data rates. Time, energy and position for every singles event are stored in list mode format, and coincidences are sorted post-acquisition in software. The output count rate (singles) of a 128 channel prototype responded linearly with activity up to 3 mCi (R²=0.999), indicating a very low dead time per channel. We present results from our investigations into the implications of this design. For example, coincidence time window and energy threshold can be adjusted post acquisition, and multiple coincidence events can be identified and treated independently. As the energy threshold is lowered from 400 keV to 160 keV, the relative number of triple coincidence events to double coincidence events increases from 0.5% to 28% for a wide coincidence timing window. We have the option to lower the energy- threshold, and to include (or exclude) these events, improving sensitivity while preserving resolution. The intrinsic axial spatial resolution was measured to be 1.15 mm FWHM by stepping a point source of ¹⁸FDG along the central axis and plotting coincidences in direct planes, and the mean energy and timing resolutions were measured to be 20.7% FWHM and 10.8 ns FWMH, respectively.