Performance characteristics of a planar positron camera with slot collimators

The UNICON, in positron mode, consists of two large field-of-view scintillation cameras in coincidence, on a rotating gantry. With bare crystals the sensitivity to singles counts and chance coincidences from activity beyond the system field of view causes increased dead time and very low maximum detected coincidence rate. Use of a collimator for each detector with slots orthogonal to the axis of rotation reduces the sensitivity to outlying activity. A clinically realistic distribution of sources and absorbers produced a maximum coincidence rate of 300/sec with bare crystals. This was increased to 4000/sec using collimators with 1 cm-wide slots, 1 cm-thick septa, 7.5 cm deep. A graded filter of lead and copper between collimator and crystal face increased the maximum detected rate to 6000/sec. Detector electronics are relatively standard so that compatability is maintained with UNICON operation in single photon mode. Detector dead times, detected spectra, system sensitivity and resolution, and chance coincidence fractions are presented for bare detectors, collimated detectors, and collimated detectors with graded filters. The slot collimators provide a system geometry similar to ring detector PET units, and allow use of those reconstruction algorithms as well as true 3D algorithms