Measurements on the timing stability of the MicroPET R4 animal PET scanner

The MicroPET R4 and P4 small animal PET scanners have detectors consisting of an array of 64 2.1times2.1times10 LSO crystals coupled via light guides to Hamamatsu R5900-C12 position sensitive PMTs. These have energy resolutions in the range of 25% and timing resolutions <3 nsec. The calibration software allows users to set up the crystal identification maps and voltage to keV conversion gain for each crystal in a mostly automated and highly interactive way. However until recently there was no provision for making adjustments to the inter-detector timing alignment. Our preliminary evaluation of this instrument showed that reasonable normalization sinograms could only be obtained with a timing window of 10 nsec or more in spite of a timing resolution of <3 nsec. We performed sham transmission scans with nothing in the field of view, and a range of timing windows from 2 to 14 nsec and used a 14 nsec timing blank scan to generate effective attenuation sinograms as a function of timing window. These showed trues count-rates which fit well to a ERF(tau) function. However, the effective attenuation value, which should be 1.0, changes from block to block becomes very high (<3.5 at 6 nsec.) in some blocks suggesting the need for timing alignment. In the latest software release, V5.2.2.8, timing alignment is permitted, and the timing is much better aligned and much more stable