Abstract
MADPET-II is a small animal PET tomograph that features individual lutetium oxyorthosilicate (LSO) crystal readout from avalanche photodiodes (APDs). The detector signals are preamplified by 16-channel fully integrated ASICs which are placed as close as possible to the detector in order to avoid attenuation of the signal or unwanted stray capacitance. However, the power consumption of the preamplifier (30 mW per channel) can cause heat transfer and, consequently, gain drift to temperature sensitive detectors. Temperature measurements on the front-end electronics of MADPET-II have shown a maximum increase of approximately 30 C in the area around the preamplifier and in the area around the APD-LSO detector with respect to room temperature. In the presence of this temperature gradient, energy spectra have been acquired from which a significant drift of the photopeak (3.4% per C) and a small increase of the mean energy resolution (3% over the whole temperature range studied) with increasing temperature has been observed. The effect of temperature on the time resolution is small in comparison to the effect of walk and jitter introduced by the analog processing electronics. The behavior of two 48 LSO-APD front-end detector arrays in coincidence at temperatures below ambient and at various values of the APD bias voltage in terms of energy and time resolution has also been studied. The total current drawn by the APDs (leakage current and photocurrent) has been monitored at various temperatures and APD bias and was modelled and fitted by a theoretical function demonstrating a and dependence. No significant improvement on time resolution with decreasing temperature has been observed. For temperature stabilization and monitoring, thermoelectric cooling is considered appropriate for mounting in the limited free space of a PET scanner, especially when this is inside an MR scanner for simultaneous PET/MR imaging.
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