Effects on the gains and time delays of an array of SPMs due to changing bias voltage

Most PET scanners have a large number of crystals with some form of light sharing technique to couple them to four photo-multipliers (PMT). While the resolution of the scanners was improved by using more smaller crystals, the PMT size had remained about the same since it is not practical to make PMTs very small. Avalanche photo-diodes and more recently silicon photo-multipliers (SPMs) are an attractive alternative especially in hybrid scanners. We measured the change in gain, the timing resolution and time delay between the positron´s annihilation and the apparent detection by an array of LYSO crystals coupled to a 4 × 4 array of SensL SPMs as the bias voltage changed. We demonstrated that the signal amplitude is proportional (~50%/V) to the bias voltage minus a threshold (~27.5 V) which is very similar for all pixels in the array as expected from previous work. Using the new SensL SPM4, we observed the change in the apparent detection time as the bias changed using a Scanwell Systems PET timing alignment probe. The apparent detection time appears to be quite consistent from pixel to pixel and changes by only a mean value of +0.7 ±0.08 nsec/Volt. The FWHM of the timing spectrum is 2.84 ±0.3 nsec over all channels from 27.5 to 29.5 V bias with some pixels and others decreasing, therefore there appears no bias voltage which optimizes the timing resolution in all channels.