Timing calibration method for NanoPET™/CT system

The PET component of NanoPET™/CT (Mediso) contains dual-module detectors, in which every detector module has been built up from two Hamamatsu H-9500 position sensitive photomultipliers (PSPMTs), optically coupled to the same crystal matrix using a thin BK-7 optical glass lightguide, allowing 95 mm AFOV. This arrangement allows to detect the signals of the crystal needles between the two PSPMTs, enlarging the effective area. However, the two PSPMTs of a detector module can have different transit time. The triggering signal of a module is the sum of the dynode signals of the two PSPMTs. For the higher slope of the triggering signal it can be helpful to delay one of the PSPMT dynode signals before the summation, to correct the possible transit time differences. With the use of this delay the effective transit times of the two PSPMTs will be synchronized and the time point of the triggering will be more accurate due to the higher slope, in other words the time resolution of the modules can be improved. The second step of the timing calibration is the correction of the difference of transit times of the 12 detector modules. For this purpose there is a digital timestamp delay instead of signal cables with different lengths. We describe the measurements and data processing methods we use to perform these calibrations illustrated with measured data.