Coincidence time alignment for planar pixellated positron emission tomography detector arrays

We describe the techniques used to overcome the inherent time skew associated with planar pixellated detector arrays that utilize large photomultipliers in positron emission tomography (PET) applications. The CTI PETSPECT planar detectors utilize an array of eight by eleven 2-inch photomultiplier tubes in a shared quadrant array to reduce the cost of the system. PMTs of this size exhibit a time skew on the order of 5 ns between events detected at their edge versus events detected near their center, requiring the use of a wide 10 ns coincidence time window if time alignment is not performed. The FPGA based electronics that control the detector array were constructed to be able to individually adjust the output of the time to digital converter (TDC) depending on the location of the event. A brief description of the TDC hardware time adjustment method is provided as well as the techniques used for calibration and the timing performance of the PETSPECT array.