1. A system for timing pick-off in time-of-flight positron emission tomography, the system comprising: an array of single photon avalanche diodes formed as complementary metal-oxide semiconductors; a summer connected with the single photon avalanche diodes of the array, the summer for analog summing pulses from the single photon avalanche diodes into a waveform of positron emission signal over time; a trigger circuit connected with the summer, the trigger circuit configured to generate a trigger signal as a function of the waveform; and a timing circuit connected with the trigger circuit, the timing circuit configured to determine a start time of the trigger signal; wherein the summer, trigger circuit, and timing circuit are free of a time-to-digital converter.