An apparatus and method for channel count reduction in solid-state-based positron emission tomography that multiplexes read-outs from photo-detectors using a sum delay circuit, including a sum channel and a delay-sum channel. The sum channel sums signals from sensors in an array and is digitized to extract the timing and energy information. A delay-sum channel includes a discrete delay line that introduces a known delay after each sensor, creating a time signature for the sensor, followed by a summing circuit that adds the delayed signals. The delay-sum channel is digitized using a high speed counters to extract location information. Start and Stop signals for the counter are derived when the sum channel output and the delay-sum channel output cross a pulse ID threshold, respectively. The pulse ID threshold is chosen to minimize the Compton scatter and not clip the photo-peak events.