A 64-by-64-pixel CMOS single-photon avalanche diode (SPAD) imager for time-resolved fluorescence detection features actively quenched and reset pixels, allowing gated detection to eliminate pile-up nonlinearities common to most time-correlated single-photon counting (TCSPC) approaches. Timing information is collected using an on-chip time-to-digital converter (TDC) based on a counter and a supply-regulated delay-locked loop (DLL).