A Cherenkov-based imaging system uses a radio-optical triggering unit (RTU) that detects scattered radiation in a fast-response scintillator to detect pulses of radiation to permit capture of Cherenkov-light images during pulses of radiation and background images at times when pulses of radiation are not present without need for electrical interface to the accelerator that provides the pulses of radiation. The Cherenkov images are corrected by background subtraction and used for purposes including optimization of treatment, commissioning, routine quality auditing, R&D, and manufacture. The radio-optical triggering unit employs high-speed, highly sensitive radio-optical sensing to generate a digital timing signal which is synchronous with the treatment beam for use in triggering Cherenkov radiation detection.