Imaging Cherenkov photon emissions in radiotherapy with a Geiger-mode gated quanta image sensor

The emission of Cherenkov photons from human and animal tissue can be observed during clinical x-ray or particle beam irradiation. However, imaging this weak emission with the necessary single-photon sensitivity in the clinical room is challenging because of milliwatt-level ambient room lighting and the presence of stray high-energy radiation. In this Letter, we demonstrate, to the best of our knowledge, the first Cherenkov imaging with a time-gated quanta image sensor employing a large single-photon avalanche diode (SPAD) array. Detecting single Cherenkov photons was possible with high photon avalanche gain, fast temporal gating, and moderately high ∼7% photon detection probability. Single-bit digitization and active SPAD quenching enabled stray x-ray noise suppression and photon-noise-limited imaging in a clinical environment. This type of imaging allows the knowledge of location, shape, and surface dose of the therapeutic beam radiotherapy with the stability of solid state-based detection.