  Objective: In this study, a fast imaging technique was developed for the
first in vivo Cherenkov emission imaging from an ultra-high dose rate (UHDR)
electron beam source at single pulse (360 Hz) submillimeter resolution.
  Approach: A CMOS camera, gated to the UHDR LINAC, imaged the Cherenkov
emission profiles pulse by pulse passively during the irradiation of mice on
their limbs and intestinal region. The utility of an intensifier was
investigated for its effect on image quality including signal to noise and
spatial resolution. Pulse by pulse variability in Cherenkov emission profile
were quantified spatially and temporally.
  Main results: An intensifier improved the emission profile signal to noise
ratio from 15 to 280, with reduced spatial resolution. The profile extended
beyond of the treatment field due to the lateral scattering of the electrons in
tissue and its optical properties. The CMOS camera with an intensifier detected
the changes in Cherenkov emission profile during expiration and inspiration of
the respiration cycle for the mice to be about 3 mm.
  Significance: This fast imaging technique can be utilized for in vivo
intrafraction monitoring of FLASH patient treatments at single pulse
resolution. It can display delivery differences during respiration, and
variability in the delivered treatment's surface profile, which may perturb
from the intended UHDR treatment more for pencil beam scanning systems. The
technique may leverage Cherenkov emission surface profile to gate the treatment
delivery via respiratory gating systems under FLASH conditions.
