These results suggest that CREL-based oxygen tomography is feasible and that incorporating time-domain analysis can provide an accurate, robust, and quantitative imaging paradigm. The delivery of megavoltage radiotherapy is well controlled and extensively planned for each subject and Čerenkov radiation emission will be emitted from targeted cancer tissues where the charged primary or secondary particles deposit most of their energy. Thus, CREL will often be excited in rather deep tissue regions. The emission peaks of the oxygen-sensitive phosphors are well above 750 nm and thus propagate readily through most tissues. Standard diffuse optical tomography (DOT) systems commonly measure light at these wavelengths through 10 cm of tissue without requiring invasive placement of the optical detectors. As in all forms of optical NIR tomography, tissue scattering and absorption limit depth resolution of the disclosed method. However, because of the relatively slow time scale of triplet emission, phosphorescence lifetime imaging is less affected by scattering and absorption14 than, e.g., fluorescence lifetime imaging, allowing for more accurate spatial reconstructions.