Quantifying Tissue Oxygen with CELI Imaging

The experimental setup for CELI has been detailed in the previous studies.¹⁸ (link) A PI-MAX 4 1024i ICCD time-gated intensified camera (Princeton Instruments, Acton, Virginia) was used to collect luminescence excited by the radiation pulses provided by 6 megavolt (MV) x-ray beams, generated by a clinical linear accelerator (Varian LINAC 2100C, Varian Medical Systems, Palo Alto, California). The ICCD camera was coupled to a 135-mm $f/2.0$ lens (Canon) and fixed at a distance of $\sim 2\mathrm{m}$ from the imaging object. The x-ray beam was delivered by as $3.25\mu \mathrm{s}$ -long pulses at a 360-Hz repetition rate, and four luminescence images were acquired at different delay times after the pulse: 5, 10, 20, and $30\mu \mathrm{s}$ . The intensity of the phosphorescence in each image pixel was plotted as a function of the temporal delay, and the fit to an exponential function was used to determine the lifetime and calculate the local ${\mathrm{pO}}_2$ .