EPR oxygen imaging (EPROI) is an established  noninvasive method to map absolute oxygen values within ~0.5 mm resolution. EPROI measures electron spin relaxation maps of a water-soluble oxygen-reporting trityl OX071 molecule. Trityl spin probes have an unpaired electron which directly collides with valence electrons in molecular oxygen. This interaction changes the spin-spin- and spin-lattice relaxation rate of the probe by Heisenberg spin exchange. There is a linear relationship between pO2 and trityl relaxation rates. This linear relationship is used for mapping oxygen, which is the basis of EPROI.