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Tissue oxygen (O₂) levels are among the most important and most quantifiable stimuli to which cells and tissues respond through inducible signaling pathways. Tumor O₂ levels are major determinants of the response to cancer therapy. Developing more accurate measurements and images of tissue O₂ partial pressure (pO₂), assumes enormous practical, biological, and medical importance.

We present a fundamentally new technique to image pO₂ in tumors and tissues with pulse electron paramagnetic resonance (EPR) imaging enabled by an injected, nontoxic, triaryl methyl (trityl) spin probe whose unpaired electron's slow relaxation rates report the tissue pO₂. Heretofore, virtually all in vivo EPR O₂ imaging measures pO₂ with the transverse electron spin relaxation rate, R_2e, which is susceptible to the self‐relaxation confounding O₂ sensitivity.

We found that the trityl electron …