Superoxide free radical spin‐lattice relaxivity: A quench‐assisted MR study

Purpose QuEnch‐assiSTed (QUEST) MRI provides a unique biomarker of excessive production of paramagnetic free radicals (oxidative stress) in vivo. The contribution from superoxide, a common upstream species found in oxidative stress‐based disease, to the QUEST metric is unclear. Here, we begin to address this question by measuring superoxide spin‐lattice relaxivity (r1) in phantoms. Methods Stable superoxide free radicals were generated in water phantoms of potassium superoxide (KO2). To measure r1, 1/T1 of different concentration solutions of KO2 in the presence and absence of the antioxidant superoxide dismutase were measured. The 1/T1 confounding factors including acquisition sequence, pH, and water source were also evaluated. Results The T1‐weighted signal intensity increased with KO2 concentration. No contribution from pH, or reaction products other than superoxide, noted on 1/T1. Superoxide r1 was measured to be 0.29 mM−1 s−1, in agreement with that reported for paramagnetic molecular oxygen and nitroxide free radicals. Conclusion Our first‐in‐kind measurement of superoxide free radical r1 suggests a detection sensitivity of QUEST MRI on the order of tens of μM, within the reported level of free radical production during oxidative stress in vivo. Similar studies for other common free radicals are needed.