Electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPR) and spectroscopy is a preclinical imaging modality with potential to be translated into a clinical imaging technique in the future. In brief, electron paramagnetic resonance imaging allows detection and quantification of free radical molecules with unpaired electrons, thus allowing measurement of free radical production and redox status in living subjects with manifold practical implications (e.g. assessment of tumor hypoxia and viability). As of 2021, no clinical EPR scanners are yet available. 

Physics

Electron paramagnetic resonance imaging is similar to nuclear magnetic resonance (NMR), the key difference is that during electron paramagnetic resonance imaging the electron spins are excited by a magnetic assembly, instead of the spins of atomic nuclei.
Most biologically-relevant free radicals, such as superoxide, hydroxyl and nitric oxide free radicals contain unpaired electrons which may undergo excitation ¹. Since the concentration and half-life of these substances is very low, their direct detection has proved to be challenging.  Therefore, efforts have been made to facilitate their detection by using redox-sensitive paramagnetic "spin trap" imaging probes ^2,3.