Characterization of a clinically used charcoal suspension for in vivo EPR oximetry.

Electron paramagnetic resonance (EPR) oximetry using particulate materials allows repeatable measurements of oxygen in tissues. However, the materials identified so far are not medical devices, thus precluding their immediate use in clinical studies. The aim of this study was to assess the magnetic properties of Carbo-Rep, a charcoal suspension used as a liquid marker for preoperative tumor localization. Calibration curves (EPR linewidth as a function of pO) were built using 9-GHz EPR spectrometry. The feasibility of performing oxygen measurements was examined in vivo by using a low-frequency (1 GHz) EPR spectrometer and by inducing ischemia in the gastrocnemius muscle of mice or by submitting rats bearing tumors to different oxygen-breathing challenges. Paramagnetic centers presenting a high oxygen sensitivity were identified in Carbo-Rep. At 1 GHz, the EPR linewidth varied from 98 to 426 µT in L-band in nitrogen and air, respectively. The sensor allowed repeated measurements of oxygen over 6 months in muscles of mice. Subtle variations of tumor oxygenation were monitored in rats when switching gas breathing from air to carbogen. The magnetic properties of Carbo-Rep are promising for its future use as oxygen sensor in clinical EPR oximetry