Oxymetry by magnetic resonance: applications to animal biology and medicine

In vivo oxygen concentration measurements are of crucial importance for monitoring the energetic metabolism from a physiological and pathological point of view. Magnetic resonance methods based on the paramagnetism of oxygen have the advantage of using great electromagnetic wavelengths, which present an important penetration in aqueous samples, and display no scattering in condensed matter. The transverse relaxation time (T-2) Of water protons is decreased by the paramagnetism of deoxyhaemoglobin, wich is inversely related to the blood oxygen concentration. A tremendous amount of work is currently being done using this blood oxygen level dependent (BOLD) effect. The exchangeable imidazole proton of the proximal histidine of haemoglobin yields a more specific signal, which is directly related to the paramagnetism of deoxyhaemoglobin and reflects the oxygen concentration more accurately than the T-2 Of water protons. Perfluorocarbon emulsions are blood substitutes which can be used to measure precisely the oxygen content of blood. Unpaired electrons of the free radical probes can also be used for sensitive oxymetry purposes thanks to bimolecular collisions between the oxygen and free radical molecules, which alter, specifically, the electron paramagnetic resonance of the radical. Dynamic nuclear polarisation, which combines the sensitivity to oxygen of EPR and the tractability of NMR imaging, is a potential means of achieving localised oxymetry. The principles and recent applications of these magnetic resonance methods to the measurement of oxygen concentrations in biology are reviewed here, in order to underline the advantages and difficulties of each method. (C) 2000 Elsevier Science B.V. All rights reserved.