Modification of Relaxation of Lipid Protons by Molecular Oxygen and Nitroxides

Measurement of proton 1/T1 in model lipids from 0.00023 to 1.3 Tesla, at 5 degrees C and 37 degrees C, shows that both oxygen and nitroxides effectively enhance the relaxation rates of the protons of lipids. Equilibration with 1 atm of oxygen has a sixfold greater effect on lipid protons than on water protons, primarily because of the high lipid solubility of oxygen. The maximum relative relaxivity of oxygen for lipid protons occurs at 0.12-0.8 Tesla. Lipid soluble nitroxides have a four- to eightfold greater effect on lipid protons than on water protons. This high relaxivity, combined with the high lipid solubility possible with nitroxides, could lead to significant contrast in vivo in lipid environments.