Feasibility of overhauser-enhanced MR imaging with nitroxide free radicals.

Overhauser enhancement can increase the signal available in low-field imaging systems. If the signal is enhanced by a factor of 40, low-field Overhauser-enhanced imaging may produce contrast equivalent to that achieved in subtraction MR mammography performed at 1.5 T. This article describes experiments in which Overhauser enhancement is performed with nitroxide free radicals at field strengths near 100 G. The enhanced signal is much less than 40, and less than the equilibrium signal at 0.25 T, in spite of absorbed power levels greater than 100 W/kg. Although the signal enhancement for nitroxides is disappointing, it is likely that other free radical contrast agents will be developed that offer higher Overhauser enhancement.