Magnetic field dependence of 1/T1 of protons in tissue

It is well established that the spin-lattice magnetic relaxation rate 1/T1 of solvent protons in homogeneous protein solutions increases dramatically as the magnetic field is reduced well below the traditional NMR range. For a 5% solution of protein of 10(5) Daltons, for example, 1/T1 increases from about 50% above the pure solvent rate at 20 MHz to five times the solvent rate at 0.01 MHz. At higher fields, the effect of protein on the relaxation rate decreases progressively toward zero. 1/T1 of solvent in erythrocyte suspension behaves similarly, indicating that extracellular water has ready access to intracellular protein. We now report analogous data for samples of various mammalian tissues: we find that the data can be accommodated within the conceptual framework developed earlier for analyzing homogeneous protein solutions. It appears that tissue water probes the macromolecular composition and structure in a tissue-specific fashion. The variation of 1/T1 with field differs for each tissue, and its magnitude at low fields varies by more than a factor of three, far more than does the water content of the tissues. The relevance to contrast in NMR imaging is discussed.