Strong field behavior of the NMR signal from magnetically heterogeneous tissues

A theory for the behavior of the nuclear magnetic resonance (NMR) signal ob tained from magnetically heterogeneous tissues is developed for the limit o f a strong external magnetic field. If B-0 is the magnitude of the external magnetic field, it is found that a free-induction signal decays in a time scaling as 1/B-0, a single-spin echo signal decays in a time scaling as 1/B -0(2/3), and a multiple-spin echo signal decays in a time scaling as 1/B-0( 2). Moreover, it is shown that the form of the signal decay for a multiple- spin echo sequence may deviate significantly from an exponential. Numerical results for a model consisting of randomly distributed magnetic spheres ar e used to confirm the theory. In addition, good agreement is demonstrated b etween the theory and experimental measurements obtained with particle susp ensions. The validity and application of the theory to biological tissues a re discussed. (C) 2000 Wiley-Liss, Inc.