Pulsed field gradient nuclear magnetic resonance methods combined with nuclear magnetic resonance imaging were used to determine the water diffusion anisotropy in perfused rat hearts at 37°C. It was found that the observed diffusion coefficient DaPP (apparent diffusion coefficient) depends on the orientation of the applied gradient g. When g is parallel to the epicardial surface, the observed diffusivity is DaPP1I=1.8±0.4x10-9 m2 s-1, whereas when g is perpendicular to it, diffusivity is

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... .5±0.5x10-9 m2 s'l. To better characterize this directional dependence, images of the second-order diffusion tensor D of the myocardium were obtained. These data demonstrate several essential features of cardiac myoarchitecture, including the helicity of fiber orien-tation with respect to the ventricular axis and the variation of fiber pitch angle with transmural depth. Diffusion anisotropy may be quantified in a coordinate-independent manner by the eigenvalues of the diffusion tensor. In the myocardial midwall, these eigenvalues were El=3.29+0.57, E2=2.01±0.42, and E3=0.77±0.58x 10-9 m22 1-(mean±SD). These data suggest that myocardial water diffusion is essentially unrestricted parallel to the myofibers. They further show that failure to measure the complete diffusion tensor may lead to substantial underestimates of diffusion anisotropy in the myocardium. (Circ Res. 1994;74:789-793.)