AbstractA new method for mapping diffusivity profiles in tissue is presented. The Bloch‐Torrey equation is modified to include a diffusion term with an arbitrary rank Cartesian tensor. This equation is solved to give the expression for the generalized Stejskal‐Tanner formula quantifying diffusive attenuation in complicated geometries. This makes it possible to calculate the components of higher‐rank tensors without using the computationally‐difficult spherical harmonic transform. General theoretical relations between the diffusion tensor (DT) components measured by traditional (rank‐2) DT imaging (DTI) and 3D distribution of diffusivities, as measured by high angular resolution diffusion imaging (HARDI) methods, are derived. Also, the spherical tensor components from HARDI are related to the rank‐2 DT. The relationships between higher‐ and lower‐rank Cartesian DTs are also presented. The inadequacy of the traditional rank‐2 tensor model is demonstrated with simulations, and the method is applied to excised rat brain data collected in a spin‐echo HARDI experiment. Magn Reson Med 50:955–965, 2003. © 2003 Wiley‐Liss, Inc.