Susceptibility tensor imaging.

Heterogeneity of magnetic susceptibility within brain tissues creates unique contrast between gray and white matter in magnetic resonance phase images acquired by gradient echo sequences. Detailed understanding of this contrast may provide meaningful diagnostic information. In this communication, we report an observation of extensive anisotropic magnetic susceptibility in the white matter of the central nervous system. Furthermore, we describe a susceptibility tensor imaging technique to measure and quantify this phenomenon. This technique relies on the measurement of resonance frequency offset at different orientations with respect to the main magnetic field. We propose to characterize this orientation variation using an apparent susceptibility tensor. The susceptibility tensor can be decomposed into three eigenvalues (principal susceptibilities) and associated eigenvectors that are coordinate-system independent. We show that the principal susceptibilities offer strong contrast between gray and white matter, whereas the eigenvectors provide orientation information of an underlying magnetic network. We believe that this network may further offer information of white matter fiber orientation.