Detection of microscopic diffusion anisotropy on a whole-body MR system with double wave vector imaging.

Double-wave-vector diffusion-weighting experiments can detect diffusion anisotropy on a microscopic level which, e.g., could distinguish lower fiber densities from reduced fiber coherence. The underlying signal difference between parallel and orthogonal wave vector orientations has been observed on vertical-bore MR systems (≥500 mT m(-1) ); however, numerical simulations reveal that it is expected to be considerably reduced for typical whole-body MR gradient pulse durations. Here, pig spinal cord tissue and a reference fluid phantom were investigated on a 3 T clinical MR system (40 mT m(-1) ). By averaging over different absolute wave vector orientations, signal variations caused by experimental imperfections like background gradient fields and eddy currents were minimized and a rotationally invariant anisotropy measure could be assessed. A significant microscopic anisotropy was observed in gray and white matter tissue even in the plane perpendicular to the cord which is consistent with previous vertical-bore experiments. Thus, it is demonstrated that double-wave-vector experiments can investigate the microscopic anisotropy on whole-body MR systems.