High field diffusion

Diffusion weighted imaging at high and ultra-high field strengths

Diffusion Tensor Imaging (DTI) is a powerful, non-invasive technique to study white matter in the human brain in vivo. Typically, image acquisition relies on singleshot EPI (sshEPI) due to its speed and its immunity to motion artifacts. Critical shortcomings of sshEPI are image blurring due to T2* decay and distortions related to B0 inhomogeneities. These problems are even worse at high field strengths. Thus, the recent introduction of ultra-high field scanners with 7T and beyond poses serious challenges to diffusion imaging, particularly with respect to field inhomogeneity and T2 as well as T2* decay. Both challenging problems can be addressed quite effectively by parallel imaging which permits to reduce the echo train. It could be shown that the main issues of susceptibility-induced distortion and T2*-related blurring can be strongly mitigated with SENSE acceleration, indicating that the inherent sensitivity benefit of 7T can actually be tapped for diffusion applications. The related boost in potential SNR is promising, but on the other hand, there are still many open questions which have to be addressed in the new future. Even with high reduction factors, diffusion-weighted imaging at 7T is still severely hampered by increased inhomogeneities of B1 and B0 as well as reduced T2 and T2*.