Investigations of T2 anisotropy in ex vivo WM using direct sample reorientation

Motivation: There are ambiguous and partly contradictory results regarding the observed anisotropy of transverse relaxation in WM, which need to be clarified and traced back to their true core. Goal(s): To obtain reliable data demonstrating the true effect of T2 anisotropy -reflecting only tissue properties- in ex-vivo white matter. Approach: A fixated sample of pig spinal cord (WM) was examined using a dedicated setup that allows sample rotation in the scanner. Data from an optimized CPMG sequence were model-free analyzed. Results: The anisotropy of T2 was clearly demonstrated. Contributions from myelin water and intra/extracellular water could be identified beyond doubt by direct bi-exponential fitting. Impact: This study shows that T2 anisotropy observed under optimized ex vivo conditions cannot be described by the current models used in the MRI community.