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.
Pampel et al. (Tue,) studied this question.