Origins of R 2 ∗ orientation dependence in gray and white matter

Significance Differences in the apparent transverse relaxation rate () between tissues are exploited in numerous magnetic resonance imaging (MRI) techniques from functional MRI to susceptibility weighted imaging. Recent results show a surprising dependence of tissueon orientation. This study demonstrates that the orientation dependence ofin both white and cortical gray matter has a sinusoidal dependence on tissue orientation and a linear dependence on the perturber volume fraction (measured by quantitative histology). A biophysical model is used to relate the observed orientation dependence to the local Larmor frequency shift and volume magnetic susceptibility of the tissue.