Is R2* in Human Brain White Matter Dependent on B0 Orientation?

Introduction: The observation of T2* heterogeneity in white matter at high magnetic field strength [1] has invited speculation that white matter fiber orientation with respect to the main magnetic field (B0) might contribute to this contrast. A possible mechanism is the orientation dependence of microscopic field distributions in oriented structures such as the brain’s major axonal fiber bundles. Orientation dependence of T2 in peripheral nerves [2] and of dipolar splitting in spectroscopy of muscle fibers can be interpreted as evidence for this and suggest an angular dependence according to 3cos(θ)-1. A preliminary study on a non-human primate suggest this mechanism indeed affects white matter T2* in high field MRI of brain [3]. To further investigate this, we performed quantitative R2* measurements at different orientations in vivo and in fixed brain tissue samples.