The in vivo influence of white matter fiber orientation towards B(0) on T2* in the human brain.

It is well known that, at higher field strength, T2*-weighted images show an extensive heterogeneity in white matter fiber bundles. The basis of this phenomenon is still under discussion, as many factors, such as iron concentration, myelination and tissue microstructure, could influence relaxation times. Furthermore, fiber direction in relation to the main magnetic field seems to influence T2* relaxation times. In this study, diffusion tensor imaging and T2* measurements were combined in seven subjects with the head in a normal and tilted position. It was shown that fiber orientation has a strong influence on T2* in the human brain, with the average T2* value changing from 49 ms for a perpendicular orientation to B(0) to 57 ms for a parallel orientation to B(0). Nevertheless, T2* times showed a wide variety of values at any orientation towards B(0).