The impact of fibre orientation on T 1-relaxation and apparent tissue water content in white matter

Recent MRI studies have shown that the orientation of nerve fibres relative to the main magnetic field affects the R (2)*(= 1/T (2)*) relaxation rate in white matter (WM) structures. The underlying physical causes have been discussed in several studies but are still not completely understood. However, understanding these effects in detail is of great importance since this might serve as a basis for the development of new diagnostic tools and/or improve quantitative susceptibility mapping techniques. Therefore, in addition to the known angular dependence of R (2)*, the current study investigates the relationship between fibre orientation and the longitudinal relaxation rate, R (1) (= 1/T (1)), as well as the apparent water content. For a group of 16 healthy subjects, a series of gradient echo, echo-planar and diffusion weighted images were acquired at 3T from which the decay rates, the apparent water content and the diffusion direction were reconstructed. The diffusion weighted data were used to determine the angle between the principle fibre direction and the main magnetic field to examine the angular dependence of R (1) and apparent water content. The obtained results demonstrate that both parameters depend on the fibre orientation and exhibit a positive correlation with the angle between fibre direction and main magnetic field. These observations could be helpful to improve and/or constrain existing biophysical models of brain microstructure by imposing additional constraints resulting from the observed angular dependence R (1) and apparent water content in white matter.