Orientation Dependence of Magnetization Transfer and “Inhomogeneous Magnetization Transfer” in Spinal Cord

Motivation: Magnetization transfer (MT) and inhomogeneous MT (ihMT) are assumed to report on myelin content. Recently, anisotropy of MT and ihMT have been demonstrated in model systems and in white matter. Goal(s): Our goal was to quantify orientation effects in (ih)MT, as they closely relate to the microstructure and serve to confirm assumptions about the relaxation mechanism. Approach: Comprehensive (ih)MT investigations were performed in fixed spinal cord with variation of the fiber-to-field angle (θFB). Results: Unambiguous orientation dependence was observed for (ih)MT. The variation depends strongly on the offset frequency, which was quantitatively predicted in simulations of the BSBM with a realistic fiber model. Impact: This study investigates the orientation dependency of magnetization transfer and related model parameters. Some subtle orientation effects are observed for the first time and are quantitatively explained by using a reasonable model for the RF saturation lineshape.