Departmental Oral Examination (Thesis Title: “T 1 Relaxation and Inhomogeneous Magnetization Transfer in Brain: Physics and Applications")

Abstract:
A major goal of the Magnetic Resonance Imaging (MRI) community is myelin quantification. MRI contrast depends on tissue microstructure, so quantitative models require good understanding of Nuclear Magnetic Resonance (NMR) physics in these complex, heterogeneous environments. In this thesis, we study the underlying physics behind two different 1 H contrast mechanisms in white and grey matter tissue: T 1 relaxation and the recently developed Inhomogeneous Magnetization Transfer (ihMT).

Using /ex-vivo /white and grey matter samples of bovine brain, we performed a comprehensive solid-state NMR study of T 1 relaxation under six diverse initial conditions. For the first time, we used lineshape fitting to quantify the non-aqueous magnetization during relaxation. A four pool model described our data well, matching with earlier studies. We also showed examples of how the observed T 1 relaxation behaviour depends upon the initial conditions.

ihMT's sensitivity to lipid bilayers like those in myelin was originally thought to rely upon hole-burning in the supposedly inhomogeneous-broadened lipid lineshape. Our work showed that this is incorrect and that ihMT only requires the presence of dipolar coupling, not a specific kind of line broadening. We developed a simple explanation of ihMT using a spin-1 system. We then performed ihMT and T 1D measurements (dipolar order relaxation time) using solid-state NMR on four samples: a multilamellar lipid system (Prolipid-161), wood, hair, and bovine tendon. ihMT was observed in all samples, even those with homogeneous broadening (wood and hair). Moreover, we saw no evidence of hole-burning.

Lastly, we present results from ihMT experiments with CPMG acquisition on the same bovine brain samples. We show that myelin water has a higher ihMT signal than water outside the myelin. It was determined that this was due to the unique thermal motion in myelin lipid. In doing so, we developed a useful metric for determining magnetization transfer's and dipolar coupling's relative contributions to ihMT. Also, we applied a four pool model with dipolar reservoirs as a qualitative model. Together, our results were consistent with myelin lipids having a uniquely long T 1D , despite recent measurements to the contrary.