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UBC Theses and Dissertations

Investigating the myelin water fraction as a function of TR and the intra/extra cellular water geometric mean T2 as a function of refocusing interval Ahmadi, Tara

Abstract

In this thesis two studies were done using MRI. In chapter two, in vivo 3.0 T MRI data from white matter and grey matter in brain from 4 healthy volunteers was studied using a multi component T₂ relaxation analysis. The goal of this study was to find the dependence of myelin water fraction (MWF), the ratio of water in myelin bilayers to the total water component, with the repetition time of the MR sequence TR. Results showed that MWF increased with decreasing TR time. This behavior is believed to be influenced by the exchange of water between the myelin water and the intra/extracellular water pools. Several models were explored to explain this result, including a fast exchange model, a slow exchange model and a hybrid model in which myelin was proposed to contain regions of fast exchange and regions of slow exchange. In chapter three, we addressed the questions: Does the intra/extra cellular (IE) water geometric mean T₂ (gmT₂) of white and grey matter depend upon the refocusing interval? To answer these questions IE water gmT₂ times for different white and grey matter regions of interest were obtained from 5 healthy subjects. It was found that IE water gmT₂ times from both white matter and grey matter tissue decreased by approximately the same amount with refocusing interval prolongation from 10ms to 40 ms. Several mechanisms for this dependence were considered, including water exchange, existence of myelin, non-heme iron accumulation, or the effect of blood oxygenation. In this case, based on our simulations, exchange did not appear to play a role. Non-heme iron accumulation was related to T₂ time but not to the change in T₂ with echo spacing. Deoxygenation of blood results in the presence of paramagnetic deoxyhemoglobin (dHb), which increases the magnetic susceptibility, and thus the local magnetic field, which can shorten the transverse relaxation time; however, this mechanism would be expected to affect grey matter T₂’s more than white matter T₂s.

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Attribution-NonCommercial-NoDerivs 2.5 Canada