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UBC Theses and Dissertations

Characterizing T₂ distributions in healthy white matter Russell-Schulz, Bretta Adrianne


Quantitative T₂ measurements in Magnetic resonance imaging (MRI) can provide information about water environments in biological structures. Here, an extended Carr-Purcell-Meiboom-Gill sequence (CPMG) with echoes out to 1120ms was used to characterize Long-T₂ times of healthy white matter in brain. One of the white matter structures, the corticospinal tract (CST), was previously found to be bright on T₂-weighted images and myelin water fraction (MWF) images. The intra-/extra- cellular water (IE) T₂ peak of the CST was found to be broadened in comparison to that from other white matter structures and often split into two distinct peaks. In the CST, it appeared that the intracellular and extracellular water environments had different T₂ times, causing the intracellular water peak to be pushed down into the myelin water T₂ regime and the extracellular peak to be pushed up to higher T₂ times. The conventional T₂ limits of 10−40ms used for the MWF at 1.5T result in an artificial increase in MWF, which causes the CST to be bright on myelin water images. When the upper limit of the MWF range was decreased to 25ms, the CST exhibited MWF values similar to those found for adjacent anterior and posterior regions. Using T₂ time of 25ms for the myelin water (MW) upper limit and IE lower limit, a moderately strong relationship between IE geometric mean T₂ (GMT₂) and MW was found across all structures and subjects. This relationship did not necessarily hold when examined across subjects within individual structures The relationship between IE GMT₂ and MWF could arise from a non-biological source, such as the algorithm used in calculating T₂ or from a biological source, such as exchange between the water environments or increased extracellular water. Based on our results the fitting algorithm does not appear to be responsible for this relationship based on our results. However, either varying amounts of extracellular water or exchange between MW and IE could explain this relationship.

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