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Characterizing magnetization exchange in healthy human brain and bovine brain Kalantari, Saeed


Multi component T₂ relaxation imaging is an established MRI technique for measuring myelin water (MW, water molecules trapped between myelin sheath bilayers). Myelin water fraction (MWF, the fraction of central nervous system water with a short T₂) has been quantitatively correlated to histological staining for myelin in central nervous system tissue and hence is considered an in vivo measure of myelin content. Various studies have reported on the measurement of MWF with a diverse range of neurological diseases such as Multiple Sclerosis (MS), Schizophrenia, epilepsy, and Phenylketonuria (PKU). Although T₂ relaxation is the main probe for measuring MWF, understanding longitudinal relaxation, T1, is essential in a number ways such as the following: 1) Estimation of the corrections for myelin water fraction that need to be taken into account due to water exchange processes in white matter in vivo is highly dependent on T₁ relaxation. 2) Investigating the effect of T1-weighting in MWF measurements at short TR. This is especially important due to recent breakthroughs in developing rapid 3-D whole brain approaches to MWF measurements that are pushing towards shorter and shorter TR times in order to make this technique a valuable clinical imaging tool. First, in vivo MRI data from multi-component T₂ relaxation from 57 healthy subjects collected at 3.0 T was analyzed to estimate the corrections which have to be taken into account due to magnetization exchange in white matter. These results showed that these MWF corrections were less than 15% and are uniform across various white matter structures. Next, the variation of MWF as function of repetition time (TR) was investigated using in vivo MRI data collected at 3.0 T from healthy subjects. These results clearly showed that the measured MWF increased as the TR decreased. Finally, in order to measure T₁ as well as the rate of magnetization exchange with higher precision, data from bovine brain white matter was collected using a 4.7 T NMR spectrometer. The results from this study clearly showed that the T₁ had two components; therefore magnetization in bovine white matter is not in a fast exchange regime on the T₁ timescale.

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