Quantitative Measurement Of Myelin Sheath And Iron In The Brain Based On MRI

Myelin sheath and iron play an important role in normal neurophysiological functions.For the studies of the normal development of brain and the pathogenesis of neurological diseases in vivo,the quantitative measurement of myelin and iron is of great clinical significance.Magnetic resonance(MR)is the non-invasive technique that can measure myelin and iron content in vivo.MR parameters are commonly used to evaluate myelin content,including T1w/T2w ratio,magnetization transfer ratio(MTR),myelin water fraction(MWF)and diffusion kurtosis imaging(DKI)indices.However,these MR parameters are indirect measurements of myelin,and the sensitivity to myelin varies,leading to inconsistent results when different MR parameters are used to study myelin.In addition,quantitative susceptibility mapping(QSM)is a MR parameter used to evaluate iron content in gray matter commonly.Notably,iron is involved in the synthesis of myelin,but excess iron will cause damage of myelin sheath.Current studies on the relationship between myelin and iron are mainly focused on local areas.Although iron can be transported through axons and dendrites among different brain areas,few studies have been done in evaluating the relationship between myelin and iron in the associated brain regions.To fill the gap,this work mainly carried out two tasks:(1)We analyzed the trends of myelin measurements along the corpus callosum in vivo human,and performed correlation analysis in the whole brain.Comparisons were made for sensitivity of T1w/T2w,MTR,MWF and DKI parameters(AK,RK and MK)for estimating myelin.It was found that T1w/T2w and MTR were less specific to myelin,but provided stable measurements in whole brain.MWF was highly specific to myelin,but it only provided an effective measure in white matter.RK cannot provide a reliable estimate of myelin in the subcortical gray matter region due to other factors such as cell density.(2)We measured the MWF and QSM values of the whole brain in macaques,and investigated the relationship between the iron in basal ganglia and the myelin of anatomically connected internal capsule.It was found that a moderate to high positive correlation existed between the QSM of basal ganglia and the MWF value of internal capsule during the synthesis and maintenance of myelin,indicating that the iron of basal ganglia had an effect on the myelin of internal capsule.Overall the results from this thesis work provides new insights in brain pathophysiology,and suggests that the interaction between myelin and iron in separate regions should not be overlooked in future studies,especially in areas with anatomical connections.