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Abstract:
In the confined environment between adjacent myelin membrane layers, water protons experience frequent interactions with non-aqueous components. This accelerates relaxation and induces magnetization transfer (MT) between water and macromolecular protons. For a quantitative analysis, models of varying complexity have been developed. Relatively comprehensive is a model that considers water in an intra-axonal, an extra-axonal and a myelin compartment as well as corresponding macromolecular pools with magnetization transfer and inter-compartmental water exchange as coupling pathways. However, it is far too complex for practical MRI applications, and simpler models are typically used to appreciate the role of myelin for proton relaxation. We will discuss some more frequently employed models in the context of T1, T2, T2* and MT experiments, underlying simplifications and potential limitations.
