Quantitative myelin imaging with MRI and PET: an overview of techniques and their validation status

Myelin is the protective sheath wrapped around axons, consisting of a phospholipid bilayer with water between the wraps. The measurement of damage to the myelin sheaths, the evaluation of the efficacy of therapies aiming to promote remyelination and monitoring the degree of brain maturation in children all require non-invasive quantitative myelin imaging methods. To date, various myelin imaging techniques have been developed. Five different MRI approaches can be distinguished based on their biophysical principles: (i) imaging of the water between the lipid bilayers directly (e.g. myelin water imaging); (ii) imaging the non-aqueous protons of the phospholipid bilayer directly with ultra-short echo-time techniques; (iii) indirect imaging of the macromolecular content (e.g. magnetization transfer; inhomogeneous magnetization transfer); (iv) mapping of the effects of the myelin sheath's magnetic susceptibility on the MRI signal (e.g. quantitative susceptibility mapping) and (v) mapping of the effects of the myelin sheath on water diffusion. Myelin imaging with PET uses radioactive molecules with high affinity to specific myelin components, in particular myelin basic protein. This review aims to give an overview of the various myelin imaging techniques, their biophysical principles, image acquisition, data analysis and their validation status. van der Weijden et al. review myelin imaging techniques and discuss their differences on a biophysical level. They conclude that the most promising techniques are quantitative susceptibility mapping and inhomogeneous magnetization transfer for MRI, and C-11-MeDAS for PET.

Automated summary

Myelin is a protective sheath made of phospholipid bilayer and water that wraps around axons. Non-invasive quantitative myelin imaging methods are needed for evaluating myelin damage, assessing the efficacy of remyelination therapies, and monitoring brain maturation in children. Different MRI and PET techniques have been developed for myelin imaging based on various biophysical principles. This review provides an overview of these techniques, including image acquisition, data analysis, and validation status, and concludes that quantitative susceptibility mapping, inhomogeneous magnetization transfer for MRI, and C-11-MeDAS for PET are the most promising techniques.