This project focuses on developing techniques for quantifying and characterizing the myelin layer in the central nervous system, and using them to investigate the processes underlying the pathophysiology of multiple sclerosis and other demyelinating diseases. One of the most effective MR biomarkers for myelin, are the water trapped between myelin sheaths. The signal emitted from this water compartment can be distinguished through its fast T2 decay time, due to magnetic dipole-dipole interactions between the mobile water protons and the bound protons in the myelin macromolecules. To probe this water compartment we emply a signal model that incorporates the tissue microarchitecture and the various mechanisms that are at play during an MRI experiment. We then use this model to extract meaningful tissue parameters.
Magnetic Resonance mechanisms and interactions in a model of myelinated axons.(a) Graphical illustration and (b) electron micrograph showing the three main tissue compartments: extracellular space (ec), water trapped between the layers of myelin sheaths (m) and intra-axonal space (ia). (c) Various mechanisms affect the MRI signal including: compartment-specific relaxation times (T1, T2), magnetic susceptibility (χ), molecular diffusion, magnetization transfer (MT), and chemical exchange.