Correlation of potential myelin measures from quantitative Magnetisation Transfer (qMT) and multi-component Driven Equilibrium Single Pulse Observation of T1 and T2 (mcDESPOT)

Introduction: The Magnetisation Transfer (MT) effect is based on the exchange of magnetisation between two proton pools; ‘free’ (‘liquid’) and ‘restricted’ (macromolecular). This property allows the MR characteristics of the macromolecular component (usually ‘invisible’ using conventional MRI due to its very short T2) to be probed via selective saturation. Analysis is based on a 2-pool quantitative model for MT and several physical quantities associated with both pools can be extracted from the fitting process. The restricted proton fraction (fb) is thought to be related to myelin content in the brain (since restricted protons are attached to macromolecules such as myelin), and was previously shown to correlate with myelin content in excised MS lesions (2) and to depend on brain location. The myelin water fraction (fm) measured using multi-component relaxometry is thought to correspond to water trapped within bilayers of the myelin sheath (3). The mcDESPOT (multi-component Driven Equilibrium Single Pulse Observation of T1 and T2) (4) technique provides a new method for investigating tissue relaxation. The SPGR and SSFP-based technique is rapid and exhibits high signal-to-noise ratio (SNR) efficiency. Another major advantage is the large relative contribution of the fast relaxing species to the measured data, as both TE and TR are short (<< 10ms) and constant throughout the experiment, whereas in conventional multi-echo approaches the fast relaxing component contributes measurably over only a few of the (shortest) TE times sampled. As it is believed that both fb and fm are related to myelin in the brain, naively we would expect these measures to show strong correlation. Here we present a preliminary study comparing fm measured via the mcDESPOT technique with fb in three healthy subjects.