Fast, quantitative myelin maps: Macromolecular pool fraction (MPF) using an optimized protocol

The macromolecular pool fraction (MPF) has been shown to correlate with myelin (Khodanovich 2017). Whole brain MPF maps can be generated efficiently from T1, B1 and B0 maps and the collection of two additional volumes: a reference with no MT pulse (MT₀) and an MT-weighted volume (MT_Δ) acquired at an optimal frequency offset, Δ (Yarnykh 2012). Further time efficiency was proposed by using the T1 and B1 maps to create a synthetic MT₀, synMT₀ (Yarnykh 2016). In the original work, T1 maps were generated using a variable-flip-angle method from two images that were acquired with the same sequence (spoiled-gradient echo, SPGR) and resolution as MT_Δ. Restrictions on TR for MT-prepared scans makes matching scans across MT and T1 mapping acquisitions not optimal. Recently, we showed that synMT can be created using a T1 mapping method relying on fastSPGR (FSPGR) scans if a calibration procedure is used to account for systematic differences across sequences (Chavez 2019). The calibration was found to be subject-independent and thus the scaling factor, f, was determined a priori and applied on subsequent synMT. However, discrepancies in bandwidth across sequences led to inaccuracies in synMPF₀ in regions of large susceptibility gradients (Δχ). In this work, we optimize T1 and B1 mapping acquisitions to reduce scan time for synMT generation and we match sequences to reduce inconsistencies in regions of large Δχ.