Quantitative magnetization transfer model validation at ultra-low field by combining saturation and inversion methods with simultaneous fitting

Portable low-field MRI is expanding access to imaging, but its diagnostic impact will depend in part on developing quantitative contrast mechanisms that remain informative at ultra-low magnetic field strengths. Magnetization transfer provides one such opportunity by probing interactions between free water and semisolid macromolecular pools, potentially adding molecular specificity to low-field MRI. Quantification of semisolid tissue parameters can further improve disease specificity but requires validated mathematical models applicable to ultra-low magnetic field strengths. Using custom semisolid samples that mimic in vivo brain tissue, we perform some of the first studies of quantitative magnetization transfer spectroscopy at an ultra-low field strength of 6.5 mT, combining saturation-based Z-spectroscopy methods with dynamic water-selective inversion-recovery and simultaneous model fitting to improve semisolid parameter isolation. Our results strongly suggest that the quantitative models developed for high-field magnetic resonance are also applicable at ultra-low field, laying the groundwork for molecularly informed quantitative imaging on future low-field portable MRI systems.