title: The physical mechanism of "inhomogeneous" magnetization transfer MRI.
abstract: Inhomogeneous MT (ihMT) is a new magnetic resonance imaging technique that shows promise for myelin selectivity. Materials with a high proportion of lipids, such as white matter tissue, show a reduced intensity in magnetic resonance images acquired with selective prepulses at positive and negative offsets simultaneously compared to images with a single positive or negative offset prepulse of the same power. This effect was initially explained on the basis of hole-burning in inhomogeneously broadened lines of the lipid proton spin system. Our results contradict this explanation. ihMT in lipids can be understood with a simple spin-1 model of a coupled methylene proton pair. More generally, Provotorov theory can be used to consider the evolution of dipolar order in the non-aqueous spins during the prepulses. We show that the flip-angle dependence of the proton spectrum of a model lipid system (Prolipid-161) following dipolar order generation is in quantitative agreement with the model. In addition, we directly observe dipolar order and ihMT signals in the non-aqueous components of Prolipid-161 and homogeneously-broadened systems (hair, wood, and tendon) following ihMT prepulses. The observation of ihMT signals in tendon suggests that the technique may not be as specific to myelin as previously thought. Our work shows that ihMT occurs because of dipolar couplings alone, not from a specific type of spectral line broadening as its name suggests.
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada.
authors.forename: Alan P
authors.initials: AP
authors.lastname: Manning
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada.
authors.forename: Kimberley L
authors.initials: KL
authors.lastname: Chang
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada; Radiology, University of British Columbia, 2211 Wesbrook Mall, Vancouver, BC V6T 2B5, Canada.
authors.forename: Alex L
authors.initials: AL
authors.lastname: MacKay
authors.affiliation: Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada. Electronic address: michal@phas.ubc.ca.
authors.forename: Carl A
authors.initials: CA
authors.lastname: Michal
keywords: Dipolar MT; Inhomogeneous MT; Lipid NMR lineshape; Myelin in-vivo; Provotorov theory