Orientation dependence of inhomogeneous magnetization transfer and dipolar order relaxation rate in phospholipid bilayers

Sarah R Morris; Rebecca Frederick; Alex L MacKay; Cornelia Laule; Carl A Michal

doi:10.1016/j.jmr.2022.107205

Orientation dependence of inhomogeneous magnetization transfer and dipolar order relaxation rate in phospholipid bilayers

J Magn Reson. 2022 May:338:107205. doi: 10.1016/j.jmr.2022.107205. Epub 2022 Mar 25.

Authors

Sarah R Morris¹, Rebecca Frederick², Alex L MacKay³, Cornelia Laule⁴, Carl A Michal²

Affiliations

¹ International Collaboration on Repair Discoveries (ICORD), University of British Columbia, Canada; Dept. of Radiology, University of British Columbia, Canada; Dept. of Physics & Astronomy, University of British Columbia, Canada.
² Dept. of Physics & Astronomy, University of British Columbia, Canada.
³ Dept. of Radiology, University of British Columbia, Canada; Dept. of Physics & Astronomy, University of British Columbia, Canada; UBC MRI Research Centre, University of British Columbia, Canada.
⁴ International Collaboration on Repair Discoveries (ICORD), University of British Columbia, Canada; Dept. of Radiology, University of British Columbia, Canada; Dept. of Physics & Astronomy, University of British Columbia, Canada; Dept. of Pathology and Laboratory Medicine, University of British Columbia, Canada.

PMID: 35390716
DOI: 10.1016/j.jmr.2022.107205

Abstract

Inhomogeneous magnetization transfer (ihMT) is a novel MRI technique used to measure white matter myelination in the brain and spinal cord. In the brain, ihMT has a strong orientation dependence which is likely to arise from the anisotropy of dipolar couplings between protons on oriented lipids in the myelin bilayers. We measured the orientation dependence of the second moment (M₂) of the lineshape, dipolar order relaxation rate (R_1D), and ihMT ratio (ihMTR) in an oriented phospholipid bilayer at 9.4 T. We found a strong orientation dependence in all three parameters. ihMTR and R_1D were maximized when the bilayers were aligned perpendicular to B₀ and minimized near the magic angle (∼54.7°). M₂ followed an orientation dependence given by the second Legendre polynomial squared as predicted by the form of the secular dipolar Hamiltonian. These results were used to calculate the orientation dependence of R_1D and ihMTR in a diffusionless myelin sheath model, which showed ihMTR was maximised for fibers perpendicular to B₀ and minimised at 45°, similar to ex-vivo spinal cord with a larger prepulse frequency offset, but in contrast to in vivo brain findings. Adding fiber dispersion to this model smoothed the orientation dependence curve as expected. Our results suggest the importance of the effects of lipid diffusion and prepulse offset frequency on ihMTR.

Keywords: Brain; Dipolar coupling; Dipolar order; Dipolar order relaxation time; Dipolar reservoir; Magnetic resonance imaging; Myelin; Phospholipid bilayer; Provotorov theory; Second moment; Spinal cord; White matter; ihMT.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Brain
Magnetic Resonance Imaging / methods
Myelin Sheath
Phospholipids*
White Matter*

Substances

Phospholipids