In vivo characterization of brain ultrashort-T2 components

Abstract

© 2017 International Society for Magnetic Resonance in Medicine Purpose: Recent nuclear magnetic resonance and MRI studies have measured a fast-relaxing signal component with T2*< 1 ms in white matter and myelin extracts. In ex vivo studies, evidence suggests that a large fraction of this component directly arises from bound protons in the myelin phospholipid membranes. Based on these results, this ultrashort-T2component in nervous tissue is a new potential imaging biomarker of myelination, which plays a critical role in neuronal signal conduction across the brain and loss or degradation of myelin is a key feature of many neurological disorders. The goal of this work was to characterize the relaxation times and frequency shifts of ultrashort-T2components in the human brain. Methods: This required development of an ultrashort echo time relaxometry acquisition strategy and fitting procedure for robust measurements in the presence of ultrashort T2relaxation times and large frequency shifts. Results: We measured an ultrashort-T2component in healthy volunteers with a median T2*between 0.5–0.7 ms at 3T and 0.2–0.3 ms at 7T as well as an approximately −3 ppm frequency shift from water. Conclusion: To our knowledge, this is the first time a chemical shift of the ultrashort-T2brain component has been measured in vivo. This chemical shift, at around 1.7 ppm, is similar to the primary resonance of most lipids, indicating that much of the ultrashort-T2component observed in vivo arises from bound protons in the myelin phospholipid membranes. Magn Reson Med 80:726–735, 2018. © 2017 International Society for Magnetic Resonance in Medicine.