In vivo characterization of brain ultrashort‐T₂ components

Purpose Recent nuclear magnetic resonance and MRI studies have measured a fast‐relaxing signal component with 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‐T 2 component 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‐T 2 components 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 relaxation times and large frequency shifts. Results We measured an ultrashort‐T 2 component in healthy volunteers with a median 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‐T 2 brain 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‐T 2 component observed in vivo arises from bound protons in the myelin phospholipid membranes. Magn Reson Med 80:726–735, 2018.