In vivo characterization of brain ultrashort‐T 2 components

Purpose Recent nuclear magnetic resonance and MRI studies have measured a fast‐relaxing signal component withT 2 ∗ < 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‐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 ultrashortT 2 ∗relaxation times and large frequency shifts. Results We measured an ultrashort‐T 2 component in healthy volunteers with a medianT 2 ∗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. © 2017 International Society for Magnetic Resonance in Medicine.