title: Nonexponential T₂ decay in white matter.
abstract: Visualizing myelin in human brain may help the study of diseases such as multiple sclerosis. Previous studies based on T(1) and T(2) relaxation contrast have suggested the presence of a distinct water pool that may report directly on local myelin content. Recent work indicates that T(2) contrast may offer particular advantages over T(1) and T(2) contrast, especially at high field. However, the complex mechanism underlying T(2) relaxation may render interpretation difficult. To address this issue, T(2) relaxation behavior in human brain was studied at 3 and 7 T. Multiple gradient echoes covering most of the decay curve were analyzed for deviations from mono-exponential behavior. The data confirm the previous finding of a distinct rapidly relaxing signal component (T(2) ∼ 6 ms), tentatively attributed to myelin water. However, in extension to previous findings, this rapidly relaxing component displayed a substantial resonance frequency shift, reaching 36 Hz in the corpus callosum at 7 T. The component's fractional amplitude and frequency shift appeared to depend on both field strength and fiber orientation, consistent with a mechanism originating from magnetic susceptibility effects. The findings suggest that T(2) contrast at high field may be uniquely sensitive to tissue myelin content and that proper interpretation will require modeling of susceptibility-induced resonance frequency shifts.
authors.affiliation: Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA. gelderen@nih.gov
authors.forename: Peter
authors.initials: P
authors.lastname: van Gelderen
authors.forename: Jacco A
authors.initials: JA
authors.lastname: de Zwart
authors.forename: Jongho
authors.initials: J
authors.lastname: Lee
authors.forename: Pascal
authors.initials: P
authors.lastname: Sati
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Reich
authors.forename: Jeff H
authors.initials: JH
authors.lastname: Duyn
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009413:Nerve Fibers, Myelinated / Q000648:ultrastructure*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D055815:Young Adult