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 T1 and T2 relaxation contrast have suggested the presence of a distinct water pool that may report directly on local myelin content. Recent work indicates that T2* contrast may offer particular advantages over T1 and T2 contrast, especially at high field. However, the complex mechanism underlying T2* relaxation may render interpretation difficult. To address this issue, T2* relaxation behavior in human brain was studied at 3 and 7 tesla. 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 (T2* ~ 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 T2* 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.

Exploring the utility of GRE-MRI signal compartments for temporal susceptibility mapping and investigation of neural microstructure

“…primary magnet defects), or thermal noise. These magnetic field inhomogeneities, alongside spin-spin (T2) relaxation, contribute in adjunct to the overall transverse relaxation of the bulk MRI signal measured, and can result in non-intuitive (deviations from mono-exponential decay and non-linear phase evolution) signal behaviour (Cronin et al, 2017;Schweser et al, 2011a;van Gelderen et al, 2012). For this reason, the dephasing effects of magnetic field inhomogeneities are conveniently labelled as T2 ' .…”

“…Beyond the spatial influence of magnetic susceptibility, non-linear variations in the temporal GRE-MRI signal have also been attributed to tissue-specific susceptibility effects (Cronin et al, 2017;Schweser et al, 2011;Schweser et al, 2016;Sood et al, 2017;van Gelderen et al, 2012). For this reason, the multi-echo GRE-MRI (mGRE-MRI) sequence has increasingly been used to visualize and quantify magnetic susceptibility in brain tissue from echo-time dependent data.…”

“…field strengths (Hernando et al, 2008;Nam et al, 2015;Sati et al, 2013Sati et al, , 2013van Gelderen et al, 2012) and, it was concluded that fitting quality improves with the addition of signal phase in the model. Nam et al compared various models and showed spatially resolved maps of model parameters based on a model in which all three signal compartments were complex valued.…”

“…, for compartments = 1 6; T2 * values were set within realistic ranges as reported previously by Peters et al (2007); and ∆f in view of previous studies on GRE-MRI signal compartments (Alonso-Ortiz et al, 2017;Sati et al, 2013;van Gelderen et al, 2012;Wu et al, 2017 increases in search space dimensionality (e.g. three search dimensions introduced through the addition of each compartment).…”

“…using complex terms to define signal compartments (Hernando et al, 2008;Nam et al, 2015;Sati et al, 2013Sati et al, , 2013van Gelderen et al, 2012). When examining the T2 * signal behaviour in white matter, Van Gelderen et al (2012) report higher residual errors in areas exhibiting greater structural order (e.g.…”