Abstract
To investigate the effect of varying levels of -weighting on the evolution of the complex signal from white matter in a multi-echo gradient-recalled echo (mGRE) saturation-recovery sequence. Analysis of the complex signal evolution in an mGRE sequence allows the contributions from short- and long- components to be separated, thus providing a measure of the relative strength of signals from the myelin water, and the external and intra-axonal compartments. Here we evaluated the effect of different levels of -weighting on these signals, expecting that the previously reported, short of the myelin water would lead to a relative enhancement of the myelin water signal in the presence of signal saturation. Complex, saturation-recovery mGRE data from the splenium of the corpus callosum from 5 healthy volunteers were preprocessed using a frequency difference mapping (FDM) approach and analyzed using the 3-pool model of complex signal evolution in white matter. An increase in the apparent as a function of echo time was demonstrated, but this increase was an order of magnitude smaller than that expected from previously reported myelin water -values. This suggests the presence of magnetization transfer and exchange effects which counteract the -weighting. Variation of the amplitude in a saturation-recovery mGRE sequence can be used to modulate the relative strength of signals from the different compartments in white matter, but the modulation is less than predicted from previously reported -values.
Highlights
White matter microstructure can be probed using gradient echo techniques, which are sensitive to the variations of NMR signal frequency produced within and across the different compartments by the myelin sheath
A robust method for removing the nonlocal field perturbation effects, which does not corrupt the local signal evolution is needed to access the effects of microstructure on the phase.2-6 Frequency difference mapping (FDM) uses the phase information from the first and second echoes of a multi-e cho dataset to eliminate any phase effects that are constant or linearly varying with time down the echo train, leaving only phase offsets which result from a variation of apparent frequency with echo time, such as those due to microstructural compartmentalization.[6]
As a result of the rapid decay of the myelin water (MW) signal and its positive resonance frequency offset, we expect the frequency difference to be negative at longer echo times and the FDM contrast to increase with echo time while the MW signal is present
Summary
White matter microstructure can be probed using gradient echo techniques, which are sensitive to the variations of NMR signal frequency produced within and across the different compartments by the myelin sheath. As a result of these effects, analysis of the evolution of the phase and magnitude of the gradient echo signal from white matter allows the signal contributions from the different compartments to be separated out, and potentially provides a new method of probing microstructure. This is best done by using a multi-echo gradient-recalled echo (mGRE) sequence. The phase variation resulting from the differences in the evolution of the signals from the different white matter compartments, which is sensitive to local microstructure, is swamped by nonlocal effects due to larger length-scale magnetic field inhomogeneities, RF-related phase offsets, physiological fluctuations, and eddy current–related field perturbations. We will refer to T1,e ≈ T1,a values as T1,long and to T1,m-values as T1,short
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