DIffusion-Prepared Phase Imaging (DIPPI): quantifying myelin in crossing fibres

Abstract

PurposeMyelin has long been the target of neuroimaging research due to its importance in brain development, plasticity, and disease. However, most available techniques can only provide a voxel-averaged estimate of myelin content. In the human brain, white matter fibre pathways connecting different brain areas and carrying different functions often cross each other in the same voxel. A measure that can differentiate the degree of myelination of crossing fibres would provide a more specific marker of myelination.Theory & MethodsOne MRI signal property sensitive to myelin is the phase accumulation, which to date has also been limited to voxel-averaged myelin estimates. We use this sensitivity by measuring the phase accumulation of the signal remaining after diffusion weighting, which we call DIffusion-Prepared Phase Imaging (DIPPI). Including diffusion weighting before estimating the phase accumulation has two distinct advantages for estimating the degree of myelination: (1) it increases the relative contribution of intra-axonal water, whose phase is related linearly to the amount of myelin surrounding the axon (in particular the log g-ratio) and (2) it gives directional information, which can be used to distinguish between crossing fibres.ResultsUsing simulations and phantom data we argue that other sources of phase accumulation (i.e., movement-induced phase shift during the diffusion gradients, eddy currents, and other sources of susceptibility) can be either corrected for or are sufficiently small to still allow the g-ratio to be reliably estimated.ConclusionsThis new sequence is capable of providing a g-ratio estimate per fibre population crossing within a voxel.

A macroscopic link between interhemispheric tract myelination and cortico-cortical interactions during action reprogramming

“…Recent work has also highlighted the contributions that NoRs may play in dynamically regulating functional axonal properties (Arancibia-Carcamo et al, 2017;Dutta et al, 2018;Ford et al, 2015;Lazari et al, 2018), which our results are compatible with. To further the previous example, two voxels with the exact same number of myelin sheets and oligodendrocytes may have different MT and R2* values if one of the voxels has shorter NoRs, therefore allowing more myelin to cluster within the same 3D volume (Andersson et al, 2020;Cottaar et al, 2020;Walhovd et al, 2014). In summary, our study is compatible with multiple morphological interpretations within white matter, but cannot distinguish between them.…”