T 1 relaxometry of crossing fibres in the human brain

Abstract

A comprehensive tract-based characterisation of white matter should include the ability to quantify myelin and axonal attributes irrespective of the complexity of fibre organisation within the voxel. Recently, a new experimental framework that combines inversion recovery and diffusion MRI, called inversion recovery diffusion tensor imaging (IR-DTI), was introduced and applied in an animal study. IR-DTI provides the ability to assign to each unique fibre population within a voxel a specific value of the longitudinal relaxation time, T1, which is a proxy for myelin content. Here, we apply the IR-DTI approach to the human brain in vivo on 7 healthy subjects for the first time. We demonstrate that the approach is able to measure differential tract properties in crossing fibre areas, reflecting the different myelination of tracts. We also show that tract-specific T1 has less inter-subject variability compared to conventional T1 in areas of crossing fibres, suggesting increased specificity to distinct fibre populations. Finally we show in simulations that changes in myelination selectively affecting one fibre bundle in crossing fibre areas can potentially be detected earlier using IR-DTI.

Investigating white matter fibre density and morphology using fixel-based analysis

“…FD). However, we note that the proposed fixel-based analysis framework can be used to investigate other measures of interest, such as fixel-specific diffusivity measures (Scherrer et al, 2016, Scherrer and Warfield, 2012), or relaxometry (De Santis et al, 2016). We clarify that for measures that do not relate directly to fibre density, modulation of such measures by FC may not be appropriate.…”

“…A major benefit of these so-called mixture models (Tournier et al, 2011) is that quantitative measures can be associated with a single fibre population within a voxel (Assaf and Basser, 2005, De Santis et al, 2016, Dell’Acqua et al, 2013, Raffelt et al, 2012b, Riffert et al, 2014, Scherrer et al, 2016, Scherrer and Warfield, 2012). We refer to such a single fi bre population within a vo xel as a fixel, 1 as introduced in Raffelt et al (2015).…”

“…Recent work estimates fixel-specific myelin content via T1 relaxometry (De Santis et al, 2016), which would provide useful additional information when investigating fibre density. However, the current acquisition time for the required inversion recovery diffusion weighted imaging sequence is ~1 h (for whole-brain coverage), which is not suitable for most clinical populations.…”

A diffusion model‐free framework with echo time dependence for free‐water elimination and brain tissue microstructure characterization

“…Other alternatives combine diffusion models with multicompartmental relaxation. For instance, inversion recovery diffusion weighted imaging has been used to identify fiber populations, and WM integrity has been characterized using the axonal stick model and multiple echo times (TE) …”

“…Other alternatives combine diffusion models with multicompartmental relaxation. For instance, inversion recovery diffusion weighted imaging has been used to identify fiber populations, 25,26 and WM integrity has been characterized using the axonal stick model and multiple echo times (TE). 27 Compartmental analysis of the diffusion signal is intimately related to a recurring issue: cerebrospinal fluid (CSF) or free-water contamination.…”