Impact of b-value on estimates of apparent fibre density

Abstract

Recent advances in diffusion magnetic resonance imaging (dMRI) analysis techniques have improved our understanding of fibre-specific variations in white matter microstructure.Increasingly, studies are adopting multi-shell dMRI acquisitions to improve the robustness of dMRI-based inferences. However, the impact of b-value choice on the estimation of dMRI measures such as apparent fibre density (AFD) derived from spherical deconvolution is not known. Here, we investigate the impact of b-value sampling scheme on estimates of AFD.First, we performed simulations to assess the correspondence between AFD and simulated intra-axonal signal fraction across multiple b-value sampling schemes. We then studied the impact of sampling scheme on the relationship between AFD and age in a developmental population (n=78) aged 8-18 (mean=12.4, SD=2.9 years) using hierarchical clustering and whole brain fixel-based analyses. Multi-shell dMRI data were collected at 3.0T using ultrastrong gradients (300 mT/m), using 6 diffusion-weighted shells ranging from 0 -6000 s/mm 2 . Simulations revealed that the correspondence between estimated AFD and simulated intra-axonal signal fraction was improved with high b-value shells due to increased suppression of the extra-axonal signal. These results were supported by in vivo data, as sensitivity to developmental age-relationships was improved with increasing bvalue (b=6000 s/mm 2 , median R 2 = .34; b=4000 s/mm 2 , median R 2 = .29; b=2400 s/mm 2 , median R 2 = .21; b=1200 s/mm 2 , median R 2 = .17) in a tract-specific fashion. Overall, estimates of AFD and age-related microstructural development were better characterised at high diffusion-weightings due to improved correspondence with intra-axonal properties.

Reduced apparent fiber density in the white matter of premature-born adults

“…Thirdly, our diffusion weighted data were acquired with 32 diffusion directions and a single b-value of 1000 s/mm 2 which is not optimal for CSD or apparent fiber density modelling. Indeed, it has been shown that the best conditions to perform CSD were higher b values and higher angular resolution 64 and that both parameters influence apparent fiber density measure 65 . Thus, our measure of apparent fiber density might not be optimal and might influence our finding of wide overlap between FD and FA changes.…”

Longitudinal patterns of white matter fibre density and morphology in children are associated with age and pubertal stage

“…Note: See Raffelt et al (2017) for complete descriptions of these measures. *The interpretation of these measures as related to the microstructure of ‘axons’ depends on the strength of the diffusion weighting ( Genc et al, 2020 ). Alt-text: Box 1 …”

“…Compared to commonly used DTI metrics such as FA or mean diffusivity (MD), fixel-based FD is a stronger candidate for signifying microstructural organisation ( Kelley et al, 2019 ), as it is both sensitive (at high b-values) to the intra-axonal signal ( Genc et al, 2020 ), and specific to fibre populations ( Raffelt et al, 2015 ). By comparison, FA measurements can be conflated by crossing fibres and extra-axonal signal contamination ( Jones et al, 2013 ; Bach et al, 2014 ; Beaulieu, 2009 ), making biophysical interpretations challenging.…”

Fixel-Based Analysis of White Matter Alterations in Early- Stage Cerebral Small Vessel Disease

“…In TBSS, false positives may be responsible for our ndings, having been shown to arise from method-inherent issues like considerable noise dependency and projection inaccuracies in regions of crossing bres 43 . Similarly, false-negative ndings by FBA have to be considered as a cause, potentially arising from the relatively low b-value (b = 1000mm/s²) leading to insu cient suppression of the extraaxonal compartment 52 .…”