Cited by 253Advanced Neuroimaging Techniques and ApplicationsFunctional Brain Connectivity StudiesAdvanced MRI Techniques and Applications

Potential Pitfalls of Using Fractional Anisotropy, Axial Diffusivity, and Radial Diffusivity as Biomarkers of Cerebral White Matter Microstructure

Open access
Figley, Chase R., Uddin, Md Nasir, Wong, Kaihim, Kornelsen, Jennifer, Puig, Josep, Figley, Teresa D.·2022·Frontiers in Neuroscience

Abstract

Fractional anisotropy (FA), axial diffusivity (AD), and radial diffusivity (RD) are commonly used as MRI biomarkers of white matter microstructure in diffusion MRI studies of neurodevelopment, brain aging, and neurologic injury/disease. Some of the more frequent practices include performing voxel-wise or region-based analyses of these measures to cross-sectionally compare individuals or groups, longitudinally assess individuals or groups, and/or correlate with demographic, behavioral or clinical variables. However, it is now widely recognized that the majority of cerebral white matter voxels contain multiple fiber populations with different trajectories, which renders these metrics highly sensitive to the relative volume fractions of the various fiber populations, the microstructural integrity of each constituent fiber population, and the interaction between these factors. Many diffusion imaging experts are aware of these limitations and now generally avoid using FA, AD or RD (at least in isolation) to draw strong reverse inferences about white matter microstructure, but based on the continued application and interpretation of these metrics in the broader biomedical/neuroscience literature, it appears that this has perhaps not yet become common knowledge among diffusion imaging end-users. Therefore, this paper will briefly discuss the complex biophysical underpinnings of these measures in the context of crossing fibers, provide some intuitive “thought experiments” to highlight how conventional interpretations can lead to incorrect conclusions, and suggest that future studies refrain from using (over-interpreting) FA, AD, and RD values as standalone biomarkers of cerebral white matter microstructure.

AI snapshot

Written from the paper’s full text.

TL;DR

This paper argues that fractional anisotropy (FA), axial diffusivity (AD), and radial diffusivity (RD) are problematic as standalone biomarkers of cerebral white matter microstructure because most white matter voxels contain crossing fibers. It uses thought experiments to show that conventional interpretations can lead to incorrect reverse inferences, and recommends avoiding over-interpretation of these metrics in isolation.

Key findings

  • Most cerebral white matter voxels contain multiple fiber populations: the proportion is now thought to be at least 33% (Behrens et al., 2007) and more likely between 60 and 90% (Jeurissen et al., 2013), with many regions containing 3 or more intersecting fiber bundles.
  • Because FA is a relative, unitless measure scaled between 0 and 1, it can paradoxically increase when one or more fiber bundles degenerate while others are preserved, or remain unchanged when all three eigenvalues increase proportionally despite net tissue loss; AD and RD are confounded for similar reasons in crossing-fiber voxels.
  • Conventional reverse inferences from FA, AD, and RD can lead to Type I (false positive) and/or Type II (false negative) errors, and the paper states there is no way to control for these effects or estimate their prevalence.
  • MD and Trace are proposed as more robust DTI metrics because they equally weight diffusion along all three tensor axes; advanced dMRI methods (DKI, NODDI, tensor-valued diffusion encoding/μFA, fixel-based analysis) and non-dMRI methods (T1w/T2w ratio, ihMT, myelin water imaging) are suggested as alternatives likely more robust to crossing fibers.

Methods

  • Narrative/commentary review with no new empirical data or sample; it synthesizes prior literature and presents two intuitive “thought experiments.”
  • The thought experiments use cartoon voxels containing three crossing fibers (vertical, horizontal, and through-plane) and corresponding diffusion tensor eigenvalues to illustrate how FA can increase despite net fiber reduction, or remain constant despite proportional tissue changes.
  • Main analysis is conceptual and biophysical, supported by references to prior simulation and empirical work (e.g., Pierpaoli et al., 2001; Wheeler-Kingshott and Cercignani, 2009; Jeurissen et al., 2013).

Limitations

  • The authors state that all MRI methods have limitations and that their list of alternatives is not comprehensive; they also note that they focused on FA for simplicity, though AD and RD are confounded similarly.
  • The authors state there is no way to control for crossing-fiber effects or estimate the prevalence of Type I and Type II errors, which could even occur simultaneously in different brain regions.
  • Evident from the text: this is not an original data study or systematic review, and the recommendations are conceptual rather than validated by new experiments in this paper.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.3389/fnins.2021.799576

Cites

54 of 54 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

14 of 27 cited sources checked · 9 supported · 0 flagged · accuracy not yet measured

  • Premisecitation

    The majority of cerebral white matter voxels contain multiple fiber populations with different trajectories.

    “it is now widely recognized that the majority of cerebral white matter voxels contain multiple fiber populations with different trajectories”
    Quote in the paperSubstantial claim2 of 2 sources checked
    • cites Behrens et al., 2007 source

      The Abstract says that multiple fibre populations occur across large regions of white matter. But Results (Local fitting of the complex fibre model; page unavailable) reports that only one third of voxels with fractional anisotropy above 0.1 supported more than one orientation in the 60-direction dataset. This supports the presence of multiple populations, not the claim that they occur in a majority of cerebral white matter voxels. (LLM, conf=0.88)

      Experimental — accuracy not yet measured
    • cites Jeurissen et al., 2013 source
  • Premisecitation

    The proportion of cerebral white matter voxels containing complex fiber geometries and/or multiple fiber bundles in different orientations is more likely between 60 and 90%.

    “and is more likely somewhere between 60 and 90% (Jeurissen et al., 2013)”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Jeurissen et al., 2013 source
  • Premisecitation

    Many cerebral white matter regions contain as many as 3 or more intersecting fiber bundles with different trajectories.

    “with many regions thought to contain as many as 3 or more intersecting fiber bundles with different trajectories (Figure 1)”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Jeurissen et al., 2013 source
  • Premisecitation

    The proportion of cerebral white matter voxels containing complex fiber geometries and/or multiple fiber bundles in different orientations is at least 33%.

    “the proportion of cerebral white matter voxels containing complex fiber geometries and/or multiple fiber bundles in different orientations (i.e., crossing fibers, kissing fibers, etc.) is now thought to be at least 33% (Behrens et al., 2007)”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Behrens et al., 2007 source

      The Results (Local fitting of the complex fibre model) and Discussion report detectable complex fibre architecture in approximately one third of voxels with fractional anisotropy greater than 0.1 using a 60-direction dataset; page numbers are not supplied. This supports the approximate proportion, but not a lower bound of at least 33% for all cerebral white matter voxels. (LLM, conf=0.88)

      Experimental — accuracy not yet measured
  • Premisecitation

    ihMT and MWI likely provide more myelin-specific information than can be obtained with more general measures based on DTI or T1w/T2w ratio metrics.

    “in the case of ihMT and MWI likely also provide more myelin-specific information than can be obtained with more general measures based on DTI or T1w/T2w ratio metrics (Mädler et al., 2008; Ercan et al., 2018; Uddin et al., 2019).”
    Quote in the paperSubstantial claim1 of 3 sources checked
    • cites Mädler et al., 2008 source
    • cites Ercan et al., 2018 source
    • cites Uddin et al., 2019 source

      The Introduction calls MWF a myelin-specific measure and DTI metrics general measures of tissue microstructure; the Conclusions also say T1w/T2w ratios should not be interpreted as myelin-specific in subcortical structures. The paper mentions an earlier ihMTR comparison but does not establish that ihMT is more myelin-specific than DTI or T1w/T2w. Page numbers are unavailable in our copy. (LLM, conf=0.86)

      Experimental — accuracy not yet measured
  • May combine two claims. 5
  • Classified as probably not a substantial claim. 3
  • May combine two claims.

    Premisecitation

    Changes in AD can induce spurious alterations in RD and vice versa in voxels containing crossing fibers.

    “changes in AD can induce spurious alterations in RD and vice versa in voxels containing crossing fibers (Wheeler-Kingshott and Cercignani, 2009)”
    Quote in the paperSubstantial claim1 of 1 sources checked
    • cites Wheeler-Kingshott and Cercignani, 2009 source
  • Probably not a substantial claim: may not pass S3 (load-bearing). May combine two claims.

    Interpretationcitation

    Modeling individual fibers at the sub-voxel level could lead to more sensitive measurements and more detailed understandings about tissue degeneration in various disorders.

    “Modeling individual fibers at the sub-voxel level like this could lead to more sensitive measurements and more detailed understandings about tissue degeneration in various disorders (Finkelstein et al., 2021).”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites Finkelstein et al., 2021 source
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Premisecitation

    Fixel-based analysis approaches have been proposed to characterize a specific fiber population within a voxel (i.e., a "fixel").

    “recent fixel-based analysis approaches have been proposed to characterize a specific fiber population within a voxel (i.e., a “fixel”) (Raffelt et al., 2017).”
    Quote in the paperNot a substantial claim1 of 1 sources checked
    • cites Raffelt et al., 2017 source
  • May combine two claims.

    Interpretationreasoning

    MD and Trace values are theoretically more robust to multiple fiber populations and/or complex fiber geometries than FA, AD, and RD, and are likely the best DTI indicators of how tissues constrain diffusion within a voxel.

    “As a result, MD and Trace values are theoretically more robust to multiple fiber populations and/or complex fiber geometries, and are likely the best DTI indicators of how tissues are constraining diffusion within a voxel.”
    Quote in the paperSubstantial claim
  • May combine two claims.

    Premisecitation

    NODDI can estimate neurite density in dendrites and axons, as well as orientation dispersion.

    “NODDI can estimate neurite density in dendrites and axons, as well as orientation dispersion (i.e., fanning of neurites), which are factors that contribute to, but are distinct from DTI-based FA measures.”
    Quote in the paperNot a substantial claim0 of 1 sources checked
    • cites Zhang et al., 2012 source
  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning

    Strong biophysical interpretations and conclusions should not be drawn using FA, AD or RD measurements alone.

    “it suggests that strong biophysical interpretations and conclusions should not be drawn using FA, AD or RD measurements alone”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Premisecitation

    Orientation dispersion estimates the angular variability between neurites and provides a better measure than FA in regions with fanning or crossing fibers.

    “Orientation dispersion estimates the angular variability between neurites and provides a better measure than FA in regions with fanning or crossing fibers.”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites Zhang et al., 2012 source
7 of 7
  • The paper relies on this without support.

    FA, AD, and RD are commonly used as MRI biomarkers of white matter microstructure in diffusion MRI studies of neurodevelopment, brain aging, and neurologic injury/disease.

    “Fractional anisotropy (FA), axial diffusivity (AD), and radial diffusivity (RD) are commonly used as MRI biomarkers of white matter microstructure in diffusion MRI studies of neurodevelopment, brain aging, and neurologic injury/disease.”
  • The paper relies on this without support.

    Many diffusion imaging end-users continue to apply and interpret FA, AD, and RD as standalone biomarkers of white matter microstructure, and this has not become common knowledge among them.

    “based on the continued application and interpretation of these metrics in the broader biomedical/neuroscience literature, it appears that this has perhaps not yet become common knowledge among diffusion imaging end-users.”
  • The paper relies on this without support.

    The conventional interpretation that higher FA values generally reflect greater white matter density is widely held.

    “the conventional thinking was that all else being equal, higher FA values generally reflected greater white matter density”
3 of 3

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Figure 2A labels the λ1 increase as both large and small

    Minor

    The Figure 2 caption assigns λ1 in part A to both the bold arrow representing a relatively large increase and the thin arrows representing a relatively small increase.

  • Tissue damage is said to both decrease and increase the eigenvalues

    Minor

    The text's example says damage causes an equal reduction in all three eigenvalues, while its description of Figure 2B says the eigenvalues increase proportionally under the same scenario.

2 of 2

Cites

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Authors
Figley, C. R., Uddin, M. N., Wong, K., Kornelsen, J., Puig, J., & Figley, T. D.
Affiliations
  • Health Sciences Centre (CA)
  • University of Rochester Medicine (US)
  • University of Manitoba (CA)
Corresponding author
Figley, Chase R.
Venue
Frontiers in Neuroscience
Published
2022-01-14
Volume
15
Pages
799576-799576
ISSN
1662-453X (ISSN-L)
OpenAlex
W4205639396
Cited by
253
References
54
Citations by year
2026: 66 · 2025: 68 · 2024: 76 · 2023: 33 · 2022: 10
FWCI
24.05
Type
journal article
Language
en
Open access
Locations
  • Frontiers in Neuroscience (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • PubMed Central (submittedVersion, cc-by)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
Funders
  • Health Sciences Centre Foundation
  • Research Manitoba
  • Natural Sciences and Engineering Research Council of Canada
Topics
  • Health Sciences › Medicine › Radiology, Nuclear Medicine and Imaging › Advanced Neuroimaging Techniques and Applications
  • Life Sciences › Neuroscience › Cognitive Neuroscience › Functional Brain Connectivity Studies
  • Health Sciences › Medicine › Radiology, Nuclear Medicine and Imaging › Advanced MRI Techniques and Applications
Keywords
fractional anisotropy, axial diffusivity, radial diffusivity, white matter microstructure, diffusion MRI, crossing fibers, MRI biomarkers

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