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Fractional Anisotropy Thresholding for Deterministic Tractography of the Roots of the Brachial Plexus
Ryckie George Wade1, Irvin Teh1, Gustav Andersson2, Fang-Cheng Yeh3, Mikael Wiberg2, and Grainne Bourke1
1University of Leeds, Leeds, United Kingdom, 2Umeå University, Umeå, Sweden, 3University of Pittsburgh, Pittsburgh, PA, United States

Synopsis

We acquired high-angular resolution DTI from 10 healthy adults. We investigated how altering the fractional anisotropy threshold changed tractograms and tract-related metrics. At thresholds above 0.06, the propagation of valid tracts reduced significantly. The fractional anisotropy thresholds required to generate valid tractograms of the roots of the brachial plexus are lower than thresholds conventionally used in the brain. We provide estimates of the probability of generating valid tracts for each spinal nerve root of the brachial plexus, at different fractional anisotropy thresholds.

Introduction

In adults with a brachial plexus injury (BPI) the spinal nerve roots are the structures most commonly injured[1]. Whilst MRI is the best non-invasive test for diagnosing root injury, it misclassifies 25% of healthy roots as injured[1]. This shortfall in conventional imaging might be improved by diffusion tensor imaging (DTI), which is sensitive to myelination, axon diameter, fibre density and organisation[2–4]. Further, DTI tractography may help clinicians to evaluate changes in the diffusivity and anisotropy throughout the length of the roots, which might help to identify areas of pathology or targets for surgery. Altering the fractional anisotropy (FA) threshold for tractography has a significant effect on tractography in the brain, but this topic has not been studied in peripheral nerves and forms the rationale for this study in the brachial plexus.

Methods

Ten healthy adults (8 males, 2 females; mean age 28 years) were scanned using a Siemens Prisma (3 Tesla) and single-shot echo-planar imaging, as follows: b-value 1000 s/mm2, 64 bipolar diffusion directions (using twice refocused spin echo with 16 interleaved non-diffusion weighted images), 2.5mm isotropic resolution with 4 signal averages. The sequence was repeated in opposing phase encoding direction. Data were combined and corrected for susceptibility artefacts. We propagated 250 tracts per root from a seeding region covering the spinal cord, via a 5mm3 region of interest covering the extra-foraminal spinal nerve roots. Tractography was performed by two independent raters, varying the FA thresholding at increments of 0.01 (from 0.04 to 0.1). DTI metrics were analysed using mixed-effects linear regression.

Results

The mean FA varied between subjects by 2% (95% CI 1%, 3%). At higher FA thresholds fewer tracts were propagated (Figure 1) which was largely due to the failure to render tracts representing the T1 spinal nerve root (Figure 2). FA thresholds of 0.04, 0.05 and 0.06 all propagated 96% of tracts representing the roots. Increasing the FA threshold to 0.07 yielded 4% fewer tracts (p=0.2), 0.08 yielded 11% fewer tracts (p=0.008), 0.09 yielded 15% fewer tracts (p=0.001) and 0.1 yielded 20% fewer tracts (p<0.001). There was <0.1% inter-rater variability in the measured FA (mean difference 0.008 [95% CI -0.004, 0.01]; ICC 0.001) and 99% agreement for tractography (Cohen’s kappa 0.92, p<0.001; Figures 3-5).

Discussion

We show that the FA thresholds conventionally used for tractography in the brain are too high for the brachial plexus roots. The normal FA of white matter pathways in the brain are 0.36 to 0.54[5], depending on the structure, which is substantially higher than the anisotropy estimates we provide. The discrepancies in FA between central white matter pathways and the roots of the brachial plexus are likely to be (at least in part) due to the 7-fold lower axon density of the roots (38,000 fibres mm-2 in the corpus callosum vs. 7914 mm-2 in the roots of the brachial plexus) and substantial fascicular bifurcation, merging, weaving and exchange throughout the length of the brachial plexus[6–8]. Our FA estimates are also slightly lower than those found in other studies[9–15]; it likely that other studies report upwardly biased estimates of FA because they used lower b-values and fewer diffusion encoding directions[16–19].

Conclusions

The fractional anisotropy threshold required to generate tractograms of the roots of the brachial plexus appears to be lower than those used in the brain. We provide estimates of the probability of generating true tracts for each spinal nerve root of the brachial plexus, at different fractional anisotropy thresholds.

Acknowledgements

No acknowledgement found.

References

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Figures

Figure 1. The overall proportion of visualised tracts representing roots of the brachial plexus at different FA thresholds.

Figure 2. The proportion of tracts visualised representing specific roots of the brachial plexus at different FA thresholds.

Figure 3. Deterministic tractography of the roots of the brachial plexus at different FA thresholds, from independent raters. The colour of the tract is determined by the local FA whereby yellow denotes a high FA (0.5), scaled to red or blue (for rater 1 or 2, respectively) which denotes low FA

Figure 4. Deterministic tractography of the roots of the brachial plexus at different FA thresholds, from independent raters. The colour of the tract is determined by the local FA whereby yellow denotes a high FA (0.5), scaled to red or blue (for rater 1 or 2, respectively) which denotes low FA

Figure 5. Deterministic tractography of the roots of the brachial plexus at different FA thresholds, from independent raters. The colour of the tract is determined by the local FA whereby yellow denotes a high FA (0.5), scaled to red or blue (for rater 1 or 2, respectively) which denotes low FA

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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