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Diffusion Tensor Imaging of the Roots of the Brachial Plexus: A Systematic Review and Meta-Analysis of Normative Values1University of Leeds, Leeds, United Kingdom, 2Sheffield Teaching Hospitals, Sheffield, United Kingdom, 3Umeå University, Umeå, Sweden, 4University of Pittsburgh, Pittsburgh, PA, United States
Synopsis
In this systematic review and meta-analysis we summarise the normal diffusion tensor imaging (DTI) metrics for roots of the brachial plexus. We included 9 studies describing 316 adults (1:1 male:female) of mean age 35 years (SD 6). The normal fractional anisotropy was 0.36 (95% CI 0.34, 0.38; Figure 4). The normal mean diffusivity was 1.51 x10-3 mm2/s (95% CI 1.45, 1.56; Figure 5). DTI metrics varied according to experimental conditions and participant factors. Our summary estimates from different conditions which may be valuable to researchers and clinicians alike.
Introduction
Magnetic resonance imaging (MRI) is the best non-invasive imaging modality for diagnosing various pathologies affecting the brachial plexus (Figure 1)[1–6]. The roots of the brachial plexus are the most common site of injury[7] and typically, the status of the root dictates the prognosis and surgical reconstruction. However, the diagnostic performance of conventional cross-sectional MRI for assessing the roots remains suboptimal[7] and consequently, there has been a recent surge of research into diffusion tensor imaging (DTI), which characterises tissue microstructure and provides proxy measures of myelination, axon diameter, fibre density and organisation. Before DTI can be used clinically to assess the ‘health’ of the roots of the brachial plexus, there is a need to estimate the normal values.Methods
This review is registered with PROPSERO (ID CRD42019155788), it was designed and conducted in accordance with the Cochrane Handbook of Systematic Reviews[8] and has been authored in accordance with the PRISMA checklist[9]. We systematically searched the literature for studies of asymptomatic adults who underwent DTI of the brachial plexus. Participant characteristics, scanning protocols, and measurements of the fractional anisotropy (FA) and mean diffusivity (MD) of each spinal root were extracted by two independent review authors. Generalised linear modelling was used to estimate the effect of experimental conditions on the FA and MD. Meta-analysis of root-level estimates were performed using Cohen’s method with random-effects.Results
After reviewing 27 full texts, 15 were excluded. We included 9 studies[10–18], describing 316 adults (1:1 male:female) of mean age 35 years (SD 6) were included. Multivariable modelling showed that the angular resolution was strongly associated with FA, whereby every additional 10 diffusion sensitising gradient directions sampled reduced the FA by 0.01 (95% CI 0.01, 0.03; Figure 2). Furthermore, multivariable modelling showed that each year of life reduced the MD by 0.03 x10-3 mm2/s (95% CI 0.01, 0.04; Figure 3). The pooled normal fractional anisotropy of the roots of the brachial plexus at 3 Tesla[10,11,13,16–19] was 0.36 (95% CI 0.34, 0.38; Figure 4). There were no statistically significant differences between the C5-T1 roots. The pooled normal mean diffusivity of the roots of the brachial plexus at 3 Tesla [11,13,16–19] was 1.51 x10-3 mm2/s (95% CI 1.45, 1.56; Figure 5).Discussion
We have shown that the roots of the brachial plexus in adults have a mean fractional anisotropy of 0.36 (95% CI 0.34, 0.38) and mean diffusivity of 1.51 x10-3 mm2/s (95% CI 1.45, 1.56). However, there is substantial variation between studies and the estimates are shown to be sensitive to experimental factors[20], such as: the SNR[21], components of the b-value[22,23], the number of diffusion sensitising gradient directions[24,25]; software pipelines for denoising and correcting artefacts arising from susceptibility, motion or eddy-currents[26,27], tensor fitting methods[21], and the size and position of ROIs[28]. Whilst the TraCED challenge[29] and several phantom studies[30–32] have demonstrated that very high reproducibility across scanners, sequences and sessions for DTI, we suggest that researchers and clinicians interpret our summary values with respect to their particular circumstances.Conclusions
The FA and MD of the roots of the brachial plexus vary according to experimental conditions and participant factors. We provide summary estimates of the normative values in different conditions which may be valuable to researchers and clinicians alike.Acknowledgements
No acknowledgement found.References
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