The Geometry of the Roots of the Brachial Plexus
Emily Rose Bligh, BSc (Hons)1, Kieran Nar, MEng1, Rebecca S Stone, BSc MSc2, David J Roberts, BSc PhD2, Irvin Teh, BE MBiomedE PhD2, Grainne Bourke, MB BCh BAO FRCSI FRCS(Plast)3 and Ryckie G Wade, MBBS MClinEd MRCS FHEA2, (1)University of Sheffield, Sheffield, United Kingdom, (2)University of Leeds, Leeds, United Kingdom, (3)Leeds Teaching Hospitals Trust, Leeds, United Kingdom
Introduction: Diffusion tensor magnetic resonance imaging (DTI) can be used to reconstruct the brachial plexus in 3D via tracts connecting contiguous diffusion tensors with similar primary eigenvector orientations. When creating DTI tractograms, the turning angle of connecting lines ('step angle') must be prescribed by the user; however, the literature is lacking detailed geometry of brachial plexus to inform such decisions.
Materials & Methods: The spinal cord and brachial plexus of 10 embalmed adult cadavers were exposed bilaterally by posterior dissection. Photographs were taken under standardised conditions and spatially calibrated in Matlab. The roots of the brachial plexus were traced from the dorsal root entry zone for 5cm laterally using a 2.5mm2 Cartesian grid overlay. The trace was composed of points connected by lines and the turning angle between line segments (the 'step angle') was resolved.
Results: Our data shows that the geometry of the roots increased in tortuosity from C5 to T1, with no significant differences between sides. The 1st thoracic root had the most tortuous course, turning through a maximum angle of 56o per 2.5mm (99% CI 44o to 70o). Significantly higher step angles and greater variability were observed in the medial 2cm of the roots of the brachial plexus, where the dorsal and ventral rootlets coalesce to form the spinal root. Throughout the brachial plexus, the majority of step angles (>50%) were smaller than 20o and <1% of step angles exceeded 70o.
Conclusions: The geometry of the brachial plexus increases in tortuosity from C5 to T1. To reconstruct 99% of tracts representing the roots of the brachial plexus by DTI tractography, users can either customise the step angle per root based on our findings or select a universal threshold of 70o.
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