Developing Clinically Orientated Diffusion-Weighted Magnetic Resonance Imaging of the Brachial Plexus in Adults

Introduction (Part 1): The nerves of the brachial plexus control movement and feeling in the upper limb. The most common form of traumatic brachial plexus injury (BPI) is root avulsion. Morphological magnetic resonance imaging (MRI) is used clinically to diagnose root avulsion, but its accuracy remains unclear. Diffusion MRI (dMRI) techniques characterise tissue microstructure and generate proxy measures of nerve ‘health’ which are sensitive to myelination, axon diameter, fibre density and organisation. Part 2: Chapter 1 describes a meta-analysis of 11 studies, showing that conventional (morphological) MRI has modest diagnostic accuracy for diagnosing root avulsion in adults with BPI. This represents the rationale for developing dMRI. Part 2: Chapter 2 shows a clinically viable dMRI sequence which is sensitive to established root avulsion in adults, highlighting uncertainties which warrant investigation before the technique is applied to acutely injured patients. Part 2: Chapter 3 is concerned with modelling the geometry of the brachial plexus in fixed cadavers, to inform the step angle in dMRI processing. Part 2: Chapter 4 is a meta-analysis of 9 studies which defines the normal fractional anisotropy and mean diffusivity values in healthy adult the brachial plexus, and how experimental factors influence dMRI parameter estimates. Part 2: Chapter 5 explores the effect of fractional anisotropy thresholding on deterministic tractography in the brachial plexus, identifying areas of uncertainty in the intrathecal and intraforminal areas. Part 2: Chapter 6 shows that two most common pre-processing pipelines worldwide generate important differences in dMRI parameters and tractograms. Part 2: Chapter 7 deploys high b-value multishell dMRI to show that up to 44% of the brachial plexus has multiple fibre orientations.