Advances in peripheral nerve MR imaging: The application of diffusion MRI in neurological disorders

Abstract

The peripheral nervous system is a network of nerves which transmits signals from the central nervous system to the body and vice versa. It regulates and controls body functions and activity. Damage to (part of) the nerves may cause distortion in transmission of the signal from the central nervous system to the innervated area and vice versa, which in turn may lead to muscle weakness or muscle wasting. The underlying pathophysiological processes leading to nerve degeneration are not always evident. New insights regarding these processes are needed to better understand disease mechanisms. Furthermore, there is a need for non-invasive imaging modalities that can diagnose and monitor disease progression, or therapeutic effects in neurological disorders. A non-invasive imaging technique that allows for the visualization of nerve tissue is magnetic resonance imaging (MRI). Diffusion tensor imaging (DTI) is an MRI technique which is sensitive to the random motion of water molecules and can be used to measure the diffusion of water molecules in tissue. In nerve tissue this diffusion is more oriented along the nerve than perpendicular to it (anisotropic diffusion). This makes it a potential valuable method to investigate peripheral nerve tissue in neurological disorders. In this thesis MRI and DTI are applied in different neurological disorders to explore the potential value in a clinical setting. To determine the potential value of DTI to investigate peripheral nerves in a clinical setting, first the reproducibility of DTI in the lumbosacral nerves was investigated. DTI was found to be reproducible and can therefore be reliably used in cross-sectional studies in a clinical setting. Small differences in inter-week comparison highlight that one needs to be careful when interpreting diffusion measures in longitudinal studies, since small differences can also be caused by factors other than disease progression or therapy response. Then DTI was applied in multiple neurological disorders including spina bifida, compressed nerves of lumbar disc herniated patients, multifocal motor neuropathy, spinal muscular atrophy and single-sided deafness. In general DTI showed differences in diffusion values such as in the fractional anisotropy, mean diffusivity, axial diffusivity and radial diffusivity. With fiber tractography it was possible to reconstruct the nerves in 3D. In for example the spina bifida patients the lumbosacral nerves showed asymmetry and disorganization compared to healthy controls. The various studies in this thesis show that MRI and DTI are promising techniques in the evaluation of peripheral nerve tissue in neurological disorders. The evaluated methods may contribute to the improvement of diagnosis and prognosis of different neurological disorders. However, in some patient groups the hypothesized intra-subject differences were not found. Furthermore, in the longitudinal follow-up of patients, one should be careful with the interpretation, since differences in diffusion values may also be caused by other factors than disease progression or therapeutic effects.