Dayan, M; (2015) Reconstruction and maturation analysis of the optic radiation as assessed by diffusion magnetic resonance imaging and tractography. Doctoral thesis , UCL (University College London).

Abstract

Magnetic Resonance Imaging (MRI) plays a prominent role in neuroimaging, as it pro¬vides an exquisite in-vivo 3D millimetre-scale depiction of the brain, as well as stark contrast between its unmyelinated and myelinated parts, respectively grey and white matter (WM). The limitations of conventional MRI sequences in differentiating between WM pathways have been overcome with diffusion MRI, a technique estimating the diffusion profile of wa¬ter molecules within tissue. By measuring this profile in each voxel, one can obtain diffusion metrics characterizing the underlying microstructure and the principal diffusion direction. Con¬necting diffusion directions from all voxels to reconstruct WM pathways is a process called tractography, unique in its ability to perform virtual dissection of the brains WM tracts. Among the principal senses is vision, a function supported by the visual pathway. This pathway includes a particular WM bundle called the optic radiation (OR), connecting the thala¬mus to the primary visual cortex. Localization of the OR is of particular importance in epilepsy surgery, during which temporal lobe resection can result in OR damage and visual field defects. Although known to be myelin-mature very early in life, little has been documented on OR mat¬urational changes, in terms of its microstructure and dimensions, or how these aspects differ according to gender and hemisphere. In this work we evaluated the reliability of an OR tractography protocol based on manually-drawn regions-of-interest, which was tested with different diffusion profile recon¬struction algorithms. Good reproducibility and repeatability of the tract dimensions and dif¬fusion metrics was demonstrated. These parameters were then estimated in a large cohort of children and adolescents, revealing that age, gender and hemisphere all significantly influenced these measurements. Our work provided, for the first time, the anatomical dimensions of the OR in a paediatric population. Finally, we designed a novel automated OR segmentation scheme that obviated the usual reliance on expert anatomical input. This method was shown to provide robust reconstruction of the OR and may be particularly useful for neurosurgical planning involving lesions or resections in close proximity to the OR.

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