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Abstract
This work is concerned with the challenge of obtaining precise and reliable measurement of the vestibular anatomy in living human subjects at the limits of resolution of three dimensional (3D) computed tomography (CT). The structure and biomechanics of the vestibular system of the inner ear are responsible for providing the sensory information regarding the translational and angular motion of the head, used by the brain for stabilisation of posture and gaze. The measurement of this anatomical geometry is thus of the utmost importance for interpreting the function of the balance system in both normal and disease cases.
In this project we investigate applying an active surface model for the recovery of the semicircular canals (SCC). The current literature provides very limited real results presented with respect to a ground truth. In contrast, we evaluate model performance using real clinical CT data with comparisons made to an objective ground truth acquired using micro-CT. At small sub-pixel scales the role of regularization is especially important, where the application of conventional active contour methods leads to non-optimal results. First of all, we study the influence of various curvature formulations, and show that conventional terms introduce a tendency to shrink or expand the solution. We proposed a new scale invariant smoothing term that eliminates this bias. Secondly, we show that the blind use of a gradient magnitude based energy performs poorly and we recognise the role played by the point spread function (PSF) interacting with small features. Thirdly, we address the influence of the PSF by devising an augmented active surface model by incorporating the PSF into the external energy. We also propose augmenting our model with further prior knowledge, that of reasonably constant underlying CT numbers.
The final contribution of this thesis is a precise measurement of the SCC morphology in 34 human ears, combined with a detailed comparison and analysis within and between subjects in terms of their physiological implications. Finally, we update the mean anatomical data using Fourier series equations to produce a precise, simple and accessible mathematical model of the 3D geometry of the SCCs. This work has been incorporated into routine clinic testing at Royal Prince Alfred Hospital, including that for benign proximal positional vertigo and canal dehiscence syndrome.