Kotecha, Aachal; (2003) In vivo imaging of the lamina cribrosa. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This thesis describes the development of techniques to image the lamina cribrosa within the optic nerve head of the living human eye using a confocal scanning laser ophthalmoscope. The axons from the retinal ganglion cells pass through the pores of the lamina cribrosa to synapse in the higher levels of the brain. The lamina cribrosa has been implicated as one of the main sites of damage to the ganglion cell axons in one of the leading causes of blindness, glaucoma, where it is thought that the raised intraocular pressure results in a backward bowing of the lamina, with consequent deformation of the ganglion cell axons. Most of our understanding of this process to date has come from investigations in post-mortem tissue, since the structure is difficult to visualise due to the optical scattering properties of the overlying neural tissue. The development of new imaging technologies has enabled the investigation of the optic nerve in vivo to quantify its structure in the living human eye. These make use of the optical sectioning effects and improved depth resolution of confocal laser scanning ophthalmoscopy. The purpose of this study was to use these advances to image the lamina cribrosa in vivo, and determine whether using these techniques its structure could be reasonable quantified. Two confocal scanning laser ophthalmoscope systems were used to obtain images of the lamina cribrosa area. These instruments used different lasers, and had different depth and spatial resolution, and were compared for image quality. A comparison was made between imaging with a near infrared laser and the helium neon laser, and it was predicted that the former might give better pore visibility due to greater tissue penetration. However, it was found that using the near infrared laser gave no benefit to lamina pore visibility. It was also found that lamina cribrosa images were best obtained with one of the two instruments- the Zeiss cSLO. Images obtained by these two instruments were digitised, and these digitised in vivo images of the lamina cribrosa underwent a series of image processing techniques to maximise the visibility of the pore-like structure. A method was developed which incorporated the use of Fourier analysis to select the optimum spatial frequency components to reduce the effects of image degradation. The result was a method that, for the first time, provided adequate image quality that allowed quantification of the morphology of the pores of the lamina cribrosa and without operator intervention. This has the advantage of removing possible sources of error due to the operator. In addition, unlike earlier studies, an unselected series of eyes were imaged without pre-selection for pore visibility. A further study was performed to determine the relationship between the lamina pore characteristics and the optic nerve head topography using three dimensional reconstruction, to determine whether any differences existed between pore characteristics of normal and glaucomatous eyes. It was found that with increasing acquired neuroretinal rim loss, the average pore area and pore number increased. This agreed with post-mortem studies.Histological studies were performed on hydrated post-mortem tissue in a range of aged eyes. The purpose of this was to establish the longitudinal thickness of the lamina cribrosa in order to evaluate the imaging depth of the cSLO. This study found that the longitudinal thickness of the lamina cribrosa was approximately two thirds larger than that previously found. This was attributed to the fact that most previous studies used dehydration and digestion techniques during tissue preparation. This work has shown that it is possible to image the lamina cribrosa in vivo and obtain quantitative information about the pore structure without operator intervention in unselected eyes. An image processing routine was developed to highlight the lamina pore structure using a technique based on Fourier analysis.

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