Wade, Alex R.; (1999) High-resolution, in vivo imaging of the human cone photoreceptor mosaic. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes a project to image the human retina in vivo using a modified confocal laser scanning ophthalmoscope (cLSO). An optical attachment to an existing prototype cLSO was designed and constructed. The attachment reduced the field of view of the cLSO to approximately three degrees of visual angle at the retina but also reduced the amount of light returning to the imaging detector and consequently lowered the signal to noise ratio. The video output of the cLSO was digitised using a high-speed, low-noise video frame grabber. Many individual frames from a single imaging sequence were aligned and averaged using novel image processing software to improve the signal to noise ratio. Arrays of point-like structures could then be identified in the final, processed images. Experiments were undertaken to confirm the point structures corresponded to cone photoreceptor apertures. The point sizes and spacings were compatible with those found in anatomical studies and it was shown that point spacing increased with increasing retinal eccentricity—a fact which was also predicted from anatomical studies. By fitting an additional, high-intensity fixation target to the cLSO it was shown that the point structures were located at the focal plane at which light is trapped into the visual system. The above results were evidence that the point sources were produced by light reflecting from the cone photoreceptor apertures: the LSO/MSA combination was imaging the cone mosaic. An additional experiment was conducted to study the time course of reflectivity changes in individual photoreceptors. It was shown that at least one factor contributing to the variation in imaged photoreceptor brightness was a signal correlated to the arterial pulse. This project has shown that it is possible to routinely image the cone photoreceptor mosaic in vivo at retinal eccentricities of between two and four degrees using a cLSO. It is also possible to use this device to obtain physiological imaging data from the cone mosaic with a temporal resolution of 1/25 second.

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