Abstract
Purpose: :
To understand the factors limiting resolution in the adaptive optics scanning laser ophthalmoscope (AOSLO) and how they contribute to the lack of visibility of central foveal cones.
Methods: :
Cones within the central 0.1º of the foveola remain unresolved with AOSLO and it is typical to resolve cone mosaics only at distances greater than 0.5º. We proposed to overcome the limits caused by source coherence and foveal cone structure by actively generating random phase differences between cones through visible light stimulation (presented at ARVO 2008). However, remaining limits such as sampling errors, resolution limits, registration errors and foveal cone reflectivity changes need to be overcome for this method to work. A revised foveal cone model explores these limits in an effort to establish the best imaging conditions that will facilitate visualization of foveal cones.
Results: :
Foveal cones are approximately 0.5arcmin in diameter and have been resolved in an AO flood-illuminated ophthalmoscope in some subjects using 550nm light and a 6mm pupil (0.38arcmin resolution). Foveal imaging with the Berkeley AOSLO was performed over a 6mm pupil 680nm (0.48arcmin resolution) and 840nm (0.59arcmin resolution) using field size of 0.6 º (7.1pixels/foveal cone) chosen to minimize sampling errors. Careful registration of 840 nm images of the fovea taken during the time course of recovery from bleach and repeated over time shows benefits similar to that predicted by the model. However, close inspection of flood-illuminated images of foveal cones suggests that some foveal cones may still be too dim to see, and that compiling data from more imaging sessions may be necessary. Additionally, more precise intra-frame image stabilization and addition will be necessary to increase the signal to noise ratio.
Conclusions: :
In order to resolve every cone in the foveola, care must be taken to optimize imaging conditions due to their dim nature and proximity to the system’s optical resolution. Changes such as using a shorter imaging wavelength, a smaller field size, and a larger pupil size will increase the system resolution. Better image stabilization would help preserve the resolution and multiple imaging sessions may aid in the detection of dim central cones.
Keywords: imaging/image analysis: non-clinical • retina