Abstract
Annular apodization of the illumination and/or imaging pupils of an adaptive optics scanning light ophthalmoscope (AOSLO) for improving transverse resolution was evaluated using three different normalized inner radii (0.26, 0.39 and 0.52). In vivo imaging of the human photoreceptor mosaic at 0.5 and 10° from fixation indicates that the use of an annular illumination pupil and a circular imaging pupil provides the most benefit of all configurations when using a one Airy disk diameter pinhole, in agreement with the paraxial confocal microscopy theory. Annular illumination pupils with 0.26 and 0.39 normalized inner radii performed best in terms of the narrowing of the autocorrelation central lobe (between 7 and 12%), and the increase in manual and automated photoreceptor counts (8 to 20% more cones and 11 to 29% more rods). It was observed that the use of annular pupils with large inner radii can result in multi-modal cone photoreceptor intensity profiles. The effect of the annular masks on the average photoreceptor intensity is consistent with the Stiles-Crawford effect (SCE). This indicates that combinations of images of the same photoreceptors with different apodization configurations and/or annular masks can be used to distinguish cones from rods, even when the former have complex multi-modal intensity profiles. In addition to narrowing the point spread function transversally, the use of annular apodizing masks also elongates it axially, a fact that can be used for extending the depth of focus of techniques such as adaptive optics optical coherence tomography (AOOCT). Finally, the positive results from this work suggest that annular pupil apodization could be used in refractive or catadioptric adaptive optics ophthalmoscopes to mitigate undesired back-reflections.
Highlights
Current clinical ophthalmoscopes form images of the retina by delivering and collecting light through non overlapping portions of the eye’s pupil in order to avoid undesired reflections from the cornea and crystalline lens
Careful examination of the image corresponding to ε = 0.52 and annular pupil illumination reveals that the intensity profiles of the cone photoreceptors appear multi-modal
The change in full-width half-maximum (FWHM) values of the central lobe of the autocorrelation as a result of using annular pupils for all the images in Fig. 4 are shown in Table 1 below. These numbers loosely agree with the theoretical predictions that apodization of the illumination pupil translates to a sharpening of the cone photoreceptor intensity profiles and apodization of the imaging pupil does not
Summary
Current clinical ophthalmoscopes form images of the retina by delivering and collecting light through non overlapping portions of the eye’s pupil in order to avoid undesired reflections from the cornea and crystalline lens. AO wavefront correction over a large pupil has resulted in an increase in transverse and axial resolution that has allowed in vivo visualization of multiple microscopic retinal structures in human and animal models, including but not limited to: the cone photoreceptor mosaic [1,6,16], the retinal pigment epithelial cell mosaic [8,17,18], leukocytes [19], the retinal capillary vasculature [19,20,21], fluorescently labeled retinal ganglion cells [8], the lamina cribrosa [22,23,24], and more recently, the rod photoreceptor mosaic [25,26] These structures cannot be resolved in all subjects due to a number of factors, one of them being the reduction in pupil diameter that comes with aging [27] which results in a proportional reduction in transverse resolution and a quadratic reduction in signal.
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