Functional imaging of the retinal pigment epithelium using high fidelity retinal densitometry

Abstract

Purpose : Retinal densitometry probes the functional integrity of the RPE by measuring reflectance changes associated with the ‘visual cycle’. However, persistent and intractable technological hurdles have prevented the full potential of the technique from being realised till now. Using expertise developed in the field of astronomy we describe a new approach to imaging retinal densitometry (IRD) that measures visual pigment synthesis rates with high fidelity. This study highlights the power of the technique in healthy controls and in people with macular disease.

Methods : Our multispectral imaging densitometer employs reflective relay optics controlled by a 3D infra-red eye tracking system to maintain precise optical alignment with the pupil within a 8mm cube in space. Multispectral illumination, provided by LEDs driven sequentially at 5Hz, and a scatter correction based on the analysis of data from a non-illuminated retinal strip facilitates the characterisation of visual pigment spectra over space and time. Twelve participants, including 5 healthy controls, 2 dichromats and 5 people with AMD volunteered to take part. Data were collected over a 10 minute recording period and processed to produce heat maps that describe topographical differences in rod and cone visual pigment synthesis rates across the central 20°.

Results : Relative to a conventional head rest, the novel tracking system improved the temporal stability of retinal reflectance measurements ~30 fold (coefficient of variation 8%) and the scatter correction brought measured visual pigment spectra into register with published microspectrophotometric results. Cone visual pigment was typically synthesised at a peak rate of 1.5mODs^-1 at the fovea but this dropped off rapidly with increasing eccentricity. Conversely, rod pigment synthesis rates were negligible at the fovea and peaked at 0.2 mODs^-1 towards the edge of the measurement area. Heat map images of visual pigment synthesis rates in people with macular disease showed substantial topographical variations that were distinct from comparable control images.

Conclusions : IRD facilitates functional imaging of the RPE by mapping visual pigment synthesis rates across the central retina. The technique provides new insights into macular disease phenotypes and may be a useful functional biomarker.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Fig. 1. Visual pigment synthesis rates in a healthy eye (top) and for a person with intermediate AMD (bottom).