Adaptive optics (AO) imaging is a powerful tool for studying photoreceptor packing and distribution in the human retina. Cone density and spacing derived from AO scanning laser ophthalmoscopy (AOSLO)
9 and AO flood-illumination ophthalmoscopy
10 are comparable to histologic findings.
10 Distance from the FPC has been the strongest determinant of cone density and spacing in normal eyes.
11 In addition, cones are more densely packed along the horizontal meridian compared to the vertical meridian.
10 Definitions used to define the foveal center include the fixation point,
12,13 location of peak cone density,
14 center of the iso-density ellipse (the ellipse that connects locations with the same cone density),
10,15 and anatomical center of the foveal pit.
16 AO cameras, including the rtx1 (Imagine Eyes, Orsay, France), often select the first retinal fixation locus used during an AO imaging session as the reference coordinate (
x,
y) of (0,0) for assigning retinal eccentricity within the single image frame. We recently demonstrated that foveal centers determined by co-registration between spectral-domain optical coherence tomography (SD-OCT) B-scan and infrared (IR) fundus images may not coincide with the (0,0) locus of the AO image frame acquired on the rtx1 device in healthy individuals.
17 This disparity may result in inaccurate and inconsistent localization of the ROI and cone mosaic measurements using the coordinates provided by the device. Given that clinical trials (
www.ClinicalTrials.gov) are already using AO imaging to study the natural history of cone photoreceptor degeneration such as rod–cone dystrophy (RCD; e.g. NCT03349242, NCT00254605, and NCT01866371), there is an unmet clinical need to examine the disparity in location between the FPC and the AO montage center (AMC) and its impact on AO image analysis in future clinical trials using the commercial AO imaging device.