True Outer Nuclear Layer Volumes Using Directional Optical Coherence Tomography

Abstract

 

Purpose

 

Standard optical coherence tomography (OCT) acquisition does not reliably differentiate between the true photoreceptor nuclei-containing outer nuclear layer (ONL) and the overlying directionally reflective Henle fiber layer (HFL). We sought to apply Directional OCT (D-OCT) and novel image processing techniques to derive accurate macular ONL volumes.

 

Methods

 

Two dilated eyes of two subjects were imaged using Cirrus HD-OCT (Carl Zeiss Meditec, Inc.) macular cubes through a central position such that the horizontal and vertical B-scans appeared “flat”. Macular cubes were then taken through eight different pupil positions each approximately 2mm away from the central position and offset by approximately 45 degrees from each other. Each SDOCT volume was automatically segmented using proprietary software (Voxeleron) which first identified the ONL/HFL interface on each non-central scan (Fig 1A) and subsequently applied a feature-based registration algorithm (Voxeleron) with sub-voxel interpolation to align the segmented volumes into a common reference frame. Thickness measurements were then computed within ETDRS regions for the ONL derived from the D-OCT reference frames and the ONL measured from standard central acquisitions.

 

Results

 

The ONL/HFL boundary could not be detected on the central OCT scans (Fig 1B). Individual non-central volumes demonstrated a petaloid zone of hyper-reflective HFL contralateral to the OCT beam pupil entry position. Alignment of the volumes into a common reference frame allowed visualization of the true ONL thicknesses (Fig 2A) compared to the ONL thicknesses measured by standard SDOCT central scans (Fig 2B). Within the central 1mm ETDRS region, the true ONL thickness was an average 13.2% (SD 1.7%) less after accounting for HFL. In the ETDRS annulus spanning 1mm-3mm, isolation of the ONL decreased the reported thickness by an average of 31.6% (SD 4.6%).

 

Conclusions

 

True volumetric ONL measurements can be obtained by Directional OCT through multiple pupil positions and automated ONL/HFL segmentation and registration techniques. Clinical studies of retinal degenerations should employ this technique to account for HFL and monitor photoreceptor loss within the macula more precisely and accurately.

 

 

 

Keywords: 552 imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • 550 imaging/image analysis: clinical • 549 image processing