Visualizing the invisible: inner plexiform layer stratification with conventional spectral-domain optical coherence tomography

TL;DR

This study shows that the inner plexiform layer of the retina can be visualized using standard OCT technology with optimized imaging and grayscale inversion.

Contribution

The first demonstration of IPL stratification using commercial SD-OCT with grayscale inversion, without requiring experimental equipment.

Findings

Inverted grayscale revealed five IPL sub-bands in all cases, especially in the parafoveal region.

The 3-mm scan protocol provided better sub-layer differentiation than 12-mm scans.

SS-OCT images did not allow for the distinction of the five IPL strata.

Abstract

The inner plexiform layer (IPL) of the retina plays a key role in visual processing, consisting of five stratified sub-bands (S1-S5) that segregate ON and OFF visual pathways. Until now, resolving these IPL sub-layers was only possible with experimental high-resolution (HR-OCT) or visible-light OCT (VIS-OCT), which remain inaccessible for clinical use. This study provides the first demonstration that IPL stratification can be visualized using commercially available spectral-domain OCT (SD-OCT) with optimized imaging and grayscale inversion.

This retrospective, cross-sectional image analysis study included three healthy individuals who underwent macular OCT imaging. Two subjects were imaged with SD-OCT devices (Nidek RS3000 Advance and Zeiss Cirrus 6000), while one subject was imaged with a swept-source OCT (SS-OCT) device (Topcon Triton DRI). High-density B-scans (1024 A-scans per B-scan) with 120 repetitions for noise reduction were analyzed in both standard and inverted grayscale display modes. The impact of scan size (12 mm, 6 mm, and 3 mm) on IPL visualization was also evaluated.

In conventional grayscale, IPL stratification was indistinct. However, inverted grayscale revealed five IPL sub-bands in all cases, particularly in the parafoveal region where the IPL is thicker. Hyperreflective dots near IPL-1, likely representing the superficial capillary plexus, were also identified. The 3-mm scan protocol provided superior sub-layer differentiation compared to 12-mm scans. However, SS-OCT images did not allow for the distinction of the five IPL strata.

This study challenges the belief that IPL stratification cannot be identified with conventional SD-OCT. By refining imaging parameters and using grayscale inversion, this approach enhances retinal circuit analysis with standard technology. While SD-OCT enables detailed IPL visualization under specific conditions, SS-OCT does not appear to be well-suited for this purpose. These findings redefine SD-OCT’s diagnostic capabilities, opening avenues for research in ophthalmology and neurodegenerative disease monitoring. Further studies should establish best practices and expand clinical applications for this novel methodology.

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