Surpassing the diffraction limit for improved resolution in adaptive optics optical coherence tomography in the living human eye

Abstract

Purpose : To achieve resolution beyond the diffraction limit of light in the lateral dimension of adaptive optics optical coherence tomography (AOOCT), using a combination of annular illumination and sub-Airy disk diameter (sub-ADD) detection (PMID 34504903) together with decoupled illumination and detection.

Methods : A custom-built AOOCT instrument was designed and assembled with illumination and detection light paths decoupled such that an annular mask and a sub-ADD pinhole could be separately inserted into the illumination and detection paths, respectively. To obtain higher pixel sampling desirable for enhanced lateral resolution, a 1060 nm, 3 MHz Fourier domain mode locked light source was used to enable volume acquisition at 12.7 Hz with 512x512 lateral pixel sampling. Resolution enhancement was assessed by acquiring images of cones in the living human eye and validated with a 1951 USAF resolution test target.

Results : Using a decoupled illumination/detection scheme, AOOCT images of the human eye obtained without either the annular mask or the sub-ADD pinhole (“conventional scheme”) were similar in resolution to previously reported AOOCT images collected using a shared illumination/detection scheme (PMID 33796365). The use of sub-ADD alone resulted in improved lateral resolution which was further improved with the combination of both annular illumination and sub-ADD detection, sufficient to reveal a contiguous cone mosaic approximately 0.5° from the fovea, not resolvable in images acquired under the conventional scheme. Consistent with qualitative observations of improved resolution near the fovea, a clear peak at the expected cone frequency (corresponding to 4.6 µm spacing) was revealed in 2D power spectra when using the combination of annular illumination and sub-ADD detection (not visible in the conventional scheme). This improvement was also confirmed with a USAF target showing increased resolution of a 2.46 µm line period element.

Conclusions : Whereas most AOOCT instruments are designed with a shared illumination/detection path, here, we demonstrate an advantage of decoupling these paths – enabling insertion of unique optical elements into each path. Despite the longer wavelength of this implementation (~33% worse resolution than 800 nm), a considerable improvement in lateral resolution was realized in the living human eye.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.