High resolution imaging and computational adaptive optics using a commercial OCT device

Abstract

Purpose : Modern laboratory OCT systems have made great progress regarding resolution and imaging speed. This has enabled both functional imaging of individual photoreceptor cells, as well as high-resolution imaging by numerical aberration correction. However, these benefits so far are limited to very complex and/or expensive setups such as Full-Field swept source OCT, that allows for phase stable imaging. Here, we investigate in how far a clinical OCT-System can achieve similar results.

Methods : A Heidelberg Engineering Spectralis OCT System was modified as follows: The A-scan rate was increased to 250kHz. The tracking-rate of the on-board SLO based eye tracker was increased from 12.5 Hertz to 500 Hertz. The active tracking of the scanning mirrors was then disabled. Instead, the tracking data was exported to be used in the postprocessing.

The fixation target of the Spectralis was replaced by a combined fixation and stimulation unit.
The recorded spectra were then reconstructed to OCT Volumes. Motion artefacts were corrected in two steps: first, axial shifts of A-scans were corrected using the segmented inner/outer segment membrane as a reference. Then lateral shifts were compensated by evaluating the tracking data and shifting A-scans to their correct position. To establish phase stability, several algorithms for phase correction were tested. Most powerful proved a modified version of an algorithm proposed by Oikawa et al. in combination with a gradient descent optimization.

Results : After applying the phase correction algorithms on the motion-corrected volumes, the average phase-error was reduced to ~π/60. Therefore, it was possible to apply basic aberration correction for lower order aberrations such as defocus and astigmatism. The Image-quality was drastically increased. Photoreceptor cells are clearly visible and reproducibly identifiable.

Conclusions : It was shown that a modern clinical OCT system like the Heidelberg Engineering Spectralis can deliver higher resolution then commonly expected. Furthermore, it was shown that by applying appropriate motion- and phase correction algorithms, the use of computational adaptive optics becomes possible, and the resolution can be further increased.

This abstract was presented at the 2023 ARVO Imaging in the Eye Conference, held in New Orleans, LA, April 21-22, 2023.