Abstract
Purpose :
We previously presented the very first full-field OCT (FFOCT) images of in vivo human retina. To obtain these images, it was necessary to equalize the optical path difference (OPD) of the reference arm and of the retinal layer of interest. However, as the retina moves constantly in the axial direction due to pulsation and fixation, this is a challenging task and several images have to be acquired to increase the chances of a matched OPD. In this work, we implement a real-time axial retinal motion tracker in the FFOCT, which we use to cancel the OPD and consistently obtain diffraction-limited high-resolution en-face retinal images without using adaptive optics.
Methods :
A customized FFOCT system was developed and combined with a spectral-domain OCT (SDOCT) system. The SDOCT B-scan is used to estimate the retina axial position using centroid algorithm. A fast voice-coil motor is installed in the reference arm giving the precise position of the reference mirror. For each iteration, the OPD is computed and a new command is sent to the voice coil to equalize the OPD. With OPD-locked, FFOCT images were acquired at 350Hz with a high-speed camera for various healthy subjects. Image stacks acquired were registered and averaged to improve signal to noise ratio (SNR).
Results :
The real-time OPD-locking system presents an axial range of 2.5 mm, a loop rate of 25Hz and a precision of 5µm rms. These specifications are more than sufficient to lock the OPD during image acquisition, as the FFOCT axial resolution is 8µm. In this way, we were able to consistently image the inner/outer segment (IS/OS) junction at the foveal and perifoveal regions for multiple subjects. Figure 1 shows images of the photoreceptor mosaic resolved without using adaptive optics for a 5mm diameter pupil, where the signal spatial variation is mainly due to the alignment of the subject.
Conclusions :
We have successfully demonstrated retinal axial motion tracking and correction enabling consistent high-resolution FFOCT retinal imaging without adaptive optics. In this way, the photoreceptor mosaic was resolved for a 5mm diameter pupil for multiple subjects. This paves the way for a straightforward implementation of a compact FFOCT ultrahigh resolution system with a transmissive adaptive lens for low order aberration correction in order to increase SNR in clinical studies.
This abstract was presented at the 2019 ARVO Imaging in the Eye Conference, held in Vancouver, Canada, April 26-27, 2019.