Pupil Tracking for Real-Time Motion Corrected Anterior Segment Optical Coherence Tomography.

Volumetric acquisition with anterior segment optical coherence tomography (ASOCT) is necessary to obtain accurate representations of the tissue structure and to account for asymmetries of the anterior eye anatomy. Additionally, recent interest in imaging of anterior segment vasculature and aqueous humor flow resulted in application of OCT angiography techniques to generate en face and 3D micro-vasculature maps of the anterior segment. Unfortunately, ASOCT structural and vasculature imaging systems do not capture volumes instantaneously and are subject to motion artifacts due to involuntary eye motion that may hinder their accuracy and repeatability. Several groups have demonstrated real-time tracking for motion-compensated in vivo OCT retinal imaging, but these techniques are not applicable in the anterior segment. In this work, we demonstrate a simple and low-cost pupil tracking system integrated into a custom swept-source OCT system for real-time motion-compensated anterior segment volumetric imaging. Pupil oculography hardware coaxial with the swept-source OCT system enabled fast detection and tracking of the pupil centroid. The pupil tracking ASOCT system with a field of view of 15 x 15 mm achieved diffraction-limited imaging over a lateral tracking range of +/- 2.5 mm and was able to correct eye motion at up to 22 Hz. Pupil tracking ASOCT offers a novel real-time motion compensation approach that may facilitate accurate and reproducible anterior segment imaging

Controlling for Artifacts in Widefield Optical Coherence Tomography Angiography Measurements of Non-Perfusion Area

The recent clinical adoption of optical coherence tomography (OCT) angiography (OCTA) has enabled non-invasive, volumetric visualization of ocular vasculature at micron-scale resolutions. Initially limited to 3 mm × 3 mm and 6 mm × 6 mm fields-of-view (FOV), commercial OCTA systems now offer 12 mm × 12 mm, or larger, imaging fields. While larger FOVs promise a more complete visualization of retinal disease, they also introduce new challenges to the accurate and reliable interpretation of OCTA data. In particular, because of vignetting, wide-field imaging increases occurrence of low-OCT-signal artifacts, which leads to thresholding and/or segmentation artifacts, complicating OCTA analysis. This study presents theoretical and case-based descriptions of the causes and effects of low-OCT-signal artifacts. Through these descriptions, we demonstrate that OCTA data interpretation can be ambiguous if performed without consulting corresponding OCT data. Furthermore, using wide-field non-perfusion analysis in diabetic retinopathy as a model widefield OCTA usage-case, we show how qualitative and quantitative analysis can be confounded by low-OCT-signal artifacts. Based on these results, we suggest methods and best-practices for preventing and managing low-OCT-signal artifacts, thereby reducing errors in OCTA quantitative analysis of non-perfusion and improving reproducibility. These methods promise to be especially important for longitudinal studies detecting progression and response to therapy.National Institutes of Health (U.S.) (NIH 5-R01-EY011289-31)National Institutes of Health (U.S.) (AFOSR FA9550-15-1-0473)Fundação de Amparo à Pesquisa do Estado de São Paulo (Grant No. 2016/17342-0)Fundação de Amparo à Pesquisa do Estado de São Paulo (Grant No. 2015/15775-3

Optical performance of the anterior segment OCT system with and without tracking.

<p>(A-B) System diagram without tracking (A) and after 2.5 mm of lateral tracking (B). (C-D) Spot diagrams of the extrema of the +/- 15 mm tracking FOV. The airy radius was 39.5 μm as denoted by the black circles and the optical performance was diffraction limited.</p

Characterization of motion correction bandwidth.

<p>(A) 2D M-scan acquired during simulated pupil phantom motion without pupil tracking correction. The red box shows the segmented pupil edge. The segmented portion was then smoothed with a Gaussian filter (B) and intensity thresholded (C). Edge detection (D) yielded the pupil motion trace. Motion traces without (E) and with (F) pupil tracking are shown for 1 Hz simulated sinusoidal motion. (G) Motion correction percentage as a function of simulated motion frequency.</p

Averaged B-scans generated by summing 125 B-scans acquired at the same location in 0.75 seconds with and without tracking.

<p>(A-B) Averaged, untracked and tracked B-scans after axial registration only. (C-D) Averaged, untracked and tracked B-scans after full (lateral+axial) registration. (E-F) Digitally zoomed images of corneal stroma for untracked and tracked fully registered B-scans. (G-H) Digitally zoomed images of the anterior chamber angle. Scleral striations (red arrow) and Schlemm’s canal (blue arrow) were better resolved with tracking. Scale bars are 1 mm.</p

Optical coherence tomography system design and performance.

<p>(A) Diagram of swept-source anterior segment OCT system. G: galvanometer scanning mirrors, L: lens, RR: retro-reflector, DM: dichroic mirror, BR: balanced receiver. (B) OCT sensitivity fall off performance. Fall off of -6dB was measured at 4.6 mm. The red dashed line denotes -6 dB.</p

Representative anterior segment OCT motion images acquired from a healthy volunteer without tracking.

<p>(A) Single frame B-scans composed of 1000 A-lines. (B) Registered and averaged (10x) B-scan acquired in repetitive B-scan mode at a frame rate of 100 Hz. (C) Volumetric image composed of 1000 A-lines and 200 B-scans, corresponding to a volume frame rate of 0.5 Hz. Red arrow denotes significant motion artifacts due to patient motion. Scale bars are 1 mm. Yellow arrow denotes an artifact from specular reflection.</p

Anterior segment volumetric time series acquired with volumes composed of 500 A-lines/B-scans and 200 B-scans/volume with and without tracking.

<p>(A) Volume corrupted by prominent motion artifacts before activating tracking. (B) Volume during which tracking was activated; image artifact caused by repositioning of the scanners is denoted by the red arrow. (C) Volume acquired with tracking activated. Scale bars are 1 mm.</p

Characterization of pupil tracking system for a 3 mm step response.

<p>(A) Generated FSM driving waveform (input) and response waveform generated by tracking algorithm (output), and the fit to the output step function. (B) Impulse response after calculating the derivative of the step response. (C) Magnitude of the frequency response of the system estimated by calculating the Fourier transform of the impulse response. The -3 dB amplitude was measured at 58 Hz, as denoted by the dashed red line.</p