Dynamic Focusing Spiral Scan OCT for High Resolution, Wide Field, Corneal and Anterior Chamber Imaging

Abstract

Purpose : Imaging of the ocular anterior segment with optical coherence tomography (OCT) typically requires trade-offs between resolution and depth-of-focus due to corneal curvature and the deep anterior chamber. We have developed a dynamically focusing OCT system coupled with constant linear velocity (CLV) spiral scanning that maintains focus from the corneal apex to the anterior lens during a scan.

Methods : Because scanning mirrors must constantly change velocity during a typical OCT raster scan, they generate non-usable data at the scan edges. CLV spiral scanning samples at constant linear velocity, optimally utilizing all imaging time by spiraling from the center to the periphery. Dynamic focusing further allows for depth tracking of radially symmetric features, such as the corneal contour. Choosing CLV parameters for isotropic sampling allows for reconstruction of full resolution, arbitrarily orientated B-scans in post-processing.
We designed a custom swept-source OCT system (λ₀ = 1045nm, 200 kHz scan rate) to have a diffraction limited spot of 12.8µm (depth-of-focus 833µm) at the corneal apex. Focusing objective was synchronized with CLV spiral scan to shift the imaging plane to follow corneal radial profile. Scan volumes were 785840 A-scans isotropically distributed over a 13mm diameter zone. Optical resolution and ratio between focus lens motion and image plane depth was verified with a USAF 1951 test chart. A volunteer subject was imaged under approved IRB protocol.

Results : Imaging the subject at a fixed focus during volume acquisition provided high contrast at the corneal apex but low contrast at the lens, iris, and periphery (Fig. 1A). Applying dynamically focusing CLV spiral scan generated a well-resolved image of the entire cornea, anterior lens, and iris processes. A temporal-nasal reconstructed B-scan (Fig. 1B) and volumetric rendering with a cut through the superior-inferior plane (Fig. 1C) are shown.

Conclusions : We have developed a high-speed OCT system to image the entire anterior chamber with high resolution and contrast over a large field of view in depth and transverse dimensions.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

A) Temporal-nasal slice of OCT volume taken without adjusting the image plane at corneal apex. Periphery, iris and lens are all low contrast B) Temporal-nasal slice of dynamic focusing CLV spiral scan OCT volume C) Volumetric rendering of B cut through at the superior-inferior slice plane of volume