Abstract
Purpose :
Optical coherence tomography angiography (OCTA) of the anterior vasculature has the potential to provide a non-invasive alternative to dye based angiography for diagnosing and monitoring a range of corneal, iris and scleral pathologies. Current anterior OCTA approaches have a limited field of view and rely on eye tracking optimised for posterior imaging and is susceptible to motion artefacts. In this paper we demonstrate anterior OCTA using the Hyper-parallel (HP-OCT) spectral domain system. HP-OCT exploits highly parallel sampling to enable robust OCT image based registration over a wide field of view.
Methods :
The HP-OCT system was primarily designed for anterior segment metrology; a parallel sampling scheme enables accurate measurement of surfaces in the presence of patient movement. A grid of 1008 simultaneous beams is generated from an 840nm SLED and a 2D micro-lens array of 0.4mm pitch. A grating based spectrometer disperses all beams on to a 2D CMOS sensor with a frame rate of 300Hz (300k A-scans/s). A single frame samples data from lateral area of 9.6x16.8mm and depth of 11mm. A full volumetric dataset consists of 256 frames, captured in less than 1s, whilst scanning over a range of only 0.4x0.4mm. OCTA data is obtained from 4 sequential frames at each scan position. An OCTA volume generated from 4x256 frames is captured in 4s. At each scan position, the 4 frames are axially registered and flow determined from amplitude based decorrelation. OCTA frames are registered using associated OCT volume frames and resampled into a refraction corrected 3D angiogram.
Results :
Sample OCTA images are shown in Figure 1. The axial and lateral resolution are 9.4mm and 33mm respectively. We note that volumetric registration enables us to identify and discard frames with excessive lateral movement. We also note evidence of polarisation fading, this may be avoided in a future polarisation diverse implementation.
Conclusions :
The HP-OCT system is capable of anterior OCTA over a wide field of view without the need for eye tracking. Parallel sampling enables robust registration of frames and minimal motion artefacts. The system has an optional high resolution anterior lens which is the subject of ongoing anterior OCTA development.
This is a 2020 Imaging in the Eye Conference abstract.