Motion-free optical coherence tomography angiography by Lissajous scan

Abstract

Purpose : Optical coherence tomography angiography (OCTA) is frequently suffered by eye motion and appeared with motion artifacts. This paper aims at describing a new OCTA method which is free from motion artifacts. This method does not use an eye tracker, but uses a special scanning protocol, so called as Lissajous scan.

Methods : We adopted Lissajous OCT (not OCTA) scan pattern previously demonstrated by the authors to the OCTA. The Lissajous OCT drives each galvanometric scanning mirror with sinusoidal pattern. The probe beam draws a continuous Lissajous curve, and the whole scan covers a rectangle region on the retina. Particularly for angiographic measurement, the scan pattern is customized, so that two consecutive Lissajous curve cycles scan the same position. A complex decorrelation based OCTA algorithm is then applied to the cycle pair.

By the nature of Lissajous scan, any combinations of subsets of Lissajous cycles have four overlapped regions. So the eye motion is automatically corrected by using the overlapped regions as landmarks.

Optic nerve heads and maculae of 7 eyes of 7 normal subjects were scanned. A volumetric scan consists of 528,520 A-lines and its acquisition takes 5.2 s. The OCT system is a custom-made 1.0-µm swept-source OCT with a scan speed of 100,000 A-line/s.

Results : Fig. 1(a) shows OCTA (yellow) overlaid on SLO (cyan). Although only rigid registration is used, it demonstrates perfect agreement as the vessels appeared as white (yellow + cyan). Since SLO can be regarded as motion-free, it demonstrates high motion correction performance of Lissajous OCTA.

Figure 1(b), (c) are mosaic OCTA, which is generated from two independent Lissajous OCTA measurements. Perfect continuation of vessels are evident in the zoomed-up images (c), and it demonstrates high repeatability of the motion correction.

Figure 2 compares raster (a) and Lissajous scan OCTA (b), where color encodes the depth. The scan area is 3 × 3 mm². While the raster image exhibits several horizontal lines caused by eye motion, the Lissajous scan does not show any motion artifacts.

Conclusions : Lissajous scan OCTA provides motion-free OCTA image without any special hardware. It can be applicable to standard OCT devices to enable motion-free OCTA.

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

 

(a) Motion-free Lissajous OCTA (yellow) overplayed on SLO (cyan). (b) Mosaic fusion of two Lissajous OCTA images and its zoomed-ups (c).

 

OCTA images acquired with standard raster scan (a) and Lissajous scan (b).