Recent developments in optical coherence tomography for imaging the retina

Abstract

Optical coherence tomography (OCT) was introduced in ophthalmology a decade ago. Within a few years in vivo imaging of the healthy retina and optic nerve head and of retinal diseases was a fact. In particular the ease with which these images can be acquired considerably changed the diagnostic strategy used by ophthalmologists. The OCT technique currently available in clinical practice is referred to as time-domain OCT, because the depth information of the retina is acquired as a sequence of samples, over time. This can be done either in longitudinal cross-sections perpendicular to, or in the coronal plane parallel to the retinal surface. Only recently, major advances have been made as to image resolution with the introduction of ultrahigh resolution OCT and in imaging speed, signal-to-noise ratio and sensitivity with the introduction of spectral-domain OCT. Functional OCT is the next frontier in OCT imaging. For example, polarization-sensitive OCT uses the birefringent characteristics of the retinal nerve fibre layer to better assess its thickness. Blood flow information from retinal vessels as well as the oxygenation state of retinal tissue can be extracted from the OCT signal. Very promising are the developments in contrast-enhanced molecular optical imaging, for example with the use of scattering tuneable nanoparticles targeted at specific tissue or cell structures. This review will provide an overview of these most recent developments in the field of OCT imaging focussing on applications for the retina.

Introduction

In 1991, optical coherence tomography (OCT) was reported for the first time (Huang et al., 1991). Owing to the clear optical system of the eye itself, OCT soon found its way to ophthalmology. Within a few years in vivo imaging of the healthy retina and optic nerve head and of retinal diseases and in particular the ease with which these images can be acquired considerably changed the diagnostic strategy used by ophthalmologists (Hee et al., 1995a, Hee et al., 1995b, Hee et al., 1995c, Hee et al., 1995d, Hee et al., 1996). Today, OCT is widely used for imaging the vitreoretinal interface, for monitoring (cystoid) macular oedema (CMO) and retinal thickness in a wide variety of patients and for monitoring retinal nerve fibre layer (RNFL) thickness and optic nerve head parameters in glaucoma patients.

OCT provides cross-sectional images of the macula or optic nerve head, analogous to ultrasonography. Instead of sound OCT uses light waves to obtain a reflectivity profile of the tissue under investigation (Huang et al., 1991). The use of light inherently leads to a one-to-two order magnitude improvement in axial resolution. In the eye it allows for visualization of structures and lesions at all levels of the retina (Hee et al., 1995a), obtained in a non-contact mode.

The OCT technique currently available in clinical practice is also referred to as time-domain OCT (TD-OCT), because the depth information of the retina is acquired as a sequence of samples, over time. Several efforts have been made to improve TD-OCT imaging quality. Some have tried to optimize the image quality post-acquisition (Adler et al., 2004; Sander et al., 2005), others by using different light sources to increase real-time image resolution (Drexler et al., 2001). Modifying image acquisition was also attempted using single flying spot raster scanning in the coronal plane parallel to the retinal surface (Podoleanu et al., 1997, Podoleanu et al., 1998a; Hitzenberger et al., 2003), or using full field capturing of a sample (Dubois et al., 2004a). Only recently, major advances in both imaging speed and sensitivity have been achieved with the introduction of spectral-domain (SD) OCT (Nassif et al., 2004; Wojtkowski et al., 2004). This review will provide an overview of the most recent developments in the field of OCT imaging of the eye, focussing on applications for the posterior segment of the eye.