Optical coherence tomography: seeing the unseen

Abstract

Optical coherence tomography (OCT) is an imaging technology that has revolutionised the way we detect and manage ocular disease. Quite simply, it allows the clinician to visualise ocular pathology previously not detectable or only evident in advanced disease. OCT has its roots in ultrafast laseractivated shutters to photograph ‘light echoes’, which allowed the depth of objects to be captured. Duguay and Mattick at AT&T Bell Laboratories in 1971 suggested this technique could be applied to ‘see inside’ biological tissue. In the 1980s, low-coherence interferometry was used to measure ‘light echoes’ and had its first in vivo ophthalmic application for measurements of the axial length of the eye. In 1989, John Apostolopoulos (unpublished) at the Massachusetts Institute of Technology in the USA used a low-coherence laser diode split into a reference path with a known distance/time delay, while a second beam was directed onto the tissue. ‘Light echo’ time delays were measured by interference between the tissue beam and reference beam; matching the echo delay with the reference time delay generated an A-scan. In 1991, Huang et al. in their paper published in Science, demonstrated that OCT provided exquisite images of individual retinal layers in an ex vivo bovine eye, with 10 μm axial resolution. The first OCT images of the human retina were then reported in 1993 by two independent groups. The first clinical OCT instruments were released in the late 1990s; however, it was not until the mid-2000s when – in part assisted by the development of antivascular endothelial growth factor (VEGF) therapy for exudative age-related macular degeneration (AMD) – there was wider clinical acceptance of this new technology. In this special issue, a range of research studies and review articles explore the enormous scope of OCT applications in research and clinical practice. Structural information from OCT has been an outstanding addition to the range of tools available for glaucoma diagnosis and monitoring. OCT-derived information is often used in association with functional assessments, such as automated perimetry. In the future, it is possible to envisage that structural information for an individual could guide and personalise visual field testing, making structure-function maps more informative. Denniss et al. explore advances in this area, and how they might impact glaucoma diagnosis and monitoring. The robust quantitative measures that OCT provides of the optic nerve head neuroretinal rim in glaucoma have now superseded the more traditional assessment of retinal nerve fibre layer thickness, and Fortune, in his review of this topic, highlights the evidence supporting their clinical adoption. There are still key limitations with using OCT imaging in glaucoma, such as the so-called ‘red-green disease’, where the likelihood of false positives (red disease) or false negatives (green disease) may be increased relative to traditional testing, as the indication of possible structural loss in glaucoma is contingent upon the underlying OCT normative database; Ly et al. discuss these factors in their review. In addition to demonstrating how OCT can play an important role in the management of glaucoma, Ly et al. provide practical, illustrative guides on the interpretation of OCT for AMD and diabetic retinopathy. They also outline how OCT has virtually become indispensable in clinical practice, due to its high sensitivity for diseases, such as neovascular AMD and diabetic macular oedema. This, in turn, means that treatment for these conditions – such as anti-VEGF therapies – can be initiated earlier, with OCT then used to monitor the response to the treatment. For many years, fluorescein angiography was the primary method for detecting abnormalities in the retinal microvascular network. Increased scan speeds and better compensation for eye movements has allowed the development of OCT angiography (OCT-A). This exciting advancement in OCT technology provides the potential for a non-invasive view of the vasculature in the retina, choroid and anterior eye. This technology has a wide range of potential applications, including the detection and monitoring of retinal and optic nerve head diseases. Wang et al. consider the advantages afforded by OCT-A to assess changes to the choroidal vasculature in a case of adult-onset vitelliform dystrophy. Chua et al. note that widespread use of OCT-A in clinical practice is still emerging, but we predict that, like its predecessor (OCT), the coming years will see researchers and clinicians push the boundaries of its application in eye care. In addition to these established applications of OCT for evaluating ocular health, this technology has been used to develop a range of ophthalmic markers for neurodegenerative disease. In this issue, Srinivasan and Efron review OCT-derived retinal measures, and their use, across a range of neurodegenerative disorders, including Alzheimer’s disease, multiple sclerosis, Parkinson’s disease and diabetic peripheral neuropathy. These authors note that if ophthalmic measures – including those obtained from OCT imaging – are confirmed as markers of neurodegenerative disease, then eye-care practitioners may play an important role in identifying and monitoring patients with systemic neurodegenerative conditions. Ocular examination of the paediatric patient, including biomicroscopy and