Optical coherence tomography-angiography

OCT-A is a new, depth-resolved method to analyse retinal and choroidal vascular structure

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Optical coherence tomography-angiography (OCT-A) has evolved quickly from an experimental technique to an essential tool, according to Eric Souied MD of the Centre Hospitalier Intercommunal Creteil, France. “Combining en-face OCT with movement detection within blood vessels, we get OCT-A, and with it a new imaging era,” said Dr Souied. Dr Souied described OCT-A as “a new, depth-resolved method to analyse the retinal and choroidal vascular structure.” Although it is an angiographic method, many characteristics set it apart from traditional fluorescein angiography (FA). It is non-invasive, due to its lack of dye. Because it is dye-less, there is no leakage, and imaging is thus also phase-independent. The scans are also three-dimensional, unlike FA and indocyanine green angiography. Furthermore, because of its phase- and time-independence, it is a rapid technique. Disadvantages include a small (narrow) scanning window and the development of artefacts, which must be distinguished from actual structural finding. Dr Souied was most interested in the ability to view individual layers of the retina and choroid. Visible layers include the superficial and deep vascular plexus, the outer retina, the choriocapillaris, and the Sattler and Haller layers. This layered segmentation allows visualisation of very specific lesions and patterns not visible on FA. With FA, identification of the axial location of pathology requires an understanding of patterns of blockage and leakage, whereas OCT-A simplifies this identification by displaying typical patterns in their ‘correct’ layer. “Type 1 choroidal neovascularisation (CNV) shows up as either a ‘blossoming tree’ or a ‘lasso’, while Type 2 CNV is alternately described as a ‘glomerulus’, a ‘jellyfish’ or a ‘medusa’,” he said, while showing reassuringly similar images from each of the five OCT-A devices currently available. These are the Angiovue (Optovue), the OCT2 (Heidelberg), the RS 300 Angioscan (Nidek), the Triton (Topcon) and the Cirrus HD OCT (Zeiss). Retinal angiomatous proliferation appears as a 'tuft' or small plexus, whereas the polyps in polypoidal choroidal vasculopathy present as round, dark, hypo-intense lesions, sometimes surrounded by a hypo-intense halo. In contrast, the branching vascular network associated with the polyps appears as hyper-intense lesions. Why the strong discrepancy between the signal intensity of the branching vascular network and the polyps? The answer is revealing, and made it clear how OCT-A works. “Polyps have turbulent blood flow, which goes undetected by the ‘motion contrast’ movement detection of OCT-A. Alternately, the linear blood flow present in the branching vascular network is perfectly visible,” explained Dr Souied. Colourisation is also possible. This has two potential uses. “First, by giving each vascular layer, its own, standardised colour, atypical lesions can be more easily identified and located within a particular layer,” said Dr Souied. “Second, colourisation of the lesions can be utilised for both qualitative (vascular lesion shape and appearance) and quantitative (vascular lesion area) analysis of CNV follow-up during treatment.” The colourisation tends to visually distinguish a particular lesion from the surrounding tissue, making it stand out. Despite his enthusiasm for this new technology, Dr Souied indicated that, early on, he had considered the possibility that it would lead to nothing. “OCT-A has proven to be great for publishing papers and printing textbooks over the past few years. But are we simply impressing each other with fancy, yet useless information?” he said. “No, I believe the that the detailed findings will become increasingly useful in the treatment of patients.” Eric Souied: eric.souied@chicreteil.fr