OCT Angiography OCT Angiography (2017) by Chow, de Oliveira et al., 190 pp., 161 illustrations, €89.99/$99.99 Hardback ISBN: 9781626234734 Thieme Publishers New York/Stuttgart
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The present book is a clearly written and documented review on Optical Coherence Tomography-Angiography (OCT-A). The many illustrations show the level of detailed information than can be found in the OCT-A images in representative diseases.
The book is intended for ophthalmologists, especially medical retina specialists, with an interest in this promising and still developing technology, and all those with an interest in the pathophysiology of diseases afflicting the retinal vaculature.
At present, Optical Coherence Tomography (OCT) has become an indispensable imaging tool in every ophthalmic practice. Since the invention of the technology in the 90s, and especially since the invention of Spectral-Domain OCT the three dimensional representation of the complex anatomy of the central retina has become the standard in the evaluation of the central retina and the optic disc. The simplicity of acquiring the images, the minimal burden for the patients, and the objective, highly accurate, and reproducible quantified measurements has made OCT in a relatively short period the most frequent performed additional examination in ophthalmology, besides the slitlamp examination.
Recently, a new modality has been added to the OCT, the OCT angiography (OCT-A). With OCT-A, movement is detected within the retina, by comparison of two (or more) images made at the same location in the retina almost at the same timepoint. Differences between these images are caused by “movement”. Detectable movement is generated by moving blood cells within the vessels of the retina. Translation of this movement into a positive signal delineates the blood vessels in the retina. These images have almost the same resolution of the standard OCT and will produce a three dimensional image of the retinal vessels up to the level of the capillaries. Vessels are shown in transversal images reconstructed from the individual b-scans.
The incredibly detailed images of the complete retinal and the choroidal vascularisation add a dimension to the classical images we know from fluorescein and ICG angiography. For example OCT-A produces images of the superficial and the deep retinal capillary plexus separately, while the FA only shows the superficial plexus. Also the choroidal vascular plexus can be imaged, and adds detail compared to the ICG images. Information about flow dynamics, staining and leakage is lacking in OCT-A and for this information FA and ICG are still necessary imaging modalities.
Although OCT-A can be acquired with almost the same ease as the traditional OCT, the interpretation of the images is much more complicated. To discriminate clearly between the different vascular plexus, the total transversal image is divided into thin slabs at different axial levels of the retina. This is to prevent artefacts and unwanted mixing of signals of vessels of different retinal levels. For the normal retina, the software allows to use prefixed slab definitions, defined to show the superficial, and deep retinal plexus, the avascular region below the outer plexiform layer and the choroidal vascular layer. However, in the case of a disturbed anatomy of the retina, like in exudative age related maculopathy (AMD), the examiner needs to define his own slab definition to acquire the wanted information about the neovascularisation. In theory, such a neovascularisation should be visible on OCT-A, and open for detailed follow-up, in reality, it still is a time-consuming enterprise, and in many cases, the OCT-A fails to picture a neovascularisation clearly.
The present book has been written by pioneers in the field of OCT-A, with much experience in OCT-A imaging. It provides a clearly written introduction into the technology, even comparing the differences between the systems known at the time of writing. Although it is to be expected that the different devices will differ in the quality of the OCT-A images that can be made, a head to head comparison is still lacking in the literature, and is also not part of this book. The book also provides insight into the many artefacts and pitfalls still present in OCT-A today. The authors have gathered an overview of the OCT-A images of a large number of retinal diseases. The experience of the authors guarantees the best possible images. The many detailed images of the retinal vasculature acquired after a careful and correct interpretation of the images show many new findings and/or confirm knowledge already known from angiographic examinations. A new terminology evolves with the interpretation of these new OCT-A images. Reading the different chapters of the book one encounters often the notion of the experts that OCT-A provides not essentially new information, but in the future might be able to add in the diagnosis and follow-up of the different entities described (adult onset foveal macular vitelliform degeneration, MacTel, high myopia, uveitis, retinal tumours). At the same time, OCT-A is much easier to perform, and can easily be repeated. Not unexpectedly, especially in those diseases with abnormal new vessels and/or loss of normal vasculature, the OCT-A really demonstrates its value, like in exudative AMD, DR, retinal vascular occlusions and radiation retinopathy. Still, even in these diseases, in contrast to the ease of acquirement, the correct interpretation is time consuming, and needs an experienced examiner. A first attempt has been made to describe features of the neovascularisation in exudative AMD during the course of treatment with anti-VEGF in the hope to define biomarkers helping in retreatment decision making. Another problem of OCT-A is the limited field of view which makes OCT-A less suitable to detect radiation retinopathy or diabetic retinopathy in the retinal periphery.
Glaucoma is another disease where OCT-A seems to increase our diagnostic abilities. Measurement of vessel density (the number of vessels in a certain area) in sectors around the optic disc demonstrates a loss of small vessels that precedes structural changes of the retinal nerve fibre layer (RNFL) measured on standard OCT, and could be an important early diagnostic sign.
Small vessels of the anterior segment, conjunctiva, cornea and iris have also been subject of examinations with OCT-A technology, and clearly show for example neovascularisations of cornea and iris.
The authors see a bright future for OCT-A, and future developments, like new devices like swept source OCT, improvement of automated quantitative analysis software and adding of Doppler OCT to measure flow, will definitely increase the possibilities of this technology. They also state: “improvement in the hardware and software components of OCT-A, as well as a standardized interpretation of these images and their artefacts is important for future development of this imaging modality”. OCT-A is still a promising technology in development.
Standard OCT once was a completely new technology in ophthalmology. The first images made with time domain OCT technology looked primitive compared to present day state of the art SD-OCT 3D images. Those first images were considered by many to be nice and colourful, but did not add much to existing diagnostic modalities. It is to be expected that OCT-A will likewise develop in the coming years, and probably will help the ophthalmologist in the future in many ways. For now, OCT-A is still a (too) difficult modality to use in daily practice. This book helps the reader to understand the technology, to see the astonishing detailed pictures of the retinal vasculature that can be acquired, to learn about the difficulties in interpretation of the images, to see retinal vessel abnormalities in many diseases shown on OCT-A images. It provides a glimpse to the bright future of OCT-A.