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Imaging Optical Coherence Tomography – Segmentation Performance and Retinal Thickness Measurement Errors Dessislava Nikolaeva Koleva-Georgieva Assistant Professor, Department of Ophthalmology, University Hospital ‘St George’, Plovdiv, Bulgaria Abstract Optical coherence tomography (OCT) has become an indispensable tool in the assessment of macular pathology in clinical settings and an integral part of many clinical trials. However, as with any imaging technology, some limitations exist. In this review, the author describes and discusses the various causes that might compromise automated retinal thickness measurements. The segmentation software might perform less accurately in the presence of scan artefacts (e.g. ‘out-of-range’, mirror, blink and motion artefacts), a low signal:noise ratio, dense media opacities and specific retinal pathological features (e.g. pigment epithelial detachment, subretinal fluid, fibrotic tissue, hard exudates and full-thickness macular holes). The awareness of the clinician and the particular search for, and recognition of, measurement errors would improve the accuracy of OCT interpretation and should be an integral part of OCT scan analysis. Keywords Optical coherence tomography, retinal thickness measurements, segmentation break-down Disclosure: The author has no conflicts of interest to declare. Received: 7 November 2011 Accepted: 13 December 2011 Citation: European Ophthalmic Review, 2012;6(2):78–82 Correspondence: Dessislava Nikolaeva Koleva-Georgieva, Department of Ophthalmology, University Hospital ‘St George’, Pestersko Schose Blvd 66, Plovdiv 4001, Bulgaria. E: dr_desikoleva@yahoo.com Optical coherence tomography (OCT) was introduced by Huang and colleauges in 1991 1 and became commercially available in 1995. It is a fast, non-invasive, non-contact method that enables in vivo visualisation of the retinal and vitreoretinal microstructure on a high-resolution cross-section (2D) or 3D image. 2–5 OCT is also a powerful method for obtaining retinal thickness measurements. Owing to its ability to quantify changes in retinal thickness, OCT has become an indispensable tool for assessing treatment initiation, the response to treatment and the need for retreatment in many retinal diseases. OCT is now an integral part of both retinal clinical practice and many clinical trials. As clinicians are becoming increasingly reliant on OCT retinal thickness measurements, it is important to determine their accuracy. Several studies have presented good reproducibility of OCT retinal thickness measurements and have shown that it is currently the most precise and reliable instrument for retinal thickness measurements. 6–9 However, all imaging techniques have some limitations and are subject to artefacts. Several studies have uncovered and analysed multiple sources of errors that decrease the accuracy of retinal thickness measurements. 10–12 Over the past few years, increasing evidence has accumulated, based on the concern of many ophthalmologists, of the limitations of current OCT devices and software. Knowledge, anticipation and recognition of image artefacts and erroneous thickness measurements by both OCT technicians and physicians are needed to reduce the influence of such errors on OCT interpretation. Retinal thickness is defined as the distance between the inner retinal boundary (vitreous–retina interface) and the outer retinal boundary 78 (retina–retinal pigment epithelium interface). 3,13 The automated retinal thickness measurement is based on the ability of the software to detect the inner and outer boundaries based on the change in reflectivity at each of these interfaces. If the software fails to delineate the retinal boundaries correctly (i.e. segmentation break-down), it will result in thickness measurement error. Retinal thickness is displayed on a topographic map and numerical values are given for centre-point thickness, for each of the nine Early Treatment Diabetic Retinopathy Study (ETDRS)-like macular subfields and total macular thickness. Incorrect retinal thickness maps might be the result of segmentation break-down owing to scan artefacts, insufficient image quality or in the presence of specific retinal pathological features, in the case of good-quality scans. Poor patient fixation can also hinder obtaining correct retinal thickness maps. In this review, the author describes and discusses the various causes that can compromise retinal thickness measurements. The author also stress that the search and recognition of thickness measurement errors should be an integral part of OCT scan analysis. Scan Artefacts Several scan artefacts have been described to cause segmentation break-down: ‘out-of-range’ images, mirror artefacts, and blink and motion artefacts. 10–12,14,15 The out-of-range image artefact appears if the OCT scan is moved out of the scanning range during image acquisition; thus, the OCT image is ‘cut’ in its upper or lower part. It might also be present if there are pathological high retinal elevations that are out of reach. This artefact can occur more often in highly myopic eyes, whereas in non-myopic eyes, it is usually a result of © TOUCH BRIEFINGS 2012