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Editors Selection IGR 9-4

Posterior Segment Imaging: OCT errors in glaucoma

Comment by David Greenfield on:

26071 Optical coherence tomography errors in glaucoma, Asrani S; Edghill B; Gupta Y et al., Journal of Glaucoma, 2010; 19: 237-242

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In the present study, Asrani and colleagues (707) characterize the prevalence and type of scanning errors associated with Stratus OCT imaging of the macula and papapillary RNFL in 89 persons with glaucoma during a three-month period. Macular thickness artifacts were present in 17% of eyes and failure to correctly segment the inner and outer borders of the retinal edge was the most common type of artifact. Signal strength was the most important determinant of high quality macular and RNFL scans. Eyes with signal strength (SS) ≥ 8 were 94% artifact free; eyes with SS ≤ 4 had a 64% incidence of artifacts. RNFL thickness artifacts were present in 16% of eyes and were also strongly influenced by SS. Eyes with SS ≥ 6 were 96% artifact free; eyes with SS ≤ had a 86% incidence of artifacts.

Signal strength was the most important determinant of high quality macular and RNFL scans

Clinicians and scientists recognize the importance of assessing image quality. Various factors should be examined when interpreting Posterior Segment Imaging 88 Editor's Selection the quality of Stratus OCT images. Studies have demonstrated that for every one-unit reduction in SS, the average RNFL thickness is reduced by approximately two microns. Secondly, failure of the RNFL segmentation algorithm will occur more commonly in eyes with weak SS. This has critical implications on the determination of glaucoma progression over time. The alignment and centration of the peripapillary scanning circle around the optic nerve head will also impact the assessment of RNFL thickness such that sectors imaged closer to the disc margin will be falsely thicker; sectors imaged further from the disc will be falsely thinner. Scanning should be avoided in high myopes or eyes with large parapillary atrophy due to scanning artifact within the regions of parapapillary atrophy. As highlighted by the authors, other factors such as ocular surface disruption and senile cataract may also contribute to artifact. Fourier-domain OCT continues to dominate the imaging market and questions will undoubtedly be raised with regard to quality assessment using these instruments. Motion artifact produces drop out zones are not uncommon given longer scan acquisition times. Some instruments have utilized ocular tracking in order to circumvent this limitation. Asrani and colleagues have made an important contribution by highlighting the significance of image quality when employing technologies for quantitative measurements.

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