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Editors Selection IGR 17-4

Structure: Influence of signal strength on Stratus OCT

Comment by Chris Leung on:

26388 Influence of OCT signal strength on macular, optic nerve head, and retinal nerve fiber layer parameters, Samarawickrama C; Pai A; Huynh SC et al., Investigative Ophthalmology and Visual Science, 2010; 51: 4471-4475

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Obtaining high quality scans is a prerequisite for interpretation of retinal nerve fiber layer (RNFL), optic disc and macular measurements obtained with optical coherence tomography (OCT). In a population study on 2092 children, Samarawickrama et al. (1207) examined the influence of signal strength, an image quality index from 1 to 10 representing the signal-to-noise ratio, on Stratus OCT (Carl Zeiss Meditec, Dublin, CA) measurements. They collected multiple scans for each eye and the means were calculated for each parameter and for signal strength. Mean signal strength in the range between 5 and 7.49 was considered as 'moderate', 7.5-9.49 was considered 'good', and &etgt;9.5 was considered 'excellent'. They showed that macular and optic disc measurements obtained at 'excellent' signal strength were significantly greater than those obtained at 'moderate' and 'good' signal strength after adjusting for age, gender, ethnicity, axial length and spherical equivalent. Different from the previous studies (Wu et al. Ophthalmology 2007; 114: 1505-1512; Cheung et al. Ophthalmology 2008; 115: 1347-1351; Vizzeri et al. Am J Ophthalmol 2009; 148: 249-255), the authors did not find significant differences in RNFL measurements between the signal strength categories except for temporal RNFL thickness.

Obtaining high quality scans is a prerequisite for interpretation of retinal nerve fiber layer, optic disc and macular measurements obtained with optical coherence tomography

Given the fact that there are large inter-individual variations in RNFL thickness and reliable measurement of circumpapillary RNFL thickness depends on precise placement of the circle scan, investigating the influence of signal strength on RNFL thickness is best designed with measurements obtained with different levels of signal strength on the same eyes imaged by the same technician. Macular and optic disc measurements are less susceptible to the influence of scan placement and the effect of signal strength could be easier to detect. Of note, the magnitude of difference between the 'excellent' and the 'moderate' groups for average RNFL thickness (1.1%) was similar to that for macular volume (1.7%). Statistically significant differences in

There is no consensus on the level of signal strength which should be considered optimal for optic disc and RNFL imaging

RNFL measurements might be detected if they were compared at individual levels of signal strength. Arbitrary grouping the signal strengths into three categories may weaken the power of analysis. The authors correctly pointed out that there is no consensus on the level of signal strength which should be considered optimal for optic disc and RNFL imaging. This is an important issue, particularly for longitudinal analysis of glaucoma progression. Being able to register and align follow-up images, and extract serial follow-up RNFL measurements at the same retinal location, it is hopeful that the spectral-domain OCT could better address this issue.

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