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M. L. Gabriele, H. Ishikawa, R. A. Bilonick, G. Wollstein, L. Kagemann, J. Xu, J. S. Kim, V. Jurisic, J. G. Fujimoto, J. S. Schuman; The Effect of Axial Scan Location on Signal Quality and Thickness Measurements Obtained With Spectral Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4637.
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Spectral domain optical coherence tomography (SDOCT) signal level varies depending on the location of target tissue within a scanning window. Our goal was to test whether axial scan location (top of scanning window versus bottom) affects scan quality and thickness measurements.
Thirteen eyes from 13 healthy subjects were imaged using SDOCT (Cirrus OCT; Carl Zeiss Meditec, Dublin, CA). A total of six Macular Cube Combo (200x200x1024 samplings in 6.0x6.0x2.0mm volume, followed by a frame of 1000 samplings in both the horizontal and vertical directions, all centered on the fovea) images were obtained for each subject, three images located at the top of the image window and three images at the bottom of the image window (order of acquisition randomized). Two signal quality parameters (Cirrus signal strength (SS) [ordinal parameter] and quality index (QI) [continuous parameter]) were obtained for all images, while retinal nerve fiber layer (RNFL) and total retinal thicknesses (TRT) were measured on the horizontal and vertical scans. Changes in quality parameters and thickness measurements were characterized using linear mixed effects modeling.
Cirrus SS was equivalent to 10 for all top images while bottom images had an average SS of 7.69 ± 0.80. QI was statistically significantly higher for images located at the scanning window top as compared to bottom (mean difference [95% confidence interval]: 18.42[17.12-19.72]). In horizontal scans, top images showed a significantly thicker RNFL (mean difference: 3.66[2.25-5.08]µm) and TRT (mean difference: 5.06[2.77-7.25]µm) as compared to bottom images. Statistically significant differences were also seen in vertical scans (RNFL thickness mean difference: 3.00[1.53-4.48]µm, TRT mean difference: 2.71[1.32-4.10]µm).
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