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A. T. Kelmenson, R. Keshavamurthy, V. S. Brar, D. W. Hwang, K. V. Chalam, S. Grover; Comparative Difference in Macular Thickness as Measured Between Spectral-Domain OCT (Spectralis) and Time-Domain OCT (Stratus). Invest. Ophthalmol. Vis. Sci. 2009;50(13):1072.
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Advances in the technology for optical coherence tomography (OCT) have led to the development of several generations of OCT in the past few years. It ranges from the time-domain OCT (TD-OCT), which is most commonly used OCT to the most recent, spectral-domain OCT (SD-OCT), which provides high resolution cross-sectional images of the retina with registration and eye tracking. In our study we compared the macular thickness as measured by TD-OCT (Stratus) with that of SD-OCT (Spectralis) in subjects without any known retinal disease.
Inclusion criteria for normal subjects included no known ocular disease, best-corrected visual acuity of 20/20, normal intraocular pressure, and no history of prior retinal disease. SD-OCT was performed by Spectralis OCT (Heidelberg Engineering, Germany) using a 4 X 6 mm volumetric scan consisting of 20 raster lines spaced 200 µm apart. Stratus TD-OCT (Zeiss Inc.,Germany) was performed using the radial scan protocol. The same operator performed both Stratus OCT and Spectralis OCT on each subject. The central point thickness (CPT) and retinal thickness of 9 ETDRS subfields, including central subfield (CSF) were measured. Statistical significance was determined by the student’s t-test.
Forty eyes of 40 subjects were enrolled in the study. There were 26 men (65%) and 14 women (35%), with ages ranging from 20-65 years. The mean CPT on Stratus and Spectralis OCT were 165.5+17.9 µm and 225.6+16.7 µm (p<0.0001), and the mean CSF measurements were 203.6+19.1 and 273.4+21.1 µm (p<0.0001), respectively. On both the Stratus and Spectralis, the retinal thickness was highest in the nasal outer fields with mean measurements of 277.8+18.3 µm and 344.6+18.4 µm, respectively. Mean difference in the retinal thickness in the CSF and the other eight ETDRS subfields ranged from 62.6 µm to 73.5 µm. Subjects were divided into subgroups of age: 20-40 years (27 eyes), 41-60 years (9 eyes), and 61 years or older (4 eyes). Measurements with Stratus and Spectralis OCT showed no variation in CSF with age (p = 0.85 for Stratus and p = 0.22 for Spectralis).
When compared to the Spectralis OCT, the Stratus OCT significantly underestimates the macular thickness in all 9 ETDRS subfields. The difference of approximately 70 µm in the retinal thickness between Stratus and Spectralis OCT scans can be explained by the fact that whereas Spectralis measures the thickness, including the retinal pigment epithelium (RPE) layer, the Stratus OCT measures only upto the junction of outer and inner segments of photoreceptors, excluding the thickness of the RPE layer.
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