Evaluation of time domain and spectral domain optical coherence tomography in the measurement of diabetic macular edema

Abstract

Purpose: To evaluate macular thickness and volume measurements and their intrasession repeatability in two optical coherence tomography (OCT) systems: the Stratus OCT, a time domain system, and the Cirrus HD-OCT, a spectral domain system (both by Carl Zeiss Meditec, Inc., Dublin, CA), in the context of diabetic macular edema (DME).

Methods: Thirty-three eyes of 33 diabetic patients with clinically significant macular edema (CSME) were scanned in a single session by a single operator on both OCT systems. Macular thickness measurements of nine standard macular subfields and total macular volume were obtained and analyzed. Bland-Altman plots were constructed to assess agreement in macular measurements. Intraclass correlation coefficients (ICCs), coefficients of repeatability (CR(W)), and coefficients of variation (CV(W)) were used to assess intrasession repeatability.

Results: Macular thickness in nine retinal subfields and macular volume were significantly higher in the Cirrus HD-OCT system compared with the Stratus OCT system. Subfield thickness and total volume measurements, respectively, were 30 to 55 microm and 3.2 mm(3) greater for the Cirrus HD-OCT system compared with the Stratus OCT system. Both Stratus OCT and Cirrus HD-OCT systems demonstrated high intrasession repeatability, with overlapping ranges for CR(W), CV(W), and ICC. Repeatability measures (CR(W) and CV(W)) differed significantly between systems in only one of nine subfields (outer temporal subfield).

Conclusions: Absolute measures of macular thickness and volume in patients with DME differed significantly in magnitude between the Stratus OCT and Cirrus HD-OCT systems. However, both OCT systems demonstrated high intrasessional repeatability. Although the two systems may not be used interchangeably, they appear equally reliable in generating macular measurements for clinical practice and research.

Figure 1

Systems of retinal sampling used by the Cirrus HD-OCT and Stratus OCT systems (Carl Zeiss Meditec, Inc., Dublin, CA). Shown are the different sampling methods for the Cirrus HD-OCT (left) and the Stratus OCT (right). Cirrus HD-OCT samples significantly more points that are evenly distributed over the macular grid, whereas Stratus OCT samples points along six radial lines that intersect at the center of the macular grid.

Figure 2

Macular grid used by both the Cirrus HD-OCT and Stratus OCT systems (Carl Zeiss Meditec, Inc., Dublin, CA). Both systems employ the same 6-mm-diameter grid defined by the Age-Related Eye Disease Study (AREDS). The inner subfields are bounded by the 3-mm-diameter circle, whereas the central subfield is bounded by the inner-most 1-mm-diameter circle.

Figure 3

Representative Bland-Altman plots of macular measurements obtained using Stratus OCT compared to the Cirrus HD-OCT. Bland-Altman plots for central subfield macular thickness (left) and macular volume (right) are shown. Solid lines: the bias (average mean difference); dotted lines: 95% confidence limits of agreement.

Figure 4

Differences in delineation of retinal layers by the Stratus OCT compared with the Cirrus HD-OCT (Carl Zeiss Meditec, Inc., Dublin, CA). Representative OCT scans obtained using the (A) Stratus OCT and (B) Cirrus HD-OCT from a study eye are shown. Segmentation lines are shown in white. The Cirrus HD-OCT segmentation algorithm identifies the thickness of the retina from the retinal pigment epithelium (RPE) to the inner limiting membrane (ILM), whereas the Stratus OCT segmentation algorithm identifies the thickness of the retina based on the distance between the ILM and junction of the outer segments (OS) and inner segments (IS) of the photoreceptors.