Sir,

We read with interest an article by Forte et al1 in the December issue of Eye, which showed good correlation between the time-domain Stratus OCT and the spectral domain SLO/OCT, but significantly higher retinal thickness measurements with the latter. Similar findings have recently been showed when comparing the Stratus with the Topcon 3D-OCT 1000,2 and Zeiss Cirrus OCT.3

One critical aspect of thickness determination that is well recognized, but not addressed by the authors of any of the above articles, is that the selection of anatomical structure as the outer boundary for thickness measurement. Several candidate hyper-reflective lines are created by (1) the inner segment (IS)/outer segment (OS) junction, which the Stratus uses by default, (2) the internal aspect of the RPE, and (3) Bruch's membrane. Here, we would like to illustrate this, by comparing the Stratus measurements with the spectral domain Topcon 3D-OCT 1000, which allows the user to choose any of the above three structures for thickness determination. Briefly, 26 normal retinas were imaged using the automatic software algorithms provided by the respective manufacturer. We found that measurements correlated well between Stratus and 3D-OCT if identical boundaries were used. Average ILM-IS/OS measurements were essentially identical on the two instruments: 250±39 μm (Stratus), and 251±39 μm (3D-OCT). Measurements using the inner aspect of the RPE or Bruch's membrane (BM) yielded results that were greater by an average of 17±8 and 58±9 μm, respectively. As shown in the Figure 1, and as shown by Forte et al1 for their instrument, there was a linear relationship between measurements obtained with both machines (goodness of fit r2=0.9957), and values correlated well over the whole range of thicknesses (Pearson's coefficient r=0.9957).

Figure 1
figure 1

As shown by Forte et al1 for the S-SLO/OCT, there was a linear relationship between measurements obtained with the Zeiss Stratus OCT and the Topcon 3D-OCT 1000 (goodness of fit r2=0.9957), and values correlated well over the whole range of thicknesses (Pearson's coefficient r=0.9957). Measurements on both instruments yield essentially the same results when the identical outer boundary is chosen on the 3D-OCT. ILM-IS/OS measurements were 250±39 μm with the Stratus and 251±39 μm with the 3D-OCT (3D-IS/OS). Measurements using the inner aspect of the RPE or BM yielded predictably higher results that were greater by an average of 17±8 μm (3D-RPE) and 58±9 μm (3D-BM), respectively.

Full size image

In summary, if the appropriate boundaries for thickness determination were used, the older generation, time-domain Stratus OCT measurements were essentially identical to those obtained with the latest generation spectral domain Topcon 3D-OCT 1000. When interpreting retinal thickness with OCT instruments of different generations and manufacturers, measurements will vary depending on the anatomical layers delineated by each instrument, and correlation between different devices should be considered in clinical and study assessment.