Conversion Equation for Spectral and Time Domain Optical Coherence Tomography Acquired Measurements of Macular Thickness in Normal Eyes and Eyes with Diabetic Macular Edema

Abstract

 

Purpose:

 

To derive a conversion equation for time-domain (TD) and spectral-domain (SD) optical coherence tomography (OCT) acquired retinal thickness measurements of normal eyes and eyes with diabetic macular edema (DME) and to facilitate monitoring of patients with the SD technology who were measured previously with TD technology.

 

Methods:

 

The index study was a prospective investigation conducted on 80 eyes from 40 normal subjects and 130 eyes from 118 patients with DME seen in our clinic. Retinal thickness values from the central 1mm of the macula and surrounding four ETDRS subfields were acquired using TD-OCT (Stratus OCT) and SD-OCT (SPECTRALIS HRA + OCT). Measurements of the central (C) subfield from both devices were used to derive a conversion equation. The equation was subsequently used to predict SD-OCT values using measurements from TD-OCT. Agreements between predicted and actual SD-OCT measurements were then assessed.

 

Results:

 

In normal eyes, the mean difference between TD-OCT and SD-OCT measurements of the C subfield was 76µm (CI95 = 74 and 77, respectively). The conversion equation, y = (1.029x) + 72.49, was derived. In eyes with DME, using the equation, SPECTRALIS predicted values were 5% higher than actual measurements, with 95% of predicted values falling within 9% of the actual measurements. Relocating SD-OCT grids to match the location on TD-OCT resulted in predicted values falling within 7% of actual measurements.

 

Conclusions:

 

The percent difference between actual thickness measurements from SPECTRALIS and predicted thickness measurements, using the conversion equation, was within reported limits of repeatability of Stratus in eyes with DME. Our equation may help to correlate OCT values from both devices in standard care and research studies for DME.  

 

Keywords: diabetic retinopathy • edema • imaging/image analysis: clinical