title: Calculating the predicted retinal thickness from spectral domain and time domain optical coherence tomography - comparison of different methods.
abstract: OBJECTIVE
  To compare the accuracy of different methods of calculating predicted central retinal thickness values in order to allow comparison between results of spectral-domain optical coherence tomography (SD-OCT) and time-domain OCT (TD-OCT) devices.
  
  
  METHODS
  In a prospective cohort study, 100 consecutive healthy individuals without ocular disease underwent sequential scanning with SD-OCT (Spectralis OCT) and TD-OCT (Stratus OCT). A group of 60 eyes was used to generate the conversion equations, which were tested on the remaining 140 eyes. Four equations were used: 1. Mean difference between SD-OCT and TD-OCT; 2. Multiplying a ratio by the original retinal thickness; 3. Linear regression analysis using retinal thickness; and 4. Regression analysis using retinal thickness and spherical equivalent. All four methods were used to calculate predicted SD-OCT values from TD-OCT measurements, and vice versa.
  
  
  RESULTS
  For all four equations, the predicted SD-OCT central retinal thickness values were similar to the actual SD-OCT, with mean difference ranging from 0.78 to 1.01 μm, and intraclass correlation coefficients >0.88. Both regression equations and mean difference showed greater accuracy, with variation between calculated and actual retinal thickness values ≤5 μm in 60% of eyes. In contrast, the ratio method was less accurate, with 15.8 % of eyes showing differences >15 μm. Similar results were found for predicted TD-OCT values.
  
  
  CONCLUSIONS
  Several methods can be used to convert central retinal thickness values from SD-OCT to the predicted TD-OCT value, or vice versa, with high degrees of accuracy and reliability. These methods may allow comparison of OCT values from SD-OCT and TD-OCT devices in clinical trials and standard patient care.
authors.affiliation: Department of Ophthalmology, Tan Tock Seng Hospital, Singapore, 308433, Singapore, Colintan_eye@yahoo.com.sg.
authors.forename: Colin S
authors.initials: CS
authors.lastname: Tan
authors.forename: Kelvin Z
authors.initials: KZ
authors.lastname: Li
authors.forename: Tock Han
authors.initials: TH
authors.lastname: Lim
mesh_terms: D000328:Adult; D015331:Cohort Studies; D003941:Diagnostic Techniques, Ophthalmological; D005189:False Positive Reactions; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests; D011446:Prospective Studies; D015203:Reproducibility of Results; D012160:Retina / Q000033:anatomy & histology*; D041623:Tomography, Optical Coherence / Q000379:methods*; D055815:Young Adult