Effect of Optical Coherence Tomography Scan Decentration on Macular Center Subfield Thickness Measurements

PURPOSE. To investigate the effect of optical coherence tomography macular grid displacement on retinal thickness measurements. METHODS. SD-OCT macular scans of 66 eyes with various retinal thicknesses were selected. Decentration of the 1-, 3-, 6-mm-diameter macular grid was simulated by manually adjusting the distance between center of the fovea (cFovea) and center of the grid (cGrid). Center subfield thickness (CSF) between the internal limiting membrane and the top of the retinal pigment epithelium was measured along the displacement distance where the grid was displaced in eight cardinal directions from the cFovea in steps of 100 mu m within the central 1-mm subfield and then by 200 mu m within the inner subfields. One-way/mixed-effects repeated-measures ANOVA models were used to determine changes of CSF (Delta CSF) as a function of displacement distance (for alpha = 0.05, power 0.80 and effect size 0.1). The interactions between the displacement distance and direction, center point thickness (CPT), and foveal contour were also analyzed. RESULTS. The CSF measurement showed statistically significant error when the displacement distance between cFovea and cGrid exceeded 200 mu m. The direction of displacement did not affect the Delta CSF-distance relationship, while the CPT and foveal contour significantly affected the relationship, in that some subgroups showed slightly larger tolerance in the displacement distance up to 300 mu m before reaching significant Delta CSF. CONCLUSIONS. Small displacement distances of the macular grid from the cFovea affect CSF measurements throughout a broad range of thicknesses and retinal contour alterations from disease. Accurate registration of OCT scans or post hoc repositioning of the grid is essential to optimize CSF accuracy.