Question
A patient was scanned by different instruments in two different practices (Figures 1 and 2a and b), but with only three months between them; is this level of discrepancy expected or acceptable with modern machines?
Answer
A number of factors will affect the retinal nerve fibre layer (RNFL) measurements made using different OCT devices. The first thing to consider is how the scan data were derived. Was this generated from a cube scan, a raster scan or a circumferential scan of the peripapillary RNFL? Each method has its merits and is capable of generating accurate measurements, but there may be subtle variation in the values produced. A cube scan should use an isotropic configuration and have equal spacing between all A scans in the cube. A raster scan will require a high density line spacing to ensure accurate measurement with as little interpolation of data as possible. A circumferential scan must have a high A scan value and has the potential to give the most accurate measurement. This variation in scan protocol will result in variation between devices.
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