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Individual A-scan signal normalization between two spectral domain optical coherence tomography devices.

PURPOSE
We developed a method to normalize optical coherence tomography (OCT) signal profiles from two spectral-domain (SD) OCT devices so that the comparability between devices increases.


METHODS
We scanned 21 eyes from 14 healthy and 7 glaucoma subjects with two SD-OCT devices on the same day, with equivalent cube scan patterns centered on the fovea (Cirrus HD-OCT and RTVue). Foveola positions were selected manually and used as the center for registration of the corresponding images. A-scan signals were sampled 1.8 mm from the foveola in the temporal, superior, nasal, and inferior quadrants. After oversampling and rescaling RTVue data along the Z-axis to match the corresponding Cirrus data format, speckle noise reduction and amplitude normalization were applied. For comparison between normalized A-scan profiles, mean absolute difference in amplitude in percentage was measured at each sampling point. As a reference, the mean absolute difference between two Cirrus scans on the same eye also was measured.


RESULTS
The mean residual of the A-scan profile amplitude was reduced significantly after signal normalization (12.7% vs. 6.2%, P < 0.0001, paired t-test). All four quadrants also showed statistically significant reduction (all P < 0.0001). Mean absolute difference after normalization was smaller than the one between two Cirrus scans. No performance difference was detected between health and glaucomatous eyes.


CONCLUSIONS
The reported signal normalization method successfully reduced the A-scan profile differences between two SD-OCT devices. This signal normalization processing may improve the direct comparability of OCT image analysis and measurement on various devices.