The densest B-scan of the Heidelberg Spectralis has an interscan spacing of 6 µm, which due to the Nyquist sampling theory, would not provide an adequate estimation of the PSF if it were less than 12 µm wide. The Heidelberg Spectralis (Heidelberg Engineering GmbH, Heidelberg, Germany) is a spectral domain OCT device capable of an A-scan rate of 85 kHz while sampling a depth of 1.9 mm. The scan density parameters of past and present commercial OCT instruments were based on earlier translational concepts, which now appear to have been incorrect. This set of findings likely apply to all commercial clinical instruments. The clinically used A-scan spacing was derived from the calculated, and not the true resolution, and results in under sampling.
The USAF target imaging showed a lateral resolution of 4.6 µm.Īlthough a calculation of the spot size of the illumination beam was reported in the past as the lateral resolution of the OCT instrument, the actual lateral resolution is better by a factor of at least 2.5 times. From the amplitude transfer function, the FWHM of the diffraction limited PSF was calculated to be 5.0 µm. Measurement of the lateral PSF from 1215 diamond particle images yielded a full-width half maximum (FWHM) to be 5.11 µm and for 732 FeO particles, 4.9 µm. Finally, resolution was measured using the 1951 USAF target.
The amplitude transfer function was calculated from human OCT images. The lateral point spread function (PSF) was evaluated using diamond abrasive powder 0 to 1 µm in diameter in silicone elastomer and a validated target with 800 nm FeO particles in urethane. To evaluate the potential lateral resolution of the Spectralis OCT, an instrument calculated to have a 14 µm resolution. The lateral resolution of an optical coherence tomography (OCT) instrument was considered to be equal to the illumination spot size on the retina.
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