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Y. Li, M. Tang, D. Huang; LASIK Flap Reflectivity Analysis With Optical Coherence Tomography . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4333.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the evolution of flap internal reflectivity after LASIK surgery with a high–speed corneal and anterior segment optical coherence tomography (CAS–OCT) system.
The CAS–OCT prototype operated at 1.3 micron wavelength, 17 micron FWHM axial resolution in cornea and 2000 a–scan/sec. Eighteen subjects seeking refractive surgery participated in this longitudinal study. The OCT scans were performed preoperatively and at 1 day, 1 week, 3 months and 6 months postoperatively. The cornea was scanned three times at each visit with an 8–mm–long horizontal OCT pattern consisting of 512 axial scans. We measured the ratio of flap reflectivity relative to the stromal reflectivity posterior to the flap to remove the influence of instrument performance and scan orientation. The reflectivity was taken as the OCT signal strength converted from a log scale to the linear optical intensity scale. The flap region was taken as the region 50 µm above the flap interface, while the reference posterior stromal region was taken as the region 50 to 250 µm above the posterior corneal surface. For comparison, we also made a similar measurement on the preoperative corneal OCT images where the "flap" region was defined as the anterior stroma 50 µm above the 1–week postoperative flap depth. The flap/posterior stroma reflectivity ratios were compared in three zones centered at the corneal vertex: diameter (D) = 1–2 mm, D = 2–4 mm, and D = 4–6 mm.
Compared to the preoperative values, the flap/posterior stroma reflectivity ratio is decreased centrally and increased peripherally (D > 2 mm). The ratio gradual decreased with time postoperatively but the ratio remained significant higher than preoperative values in the two D > 2 mm zones at all study time points. Receiver operating curve (ROC) analysis showed that the ratio in D = 4–6 mm area is the best discriminant parameter for detecting LASIK–related reflectivity change (area under ROC = 0.95, 6 month postoperatively).
Post–LASIK flap/posterior stroma reflectivity ratio differed from the preoperative stromal values. The difference persists over time and may be useful for visualization and measurement of the LASIK flap both in the clinical setting and for detection of previous LASIK in eye bank corneas.
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