Abstract
Purpose: :
Optical Coherence Tomography (OCT) provide images of the anterior segment structures (cornea, iris, crystalline lens) based on optical path length measurements. Quantitative 2-D and 3-D geometrical reconstructions require corrections for the optical distortion. We developed 3-D ray tracing algorithms to provide quantitative measurements of anterior segment OCT images, and tested it on intraocular lenses of known geometry.
Methods: :
OCT images were obtained with a custom-developed time domain OCT system. The power of the light incident on the sample was less than 150µW. Measurements were recorded at a speed of 0.3 s per A-Scan. The system was equipped with a set of polarizer and quarter wave plate to reduce possible saturation due to the specular reflection. The images were processed using a custom-developed contrast enhancement algorithm, and denoised by customized wavelet denoising procedure. Normals to the surfaces are obtained, which (in combination with the refractive index) are used to perform a 3-D ray tracing. Correction of individual images can be performed in 2-D (sections) or 3-D (topographic maps). A PMMA intraocular lens (with nominal 17.29 and 11.04 mm anterior and posterior radii of curvature) was used to assess the accuracy of the correction. Real lens parameters were obtained using non-contact confocal profilometry (Sensofar, Spain).
Results: :
Non-contact profilometric and OCT measurements of the lens geometry agreed within more than 95%. Radii of curvature from profilometric measurements of each of the lens surfaces measured individually yielded 17.90 and 11.07 mm for the anterior and posterior surfaces, respectively. Radii of curvature from OCT (followed by surface retrieval and optical distortion correction from the preceding surface) yielded 17.88 and 11.58 mm mm for the anterior and posterior surface, respectively .
Conclusions: :
OCT can provide quantitative measurements of anterior and intraocular surface geometry, provided that optical distortion correction is applied. Our algorithms are capable of correcting the optical distortion produced by the local curvature of the surfaces and retrieve accurate 3-D surfaces and 2-D sections. While the algorithms have been applied to a laboratory-developed system, they could be applicable to other OCT systems.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • anterior segment • anterior segment