Enhancing tissue-contrast imaging of the anterior eye

Abstract

Optical coherence tomography (OCT) has become one of the most important diagnostic tools in ophthalmology. Originally used to examine the posterior eye, which includes the retina and optic nerve, it now is being applied to the anterior eye, which includes the cornea and iris.1, 2 The back of the eye has several tissue layers, each with different light-scattering properties. OCT uses these differences for image contrast that clearly distinguishes the posterior tissues. However, the scattering properties of anterior eye tissue layers are similar, so it is difficult to selectively obtain contrast with OCT. There are several ways to increase OCT exogenous contrast, such as using nanoparticles and dye molecules, but they are somewhat invasive and thus not convenient for daily clinical use. However, OCT endogenous contrasts may be useful. Doppler OCT provides selective contrast of vasculature.3 Spectroscopic OCT,4 based on tissue absorption, may also improve contrast. And polarization-sensitive OCT (PS-OCT)5 enables birefringence (double refraction), which is strongly associated with tissue composition. We are working on tissue-contrast OCT of the anterior eye using PS-OCT detection and signal processing. This method yields pseudo-color, tissue-contrast OCT images with various tissues displayed in different colors. It can be used to selectively contrast tissues of the anterior eye, including the cornea, conjunctiva, sclera, uvea, and trabecular meshwork. We used a swept-source PS-OCT system with a probe wavelength of 1.3μm. It enables 3D tomography based on standard scattering OCT and PS-OCT, which measures phase retardation.5 The depth resolution is 12μm and themeasurement speed is 20,000 depth scans/s. These specifications are superior to the corresponding quantities in ultrasound biomicroscopy, a standard modality for examining the front of the eye. Figure 1 Figure 1. Human anterior segment cross section using (a) scattering optical coherence tomography (OCT), and (b) polarization-sensitive OCT. These images were constructed from Interactive Science Publishing data sets (see Reference 6).