Optical frequency domain imaging of human retina and choroid

Abstract

Optical coherence tomography (OCT) has emerged as a practical noninvasive technology for imaging the microstructure of the human eye in vivo. Using optical interferometry to spatially-resolve backreflections from within tissue, this high-resolution technique provides cross-sectional images of the anterior and posterior eye segments that had previously only been possible with histology. Current commercially-available OCT systems suffer limitations in speed and sensitivity, preventing them from effective screening of the retina and having a larger impact on the clinical environment. While other technological advances have addressed this problem, they are inadequate for imaging the choroid, which can be useful for evaluating choroidal disorders as well as early stages of retinal diseases. The objective of this thesis was to develop a new ophthalmic imaging method, termed optical frequency domain imaging (OFDI), to overcome these limitations. Preliminary imaging of the posterior segment of human eyes in vivo was performed to evaluate the utility of this instrument for comprehensive ophthalmic examination.

(cont.) The 1050-nm OFDI system developed for this thesis comprised a novel wavelength-swept laser that delivered 2.7 mW of average power at a sweep rate of 18.8 kHz, representing a two-order-of-magnitude improvement in speed over previously-demonstrated lasers in the 1050-nm range and below. The system, with an optical exposure level of 550 gW, achieved resolution of 10 gm in tissue and sensitivity of >92 dB over a depth range of 2.4 mm. Two healthy volunteers were imaged with the OFDI system, with 200,000 A-lines over 10.6 seconds in each imaging session. In comparison to results from a state-of-the-art spectral-domain OCT system, the OFDI system provided deeper penetration into the choroid. This thesis demonstrates OFDI's capability for comprehensive imaging of the human retina, optic disc, and choroid in vivo. The deep penetration power of the system enabled the first simultaneous visualization of retinal and choroidal vasculature without the exogenous dyes required by angiography. The combined capability for imaging microstructure and vasculature using a single instrument may be a significant factor influencing clinical acceptance of ophthalmic OFDI technology.