Low coherence tissue interferometry for the depth-resolved measurement of ocular fundus pulsations

Abstract

Zsfassung in dt. SpracheA new technique called low coherence tissue interferometry is introduced, which allows for the measurement of ocular fundus pulsations (i.e., distance variations between the anterior surface of the cornea and the ocular fundus) at pre-selected axial positions of the subject's eye. Unlike previously presented systems, which only allow for observation of the strongest reflecting retinal layer, this system enables the measurement of fundus pulsations at a pre-selected ocular layer. For this purpose the sample is illuminated by light of low temporal coherence. The layer is then selected by positioning one mirror of a Michelson interferometer according to the depth of the layer. The device contains a length measurement system based on partial coherence interferometry and a line scan CCD camera for recording and online inspection of the fringe system.In vivo measurements in healthy humans at different angles to the axis of vision were performed during this work and algorithms for the enhancement of the recorded images were developed. In all subjects, fundus pulsations were measurable in various reflective layers at the posterior pole of the eye, especially in the inner limiting membrane and the retinal pigment epithelium. The contrast of the observed interference fringes was evaluated in vitro for different positions of the measurement mirror and calculated for various distances from the front surface of the cornea. This enabled, firstly, for determination of the plane of highest contrast, and, secondly, for estimation of the maximum observable fundus pulsation amplitude. Furthermore, usability of a diffractive optical element for enhancement of the signal to noise ratio was studied, and fringe contrast and spacing for a system with diffractive optical element was calculated. In one subject, comprehensive measurements of the fundus pulsation amplitudes were performed at horizontal angles ranging from -2 degree temporal to 19 degree nasal to the axis of vision, resulting in an overview graph of the topograhpic (lateral and axial) distribution of the fundus pulsation amplitude. Particularly in the surroundings of the optic nerve head, the ability of low coherence tissue interferometry to measure fundus pulsations at well-defined axial positions is valuable and can provide new insight into the biomechanical properties of the eye, especially in glaucoma patients. Further applications of the technique developed in this work include assessment of eye elongation during myopia development and blood flow related changes in intraocular volume.8