Low coherence laser Doppler velocimetry in human retinal vessels

The application of laser Doppler velocimetry for the study of retinal blood velocities has been introduced almost 30 years ago. Since then a variety of clinical and animal studies has been performed using this technique. In this technique a laser beam of high temporal coherence is focused on a retinal vessel. Fourier analysis of the photocurrent obtained from the reflected and scattered light provides a Doppler shift power spectrum. The shape of this spectrum is rectangular assuming a parabolic velocity profile in the vessel. The cutoff frequency of this spectrum is proportional to the maximum center blood velocity within the retinal vessel. Assuming a theoretical velocity distribution in the retinal vessel a mean velocity can be calculated from these data. Whereas this technique`s capable of measuring absolute flow velocities in retinal vessels no information on velocity profile is available. The introduction of light sources with high spatial, but low temporal coherence may enable to get additional information on retinal flow velocities with high depth resolution. In the present study we will investigate several approaches to gain depth resolution using super luminescent diodes for the illumination of the eye. This technique may be used to gain new insights into the pathogenesis of eye disease with vascular origin.

In the present project we developed a short-coherence laser Doppler velocimeter for the measurement of velocity profiles in retinal vessels. The present instrument is superior to all existing prototype systems with respect to sensitivity and reproducibility. With the instrument developed in this project velocity profiles were measured both in vitro and in vivo with high time resolution. Measurement of velocity profiles in arteries and veins of the retina are of relevance with respect to several aspects. Vascular diseases of the retina and the optic nerve head are common. Among these diseases are retinal arterial and venous occlusions, diabetic retinopathy, glaucoma and ischemic neuropathies of the optic nerve. These diseases lead to an irreversible loss of vision and are among the most common causes of blindness in the Western countries. The therapy of these diseases is difficult. This is among other things related to the fact that there is currently no clinically applicable instrument for the measurement of retinal blood flow available. The short short-coherence laser Doppler velocimeter appears to be a step towards a solution of the project. The aim of our future research in the field is to test the applicability of the technique in patients with vascular diseases of the eye and check for the usability with respect to early detection of retinal vascular disease. In addition, we plan to evaluate new therapeutic treatment regimen in patients with ocular vascular disease.