K 283The optics of retinal vessels - consequences for scanning laser doppler flowmetry and fundus oximetry
M. Hammer, D. Schweitzer, A. Kolb, J. Strobel
Goal: Newly developed techniques as Scanning Laser Doppler Flowmetry (SLDF) and Fundus Oximetry enable the recognition of functional changes at the retina which are invisible. However, models are needed to extract the clinical parameters blood volume, velocity, flow, and oxygen saturation from optically measured data. Since the accuracy of the clinical parameters depends on the correctness of the optical models, the description of the optics of retinal vessels and capillaries should be as true sa possible.
Method: Light propagation in biological tissues can be described by the Monte-Carlo simulation. This technique was applied to determine the contribution of different pathways of the radiation to the fundus reflection as well as it's angular distribution. The results were used in empirical models.
Results: The deflection of the light needed to produce a measurable doppler shift in SLDF is mainly due to scattering at the erythrocytes. 32.8% of the light, incident on a 0.01 mm thick layer of blood, is deflected by more than 10°. The same layer of retina deflects only 3.6% of the light. The modelling of the reflection of retinal vessels needs the consideration of the transmitted as well as the backscattered light. An appropriate model was set up. Using this model, we were able to determine the oxygenation of calibrated blood specimen from their spectra measured with the Jena ophthalmo-spectrometer with a mean error of 6%.
Discussion: A careful development and evaluation of the underlying models and algorithms is the basis to get data from SLDF and fundus oximetry correct enough for clinical diagnostics.
University of Jena, Department of Ophthalmology, Bachstr. 18, Jena