2-DIMENSIONAL MAPPING OF THE PERFUSION OF THE RETINA AND OPTIC-NERVE HEAD

Aim-To present a new non-invasive method of performing a high definiti on topography of perfused vessels of the retina and the optic nerve he ad with simultaneous evaluation of blood flow. Method-By a combination of a laser Doppler flowmeter with a scanning laser system the perfusi on of the retina and the optic nerve head is visualised. The principle s of measuring blood flow by laser Doppler flowmetry are based on the optical Doppler effect: laser light scattered by a moving particle is shifted in frequency by an amount Delta f. Our data acquisition and ev aluation system is a modified laser scanning tomograph. The technical data are retinal area of measurement 2.7 mm x 0.7 mm, 10 degrees field with 256 points x 64 Lines, measurement accuracy 10 mu m, wavelength 670 nm and 790 nm, light power 100 mu W and 200 mu W, data acquisition time 2.048 s. Every line is scanned 128 times by a line sampling rate of 4000 Hz. By performing a discrete fast Fourier transformation over 128 intensities of each retinal point the laser Doppler shift is calc ulated for each retinal point. With these data a two dimensional map w ith 256 x 64 points of the retinal perfusion is created. The brightnes s of the pixel is coded by the value of the Doppler shift. Offline cap illary blood flow is estimated in arbitrary units according to the the ory of laser Doppler flowmetry in every region of interest of the perf usion picture. We estimated the reliability and the validity of the me thod. Retinal blood how was measured by scanning laser Doppler flowmet ry (SLDF) while varying intraocular pressure by a suction cup of three healthy volunteers. Measurements of retinal blood flow performed in 4 7 eyes by the presented method (SLDF) were correlated with data gained by a commercially available laser Doppler flowmeter. Perfusion pictur es of the superficial retinal layer and of deep prelaminar layers in t he optic nerve head are presented. Results-The reliability coefficient s r(1) of 'flow', 'volume', and 'velocity' were 0.84, 0.85, and 0.84 r espectively. We found a significant linear relation between SLDF flow and the ocular perfusion pressure (r=0.84, p<0.001). Comparative measu rements of the retinal blood flow by SLDF and a commercially available laser Doppler flowmeter showed a linear and significant relation (how r=0.6, p<0.0001, volume r=0.4, p<0.01). Capillaries of the retinal su perficial vasculature or deep ciliary sourced capillaries of the optic nerve head became visible with a high resolution by the confocal tech nique dependent on the focus. Offline, the blood flow variables of are as of 100 mu m x 100 mu m were calculated. Conclusion-SLDF enables the visualisation of perfused capillaries and vessels of the retina and t he optic nerve head in high resolution by two dimensional mapping of p erfusion variables which are encoded by the Doppler signal. This metho d achieves simultaneously qualitative and quantitative evaluation of c apillary blood flow of distinct areas of the capillary meshwork.