Functional imaging of the retinal microvasculature by scanning laser Doppler flowmetry.

PURPOSE: to image functionally perfused retinal vessels and to assess quantitatively the intercapillary space of the retinal microvasculature.
METHOD: The base of functional imaging and the quantitative assessment of the retinal vasculature is the two-dimensional map of the retina encoded by the laser Doppler frequency shift. By Scanning Laser Doppler Flowmetry (HRF. Heidelberg Engineering) the laser Doppler frequency shift of 16.384 retinal sites (256 pixels x 64 lines, spatial resolution 10 mum) of a retinal area of 2.7 x 0.7 mm was gained. The image processing was performed by a recently described algorithm (AFFPIA). Using the data of the laser Doppler frequency shift of every retinal site, a color-coded retinal image was established showing perfused vessels and capillaries. By automatic pattern analysis of this image vessels and capillaries were identified and segmented. Based on this image the distances in [microm] of every retinal site to the next vessel or capillary were calculated ("distance to next capillary"). The functional imaging of the retinal perfusion was demonstrated in (1) normal retina, (2) retinal arterial occlusion, and (3) proliferative retinopathy. Intraobserver reliability of the quantitative assessment of the parameter "distance to next capillary" was estimated by measuring 10 eyes of 10 subjects at 5 different days by one observer. Interobserver reliability of the quantitative assessment was evaluated by analysing 10 perfusion maps by 5 different operators. In 93 eyes of 71 normal subjects (mean age 40.4 mu 15 years) the juxtapapillary retina was quantitatively evaluated.
RESULTS: QUALITATIVE EVALUATION: The functional images of the retinal perfusion of eyes with normal retina, with retinal arterial occlusion, and with proliferative retinopathy corresponded well with the fluorescein angiography. Perfused vessels and capillaries became visible in a high local resolution. QUANTITITATIVE ASSESSMENT: The coefficient of reliability of the introobserver and interobserver reproducibility of the parameter 'mean distance to next capillary" was 0.74, and 0.95, respectively. The quantitative assessment of the perfusion showed that the major part of the retinal sites (>700%) had distances to the next capillary lower than 30 microm 46% of the retinal area had distances to the next capillary from 0-20 microm 26% of the retina had distances from 20-30 microm, 12% of the retina had distances from 30-40 microm 7% of the retina had distances from 40-50 microm, 4% of the retina had distances from 50-60 microm, and 4% of the retinal sites showed distances to the next capillary greater than 60 mum. The mean distance to the next capillary or vessel was calculated with 21 +/- 6.5 microm.
CONCLUSION: By non-invasive Scanning Laser Doppler Flowmetry in combination with adequate software it is possible to perform a functional imaging of the retinal vasculature and to measure all index for the functional density of retinal capillaries and vessels.