LOCAL CONNECTED FRACTAL DIMENSIONS AND LACUNARITY ANALYSES OF 60-DEGREES FLUORESCEIN ANGIOGRAMS

Purpose, The retinal vascular tree exhibits fractal characteristics. T hese findings relate to the mechanisms involved in the vascularization process and to the objective morphologic characterization of retinal vessels using fractal analysis. Although normal retinas show uniform p atterns of blood vessels, in pathologic retinas with central vein or a rtery occlusions, the patterns are irregular. Because the generalized box fractal dimension fails to differentiate successfully between norm al and abnormal retinal vessels in 60 degrees fluorescein angiograms, the authors have further investigated this problem using the local con nected fractal dimension (alpha). Methods. The authors studied 24 digi tized 60 degrees fluorescein angiograms of patients with normal retina s and 5 angiograms of patients with central retinal vein or artery occ lusion. The pointwise method estimated the local complexity of the ang iogram within a finite window centered on those pixels that belong to the retinal vessels. Color-coded dimensional images of the angiograms were constructed by plotting the pixels forming the object with a colo r that corresponded to specific values of alpha +/- Delta alpha. Resul ts, The color-coded representation allowed recognition of areas with i ncreased or decreased local angiogram complexity. The alpha distributi ons showed differences between normal and pathologic retinas, which ov ercomes problems encountered when using the methods of calculating the generalized fractal dimensions. A multivariate linear discriminant fu nction using parameters from the alpha distribution and a further frac tal parameter - lacunarity - reclassified 23 of the 24 normal and 4 of the 5 pathologic angiograms in their original groups (total: 92.1% co rrect). Conclusions, This methodology may be used for automatic detect ion and objective characterization of local retinal vessel abnormaliti es.