Table 1.

Retinal vascular metrics from retinal photography

Metric	Derivation	Interpretation	Strengths	Weaknesses
CRAE	Widths of the reflective erythrocyte column within the vessel lumen from 6 largest arterioles located in a zone 0.5–1 disc diameters away from the optic disc margin	Summarized surrogate measure of internal arteriolar widths that reflect narrowing or widening	Provides insight into disease affecting arterioles Relatively easy to obtain and automate	Summarized rather than absolute values Potential for magnification and positioning errors Values are not true vessel widths nor cross-sectional area that may be more relevant to disease
CRVE	Widths of the reflective erythrocyte column within the vessel lumen from 6 largest venules located in a zone 0.5–1 disc diameters away from the optic disc margin	Summarized surrogate measure of internal venular widths that reflect narrowing or widening	Provides insight into disease affecting venules Relatively easy to obtain and automate	Summarized rather than absolute values Potential for magnification and positioning errors Values are not true vessel widths nor cross-sectional area that may be more relevant to disease
AVR	Ratio of CRAE to CRVE	Changes usually indicative of generalized arteriolar narrowing	Avoids magnification errors Dimensionless	Provides little insight into the underlying pathophysiology If used alone, it can lead to incorrect inferences: both CRVE and CRAE narrow with increasing blood pressure, producing a normal AVR masking any association
Df	Images are binarized and vessel maps are broken into short segments (skeletonization) Entire image divided into boxes, and those containing a vessel segment are counted. The process is repeated with different box sizes. The number of boxes with vessel segments is plotted against the total number of boxes in the image	Index of vessel network spatial occupancy (complexity) Reduced (sparse) or increased (dense) complexity relative to health or within a cohort reflects suboptimal vascular network geometry	Based on robust models of the optimality of vascular branching May be more sensitive than calibers in reflecting microvascular disease in other organ beds	Less widely studied than calibers Simplifies 3-dimensional vascular networks into 2-dimensional skeletonized maps

AVR, arteriole-to-venule ratio; CRAE; central retinal arteriolar equivalent; CRVE, central retinal venular equivalent; Df, fractal dimension.