Measurement of vessel tortuosity on fundus photographs

Quantitative measurement of vessel tortuosity and its variation on fundus photographs is a sensitive means of obtaining information about the course of an asphyctic event in newborns, virtually independent of bias produced by the photographic process. We subdivide a tortuous vessel into single arcs and measure the chord length and the arrow height (Pfeilhöhe) of every arc on a projected image of the film. From these figures, a fairly accurate value for the relative length increase of the arc, as compared with the chord, can be derived by a simple approximation formula [Eq. (5)]. It is shown that neglect of the third dimension, not visible on an ordinary photograph, entails only a small error. Trained observers achieve results reproducible to about 1% in relative length variation.