Determination of the Inner Vessel Diameter in Fluorescence Angiography - An Optical Model Study

Indocyanine green fluorescence angiography (ICG-FA) offers a semi-quantitative, non-contact method to intraoperatively assess blood volume flow. However, absolute quantification requires the precise measurement of both flow velocity and inner vessel diameter.

We hypothesize that vessel diameters derived from ICG-FA are affected by a systematic error caused by optical refraction at the vessel wall, rather than being directly representative of the true inner lumen diameter. To test this hypothesis, two different phantoms were investigated: silicone tubes filled with an ICG–blood analogue and solid polyurethane phantoms with varying wall-to-lumen ratios and known diameters. Imaging was performed using both a near-infrared–enhanced camera and a commercial surgical microscope. Vessel diameters were estimated using Otsu’s edge detection and a one-dimensional gradient filter. An optical model was introduced to describe refraction-induced distortions at the vessel wall. Both experimental measurements and optical model predictions revealed systematic deviations of the estimated inner diameter from the ground truth, with a predominant overestimation depending on phantom geometry and wall-to-lumen ratio. ... mehrMeasurements performed with refractive index matching using glycerin reduced these deviations in phantom experiments, confirming optical refraction at the vessel wall as the dominant source of error; however, refractive index matching is not feasible in clinical practice. Therefore, the presented optical model provides a basis for model-based correction strategies to enable reliable quantitative diameter and flow measurements in ICG-FA.