frangi

Frangi typically refers to the Frangi vesselness filter, a computer vision method for enhancing tubular structures in images, particularly blood vessels in medical imagery. It is used to improve vessel segmentation, tracing, and visualization in biomedical image analysis.

The technique was introduced by Frangi and colleagues (Frangi, Niessen, Vincken, Viergever) in 1998 with the MICCAI paper Multiscale Vessel Enhancement Filtering. It has since become a widely cited reference in vessel enhancement and analysis.

Conceptually, the filter analyzes the image at multiple scales by smoothing with Gaussians and computing the Hessian matrix. The eigenvalues of the Hessian are used to distinguish line-like (tubular) structures from blob-like or background regions, producing a vesselness score that highlights vessels while suppressing non-tubular features.

Implementation details vary, but the standard approach uses a scale-space representation and a vesselness function that combines eigenvalue ratios and sign conditions to emphasize bright tubes on dark backgrounds or vice versa. The method supports 2D and 3D images and is available in several software libraries for image processing and analysis.

Applications include retinal vessel analysis, cerebral and pulmonary vasculature imaging, and broader vascular segmentation pipelines. While effective, the method requires choosing scales and parameters and can be sensitive to noise and imaging artifacts, potentially producing false positives in non-tubular structures.