A system for determining an effective cross-sectional area of a tubular cardiovascular structure to enable assessment of blood flow through the tubular cardiovascular structure, the system comprising: - an image interface for obtaining a three-dimensional image of the tubular cardiovascular structure; - a segmentation subsystem for segmenting the three-dimensional image to obtain a segmentation of a lumen inside the tubular cardiovascular structure; - an analysis subsystem configured for: i) determining a centerline of the tubular cardiovascular structure, the centerline representing an assumed direction of blood flow through the lumen; ii) using the segmentation of the lumen, determining an apparent flow aperture of the tubular cardiovascular structure in a direction of the centerline; and iii) determining the effective cross-sectional area of the tubular cardiovascular structure based on the apparent flow aperture; characterized in that the analysis subsystem is configured for determining the apparent flow aperture by: i) projecting the segmentation of the lumen along the direction of the centerline onto a projection plane to obtain a projection of interior parts (510, 520) of the tubular cardiovascular structure forming obstacles for the blood flow parallel to the centerline; and ii) determining the apparent flow aperture by determining an area in the projection which is free of said projected interior parts. - an image interface for obtaining a three-dimensional image of the tubular cardiovascular structure; - a segmentation subsystem for segmenting the three-dimensional image to obtain a segmentation of a lumen inside the tubular cardiovascular structure; - an analysis subsystem configured for: i) determining a centerline of the tubular cardiovascular structure, the centerline representing an assumed direction of blood flow through the lumen; ii) using the segmentation of the lumen, determining an apparent flow aperture of the tubular cardiovascular structure in a direction of the centerline; and iii) determining the effective cross-sectional area of the tubular cardiovascular structure based on the apparent flow aperture; characterized in that the analysis subsystem is configured for determining the apparent flow aperture by: i) projecting the segmentation of the lumen along the direction of the centerline onto a projection plane to obtain a projection of interior parts (510, 520) of the tubular cardiovascular structure forming obstacles for the blood flow parallel to the centerline; and ii) determining the apparent flow aperture by determining an area in the projection which is free of said projected interior parts. i) determining a centerline of the tubular cardiovascular structure, the centerline representing an assumed direction of blood flow through the lumen; ii) using the segmentation of the lumen, determining an apparent flow aperture of the tubular cardiovascular structure in a direction of the centerline; and iii) determining the effective cross-sectional area of the tubular cardiovascular structure based on the apparent flow aperture; i) projecting the segmentation of the lumen along the direction of the centerline onto a projection plane to obtain a projection of interior parts (510, 520) of the tubular cardiovascular structure forming obstacles for the blood flow parallel to the centerline; and ii) determining the apparent flow aperture by determining an area in the projection which is free of said projected interior parts.