Apparatus and methods are described for use with a lumen including, without generating a virtual three-dimensional model of the lumen, and by performing image processing on two-dimensional angiographic images, determining blood velocity within the lumen, and determining geometry of the lumen at a given location. A value of a current flow-related parameter at the location is determined based upon the determined blood velocity and the geometry of the lumen at the location. An indication of a value of a second flow-related parameter of the subject is received, and a value of a luminal-flow-related index of the subject at the location is determined, by determining a relationship between the value of the current flow-related parameter and the value of the second flow-related parameter. Other applications are also described.
