Apparatus for use with an imaging device configured to acquire a set of angiographic images of a lumen of a subject's body, and a display, the apparatus comprising: at least one processor, comprising: blood-velocity-determination functionality configured, via image processing, to determine blood velocity within the lumen, by: defining at least first and second regions of interest along the lumen in one of the angiographic images; identifying the regions of interest in at least some additional angiographic images belonging to the set of angiographic images; determining a distance between the regions of interest; determining that a presence of a contrast agent appears at the first region of interest in a first one of the angiographic images and that the presence of contrast agent appears at the second region of interest in a second one of the angiographic images; and determining the time that it took for the contrast agent to travel from the first region of interest to the second region of interest, based upon an interval between an acquisition of the first angiographic image and an acquisition of the second angiographic image; geometry-indication-receiving functionality configured to receive an indication of geometry of the lumen at a given location within the lumen; current-flow-related-parameter-determination functionality configured to determine a value of a current flow-related parameter at the given location based upon the determined blood velocity and the geometry of the lumen in the vicinity of the location; flow-related-parameter-receiving functionality configured to receive an indication of the value of a second flow-related parameter, wherein said indication includes: a) an indication of an aortic blood pressure of the subject and/or b) an indication of the value of a second flow-related parameter at the given location within the lumen at a time when the lumen was healthy; index-determination functionality configured to determine a value of a luminal-flow-related index of the subject at the given location, by determining a relationship between the value of the current flow-related parameter and the value of the second flow-related parameter; and output-generation functionality configured to generate an output on the display in response to the determined value of the luminal-flow-related index. defining at least first and second regions of interest along the lumen in one of the angiographic images; identifying the regions of interest in at least some additional angiographic images belonging to the set of angiographic images; determining a distance between the regions of interest; determining that a presence of a contrast agent appears at the first region of interest in a first one of the angiographic images and that the presence of contrast agent appears at the second region of interest in a second one of the angiographic images; and determining the time that it took for the contrast agent to travel from the first region of interest to the second region of interest, based upon an interval between an acquisition of the first angiographic image and an acquisition of the second angiographic image; geometry-indication-receiving functionality configured to receive an indication of geometry of the lumen at a given location within the lumen; current-flow-related-parameter-determination functionality configured to determine a value of a current flow-related parameter at the given location based upon the determined blood velocity and the geometry of the lumen in the vicinity of the location; flow-related-parameter-receiving functionality configured to receive an indication of the value of a second flow-related parameter, wherein said indication includes: a) an indication of an aortic blood pressure of the subject and/or b) an indication of the value of a second flow-related parameter at the given location within the lumen at a time when the lumen was healthy; a) an indication of an aortic blood pressure of the subject and/or b) an indication of the value of a second flow-related parameter at the given location within the lumen at a time when the lumen was healthy; index-determination functionality configured to determine a value of a luminal-flow-related index of the subject at the given location, by determining a relationship between the value of the current flow-related parameter and the value of the second flow-related parameter; and output-generation functionality configured to generate an output on the display in response to the determined value of the luminal-flow-related index.