A system and method are provided for determining vascular velocity using non-invasively acquired medical images. The method includes reconstructing CT angiography (CTA) data into a plurality of images of the subject by producing a composite image using the CTA data corresponding to a set of the plurality of view angles, backprojecting each view angle in the CTA data and weighting a value backprojected into at image pixel by an attenuation value of a corresponding pixel in the composite image, and summing backprojected values for each image pixel to produce a CT image of the subject. The method also includes determining a flow direction or a velocity of flow within a vessel, calculating, using the flow direction or velocity, a pressure in the vessel, and generating a quantitative map of the subject indicating the flow direction, velocity, or pressure in the vessel against an image of the subject including the vessel.