1. A method of detecting blood vessels in an anterior posterior x-ray radiograph comprising the steps of: a. identifying and suppressing, by a computer, rib shadows in the radiograph; b. identifying and suppressing, by the computer, short pieces of edges in the radiograph as indicative of lung tissue shadow edges; and c. differentiating, by the computer, between nodule edges and blood vessel edges in the radiograph in which the rib shadows and the short pieces of edges are suppressed; wherein the step of differentiating between nodule edges and blood vessel edges comprises segmenting lungs into a vassel region having an upper limit at knuckle point on left lung an a lower limit at cardiac notch and weighting candidates within the vessel region as being likely blood vessel candidates based on the candidates' locations.