1. A method of detecting lung nodule shadows in an anterior posterior x-ray radiograph comprising the steps of: a. generating candidate sub areas of the radiograph showing changes in contrast above a threshold level, and b. eliminating false positives by eliminating edges assignable to organ shadows by: i. identifying edges; ii. categorizing and eliminating rib shadow edges comprising dividing the x-ray radiograph into six vertical areas corresponding to inner left zone, central left zone, outer left zone, inner right zone, central right zone and outer right zone, and identifying linear features having angles subtended to horizontal within an appropriate range as being rib edges wherein the appropriate ranges are −5° to 25°; −20° to 20°; 10° to 90°; −45° to 0°; 2° to 10° and −90° to −10° with respect to a horizontal direction at 3 O'clock, such that counter-clockwise direction is considered positive; iii. categorizing and eliminating lung tissue shadow edges, and iv. categorizing and eliminating blood vessel shadow edges.