1. A method for extracting nodule features in a chest x-ray image for detecting false positives in nodule candidates, comprising: generating a representation of vessel trees in the chest x-ray image, the representation of the vessel trees defining background contextual information in the chest x-ray image; extracting at least one feature for each of the nodule candidates based on the background contextual information; and detecting false positives in the nodule candidates based on the at least one feature detected for each of the nodule candidates.