1. A method for detecting an object in a 2D medical image, comprising: detecting candidates for the object in the 2D medical image using a hierarchical tree-structured array of trained classifiers comprising: a first classifier trained for an object class to detect object candidates in a first search space; and a plurality of second classifiers, each trained for a respective one of a plurality of sub-classes of the object class to detect object candidates of the respective one of the plurality of sub-classes in a second search space having a greater dimensionality than the first search space based on the object candidates detected by the first classifier.