1. A method for detecting target objects in a three dimensional (3D) image volume of an anatomical structure comprising the steps of: obtaining a set of candidate locations in the image volume; for each candidate location, cropping out sub-volumes of at least two different scales, each sub-volume comprising a plurality of voxels; for each of the sub-volumes, rotating each sub-volume in at least two different orientations; applying a shape classifier to each sub-volume; if the voxels in the sub-volume pass the shape classifier, computing a gradient direction for the voxels; if the gradient direction for the voxels is one of a predefined orientations, applying a probability classifier to the voxels; using a probability measure computed by the probability classifier as a confidence measure for the sub-volume; and if the confidence measure is above a predetermined threshold value, determining that the sub-volume contains the target object.