1. A method for on-line learning of landmark detection models comprising: receiving a selection of a landmark to be detected in a 3D medical image; determining a current landmark detection result for the selected landmark in the 3D medical image by: automatically determining, by a processor, whether a stored trained machine-learning based landmark detection model corresponding to the selected landmark is reliable, in response to a determination that the stored trained machine-learning based landmark detection model corresponding to the selected landmark is reliable, automatically detecting the selected landmark in the 3D medical image using the stored trained machine-learning based landmark detection model corresponding to the selected landmark, and in response to a determination that the stored trained machine-learning based landmark detection model corresponding to the selected landmark model is not reliable, receiving a manual annotation of the selected landmark in the 3D medical image; and updating the stored trained machine-learning based landmark detection model corresponding to the selected landmark based on the current landmark detection result for the selected landmark in the 3D medical image.