1. A method for defining boundaries of a region of interest (ROI) from sets of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) images acquired over a series of time periods, the method comprising: receiving a ROI slice for each time period from among the corresponding set of DCE MRI images, the ROI slice being representative of the ROI in the corresponding time period; applying an expectation/maximization (EM) algorithm by a processor to group voxels of each received ROI slice in the series of time periods according to a number of statistical classes; and applying an active contour model by the processor to each received ROI slice in the series of time periods to extract a respective boundary of the ROI for each time period, based on the respective grouped voxels.