1. A method of identifying a region of interest in a medical image of a subject, the method comprising: receiving, through a user interface of a computer, a medical image into said computer, said medical image of the subject comprising a plurality of voxels, with each of said voxels exhibiting an intensity; automatically executing, in said computer, a computerized algorithm stored in the computer to generate a hierarchy of said voxels of said image sorted in order of said intensity, with non-adjacent voxels labeled with a respective label as a local maximum or a local minimum of said intensity according to said hierarchy; receiving, after generating said hierarchy of voxels, a single user entry into said computer via said user interface, the single user entry designating an initial voxel in said image, said designated initial voxel exhibiting said intensity and label; defining, by said computer, for each label, a predetermined relationship of respective intensities of respective voxels by determining a numerical value of the intensity for the designated initial voxel; determining, by said computer, a set of candidate regions of interest in the image by selecting, for each candidate region of interest, a set of voxels from the sorted hierarchy with each voxel in said set of voxels having a numerical value for the respective intensity thereof that is equal to or higher than said numerical value of the intensity for the designated initial voxel, wherein said selecting, by said computer, said set of voxels is performed according to selection criteria requiring each voxel of said set of voxels to be adjacent to another voxel of said set of voxels, and wherein any pair of voxels in said set of voxels is connected by a path of adjacent voxels of the set of voxels; determining, by said computer, a hierarchy among the set of candidate regions of interest according to sorted hierarchy of said intensity; receiving, by said computer via said user interface, a user selection of at least one region of interest from said set of candidate regions of interest; and displaying, at a display device in communication with said computer, said selected at least one region of interest comprised of voxels from said stored hierarchy that respectively exhibit an intensity having the predetermined relationship, defined by the label of the designated initial voxel, to said intensity of said designated initial voxel.