1. A method for determining, in three dimensions, connected substructures within a body, said method comprising the steps of: (a) storing, in a memory means, three-dimensional signal patterns representing the value of at least one physical property associated with substructures within said body at regularly spaced parallelopiped grid locations within said body; (b) labeling said stored signal patterns wherein those signal patterns representing values which lie within a specified range receive a same label; (c) storing and placing on a memory stack an initial grid location, said grid location corresponding to a position within or on a desired internal substructure of said body; (d) selecting and removing the grid location from the top of said memory stack; (e) comparing the label associated with said grid location selected from said memory stack with labels associated with adjacent grid locations to identify grid locations having the same label as said label associated with the grid location from said stack; (f) marking, as connected, those grid locations for which a selected arrangement of adjacent grid locations exhibit the same label; (g) storing on said memory stack a list of any of said marked grid locations; and (h) repeating steps (d), (e), (f) and (g) for a plurality of times not to extend beyond the occurrence of an empty stack.