1. A method for determining, in three dimensions, connected substructures within a body, said method comprising the steps of: (a) storing, in memory means three-dimensional signal patterns representing the value of at least one physical property, associated with substructures within said body, at regularly spaced grid locations within said body; (b) selecting signal pattern values, from said memory means, which are associated with slices through said body; (c) determining connectivity of two-dimensional substructures within each slice independently of connectivity in other slices, said connectivity being indicated by ordered indicia associated with distinct planar substructures; (d) determining connectivity of said two-dimensional substructures between adjacent slices, said connectivity being indicated in a pair list of connected substructures; (e) permuting a list of said ordered indicia in a memory means in accordance with said pair list, said permutation being carried out only for those pairs which are not already indicated as being connected; (f) determining from said permuted list those two-dimensional substructures which are connected.