1. A method of segmenting a colon in a medical image volume data set comprising an array of voxels having associated signal values, the method comprising; defining a segmentation data structure for maintaining segmentation information; and classifying voxels in the segmentation data structure by performing a first step of classifying voxels as not belonging to the colon based on their associated signal value and/or position in the data set and performing a second step of classifying remaining voxels as belonging to the colon, thereby automatically segmenting the colon without first requiring a voxel to be identified within it.