1. A computer-implemented method comprising: receiving a digital representation for a colon, wherein the digital representation represents at least a portion of a colon wall for the colon; identifying an outer boundary for the colon wall via a level set technique; and outputting an indication of the outer boundary of the colon wall; wherein the level set technique comprises: generating a speed image; and evolving an isosurface based at least on the speed image; wherein identifying the outer boundary for the colon wall via the level set technique comprises: using a lumen segmentation of the digital representation as an initial level set boundary.