| Distribution | terminal ileum (~70%), any GI segment, skip lesions, perianal | rectum → continuous proximal; backwash ileitis (gaping IC valve) | ileocaecal valve, caecum, ascending colon |
| Depth | transmural | mucosal/submucosal | transmural, granulomatous |
| Barium | aphthous ulcers → cobblestoning, string sign of Kantor (narrow terminal ileum: spasm/stricture), rose‑thorn fissures, fistulae, separated loops (creeping fat), pseudosacculations on the antimesenteric border | granular mucosa, collar‑stud (button‑hole) ulcers, pseudopolyps, loss of haustra → "lead‑pipe" colon, shortened colon, widened presacral space | conical, shrunken, pulled‑up caecum (Fleischner / inverted umbrella sign: thickened gaping IC valve with narrow terminal ileum), Stierlin sign (rapid emptying of an inflamed terminal ileum/caecum), goose‑neck deformity, strictures |
| CT/MR enterography | mural thickening with mural stratification (target), comb sign (engorged vasa recta), fibrofatty proliferation (creeping fat), fistula/abscess, strictures | continuous colonic wall thickening, target/halo, toxic megacolon (transverse colon > 6 cm, loss of haustra, mucosal islands) | necrotic (low‑attenuation, rim‑enhancing) mesenteric nodes, high‑density ascites, omental caking, peritoneal thickening, splenic/hepatic microabscesses |
| Cancer risk | small bowel/colon (less) | colorectal cancer (extent and duration), cholangiocarcinoma (PSC) | — |