Inflammatory Bowel Disease
- Inflammatory bowel disease (IBD) is the chronic-temper-tantrum category of the gut, and it splits mainly into two siblings: Crohn disease and ulcerative colitis (UC).
- The cheat code: Crohn can hit anywhere from mouth to anus, in patchy skip lesions, full-thickness through the wall (so it makes fistulas, abscesses, and strictures). UC is continuous, starts at the rectum, climbs the colon, and stays mucosa-deep.
- On cross-sectional imaging you're hunting for a thickened, inflamed, hyperenhancing bowel wall, with the surrounding fat looking angry.
- CT shows you the emergencies; MR enterography is the workhorse for mapping and follow-up because it dodges the radiation in young patients.
- Complications are the real exam (and real-life) bait: strictures, fistulas, abscesses, and a long-term cancer risk.
Imagine your bowel is a very long garden hose that, for reasons nobody fully understands, has decided to be furious with you forever. Not a one-time clog. A chronic, relapsing grudge. That's inflammatory bowel disease in a sentence — and the imaging job is mostly figuring out which grudge, where, and whether it has gotten anyone into trouble yet.
The two siblings
IBD is dominated by two diagnoses that look superficially similar and behave completely differently. Telling them apart is most of the game.
| Feature | Crohn disease | Ulcerative colitis |
|---|---|---|
| Where | Mouth to anus, loves the terminal ileum | Colon and rectum only |
| Pattern | Patchy skip lesions (inflamed, then normal, then inflamed) | Continuous, starting at the rectum and marching up |
| Depth | Full thickness through the wall (transmural) | Mucosa and submucosa only |
| Signature complications | Fistulas, abscesses, strictures | Toxic megacolon, higher long-term cancer risk |
The phrase to tattoo on your brain: Crohn is patchy and deep; UC is continuous and shallow. Almost everything else falls out of those two facts. Crohn goes full thickness, so it can tunnel out of the bowel entirely and form a fistula (an unauthorized plumbing connection to another loop, the bladder, or the skin). UC stays shallow, so it doesn't tunnel — but the entire inner lining gets inflamed continuously, which is its own kind of misery.
What inflamed bowel looks like
Whatever the modality, inflamed bowel sends the same few signals. The wall gets thick (normal small bowel wall is thin — only a few millimeters), it enhances more brightly after contrast because inflammation means more blood flow, and the fat around it stops looking clean. Healthy mesenteric fat is uniformly dark and quiet on CT; angry fat gets hazy and streaky, the radiology equivalent of a smudged window.
That last finding — small mesenteric vessels lined up and engorged, feeding the inflamed segment — is called the comb sign, because the vessels look like the teeth of a comb pointing at the sick bowel. It's a classic marker of active Crohn inflammation.
Which test, when
CT is the first responder. Someone rolls in with belly pain and a fever, and a contrast-enhanced CT (start with the approach to the abdominal CT) quickly answers the question that matters at 3 a.m.: is there an abscess to drain, a bowel obstruction from a stricture, or free air? Those are the things that change tonight's plan.
But IBD patients are often young and will be imaged many, many times over a lifetime, and CT radiation adds up. So for mapping the disease and following it, the workhorse is MR enterography — same beautiful wall detail, no ionizing radiation, and it can distinguish active inflammation from old, burned-out scar tissue. That distinction is huge: an inflamed stricture might calm down with medication, whereas a fibrotic one is just scar and may need surgery.
On MR enterography, restricted diffusion and brisk wall enhancement point toward active inflammation; a stiff, narrowed, non-enhancing segment points toward fibrotic scar. Treating scar with anti-inflammatory drugs is like watering a plastic plant.
The complications that actually matter
This is where IBD imaging earns its keep. Watch for:
- Strictures — chronic inflammation heals with scar, the lumen narrows, and you get obstruction with upstream dilated bowel.
- Fistulas and sinus tracts — Crohn's signature tunneling, including the dreaded perianal fistulas (pelvic MRI is the tool of choice for those).
- Abscesses — a walled-off pocket of pus, often near an inflamed loop; this is the one the interventional team may need to drain.
- Toxic megacolon — a feared UC complication where the colon dilates dramatically and stops working; it's a surgical emergency, and you may first spot the dilated, edematous colon on the abdominal radiograph.
Severely active colitis can mimic — and overlap with — bowel ischemia, since both produce a thickened, edematous, poorly enhancing wall. Lean on the distribution and history: ischemia follows a vascular territory, while IBD follows the Crohn/UC patterns above. When the wall doesn't enhance at all, start worrying about dead bowel rather than just inflamed bowel.
The one thing to walk away with
Don't try to memorize a hundred findings. Anchor on the two-sibling rule — Crohn is patchy and transmural, UC is continuous and superficial — read the bowel wall for thickness and enhancement, and then ask the only question that changes management: has it caused a stricture, a fistula, an abscess, or an obstruction yet? Get that, and you've got IBD imaging.
Thick, brightly enhancing wall plus angry surrounding fat equals active inflammation. Then classify the pattern (Crohn vs UC) and hunt for the complication.