MR Enterography
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Direct answer
MR enterography images the small bowel after the patient drinks roughly 1-1.5 litres of an iso-osmolar neutral oral agent such as mannitol or polyethylene glycol, distending loops while a spasmolytic freezes motion, and a T2-weighted and contrast-enhanced sequence set reads wall thickening, mural stratification, enhancement pattern and mesenteric changes in one radiation-free study. It is the first-line cross-sectional test for Crohn's disease in young patients because it can be repeated, it grades active inflammation — wall thickening over 3 mm with mural hyperenhancement, restricted diffusion and engorged vasa recta — and it maps strictures, fistulae and extraluminal complications. In Indian practice its sharpest job is separating ileocaecal tuberculosis from Crohn's disease, the single most consequential small-bowel distinction in the country.
What you must remember
- Preparation: 1-1.5 litres of biphasic neutral oral contrast (mannitol or PEG solution) over 45-60 minutes; the lumen is bright on T2 and dark after gadolinium, so the mucosa stands out. An antispasmodic such as hyoscine butylbromide reduces peristaltic artefact.
- Core sequences: single-shot T2 (HASTE) for anatomy, balanced steady-state free precession for motility and film quality, fat-suppressed T2 for oedema, diffusion-weighted imaging, and dynamic post-contrast gradient-echo T1 with fat saturation.
- Active Crohn's: wall over 3 mm thick, marked mural enhancement with stratification (target sign), T2 mural oedema, restricted diffusion, engorged vasa recta producing the comb sign, and enhancing mesenteric nodes.
- Stricturing disease: pre-stenotic dilation over 3 cm with mural thickening and reduced enhancement; fistulae and sinus tracts track as enhancing crevices, best shown on fat-suppressed post-contrast images.
- Ileocaecal TB versus Crohn's: TB favours a short segment, a pulled-up contracted caecum, thickened ileocaecal valve, necrotic (rim-enhancing) nodes and chest radiograph changes; Crohn's favours skip lesions, linear ulcers, comb sign and perianal disease. Both can granuloma, so the chest film, colonoscopy biopsy and trial of therapy settle it.
- Perianal Crohn's: separate pelvic MRI with STIR and post-contrast sequences classifies fistulae by Parks' classification — MR enterography plus pelvic MRI is the complete Crohn's study.
- Radiation logic: Crohn's patients accumulate CT dose over decades of relapses, and young Indians diagnosed in their twenties are the population that benefits most from MR surveillance.
Walking through an ileocaecal case
A 24-year-old has chronic abdominal pain, low-grade fever and a right iliac fossa mass; MR enterography is asked to characterise the deformed ileocaecal area. The study shows a 6 cm terminal ileal segment with 6 mm wall thickening, marked stratified enhancement, mesenteric stranding with comb vasa recta, and two jejunal skip lesions. The caecum is contracted and pulled up, the ileocaecal valve is lip-shaped and thickened, and a necrotic mesenteric node shows rim enhancement. Each finding is a vote: skip lesions and comb sign say Crohn's; contracted caecum, valve deformity and necrotic nodes say tuberculosis. The chest radiograph shows an old healed infiltrate. On balance this is ileocaecal TB: antitubercular therapy with colonoscopy biopsies for caseating granulomas and PCR, and the report phrases it "favouring tuberculosis", because the distinction rests on combined evidence, never the MR alone.
Perianal fistulae, rectal aphthous skip lesions and absent nodal necrosis would swing the pendulum to Crohn's, and the study's second job begins — grading activity to decide steroids, biologics or stricture surgery.
Where students slip
Two slips recur. The first is calling any wall thickening "active": fibrotic strictures also thicken the wall, but they enhance little and lack oedema, diffusion restriction and comb vasa recta — activity is a package, not a measurement. The second is forgetting that MR enterography is a function test of the whole bowel: only MR shows motility of a strictured segment, and only MR can be repeated yearly in a 22-year-old without a thought for dose. A viva favourite is why the oral agent must be iso-osmolar — hyperosmolar agents draw water into the lumen and cause diarrhoea, while plain water is absorbed and loses distension. The 3 mm wall and 3 cm pre-stenotic dilation numbers are the quotable exam payload.
Frequently asked questions
What oral preparation is used for MR enterography and why?
About 1-1.5 litres of an iso-osmolar neutral agent such as mannitol or PEG over an hour, which distends loops without drawing fluid in or being absorbed, giving biphasic lumen signal.
Which MR features indicate active Crohn's inflammation?
Bowel wall thickening over 3 mm with stratified marked enhancement, mural T2 oedema, diffusion restriction, engorged vasa recta (comb sign) and enhancing mesenteric fat and nodes.
How does ileocaecal tuberculosis differ from Crohn's on MR enterography?
Tuberculosis shows a short ileocaecal segment, contracted pulled-up caecum, deformed valve, necrotic rim-enhancing nodes and often chest disease, whereas Crohn's shows skip lesions, comb sign and perianal fistulae.
Why is MRI preferred over CT for follow-up of young Crohn's patients?
Crohn's relapses for decades requiring repeated imaging, and MRI gives comparable information without ionising radiation in young patients.
How are strictures and fistulae assessed in MR enterography?
Strictures are identified by wall thickening with pre-stenotic dilation over 3 cm and graded by residual enhancement and motility, while fistulae appear as fluid-filled or enhancing tracts on T2 fat-suppressed and post-contrast sequences.