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AbdomenDemonstration data

MR Enterography (Small Bowel)

MR enterography distends and images the small bowel without ionising radiation, making it the modality of choice for assessing and monitoring Crohn's disease, particularly in young patients requiring repeated studies. Biphasic oral contrast provides luminal distension, while multiphase T2, diffusion and dynamic gadolinium sequences characterise mural inflammation, fibrosis and extramural complications.

Indications

  • Diagnosis and activity assessment of Crohn's disease and treatment response monitoring
  • Evaluation of small bowel strictures and differentiation of inflammatory from fibrotic stenosis
  • Detection of penetrating complications: sinus tracts, fistulae, abscess and phlegmon
  • Assessment of small bowel tumours and polyposis syndromes
  • Investigation of obscure gastrointestinal bleeding and unexplained anaemia
  • Surveillance in young patients where cumulative CT radiation is a concern

Contraindications & Cautions

  • Non-conditional pacemakers, ICDs, neurostimulators and ferromagnetic implants
  • Suspected high-grade or complete bowel obstruction (oral contrast may worsen symptoms)
  • Anti-peristaltic agents contraindicated in narrow-angle glaucoma, uncontrolled arrhythmia and, for hyoscine, significant cardiac disease
  • eGFR below threshold (<30 mL/min/1.73 m2) contraindicates group I gadolinium agents (NSF risk)
  • Pregnancy: avoid gadolinium; unenhanced MRE may be performed if clinically essential

Patient Preparation

  • Fast 4-6 hours before the examination
  • Ingest 1-1.5 L of biphasic oral contrast (e.g. mannitol or polyethylene glycol solution) over 45-60 minutes for luminal distension
  • Administer an intravenous anti-peristaltic agent (hyoscine butylbromide or glucagon) immediately before scanning to reduce motion artefact
  • MRI safety screening and intravenous cannula placement for gadolinium
  • Position prone where tolerated to compress and separate bowel loops; supine if the patient cannot lie prone

Technique & Parameters

  • Coronal and axial single-shot fast spin-echo T2 (with and without fat saturation) for mural oedema
  • Balanced steady-state free precession (true FISP/FIESTA) coronal cine to assess peristalsis and fixed narrowing
  • Diffusion-weighted imaging with ADC mapping to highlight active inflammation
  • Dynamic multiphase (enteric, portal, delayed) fat-saturated 3D T1 gradient-echo after intravenous gadolinium
  • Cine motility sequences to distinguish fixed strictures from transient contractions
  • 1.5T or 3T with a phased-array body coil; breath-hold or free-breathing radial techniques

Systematic Review

  • Assess luminal distension adequacy before interpreting mural thickness
  • Measure mural thickness (>3 mm abnormal) and grade T2 mural oedema (bright wall)
  • Evaluate mucosal and mural enhancement pattern and stratification on dynamic T1
  • Look for restricted diffusion as a marker of active inflammation
  • Identify strictures, pre-stenotic dilatation and upstream obstruction
  • Search extramural: comb sign, fibrofatty proliferation, sinus tracts, fistulae, abscess and lymphadenopathy

Key Findings & Significance

  • Active inflammation: mural thickening with T2 hyperintensity, mucosal hyperenhancement and restricted diffusion
  • Comb sign: engorged vasa recta along the mesenteric border indicating active disease
  • Fibrofatty proliferation (creeping fat) and mesenteric lymphadenopathy
  • Stricture with pre-stenotic dilatation >3 cm; fibrotic strictures show delayed homogeneous enhancement and low T2
  • Penetrating disease: sinus tracts, entero-enteric or enterocutaneous fistulae, abscess with rim enhancement
  • Skip lesions with intervening normal segments characteristic of Crohn disease

Differential Considerations

  • Segmental mural thickening: Crohn disease, infective enteritis (TB, Yersinia), NSAID enteropathy, ischaemia, radiation enteritis
  • Small bowel mass: adenocarcinoma, neuroendocrine tumour, GIST, lymphoma, metastasis
  • Mesenteric mass/lymphadenopathy: Crohn, lymphoma, carcinoid with desmoplastic reaction, TB
  • Diffuse bowel enhancement: shock bowel, vasculitis, angio-oedema
  • Fistula: Crohn disease, post-surgical, malignancy, diverticular disease

Pearls & Pitfalls

  • Inadequate distension is the commonest cause of a false-negative or false-positive study; assess distension first
  • Combining T2 oedema, restricted diffusion and mucosal hyperenhancement improves accuracy for active inflammation
  • Cine imaging differentiates a fixed fibrotic stricture from a transiently contracted loop
  • The MaRIA and Lemann indices provide reproducible scoring of activity and cumulative bowel damage
  • Fibrotic and inflammatory strictures often coexist; report the dominant component to guide medical versus surgical therapy

Structured Report

  • State field strength, oral and intravenous contrast used and distension quality
  • Report affected segments, length, mural thickness and activity markers (T2, DWI, enhancement)
  • Characterise strictures with degree of narrowing and pre-stenotic dilatation
  • Describe penetrating complications and their relationships to adjacent structures
  • Provide an activity assessment and comparison with prior studies to guide management

References

  • Bruining DH, et al. Consensus recommendations for evaluation, interpretation and utilization of CT and MR enterography in Crohn's disease. Radiology 2018
  • Rimola J, et al. MaRIA index for magnetic resonance activity in Crohn disease. Gut
  • ACR-SAR-SPR Practice Parameter for the Performance of MR Enterography
  • Sinha R, et al. Radiographics: MR enterography technique and interpretation

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.