What is MRE (Magnetic Resonance Enterography)?
MRE is a specialized MRI protocol of the abdomen and pelvis specifically designed to evaluate the small bowel by using large-volume oral contrast to distend the intestines, combined with intravenous gadolinium contrast and multiple imaging sequences to assess bowel wall inflammation, thickness, and complications. 1
How MRE is Performed
Standard Protocol Components
Oral Contrast Administration:
- Patient ingests a large volume of biphasic oral contrast agent (appears dark on T1-weighted images, bright on T2-weighted images) to distend the small bowel 1
- This distension is critical for adequate visualization of the bowel wall
Intravenous Contrast:
- Traditionally includes IV gadolinium-based contrast
- Enteric phase imaging occurs 45-70 seconds after injection to assess for mural hyperenhancement indicating active inflammation 1
Standard Imaging Sequences:
- Single-shot T2-weighted sequences
- Balanced steady-state free precession sequences
- T1-weighted fat-suppressed post-contrast images
- Diffusion-weighted imaging 1
Optional Adjuncts:
- Hypoperistaltic medications (glucagon or hyoscine butylbromide) may be administered to reduce motion artifact, though these can cause nausea 1
Special Considerations for an 84-Year-Old with Limited Renal Reserve
Critical Modifications Needed
For patients with renal impairment, consider non-contrast MRE as an acceptable alternative using T2-weighted and diffusion-weighted imaging sequences without IV gadolinium 2. Recent evidence shows that:
- Diffusion-weighted imaging alone provides comparable identification of inflammatory wall thickening and lesion detection compared to contrast-enhanced MRE 1
- Non-IV contrast MRE showed similar performance for detecting active inflammation in the terminal ileum and colon 1
However, there is a trade-off: IV contrast does improve sensitivity for detecting penetrating complications (fistulae, abscesses, sinus tracts) 1, so the decision depends on the clinical question being addressed.
Practical Algorithm for This Patient
If evaluating for penetrating complications (abscess, fistula): Consider the risk-benefit of low-dose gadolinium with appropriate renal function assessment, or proceed directly to alternative imaging if contraindicated
If evaluating for inflammatory activity only: Proceed with non-contrast MRE using diffusion-weighted imaging, which avoids gadolinium entirely 2
If patient cannot tolerate oral contrast (common in elderly with acute symptoms): Consider non-contrast MRI of abdomen/pelvis, though this has reduced sensitivity (50% for small bowel, 86% for terminal ileum) 1
What MRE Detects
Mural features of inflammation:
- Wall thickening
- Hyperenhancement
- Edema
- Ulcerations 1
Perienteric features:
- Engorged vasa recta (comb sign)
- Fibrofatty proliferation
- Lymphadenopathy 1
Complications:
- Strictures with upstream dilation
- Fistulae and sinus tracts
- Abscesses
- Bowel perforation 1
Performance Characteristics
MRE demonstrates high diagnostic accuracy with sensitivity of 66-95% and specificity of 64-97% for detecting active bowel inflammation on a segmental basis 1. It is superior to fluoroscopic small bowel follow-through (sensitivity 76%, specificity 67%) 1.
Common Pitfalls in Elderly Patients
- Oral contrast tolerance: Elderly patients may struggle with large-volume oral contrast ingestion, particularly if symptomatic 1
- Gadolinium in renal impairment: Always assess renal function before IV contrast; nephrogenic systemic fibrosis risk exists with gadolinium in severe renal dysfunction
- Motion artifact: Elderly patients may have difficulty remaining still for the 30-45 minute examination duration
- Mesenteric fat: Reduced mesenteric fat in elderly can make perienteric inflammation assessment more challenging 1