MR Enterography for Diagnosing Small Bowel Crohn's Disease
MR enterography (MRE) diagnoses small bowel Crohn's disease by detecting segmental mural hyperenhancement combined with wall thickening on contrast-enhanced imaging, with asymmetric involvement being highly specific for Crohn's disease. 1
Primary Diagnostic Imaging Findings
Core Features for Diagnosis
The combination of segmental mural hyperenhancement and wall thickening has moderately high sensitivity and specificity for small bowel Crohn's disease at MRE. 1 However, mural hyperenhancement alone without wall thickening is nonspecific and may reflect other inflammatory processes. 1
Asymmetric involvement is a specific finding for Crohn's disease, whether referring to the morphologic pattern of hyperenhancement, wall thickening, or stratification in cross-sectional or longitudinal directions. 1 The mesenteric border is typically more affected than the antimesenteric border. 1
Wall Thickening Severity Classification
Wall thickness should be measured at the thickest portion of the most distended segment: 1
- Mild: 3-5 mm
- Moderate: 5-9 mm
- Severe: ≥10 mm
Enhancement Patterns
MRE can demonstrate stratified (bi- or tri-laminar) enhancement patterns, appearing as inner-wall hyperenhancement or a "halo sign." 1 In Crohn's disease, this can result from submucosal edema, intramural fat deposition, or inflammatory infiltration. 1 The tri-laminar pattern is more frequently identified on contrast-enhanced MR due to its superior contrast resolution compared to CT. 1
MRE-Specific Sequences and Their Diagnostic Value
T2-Weighted Imaging
Hyperintense T2-weighted signal correlates with moderate to severe endoscopic inflammation. 1 Intramural edema appears as hyperintense signal on fat-saturated T2-weighted images compared to normal small bowel, and increased hyperintensity is associated with more severe inflammation. 1
Diffusion-Weighted Imaging (DWI)
Restricted diffusion at MRE correlates with moderate to severe endoscopic inflammation. 1 In regions showing Crohn's disease-related inflammation on gadolinium-enhanced images, increased diffusion-weighted signal abnormality is associated with more severe inflammation. 1
Unenhanced MRE with diffusion-weighted imaging has moderate sensitivity and specificity for detection of ileal Crohn's disease. 1 A 2016 prospective study demonstrated that DWI without intravenous contrast was noninferior to contrast-enhanced MRE for evaluating small bowel inflammation in generally well-distended bowel, with 91.8% agreement between the two techniques. 2
Contrast-Enhanced Imaging
Contrast-enhanced imaging is performed in the enteric to portal phases of enhancement. 1 This allows assessment of mural hyperenhancement patterns and helps distinguish active inflammation from other processes.
Detection of Complications
Strictures
A stricture is present when the lumen is narrowed and there is proximal small bowel dilation. 1 Strictures are classified based on upstream dilation: 1
- Without upstream dilation: Upstream lumen <3 cm
- With mild upstream dilation: Upstream lumen 3-4 cm
- With moderate to severe upstream dilation: Upstream lumen >4 cm
Most Crohn's disease strictures have both inflammation and fibrosis. 1 Location and length should be described for potential surgical or endoscopic intervention. 1 For focal stenoses >1.5 cm in diameter, evaluate for signs of tumor including mass or extension into adjacent mesentery. 1
Penetrating Disease
Visualization of ulcers at cross-sectional enterography is a marker of severe inflammation. 1 Ulcers appear as small focal breaks in the intraluminal surface with focal extension of air or enteric contrast into the inflamed bowel wall, correlating with severe endoscopic inflammation. 1
MRE can detect fistulas, sinus tracts, inflammatory masses, and abscesses. 3 A 2014 meta-analysis showed MRE had an area under the curve (AUC) of 0.936 for detecting fistulas, 0.931 for stenosis, and 0.996 for abscesses. 4
Clinical Indications and Advantages
When to Use MRE
Cross-sectional enterography should be performed at diagnosis of Crohn's disease to detect small bowel inflammation and penetrating complications beyond the reach of standard ileocolonoscopy. 1 This is a strong recommendation as MRE may detect small bowel inflammation not seen at ileocolonoscopy. 1
Cross-sectional enterography should be considered in disease monitoring paradigms when small bowel disease or penetrating disease complications are present. 1
MRE vs. CT Enterography
MRE should be used rather than CTE when possible for estimating response to medical treatment in asymptomatic Crohn's disease, as its multiparametric nature permits evaluation of multiple imaging parameters that reflect inflammation and avoids radiation. 1 This is particularly important in young patients who require prolonged follow-up. 5
A 2014 meta-analysis found MRE has diagnostic effectiveness comparable to CTE, with pooled sensitivity of 87.9% and specificity of 81.2% for detecting active small bowel Crohn's disease, compared to 85.8% and 83.6% for CTE respectively. 4 No statistically significant incremental yield was found between the two modalities. 4
Important Caveats
MRE with only oral contrast will not detect or stage colonic inflammation as well as ileocolonoscopy. 1 Colonoscopy remains the reference standard for colorectal inflammation. 1
If intravenous contrast cannot be administered, noncontrast MRE with T2-weighted and diffusion-weighted imaging should be used as an acceptable alternative. 1 However, DWI may be less accurate for diagnosing penetrating complications compared to contrast-enhanced imaging. 2
Adequate small bowel distention is essential for optimal evaluation, though endoscopic and double-contrast barium techniques remain superior for depicting early Crohn's disease changes such as aphthoid ulceration. 3