Magnetic Resonance Enterography (MRE), often called a Small Bowel MRI, is a specialized, non-invasive imaging procedure that generates detailed images of the small intestine. This technique uses a powerful magnetic field and radio waves to create cross-sectional pictures of the soft tissues within the abdomen. MRE helps medical professionals accurately diagnose, monitor, and assess various conditions affecting the digestive tract by providing an in-depth view of the bowel wall, surrounding tissues, and the intestinal lumen.
Preparing for the Scan and Understanding the Technique
Preparing for a Small Bowel MRI involves several steps focused on achieving optimal distension of the small intestine, which is necessary for clear imaging. Patients must fast for at least four to six hours before the scan to ensure the stomach is empty. The most distinct part of the preparation involves drinking a large volume (typically 1.5 to 2 liters) of a specialized oral contrast agent over a period of 45 to 60 minutes prior to the imaging.
This oral contrast, which may contain an ingredient like mannitol, travels through the small bowel to effectively distend its loops. Distension opens up the collapsed intestinal walls, allowing the radiologist to clearly see the thickness and integrity of the wall structure. Without this distension, the bowel walls lie close together, which can obscure subtle abnormalities or mimic disease. The contrast solution also provides a distinct signal on the MRI images, highlighting the bowel lumen against the surrounding tissues.
In addition to the fluid consumed orally, an intravenous (IV) line is placed to administer two different types of injections during the scan. The first is a gadolinium-based contrast material, which enhances the visibility of blood flow and active inflammation within the bowel wall. Areas of active disease will “light up” brightly on the images after this injection because of increased blood flow to the affected tissue.
The second injection often involves an anti-peristaltic agent, such as Glucagon or Buscopan, given just before the scanning begins. This medication temporarily slows down peristalsis, the natural, involuntary muscle movements of the intestines. By minimizing this motion, the scan can capture sharp, clear images, preventing blurring. The entire process of drinking the contrast and the subsequent scan usually takes about two to three hours.
Key Conditions Diagnosed by Small Bowel MRI
Small Bowel MRI is primarily utilized for the evaluation of Inflammatory Bowel Disease (IBD), specifically Crohn’s disease, throughout the small intestine. The scan is highly effective at identifying signs of active inflammation, which typically presents as abnormal thickening of the bowel wall, often exceeding three to four millimeters in thickness. The degree of enhancement of the bowel wall after the intravenous contrast injection directly correlates with the severity of active inflammation.
The technique is crucial for differentiating between inflammatory complications and chronic, non-inflammatory changes. For instance, the scan can distinguish between an inflammatory stricture, which might respond to medical therapy, and a fibrotic stricture, which represents permanent scarring and may require surgical intervention. Active inflammation often shows a layered appearance, or mural stratification, within the bowel wall, while chronic fibrotic changes typically appear as a homogenously scarred and narrowed segment.
MRE is also effective for detecting complications that extend outside the bowel wall, known as extramural disease.
Extramural Disease and Complications
This includes identifying fistulas, which are abnormal tracts connecting the bowel to other organs or the skin, and abscesses, which are collections of pus often found in the mesentery near the inflamed bowel. The presence of the “comb sign”—engorgement of mesenteric blood vessels supplying the inflamed segment—is a specific finding that indicates active disease.
Bleeding and Tumors
Beyond IBD, Small Bowel MRI plays an important role in locating the source of obscure gastrointestinal bleeding when traditional endoscopy and colonoscopy procedures are inconclusive. The high soft-tissue resolution can sometimes pinpoint small tumors or abnormal vessels that are actively bleeding within the small intestine. The scan can also identify and characterize small bowel tumors, including primary cancers, metastatic lesions, and benign masses like polyps or neuroendocrine tumors.
Other Conditions
Identifying other conditions, such as complications related to Celiac disease or small bowel ischemia, is also possible. In Celiac disease, MRE can help monitor for the development of complications like ulcerative jejunoileitis or T-cell lymphoma. Signs of ischemia, a lack of blood flow, may be seen as a segment of the bowel with reduced or absent contrast enhancement, which helps medical teams diagnose this potentially life-threatening condition.
Why Small Bowel MRI is Often Preferred
The most significant advantage of Small Bowel MRI over alternative imaging techniques, such as Computed Tomography Enterography (CTE), is the complete absence of ionizing radiation. This is especially important for patients who require frequent, long-term monitoring of a chronic condition, such as Crohn’s disease. Avoiding cumulative radiation exposure over many years significantly reduces a patient’s lifetime risk of developing radiation-induced cancers.
MRE offers superior soft-tissue contrast compared to CT, which allows for a more detailed characterization of the bowel wall layers and surrounding tissues. This enhanced detail aids in the assessment of disease activity, particularly in determining the extent of inflammation within the bowel wall and the surrounding mesenteric fat. The ability to visualize the full thickness of the bowel wall is a benefit, helping to define the depth of disease penetration more accurately.
The technique provides a comprehensive assessment of the entire abdomen and pelvis, which is beneficial for detecting and monitoring disease activity outside the bowel. This includes evaluating the mesentery for lymphadenopathy, the presence of creeping fat, and the extent of any extramural complications like abscesses or fistulas. Furthermore, the ability to acquire multi-planar images means the radiologist can view the small bowel from any angle, providing a complete spatial understanding of the disease.
While capsule endoscopy provides excellent views of the mucosal surface inside the small bowel, it cannot assess the deeper layers of the bowel wall or the surrounding structures. MRE effectively bridges this gap by providing transmural and extramural information, which is a component for staging disease and guiding treatment decisions.