Crohn’s disease is the most common inflammatory condition affecting the small bowel. Although any part of the gastrointestinal tract, from the mouth to the anus, may become involved with Crohn’s disease, it most commonly involves the terminal ileum. Involvement of the terminal ileum occurs in approximately 70% of patients often together with cecum. Twenty to 30% will have isolated colon involvement. Five percent of patients will manifest Crohn’s disease in the duodenum or jejunum. Patients with longstanding Crohn’s disease have a well-documented increased incidence of cancer (approximately 3% of patients) of the gastrointestinal tract usually involving the colon or ileum. Changes of Crohn’s disease are well shown on MRI. T2-weighted single-shot echo-train spinecho and gadolinium-enhanced T1-weighted fat-suppressed SGE images demonstrate characteristic findings:
- transmural involvement,
- skip lesions,
- mesenteric inflammatory changes.
Single-shot echo-train spin-echo and gadolinium-enhanced T1 SGE images are also useful for the evaluation of complications of Crohn’s disease (Marcos and Semelka 2000). Single-shot echo-train spinecho image is a very effective technique to demonstrate dilated obstructed bowel developing secondary to strictures and edema, whereas gadolinium-enhanced fat-suppressed SGE is useful in demonstrating inflammatory changes in bowel and mesentery. Both techniques were effective in showing wall thickening, abscess, and fistulae formation.
The MRI criteria of mild, moderate, and severe disease has been described and is a function of wall thickness, length of diseased segment, and extent of mural contrast enhancement. The extent of mural enhancement may also be determined by comparison of bowel enhancement on gadolinium-enhanced fat-suppressed SGE with that of the renal parenchyma (Marcos and Semelka 2000).
Bowel should not enhance to the same degree as renal cortex on either early capillary-phase images or > 1 min interstitialphase images. Enhancement equivalent or greater than renal cortex is abnormal and most often reflects the presence of inflammatory change. MRI also may have a role in the evaluation of acute exacerbations of Crohn’s disease. Specifically, in patients with longstanding disease, marked enhancement of the mucosa with substantially thickened wall and minimal enhancement of the outer layer is suggestive of acute-on-chronic involvement, and may have a role in the evaluation of acute exacerbations of Crohn’s disease.
In patients with non-active chronic disease, there may remain persistent thickening and abnormal enhancement seen on delayed post-gadolinium images. Acute disease will result in edema that may be best visualized on fat-suppressed single shot T2-weighted images.
MRI may play an important role in the determination of Crohn’s disease activity because of its high contrast resolution, high sensitivity to the contrast material and its safety compared to CT and barium studies.