MDCT of the Small Bowel

Grace A. Tye, MD; Terry S. Desser, MD


Appl Radiol. 2012;41(8):6-17. 

In This Article


A variety of inflammatory conditions of the bowel are well depicted on MDCT, although the manifestations can often be nonspecific, making a precise diagnosis difficult. However, a systematic approach to the interpretation of small-bowel abnormalities detected on CT can improve specificity in some cases.

Crohn's Disease

Patients with Crohn's disease who present with an acute flare often demonstrate bowel wall thickening, with mural stratification that results in a target-like or "halo" appearance of the bowel. Bowel wall thickening is defined as a wall thickness >3 mm in a well-distended loop. The"halo" appearance refers to alternating layers of hyperdense mucosa, hypodense intramural edema, and hyperdense serosa (Figure 10). The presence of low-density intramural edema suggests an active inflammatory process. A more sensitive yet less specific indicator of active disease ismucosal hyperemia, which is optimally demonstrated on a CT enterography protocol. Luminal narrowing may be seen secondary to edema or spasm and is reversible at this stage, although it can be severe enough to cause an associated SBO.[7] With chronic inflammation, intramural fatdeposition can be seen, with fat attenuation seen within the thickened bowel wall rather than the water-density intramural edema seen with acute disease. Alternatively, chronic inflammation can also result in muscular hypertrophy, collagen deposition, and fibrosis, resulting in strictures that can cause small-bowel obstruction.

Figure 10.

Crohn's disease. (A) Axial contrast-enhanced image shows a target-like appearance of small bowel resulting from encircling of acute inflamed mucosa by edematous submucosa, the "halo sign" (arrowhead). Note engorged vasa rectae (arrow). (B) Volume-rendered image from a CTE study performed on the same patient a few days later shows the "comb sign," representing engorged vasa rectae supplying the acutely inflamed small-bowel loop (arrowheads).

Extra-enteric findings can also be helpful indicators of active disease. The "comb sign" (Figure 10) describes the engorged vasa rectae within the mesentery that run perpendicular to the bowel wall and correlates with more advanced, active disease that may warrant more aggressive medical therapies, given its correlation with more frequent hospital admissions.[48] An increase in the density of the mesenteric fat surrounding an abnormal loop of bowel is a highly specific indicator of active Crohn's disease and correlates with elevated C-reactive protein levels and histopathologic severity of disease.[7] Fibrofatty proliferation, or fatty deposition along the mesenteric border of inflamed bowel segments, is highly specific for transmural inflammation secondary to Crohn's disease, but can be seen with both active and chronic disease.

Other important extra-enteric findings include abscesses and fistulas, as these may determine a need for surgical intervention. When abscesses or fistulas are suspected clinically, positive enteric contrast should be administered prior to the CT scan.

Celiac Disease

Celiac disease is a chronic intolerance to gluten in genetically predisposed patients that can lead to mucosal inflammation in the small bowel, most often affecting the duodenum and proximal jejunum. Although definitive diagnosis is made via endoscopic biopsy showing characteristic histopathologic changes, as well as resolution of these changes after institution of a gluten-free diet, MDCT is increasingly being used for evaluation. Several authors have recently advocated the use of CT enteroclysis for evaluation of suspected celiac disease. CT enteroclysis differs from the routine MDCT protocol in that a large volume of room-temperature water (generally >2 L) is infused through a nasojejunal tube during image acquisition. Detailed protocol information can be obtained from previously published literature.[49,50]

Soyer et al in a retrospective analysis of their experience with CT enteroclysis in patients with suspected celiac disease, found that reversal of the jejuno-ileal fold pattern had the highest specificity for the disease (100%), but a sensitivity of only 63.6% (Figure 11). Other findings that were strongly correlated with celiac disease included ileal-fold thickening, vascular engorgement, and splenic atrophy.[50] Intussusception can be seen in celiac sprue due to hypertrophied lymphoid tissue acting as lead-points. Lymphadenopathy is often present as well. Patients with celiac disease are also at increased risk for development of small-bowel adenocarcinomas and lymphomas, and the bowel should be carefully evaluated for evidence of tumoron MDCT.

Figure 11.

Celiac disease (sprue) with reversal of jejunal-ileal fold pattern on CT. (A) Axial contrast-enhanced image shows loops of proximal jejunum (arrowheads) with effaced, atrophic folds. (B) Loops of ileum show fold thickening and increased number of folds (arrows).

Radiation Enteritis

Radiation administered in the course of cancer therapy can cause both acute and chronic inflammatory changes in any bowel loops included within the radiation port. Radiation enteritis is characterized by segmental areas of abnormal bowel wall thickening. Chronic radiation enteritis can result in development of homogeneously enhancing scar tissue within the bowel wall, similar to that seen with fibrotic changes in chronic Crohn's disease.[32] Given the somewhat nonspecific findings, knowledge of the clinical history and note of the location of the abnormal small bowel within the site of previous radiation are critical to a confident diagnosis.


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