MDCT of the Small Bowel

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

Disclosures

Appl Radiol. 2012;41(8):6-17. 

In This Article

Technique

Many different techniques have been established for MDCT of the small bowel. At our institution, we routinely employ either a 16- × 1.25-mm or 64- × 0.625-mm detector configuration, reconstructing at 1.25- and 5-mm-thick axial sections for primary viewing. Coronal reformations are also routinely performed. Intravenous contrast is routinely administered for standard MDCT of the abdomen/pelvis, in which small bowel pathology is suspected, unless there is a contraindication. We administer 150 mL of a 300-mg/mL iodinated contrast medium intravenously at a rate of 2 to 3 mL/second. Scanning commences during the portal venous phase, after a 70-second delay.

High-density enteric contrast is not typically administered for our routine CT scans for several reasons. First, the presence of water-density fluid in the bowel lumen is preferred when intravenous contrast has been administered, as this maximizes visualization of mucosal enhancement patterns, which are of critical importance in cases of bowel ischemia and a number of inflammatory conditions. High-density contrast can also obscure intraluminal lesions, such as gallstones in cases of gallstone ileus. Second, imaging cannot commence until 1 to 2 hours following the administration of positive enteric contrast in order to ensure adequate opacification of distal small-bowel loops—a delay that is often not feasible in the emergent-care setting.

Positive enteric contrast agents, including barium sulfate suspensions and water-soluble high osmolar iodinated solutions, can be useful in a few situations, however. If intravenous contrast cannot be given, positive enteric contrast will facilitate delineation of the small-bowel loops and thus aid in distinction of bowel loops from surrounding soft-tissue structures. Patients with ovarian malignancies routinely ingest positive enteric contrast agents prior to their staging/surveillance MDCT scans, which are also performed with intravenous contrast, in order to improve our ability to distinguish peritoneal tumor implants from normal small-bowel loops. Patients in whom there is high clinical suspicion for fistulaeor abscesses are also given positive enteric contrast prior to their studies for optimal detection and characterization of these entities.

When there is high clinical suspicion for bowel ischemia, our MDCT protocol is modified to optimize visualization of the mesenteric arteries and any associated abnormalities. A CT angiogram is obtained using a bolus-tracking technique, with image acquisition occurring during the arterial phase of enhancement.

CT enterography (CTE), another modification of the MDCT protocol, is also being increasingly used in select patients in whom specific clinical questions regarding small-bowel architecture is needed, such as Crohn's disease or occult gastrointestinal (GI) bleeding. The protocol for CTE differs most prominently from routine MDCT in the administration of large volumes of neutral enteric contrast prior to theMDCT scan. Neutral enteric contrast that is not resorbed is preferred to maximize distal-bowel luminal distention and conspicuity of the enhancing bowel wall. In addition, the beam hardening artifacts that can be seen with positive enteric contrast agents are avoided. A number of studies have been performed comparing various contrast agents, including a combination of water and methylcellulose, polyethyleneglycol, mannitol, sorbitol, and low-concentration barium solutions.[1–5] The most commonly used agent now is Volumen (Bracco Diagnostics,Inc., Princeton, NJ), a low-concentration barium solution containing sorbitol, a nonabsorbable sugar alcohol that limits water resorption and thereby helps maintain luminal distention.[6]

The details of neutral enteric contrast administration for CTE vary slightly from institution to institution.[1,7] but generally involve ingestion of a total of 1350 mL of Volumen, administered in three 450 mL portions at uniform time intervals beginning 45 to 90 minutes prior to scanning.

The timing of CT image acquisition relative to intravenous contrast injection is also modified in the CTE protocol, with images acquired during the "enteric" phase of contrast enhancement, after a 45-second delay (Figure 1).

Figure 1.

Volume-rendered image from a CT enterography shows a hypervascular mass at the terminal ileum (arrow), and associated metastatic mesenteric hypervascular lymph node with surrounding desmoplastic reaction. Diagnosis was ileal carcinoid. Postprocessed images from CTE studies can provide clear overviews of primary and associated pathology.

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