MDCT of the Small Bowel

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

Disclosures

Appl Radiol. 2012;41(8):6-17. 

In This Article

Ischemia

Mesenteric ischemia can be either acute or chronic, with acute mesenteric ischemia far more common than chronic ischemia. CT, and more recently, MDCT, play a critical role in the diagnosis of patients with acute mesenteric ischemia. Acute mesenteric ischemia is an infrequent but important cause of abdominal pain, as it is associated with an average mortality rate of 71%. There has been little improvement in outcomes in the past 10 years, despite medical, surgical, and technological advances.[29]

Acute mesenteric ischemia can be due either to occlusive disease (80%) or to nonocclusive disease (20%). The majority of occlusive cases are due to thromboembolic disease affecting the SMA, with 50% secondary to embolus and 20% secondary to thrombosis. SMV occlusion is less common, responsible for 5% to 10% of cases. Vasculitis from a variety of underlying conditions is also an important, although less common, occlusive cause of acute mesenteric ischemia. The CT appearance can be quite variable, as it is in large part a reflection of the pathologic stage of ischemic injury to the bowel, which ranges from mild mural edema to transmural infarction, depending on the duration of ischemia, the cause (arterial or venous), and the extent of involvement.[30]

Arterial occlusive ischemia due to thromboembolic disease is best evaluated with a biphasic MDCT angiography technique, in which both arterial and portal venous phase scans are acquired. An occlusive thrombus can be identified as a hypodense filling defect within the proximal portion of the SMA, usually adjacent to a site of preexisting calcified atherosclerotic plaque. An occlusive acute embolus also appears as a hypodense filling defect, but is typically located at arterial branch points. Three-dimensional reconstructions and volume-rendered images of the mesenteric vasculature can often be quite helpful for confident diagnosis. Ischemic small bowel due to arterial occlusive disease is characterized by a loss of motor tone, resulting in distention of the small-bowel loops, but generally no significant bowel wall thickening. Rather, the walls appear paper-thin. Lack of mural enhancement is a highly specific sign of mesenteric ischemia,but can be difficult to detect (Figure 3).[31,32] In more advanced cases, in which ischemia has progressed to infarction, pneumatosis and portalvenous gas may be seen. However, it is important to note that these can also be seen in nonischemic conditions, in which the integrity of the intestinal mucosa has been breached.[30,33,34]

Figure 3.

Small-bowel ischemia. Axial CT image shows normally enhancing small bowel (arrowhead) and ischemic loops of small bowel in the left upper quadrant (arrow). Adjacent isodense free fluid in the mesentery causes the "disappearing loop" sign. Ischemia was due to an SMA embolus (not shown).

Findings on MDCT in cases of bowel ischemia due to venous occlusion are typically different from those due to arterial occlusion, characterized by more pronounced wall edema, mesenteric edema, and interloop free fluid. Bowel-wall thickening in ischemia is typically symmetric, but can have various patterns of attenuation, including homogeneous hypodensity or a halo/target appearance. Mural hemorrhage is frequently seen as well in cases of venous ischemia, and can cause a heterogeneous attenuation within the bowel wall due to areas of hemorrhage juxtaposed with areas of edema and hypoperfusion.[32] Venous occlusion can be seen in patients with an underlying hypercoagulable state, or may be due to a complicated small-bowel obstruction, an infiltrative neoplasm, or an infection. Occasionally, a thrombus can be directly visualized within the affected mesenteric vein as a hypodense filling defect that may expand the vein.

The gastrointestinal tract is involved in 50% of patients with vasculitis. Vasculitides involving the large vessels include Takayasu arteritis and giant cell arteritis. Medium vessels are affected in polyarteritis nodosa, typically seen in older females, and Kawasaki disease, a pediatric disease. Systemic lupus erythematosis (SLE), Henoch-Schonlein purpura, and Wegener's disease affect small vessels and are typically seen in younger patients. These vasculitides cause ischemia at unusual sites, such as the stomach and duodenum, as well as the small bowel and the colon. Infarcts in other organs, such as the kidney and spleen, are often seen as well. Bowel ischemia in these patients is secondary to narrowing and occlusion of small mesenteric arteries, resulting in long segments of diffuse, marked bowel wall edema, and hemorrhage(Figure 4).

Figure 4.

Lupus vasculitis. Coronal reformatted image shows long segment of submucosal edema/hemorrhage in the midabdomen (arrow).

Generalized states of systemic hypotension, such as heart failure and shock, may produce nonocclusive small-bowel ischemia. Reperfusion injury occurs once blood flow to the bowel is restored to normal, leading to mucosal injury and submucosal edema. On MDCT,this may produce the dramatic appearance of marked mucosal hyperemia and bowel wall thickening known as "shock bowel."

Chronic mesenteric ischemia is also known as "intestinal angina" and results from inadequate blood flow to the bowel, typically in the setting of underlying atherosclerotic disease in elderly patients causing significant occlusion or narrowing of the celiac axis, SMA, and/or inferior mesenteric artery. Symptoms typically occur in a postprandial setting, when increased splanchnic blood flow demand is greater than the blood supply available from the diseased arteries. Chronic mesenteric ischemia is often suspected based on the clinical presentation, in conjunction with arteriographic evaluation of the splanchnic vessels.

On MDCT utilizing the CT angiography technique described above, calcified and noncalcified atherosclerotic plaque is well demonstrated, as well as the extensive collateral formation that occurs due to the chronic nature of the disease. MDCT also allows the exclusion of extrinsic causes of vascular occlusion, such as tumors. MDCT images can also be used for preoperative planning for revascularization.[30]

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