MDCT of the Small Bowel

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

Disclosures

Appl Radiol. 2012;41(8):6-17. 

In This Article

Applications

Congenital Anomalies

A number of congenital conditions affecting the small bowel can cause symptoms that come to clinical attention and which can be detected on MDCT. Some of these entities, including Meckel's diverticulum, malrotation, and annular pancreas, are detailed below, along with their complications and associated MDCT findings.

Meckel's Diverticulum

Meckel's diverticulum is the most common congenital anomaly of the gastrointestinal tract and results from failure of regression of the vitelline duct, resulting in a true diverticulum arising from the distal ileum. It is estimated from autopsy series that approximately 2% to 4%of individuals have Meckel's diverticula, although the vast majority of individuals are asymptomatic.[8–10]

Meckel's diverticulum is the most common cause of gastrointestinal bleeding in patients under the age of 30 and is due to acid production and subsequent ulceration by rests of ectopic gastric mucosa within the diverticulum.[11,12] Meckel's diverticulum can also present with intestinal obstruction due to intussusception, volvulus, or hernia.[13,14] Symptoms can also result from inflammation and/or perforation of the diverticulum.[14] While symptomatic Meckel's diverticula in children are most often related to hemorrhage or intussusception, in adults,symptoms are more commonly due to obstruction or diverticulitis.[11,15–17] In such a setting, MDCT is frequently utilized for evaluation of these nonspecific symptoms. As a result, Meckel's diverticula and their associated complications have been increasingly diagnosed by CTin both the pediatric and adult populations.[8,12] The usefulness of MDCT in detecting a Meckel's diverticulum as the cause of lower gastrointestinal bleeding has also been recognized.[13]

On a CT scan, a Meckel's diverticulum typically appears as a blind-ending pouch located in the midline or in the right lower quadrant, containing air and fluid or particulate matter.[11,12] Visualization of a fibrous band connecting the diverticulum to the anterior abdominal wall is helpful for confident diagnosis. Mural thickening and infiltration of the surrounding mesenteric fat may be present if there is active inflammation.Identification of a normal appendix on CT is also quite useful for excluding the diagnosis of appendicitis in these patients, as acute appendicitis is often clinically suspected.[12] Olson et al described the appearance of symptomatic Meckel's diverticula in a pediatric population, noting 3 typical patterns: isolated small bowel obstruction (31%); intussusception with small bowel obstruction (19%); and cystic mass with surrounding inflammatory change (25%) (Figure 2).[8]

Figure 2.

Meckel's diverticulitis. (A) Axial image through the pelvis shows inflamed fat in the midline (arrowhead), with adjacent gas-filled sacculations (white arrows) and thickened, dilated loop of ileum (black arrow). (B) Cystic structure with air-fluid in midline containing calcifications proved to be a Meckel's diverticulum.

Anomalies of Midgut Rotation

The primitive midgut undergoes a number of complex rotations during the first trimester of fetal development in order to establish its finalnormal position, followed by peritoneal fixation of the broad-based small-bowel mesentery, which extends from the ligament of Treitz to the ileocecal valve. A number of anomalies can occur when these rotations do not occur normally, including nonrotation, a spectrum of malrotation,and reverse rotation. Classically, a complete midgut malrotation presents in the neonatal period secondary to volvulus, with neonatal bilious vomiting. However, many individuals, particularly those with less severe degrees of midgut malrotation, may not present with symptoms until later in life, with a range of clinical presentations, from vague abdominal pain to intermittent symptoms of obstruction to malabsorption and weight loss, at which point, these patients are often evaluated by CT scan.[18,19] Some may not develop symptoms at all, with the abnormal positioning of their bowel noted only incidentally on CT studies performed for other reasons.[20] Given the risk of volvulus associated with malrotation of any degree, however, the recognition of these anomalies by radiologists is critical, even when patients are asymptomatic.

An abnormal position of the cecum on CT can be useful for determining the presence of a malrotation, with an increasingly abnormal position generally corresponding with a more severe degree of malrotation. The duodenum may also lie in an abnormal position, anterior to the superior mesenteric artery (SMA). An important clue to the presence of a midgut malrotation or nonrotation is the reversal of the normalrelative positions of the superior mesenteric vein (SMV) and artery, first described by Nichols and Li as the "SMV rotation sign."[21] That is,if the SMV lies to the left of or directly anterior to the SMA rather than in its normal position to the right of the SMA, a midgut malrotation or nonrotation is likely present, as this results from failure of completion of the normal 270-degree counterclockwise rotation of the midgut around the axis of the SMA during fetal development. This abnormal relationship of the SMA and SMV can be demonstrated on ultrasound in patients with a favorable body habitus, but is even more easily seen on CT.[18–24] The "whirlpool sign" can be seen on both CT and ultrasound in cases of malrotation complicated by midgut volvulus and describes the twisting of the small bowel and SMV around the SMA.[18,19,25,26]

Annular Pancreas

Annular pancreas is a rare congenital anomaly that is typically diagnosed in early childhood, either due to symptoms of complete duodenal obstruction and/or frequently associated additional congenital abnormalities. In adults, however, annular pancreas is often not clinically suspected, but is increasingly diagnosed on imaging studies performed to evaluate symptoms of gastric outlet obstruction or pancreatitis.[27,28] An annular pancreas results from failure of rotation and fusion of the 2 ventral pancreatic anlages to the dorsal anlage during the first trimester of fetal development. It is a rare congenital anomaly, although the increased use of MDCT and MRCP in recent years has suggested that there may be a higher incidence than previously thought, particularly when also taking into consideration the incidence of incomplete annular pancreas, which can also cause clinical symptoms.[27] CT, particularly with imaging during the pancreatic phase of enhancement, enables visualization of the pancreatic tissue encircling the duodenum either completely or partially.[27,28] Sandrasegaran et al found that identification of pancreatic tissue posterolateral to the second portion of the duodenum is 92% sensitive and 100% specific for diagnosis of annular pancreas, whether complete or incomplete. That is, a complete ring of pancreatic tissue surrounding the duodenum is not necessary for a diagnosis of annular pancreas.[27]

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