Ultrasound (US) plays an ever-increasing role in managing children with acute abdominal pain and has replaced the plain abdominal radiograph as the modality of initial choice in many clinical situations. The major advantages of US are that it does not use ionizing radiation, is relatively inexpensive, and the abdominal viscera, including the bowel, can be well delineated in children. Therefore, many pathological entities can be easily confirmed or excluded on US.

Abdominal radiography (AXR) remains a standard method for evaluating the acute abdomen in some clinical situations. It is essential when peritonitis is present and perforation is suspected.

All neonates with an acute abdomen are evaluated with AXR. This modality is essential for evaluating conditions such as necrotizing enterocolitis and congenital bowel obstruction. In the former, the AXR may be diagnostic; in the latter, findings may guide the choice of subsequent (if necessary) contrast examinations of the GI. Radiographic views with a horizontal beam are essential to exclude the presence of free air due to bowel perforation and can be performed with the neonate in the dorsal decubitus (lateral shoot-through view) or left lateral decubitus position.

In older children, the diagnosis of intestinal obstruction can often be made on the supine film alone. A search for air-fluid levels on the upright view does not always add extra information, and a search for free air in the abdomen is often more easily achieved with a lower radiation dose in a single, upright view of the chest, which will also serve to exclude lung pathology.

However, in both the neonate and older child, the AXR findings are often nonspecific, which limits the role of this modality.

This imaging modality remains important in certain conditions. For example, in congenital low-bowel obstruction, the contrast enema is important in some neonates to establish a diagnosis; in others, it is also useful for therapy (e.g., meconium ileus or meconium plug syndrome). Contrast evaluating the upper GI may also be required to confirm or exclude anomalies of midgut rotation in symptomatic children in whom US is not conclusive (see below).

Computed tomography (CT) may be reserved for the more complicated imaging situations when US may not provide all necessary information, e.g., appendicitis when gas obscures the right lower quadrant or in older, obese children; and children with abscesses. CT without contrast injection is also extremely helpful in delineating urinary stones when these are not well shown by US.

Magnetic resonance imaging (MR) has a more limited role in children with acute abdominal pain. However, it can depict the anatomy exceptionally well in certain conditions in which it may be used to complement findings on US. These situations include biliary- and pancreaticduct anomalies and gynecological disorders such as complex anomalies associated with hydrocolpos.

In typical cases, there is gaseous distention of the duodenum, but often, appearances may be nonspecific or even normal. One should, therefore, never rely on the plain film findings to rule out malrotation. In some patients, the duodenum may be fluid filled and not visible, and only the stomach is distended with air, simulating a complete or partial gastric outlet obstruction. In children with severe vascular compromise due to volvulus, the entire small bowel may be dilated (with air-fluid levels), resembling a low-bowel obstruction or ileus, but vascular compromise may also produce a gasless abdomen. A volvulus with fluid-filled bowel may also appear as a soft tissue abdominal mass.

Because of the wide variation of appearances on plain radiographs with midgut malrotation, the clinician should never rely on plain radiographs to rule out this entity. Therefore, any child in whom there is a clinical suspicion of malrotation should be examined with modalities that will directly depict the position of the bowel and/or nature of the obstruction. Abdominal US and contrast examination of the upper GI tract are the most valuable modalities in this regard.

Over the past decade, several sonographic findings have been described in malrotation and include:

Yousefzadeh drew attention to US depiction of the third part of the duodenum (D3), in the angle between the aorta and SMA, and suggested that documentation of the presence of D3 in this normal position excludes the presence of malrotation. Further data regarding this is required.

There was much debate about whether the upper GI series or the contrast enema is the most effective imaging modality to diagnose malrotation. Most institutions now rely more heavily upon the upper GI series, but if in doubt, it is prudent to do both. On the upper GI series, the duodenojejunal flexure is absent and the proximal small bowel typically lies in the midline (Fig. 5). Much less commonly, an internal hernia may be delineated (Fig. 6). On contrast enema, the cecum is usually in the right upper quadrant and the ascending colon and hepatic flexure are not correctly positioned. The major difficulty in making or excluding the diagnosis of malrotation is the variation in position that those structures may assume normally or with malrotation. It is beyond the scope of this chapter to describe them all. Meticulous technique is critical to either type of study in order to delineate these structures accurately. The use of too much or too little contrast material may render the study undiagnostic.

At the time of diagnosis, three abnormalities may be present on AXR: bowel dilatation, intramural gas, and portal venous gas (Fig. 7).