The plain abdominal radiograph is an indispensable tool for both suspected abdominal pathology and the placement of tubes and catheters in children. Since children are more sensitive to radiation and have a longer life expectancy than adults, it is important to minimize radiation exposure. Dose reduction in diagnostic radiology must be achieved without compromising the diagnostic quality of the image. Low-dose techniques using increased tube potential (kVp) between 60 and 70 and a low milliampere-second (mAs) of 2-4 significantly decrease the dose (1).

Various radiographic views for assessment of the abdomen include supine, decubitus, and cross-table (horizontal) lateral. The left lateral decubitus view is preferred over the right lateral decubitus view for detection of a pneumoperitoneum because it is more sensitive (gas is easily seen between liver and abdominal wall). The bowel gas pattern is part of the radiographic assessment. Gas is usually present in the stomach within 10-15 minutes of birth and fills the large bowel by 12-14 hours (2). The bowel loops should be symmetric and often have a polygonal pattern (“chicken wire”) on the supine image (Fig. 102.1). A decubitus film or, less commonly, a cross-table lateral view also shows the bowel gas pattern in another plane (Figs. 102.2 and 102.3). Both views are evaluated for abdominal masses and calcifications. Free intraperitoneal fluid may be seen on a plain film if the amount is sufficient; it will cause the bowel loops to move centrally, with increased opacity in the flanks (Fig. 102.4). The abdominal radiograph may also reveal a lower lobe pneumonia or pleural effusion as the cause of abdominal symptoms. In addition, abnormalities of the soft tissues and bones can be visualized.

Abnormal plain film findings include the following:

Tubes and catheters: An abdominal radiograph is often ordered to assess the position of tubes and catheters after placement. It is important to determine the position of the tubes and catheters to prevent untoward complications associated with their misplacement, kinking, or break in continuity. Common tubes and catheters seen on an abdominal radiograph include umbilical arterial catheter (UAC), umbilical venous catheter (UVC), peripheral and central venous catheters, enteric tube, gastrostomy tube, ventriculoperitoneal shunt tubing, drainage tubes, pH probes, and temperature probes (Figs. 102.5A-C, 102.6A,B, and 102.7).

Bowel obstruction (images can be seen in later sections according to etiology): The typical clinical presentation in a neonate includes poor feeding, vomiting, abdominal distension, and delayed passage of meconium. An increase in postgastric aspirates may also be present. Differentiation between large and small bowel on plain film in the neonate or very young
child can be difficult since mucosal landmarks such as valvuli, plicae, and haustra are poorly developed. The radiologic findings in obstruction differ dep ending on the level of pathology. High intestinal obstruction implies obstruction from the stomach to the level of proximal ileum. There may be air
only in the stomach in cases of gastric outlet obstruction such as in pyloric stenosis, antral dyskinesia, or antral web. Two dilated bowel loops (double bubble) are seen with duodenal atresia (DA), annular pancreas, or duodenal web. More dilated bowel loops are present with a jejunal obstruction as in jejunal atresia (3,4). A low bowel obstruction shows multiple loops of dilated bowel in the abdomen. Low intestinal obstruction is seen in conditions involving the ileum, such as meconium ileus and ileal atresia, and pathology of the colon including Hirschsprung disease (HD), meconium plug/small left colon,
colonic atresia, and imperforate anus. Contrast enemas are helpful in determining whether the colon or distal small bowel is the cause of low intestinal obstruction. In many cases of a high obstruction, the patient is taken to surgery after the plain film is obtained. If there is a concern for malrotation, an upper GI (UGI) study should be performed. In cases of low intestinal obstruction, plain film followed by enema is recommended (5).

Pneumoperitoneum: Causes of pneumoperitoneum in the child include necrotizing enterocolitis (NEC), spontaneous perforation not associated with NEC, ruptured Meckel diverticulum, dissection from pneumomediastinum, gastric perforation (e.g., from a nasogastric tube, mechanical ventilation, indomethacin administration, or child abuse), colonic perforation (possibly secondary to an enema or a rectal thermometer), peritoneal dialysis, paracentesis, recent surgery, and abdominal trauma (6). In older children, perforation in appendicitis, Meckel diverticulitis or ischemic or infective enteritis, and penetrating and blunt abdominal trauma are common causes of pneumoperitoneum (7). Locations of gas in the abdomen include intraluminal, extraluminal, intramural, and intraparenchymal. Free intraperitoneal air (extraluminal) is seen in nondependent areas (subdiaphragmatic along the liver dome, or along the lateral margin of the liver in a left lateral decubitus view) (Fig. 102.8A,B). Portal venous gas can be seen as branching linear lucencies along the periphery of the liver (Fig. 102.9). Gas in the biliary tract is more central overlying the liver with a branching tree pattern (8).

Pneumatosis intestinalis: Pneumatosis intestinalis (PI) is when air accumulates within the bowel wall. It is a well-known radiographic finding in cases of NEC, where it portends bowel infarction. Pneumoperitoneum and portal venous gas may be accompanied with PI. PI can also appear in intestinal ischemia, bowel obstruction, graft-versus-host disease, short bowel syndrome, infection with rotavirus, decompensated cardiac disease, and nonischemic colitis (9,10). A more benign occurrence of pneumatosis can be seen in chronic steroid use, immunodeficiency, nutritional deficiencies, bronchopulmonary dysplasia (BPD), or pneumomediastinum. On radiographs, pneumatosis may appear as small linear or bubbly lucencies within the bowel wall (Fig. 102.10).

Abdominal masses: The appearance of a soft-tissue opacity or mass is a common finding in a child with abdominal pathology. Plain abdominal radiographs are the initial imaging tests performed in the evaluation of a suspected abdominal mass. The radiographs may help determine the location of the mass, whether there is associated bowel obstruction, and whether calcifications are present. Calcifications are seen in several types of tumors including neuroblastoma and teratoma. Calcifications could also represent stones in the biliary or renal tracts. Sonography is a very useful adjunct study in the workup of abdominal masses.

The UGI series is often modified so that it both answers the clinical question and limits the radiation dose (4,5). An UGI study includes the esophagus, stomach, and duodenum to the duodenojejunal junction and is adequate to assess malrotation. Barium is the most commonly used contrast agent and is excellent for delineating GI tract anatomy. A small-bowel follow-through (SBFT) is used to assess the small bowel beyond the duodenum to the cecum, using serial imaging over several hours (Fig. 102.11A,B). Low osmolar, nonionic contrast agents (such as iohexol) are important and have clinical uses, but are much more expensive. These nonionic agents are used when there is concern for possible leak, as in the case of a repaired TE fistula. Hyperosmolar contrast (i.e., gastrografin) can be therapeutic in cases of meconium ileus, meconium plug, and chronic constipation. Water-soluble agents should be used if there is a potential for perforation.

Ultrasonography is a key imaging tool with important applications in diagnosing abdominal pathology in children in the PICU. It is usually the first investigation in a child with a palpable abdominal mass after a plain film. In addition to identifying the solid or cystic nature of the mass, determination of the organ of origin may be assessed. US can be used to evaluate the relationship of the superior mesenteric artery (SMA) with the superior mesenteric vein (SMV) in cases of malrotation (11,12). High-resolution US can be used to evaluate the bowel wall and to detect pneumatosis and portal venous gas. US is the modality of choice to confirm the presence of absence of intussusception. US is also an excellent tool to assess for ascites and can be used to guide paracentesis (Fig. 102.12). The lack of ionizing radiation, easy portability, and availability make it an ideal modality for use in children. The disadvantages of US include a potentially limited window in the presence of various catheters, tubes and dressings, interference by overlying bowel gas, as well as operator dependence.

Concern for an increased risk of cancer in children exposed to radiation during CT makes it a less favored imaging modality. CT is performed only when radiographs or US do not diagnose the condition. CT is more useful than US in cases of trauma and to delineate the extent of visceral and intraperitoneal abscesses. The benefits of CT are excellent spatial and contrast resolution; newer scanners are fast enough that the patient may not need sedation. Younger patients receiving IV contrast for the CT often require sedation despite the use of fast scanning techniques because they tend to move when the contrast is administered. When ordering CT imaging, one should consider alternative imaging modalities not involving ionizing radiation such as US or MRI. Research is currently being done to provide CT scanner algorithms that decrease radiation dose while maintaining images of diagnostic quality (14,15). High scanning speed can decrease motion artifacts and at the same time decrease the radiation dose to the patient.

Improved detail due to better soft-tissue contrast and lack of ionizing radiation make MRI an important problem-solving
tool. The limitations for MRI are its cost, availability, and motion artifacts due to long scanning time. Various fast imaging techniques can reduce motion artifact. Techniques such as MR cholangiopancreatography to evaluate biliary and pancreatic abnormalities, MR enterography for gastrointestinal pathologies, and MR urography for evaluation of renal collecting system abnormalities and renal function are available. Diffusion-weighted sequences are used widely for imaging of neoplasms (16).