Imaging Techniques: Introduction

Imaging provides significant contribution in evaluation, diagnosis, and follow-ups of pediatric gastrointestinal (GI) problems. Children truly are not merely small adults, and the many unique imaging features of pediatric problems should be recognized. The imaging techniques used to evaluate the pediatric patients with GI symptoms are significantly different from those applied to adults. Radiologists and clinicians should work in concert to select the most appropriate modality, as well as optimal timing of the examination to maximize the benefits and minimize the risks and costs. When imaging modalities requiring radiation are selected, the “as low as reasonably achievable” (ALARA) principle should always be observed to minimize the radiation exposure.1

Radiographs

The technique of using X-rays to evaluate disease is over a century old, having been demonstrated first by Professor Wilhelm Conrad Roentgen in 1895, using a cathode ray tube and photographic emulsion. Despite its antiquity, radiography continues to be used as the first-line imaging technique to evaluate various abdominal conditions, providing important clues directing subsequent workups. Modern X-ray equipment is vastly different from those used during the first decades of development, and uses sensitive digital imaging techniques to minimize exposure to ionizing radiation.

The X-ray machine can be positioned above a supine patient (vertical beam), or can be directed horizontally. Radiographs using a horizontal beam technique are essential to detect a small amount of free air (pneumoperitoneum). This can be most easily accomplished in many patients by obtaining erect frontal films. Patient who cannot stand up may have either decubitus frontal or supine cross-table lateral views. Free air can be detected at the highest part of the peritoneal cavity, under the diaphragm on an erect frontal projection, along the non-dependent flank on a decubitus frontal projection (Figure 9–1), or along the anterior abdominal wall on cross-table lateral projection. Larger amounts of free air can even be diagnosed on routine supine frontal views (using a vertical beam) by outlining intestinal walls sandwiched between the intraluminal and extraluminal air (“double wall” or Rigler sign). Other structures that can be seen in this situation include the falciform ligament of the liver and occasionally the umbilical arterial ligaments. In the neonate, a large amount of free air can dissect into the scrotal cavity on one or both sides through an incompletely closed processus vaginalis.2,3

The radiological hallmark of necrotizing enterocolitis is multiple air bubbles in the intestinal wall (pneumatosis intestinalis) (Figure 9–2). It can be seen as bubbly lucency along the intestine or curvilinear/ring-like lucency.4 The intramural gas eventually finds the way to the portal veins via the mesenteric veins (Figure 9–3). When the findings are equivocal, consider sonographic observation of the liver. Air bubbles carried by the mesenteric veins circulating into portal veins can be demonstrated even before visualization of portal venous gas on radiographs. Once the diagnosis of necrotizing enterocolitis is established and treatment has initiated, sequential follow-up abdominal radiographs, usually a single right-side-up decubitus frontal view every 12 hours, are obtained to detect perforation. Free air can be seen in the non-dependent peritoneal cavity along the right lateral flank against the soft tissue window provided by the liver.

Congenital upper GI obstruction, such as duodenal and proximal jejunal atresia, is often detected by in utero ultrasound (US) demonstrating dilated, fluid-filled intestinal loops with associated polyhydramnios. In distal ileal atresia (Figure 9–4), postnatal radiographs show progressively dilating intestinal loops. Distinction from colonic obstruction or from Hirschsprung’s disease (see Figure 19–6) requires a contrast enema, because in neonates, differentiation between dilated small intestine and colon is often impossible.3 In acquired mechanical obstruction, radiographs show progressive intestinal distention with multiple air–fluid levels, with the absence of distal gas. One should be cautious that, in case of strangulated, vascular-compromised mechanical obstruction, dilated segments of affected intestine contain large amounts of fluid without significant gas (fluid-filled loops). When this condition is suspected, sonography can be used for verification.

In acute abdominal conditions, one should look for calcified lesions, including appendicoliths, gallstones, renal/ureteric stones, and calcification/ossification of pelvic masses. A typical appendicolith is seen in the right lower quadrant as a round, laminated calcification. A diagnosis of acute appendicitis can be made in these patients, when they present with appropriate clinical and laboratory findings. Many patients with incidentally found appendicolith and without symptoms of appendicitis undergo elective appendectomy because of high prevalence of rupture once appendicitis occurs. Torsion of the ovary containing dermoid or teratoma should be suspected when formed teeth or small skeletal structures are seen in the pelvis in a female with acute lower abdominal pain. Sonography can confirm the diagnosis. When gallstone and renal/ureteric stones are suspected, sonography can verify the diagnosis.

Upper Gastrointestinal Series

Preparation for a barium upper GI series for children is minimal, withholding feeding for 2–6 hours, depending on the age3 (Table 9–1). Digital low-dose pulsed fluoroscopy with “last screen capture,” small field size, and gonadal shielding minimize the radiation exposure. Barium is the contrast material of choice because of its superior contrast, mucosal coating, and biological inertness. When perforation is suspected, low-osmolar non-ionic water-soluble contrast is used. High-osmolar contrast material such as Gastrografin (osmolarity of 1900 mmol/L, which is approximately six times the normal serum osmolarity) should not be used because of its potential to cause a significant fluid shift, resulting in hypovolemia, or if aspirated, severe pulmonary irritation.

Small Bowel Follow-Through

Small bowel follow-through is performed by having the patient ingest additional barium after routine upper GI evaluation, and watching its progress as motility carries it through the entire small bowel. The most common indication is suspicion of Crohn’s disease. Other indications include evaluation of polyps, malabsorption, protein-losing enteropathy, and intestinal dysmotility.

Sequential overhead radiographs are taken every 30–60 minutes, supplemented by fluoroscopic observation and spot radiographs when abnormalities are suspected. Occasionally, continuous infusion of barium into the small bowel through a nasoduodenal or nasojejunal tube (enteroclysis and small bowel enema) is performed to evaluate subtle obstructive lesions, such as multiple strictures and adhesions, which are difficult to be elucidated otherwise. Enteroclysis is particularly helpful when surgical intervention is contemplated in patients with incomplete small bowel obstruction.

Contrast Enema

Barium provides excellent contrast and it is most commonly used unless there is a contraindication, such as the possibility of perforation. Because of its poor contrast density and high osmolarity, hyperosmolar water-soluble contrast material such as Gastrografin (sodium/meglumine diatrizoate) has only limited use in the pediatric age group.

Lower osmolarity, non-ionic water-soluble contrast agents (such as Isovue/iopamidol) are commonly used instead of barium when there is concern of perforation. Water-soluble, low-osmolar contrast should also be used to evaluate a segment that has limited function, such as colon downstream from a colostomy, or a region with diminished motility. In these situations, barium may become inspissated. When neonatal distal intestinal obstruction is suspected, a barium enema should be performed to differentiate colonic from distal ileal obstruction.3 Hirschsprung’s disease is characterized by a small rectum and dilated proximal colon above the aganglionic segment (see Figure 19–6), while distal ileal obstruction such as ileal atresia and meconium ileus would show unused small colon (microcolon) (Figure 9–4).

The traditional use of contrast enema for the evaluation of polyposis syndromes and colonic inflammatory bowel disease including Crohn’s disease and ulcerative colitis has been in major part replaced by endoscopic evaluation, including capsule endoscopy of the small intestine (see Chapter 10).

Sectional Imaging

US is a highly useful sectional imaging modality in evaluation of pediatric GI disorders. In some cases, US provides diagnostic findings, and in others it can provide information useful in formulating the differential diagnosis. US can be performed at the bedside and, in contrast to computerized tomography (CT), is free of ionizing radiation. It does require an expert operator, capable of directing the US probe at the desired structures, recognizing abnormalities, and recording the selective images. This often limits the full utility of US in many smaller facilities. Nevertheless, US should be considered as the first study when sectional imaging is required in pediatric patients.

CT provides exquisite anatomical information without the factor of operator dependence that is problematic in US studies. However, we should recognize that CT is the largest source of ionizing radiation in diagnostic imaging. Genetic effects and risk of carcinogenesis due to ionizing radiation is proportional to the dose, without any threshold effect. Further, the radiation risk for children is much greater than that of adults because of higher tissue radiosensitivity, a longer lifetime in which to manifest radiation-induced injury, and the cumulative effects of repeated examinations. The typical radiation dose from an abdominal CT study is approximately 500 times that of a single chest radiograph, and is equal to about 2 years of background radiation exposure. Thus, clinicians must be cautious about ordering CT studies. When CT is performed, appropriate pediatric technical parameters should be used, rather than using adult techniques for children.

Magnetic resonance imaging (MRI) is another imaging modality that, like US, does not use ionizing radiation. It is increasingly used in place of CT with recent advancement of faster data acquisition techniques and widespread availability. The basic principle of MRI is the interaction between applied magnetic field and hydrogen atoms in water in the body. T1 weighted and T2 weighted are two commonly used sequences. T1-weighted images best demonstrate anatomy, while T2-weighted images best demonstrate pathology. On T1-weighted images water appears dark; this is the reason for low T1 signal of most pathologies such as infection, infarction, or tumors. Only few tissues can produce high signal on T1-weighted images, for example, fat, methemoglobin, and cyst with proteinaceous contents. On T2-weighted images water appears bright; hence, most pathologies appear bright on this sequence as they have increased water content. Hemosiderin appears low signal on T2-weighted images. Air, calcification, fast-flowing blood, and cortical bone do not produce any signal on T1/T2-weighted images and appear black. Contrast material is required to increase the contrast between normal tissue and pathology. Gadolinium (Gd) is the most commonly used MRI contrast media that increased the signal intensity of water molecules on T1-weighted images. Gd is mainly used when tumors and infection are suspected. Chemical shift imaging (in-phase and opposed-phase image) is another technique mainly used for liver and adrenal imaging. On in-phase images, the signal from both water and fat protons contributes to tissue signal intensity. On opposed-phase images, signal from fat protons cancels that of water protons. In other words, signal of fat drops out on opposed-phase images. This technique is used to detect focal or diffuse fatty liver infiltration, fat-containing liver tumors, and adrenal adenomas. Because MR image acquisition is much slower than CT, requiring the patient to remain perfectly still for the best images, and because the machines tend to be noisy and can frighten young children, sedation or anesthesia is commonly required for young children undergoing MRI.

Nuclear Medicine

In the workup of lower GI bleeding the “Meckel’s scan” using 99mTc-pertechnetate is often used. 99mTc-pertechnetate is taken up by gastric mucosa and, because about one-half of the Meckel’s diverticulum also contains gastric mucosa, it can be demonstrated (Figure 9–5). When performed non-emergently, pretreatment of the patient with an H2 blocker can improve the sensitivity of this test. Intestinal duplications containing gastric mucosa can also be detected.