Overview

Evaluation of the GI system includes the hollow viscera (from the esophagus to the rectum); solid viscera (the liver, spleen, and pancreas); and the peritoneal cavity and retroperitoneal spaces, in which all entities are contained. Although considerable overlap can exist among various modalities and newer applications continue to be defined, plain films usually are the initial imaging modality used in assessment of the GI system.

Chest radiographs can point out some abnormalities of the esophagus, such as achalasia and, notably, esophageal atresia, which requires no further imaging for diagnosis. Abdominal radiographs can assess calcifications, which would be expected in cases of meconium peritonitis or can be present in abdominal masses such as a hepatoblastoma or in persons with appendicitis. Intramural air, free intraperitoneal air, portal venous air, and the double bubble sign of duodenal atresia are identified on plain films and point to the correct diagnosis in the appropriate clinical setting. Inflammatory conditions such as Crohn disease may be suspected by assessing the gas pattern. Dilated bowel loops typically direct the radiologist toward consideration of an ileus pattern or obstruction. In such cases, decubitus views and prone cross-table lateral views of the rectum can help differentiate between these possibilities and direct further diagnostic imaging.

Air is the inherent contrast medium in plain film diagnosis, and the abdominal series is based on distribution and movement of gas. The basic plain film evaluation typically consists of supine and horizontal-beam images. Left-side-down decubitus and upright views are used for evaluation of free intraperitoneal air and air-fluid levels. The left-side-down decubitus view further directs air toward the right colon for evaluation of the right lower quadrant and into the rectum for evaluation of obstruction. Obtaining a film with the patient in the prone position is more effective in directing gas to the rectum for assessment of its caliber when concern exists about the possibility of a bowel obstruction; a horizontal-beam film (cross-table lateral) of the rectum with the patient in the prone position is particularly helpful in these cases.

The mainstay of further assessment of suspected pathology of the hollow viscera remains fluoroscopy, although sonography, computed tomography (CT), scintigraphy, and, increasingly, magnetic resonance imaging (MRI) also are applicable and will be discussed later in this chapter. To limit the radiation dose, fluoroscopy should be intermittent; pulsed fluoroscopic techniques can decrease the radiation dose substantially without loss of clinical information.1,2 Capture and storage of the fluoroscopic images can be used liberally to document such findings as viscus distension, course of contrast, and peristaltic activity, with spot films reserved for areas in which greater anatomic detail is of diagnostic importance, such as mucosal abnormalities and potential perforation with contrast leaks.

Fluoroscopic studies typically require use of enteric contrast material for diagnosis.3,4 Barium, an inert substance that is not absorbed, remains the primary contrast medium used in fluoroscopic procedures, whether it is administered orally to evaluate the esophagus and upper GI tract or rectally in a contrast enema. Several barium preparations are available; barium sulfate powder (96% wt/wt) can be diluted with sterile water for infant upper GI examinations to the desired concentration of 40% to 60% wt/vol. Premixed suspensions (60% wt/vol) can be used in older children, adolescents, and adults. Enema kits containing 97% barium wt/wt can be mixed with water to a final concentration of 15% to 33% barium wt/vol for infants, older children, and adolescents. Although adverse reactions to barium products are rare—reportedly 2 per million or less⁵—they do occur and may present as a rash, loss of consciousness, and anaphylaxis, typically related to any one of several additives, such as methylparaben and carboxymethylcellulose.^5–7 Aspiration of barium in small quantities is tolerated, but aspiration of a large volume of barium can be fatal.^8,9

Barium is contraindicated in cases in which viscus perforation is suspected. In such cases, a low-osmolality, nonionic, water-soluble iodinated medium such as iohexol is used (Table 85-1). It is important that hypertonic media such as ionic or high-osmolality media (e.g., diatrizoate or iothalamate) not be used orally because of the risk of aspiration and consequent pulmonary edema.^10–12

Gastrografin (diatrizoate meglumine and diatrizoate sodium) is an ionic, markedly hypertonic iodine solution with an osmolality (mOsm/kg) of approximately 1600; a 1 : 5 dilution approximates serum osmolality (285) but also dilutes the iodine concentration. Ionic hyperosmolar media can be absorbed from the GI tract and thus pose a risk in patients with a history of hypersensitivity, particularly to iodine, and potentially in patients with thyroid disease. Hyperosmolar media cause severe pulmonary complications of edema and pneumonitis if aspirated and can cause major fluid shifts into the bowel lumen, leading to a decrease in intravascular volume, an increase in serum osmolarity, and a decrease in cardiac output. In patients with underlying bowel disease, additional injury is possible.13,14

Omnipaque (iohexol) is a nonionic water-soluble iodinated contrast medium that is available in concentrations of 140, 180, 240, 300, and 350 mg of iodine. It is poorly absorbed from the intact GI tract, with renal excretion of 0.1% to 0.5% of the administered dose. Isovue (iopamidol) also has been used in the evaluation of the pediatric GI tract, but currently only Omnipaque is officially approved for this purpose. It must be emphasized that the osmolality of both of these media is greater than that of blood and that no agent is safe in the tracheobronchial tree, and thus great care and close fluoroscopic monitoring is necessary in all patients in whom aspiration is a potential complication.¹⁵

Barium is the standard agent used in the evaluation of the colon. However, in cases of potential perforation, water-soluble agents are used and can be diluted to approximate the tonicity of serum. Higher osmolality contrast media are used rectally for therapeutic purposes in cases of uncomplicated meconium ileus after diagnosis with a low-osmolarity agent. Gastrografin (diatrizoate meglumine and diatrizoate sodium) was the original agent described for this purpose.¹⁶ However, this agent can be associated with large fluid shifts and systemic complications in severely ill infants.¹³ Full-strength iothalamate meglumine 30% also can be used successfully for this purpose. Close attention to water and electrolyte balance, along with surgical standby, are mandatory.

Air also can be used during fluoroscopic procedures. For example, air provides an excellent way to distend a viscus during fluoroscopic transpyloric tube placement without obscuring the tube or adjacent bowel loops, and it is the preferred agent in the reduction of intussusception.

Specific procedures are outlined in the following sections, and indications and imaging protocols are discussed.

Indications and Protocols

Overview

Sonography has become an extraordinarily useful modality in the evaluation of patients with GI symptoms. In addition to the availability of spectral as well as frequency shift and amplitude color Doppler imaging, advances with newer equipment include recording of motion cine images, harmonic imaging, extended field-of-view imaging, and three-dimensional (3D) capability.

Indications and Protocols

The utility and application of sonography in pediatric patients is extensive. The major advantage of sonography over CT is the obvious lack of ionizing radiation, but other very important advantages include the lack of need for sedation, and the multiplanar imaging capabilities. In general, scanning should be performed with the transducer that has the highest frequency necessary to penetrate the anatomy to be imaged and that will allow optimal spatial resolution; linear transducers are preferred if the access window will allow it. Curved array transducers allow a broader field of view; sector transducers may be necessary when the access window is limited or to image deeper structures in larger patients.

The primary role of sonography in the diagnosis of pyloric stenosis has become firmly established. Sonography is also extremely useful in the assessment of patients with clinically equivocal symptoms of appendicitis, although this setting punctuates its well-known operator dependence (e-Fig. 85-2), with published sensitivities ranging between 40% and 100%.^17–19 Sonography also is extremely useful in the evaluation of mesenteric adenopathy (e-Fig. 85-3), in highly detailed assessment of the bowel wall (Fig. 85-4),²⁰ in evaluation of small and large bowel intussusception (e-Fig. 85-5),²¹ and coupled with Doppler, in the effective estimation of disease activity in patients with Crohn disease.²²

The role of sonography in the diagnosis of solid organ pathology is likewise extensive and can be the final diagnostic tool for many abnormalities. Sonographic detail is particularly well visualized in young children, in whom high frequency and linear transducers can be used to access even the deeper abdominal structures. Indications include evaluation of biliary tract abnormalities, such as choledochal cysts, although additional imaging may be performed with MR depending on the specific clinical circumstances. Sonography may be performed as the initial modality to investigate suspected liver masses, and any subsequent CT or MRI protocols can be tailored on the basis of sonographic findings.

Although vascular structures are seen easily with contrast-enhanced CT, the direction and velocity of flow can be evaluated with Doppler sonography. Analysis of waveform pattern can identify hepatofugal flow in collateral vessels in patients with portal hypertension, along with vascular stenosis or thrombus. In patients with heterotaxy, abdominal sonography is helpful in assessing a splenic mass (located along the greater curvature of the stomach) and the associated vascular anomalies, such as interruption of the inferior vena cava, a preduodenal portal vein, and infradiaphragmatic total anomalous pulmonary venous connection23,24 (e-Fig. 85-6).

Overview

CT is a particularly useful modality in pediatric abdominal imaging. The introduction of scanners with multichannel technology and volumetric acquisition permits very rapid examinations with isotropic reconstructions in multiple planes, with decreasing need for sedation.25,26 These new capabilities require development of newer protocols to accommodate more complex and sophisticated diagnostic demands. The timing and rate of contrast administration, with the ability to scan during a specific phase of intravascular contrast distribution, demand particular attention to technical details and new approaches to image interpretation.^27,28 The pediatric radiologist is further challenged by the need to balance image detail with radiation dose and implementation of the ALARA, or “as low as reasonably achievable” concept, with the increasing recognition of the potential risks of radiation exposure for pediatric patients.^29,30 Improvements in equipment aimed at reducing radiation exposure include innovations such as improved collimators and iterative reconstructive algorithms. Although significant challenges persist, much progress has been made through educational and awareness-raising social marketing campaigns such as the Image Gently Campaign of The Alliance for Radiation Safety in Pediatric Imaging (www.imagegently.org).³¹

Indications and Protocols

Unlike sonography, CT images are sequential, standardized, and much less operator dependent, and therefore CT is particularly useful in patients with complex disease affecting multiple organ systems, because it provides reliable monitoring of change in the extent of the disease during therapy and follow-up. CT also helps solve problems in patients with unusual multi-organ abnormalities. Evaluation of both intraabdominal and extraabdominal multiorgan pathology can be accomplished with great anatomic detail (e-Figs. 85-7 and 85-8) because evaluation of solid organ, hollow viscera, and peritoneal cavity pathology is rapidly accomplished with great anatomic detail and physiologic information. The relative lack of operator dependence and high sensitivity and specificity of CT in the imaging diagnosis of appendicitis has led to its increasing use when this diagnosis is clinically equivocal, with a documented reduction in negative appendectomies.³² However, this success has led to overuse in patients with abdominal pain; therefore physical examination, followed by ultrasound when the diagnosis is clinically uncertain, is recommended by most pediatric radiologists, with CT reserved for more difficult cases.^17,18 MRI is receiving increasing attention as a viable substitute for CT scanning in many indications, such as inflammatory bowel disease.³³

CT protocols vary and undergo change with the ongoing introduction of new applications and advances in equipment capability; generalizable protocols applicable to pediatric patients can be downloaded at http://www.imagegently.org. However, some underlying principles underscore most successful pediatric examinations. Administration of intravenous contrast material is extremely important, particularly in pediatric patients in whom a paucity of intraabdominal fat decreases intrinsic intraabdominal contrast.^27,34 CT angiography requires a rapid contrast bolus injection, which in pediatrics can be challenging because of the caliber of IV access. Lowering kVp is important in patients in whom high-contrast structures are of interest, such as those undergoing angiography or bone examinations; in neonates, the kVp can be decreased to as low as 80, with some adjustment of the milliamperes-second (mAs) to produce acceptable image quality.²⁹ Precontrast images are seldom necessary and serve to increase the radiation exposure without adding diagnostic information. If necessary (e.g., to identify the presence of calcifications in an abdominal mass), the mAs of the precontrast scans can be decreased significantly and the scan should be limited to the appropriate specific area (e.g., scan only the mass, not the entire abdomen). The use of oral contrast material is usually important when outlining some types of intraperitoneal pathology, such as abscess or masses, but in other cases, its use is more controversial.³⁵ Positive oral contrast material will mask mucosal enhancement; use of water-density contrast material may be more appropriate in such cases.

Despite radiation concerns, CT remains an important life-saving modality in pediatric diagnosis. As with any other tool, it needs to be used judiciously, according to the principles of appropriateness, justification, optimization, and training.³⁶

Overview

MRI increasingly is being used for pediatric GI applications. The introduction and increasing use of imaging at 3 T and advances in parallel imaging and coil design expand the range of pediatric applications. In addition to congenital and acquired hepatobiliary abnormalities, indications for MRI increasingly include bowel abnormalities, such as inflammatory bowel disease and appendicitis.

Patient Preparation and Equipment Requirements

Adequate patient preparation is essential, because MRI often is time-consuming and requires extensive hospital resources. Considerations include the need for sedation, fasting requirements, and the need for oral contrast media.

Preparation begins with assessment of the need for sedation or anesthesia. Generally, children younger than 6 years who cannot hold their breath for 20 seconds will need sedation or anesthesia. Sedation is avoided in patients undergoing MR enterography examinations because of the need to use oral contrast media.

Patients should refrain from oral intake for 4 hours before the examination to ensure gallbladder distension in examinations of the biliary tree, as well as to minimize artifacts from bowel peristalsis. For all cases requiring vascular assessment, power injection of contrast material in appropriate-aged children is optimal and requires adequate peripheral venous access. Finally, once the patient is on the MR table, a respiratory monitoring pillow or belt should be placed with care.

Use of an oral contrast agent is essential for enterography examinations. A number of choices are acceptable, although most regimens consist of a biphasic agent, that is, one that gives the bowel lumen a long T2 and T1 relaxation time. Agents include VoLumen (E-Z-Em, New York, NY), mannitol, polyethylene glycol l, and locust bean gum (a type of galactomannan) solutions. Little difference is seen in efficacy of these choices, although patient tolerance may vary.³⁷ Most important is rapid consumption of a large volume of the contrast agent; 25 mL per kilogram of body weight over an hour is adequate. Placing the patient in the right decubitus position for the final 15 minutes before the start of imaging aids in emptying of the stomach.

Antiperistaltic agents also are required for enterography examinations. Glucagon, the more commonly used intravenous agent, can be used with either of two strategies: (1) half of the dose at the beginning of the examination and half just before administration of intravenous contrast material, or (2) the entire dose just before intravenous contrast material is administered. The dose, from 0.5 mg to 1 mg, varies between facilities. Although glucagon causes nausea and vomiting, it is well tolerated by most patients if it is given as a slow intravenous push over 1 minute and if, before it is administered, the patient is instructed to expect a brief period of nausea.

Equipment specifications are important because of children’s smaller sizes, with the consequent need for improved signal to noise and faster acquisition times to decrease the need for and length of sedation. Although the literature to date is still sparse on pediatric abdominal imaging at 3 T,38,39 increasing experience suggests that most children will benefit from the higher signal. Phased array surface coils are now standard, typically with eight to 32 channels. In the following situations, 1.5 T often provides improved image quality compared with 3 T: when the patient is very large, when ascites is present, when enterography examinations are being performed (1.5 T results in fewer banding artifacts in steady-state imaging), and during hepatic iron quantification.

Indications and Protocols

Hepatic tumors are well evaluated by MRI relative to CT,⁴⁰ with the goal of imaging being tumor characterization, staging, and assessment of resectability. For lesion characterization, determination of the T2-weighted signal and enhancement characteristics is essential.⁴¹ Staging and resectability of tumors (such as hepatoblastomas) require delineation of anatomic boundaries, lymph node involvement, vascular invasion, and delineation of the biliary tree, according to accepted staging systems such as PRETEXT (PRETreatment tumor EXTension) outlined by the International Childhood Liver Tumor Strategy Group.^42,43 Biliary and pancreatic diseases also are well assessed by MRI.^44,45 Common indications include cholelithiasis, pancreatitis, sclerosing cholangitis,^46,47 ductal plate malformations and choledochal cysts,^48–50 and biliary complications of liver transplantation. In the case of liver transplantation, assessment of vascular complications often is essential. Diffuse liver disease, such as fibrosis, steatosis,^51–53 and iron deposition, can be quantified by MRI. Fibrosis has been quantitatively assessed by elastography,⁵⁴ as well as qualitatively by T2-weighted imaging and delayed contrast enhancement.⁵⁵ Although steatosis can be assessed by spectroscopic methods,⁵⁶ more commonly steatosis, as well as iron deposition, are assessed by multi-echo gradient echo imaging.^53,57 MR enterography is most commonly performed for evaluation of inflammatory bowel disease.^58–60