Overview of Imaging of the Upper Gastrointestinal Tract
Even after a careful history and physical examination, imaging of the gastrointestinal tract is often required to make the correct diagnosis. Risk factors involved and the cost-benefit ratio should be taken into consideration before proceeding with imaging in lieu of prolonged careful observation or a therapeutic trial. The presence or absence of “alarm signs and symptoms” can aid this decision, including weight loss, nausea, vomiting, fever, age over 45, and evidence of bleeding, including a positive test for occult bleeding, gross bleeding, or anemia. Having particularly severe or worsening pain should increase consideration of further testing, as should pain that does not respond to simple measures or that interrupts sleep. Imaging the upper digestive organs can be achieved by endoscopic or radiographic techniques. Endoscopic evaluation is often the procedure of choice.
Radiographic testing includes noncontrast x-rays, contrast fluoroscopic testing, and cross-sectional imaging. Cross-sectional imaging of the chest, esophagus, and upper digestive tract is performed most commonly with CT scans. Other commonly used cross-sectional imaging technologies include magnetic resonance imaging (MRI) and positron-emission tomography (PET). These techniques are best suited for examining lesions in the wall of the digestive tract, liver, or pancreas or when there is suspicion for lesions extrinsic to the lumen. Arteriography is a commonly used diagnostic and therapeutic radiographic study valuable for evaluating tumor invasion and the site of bleeding. Therapeutically, it is used to treat persistent bleeding sources, to embolize, to deliver chemotherapy to malignant lesions, or to repair surgical anastomotic strictures.
Ultrasonography, with or without Doppler studies of blood flow, is a very valuable means of evaluating the liver, gallbladder, and pancreas and, in selected patients, the intestines. It is rarely of value, however, in examining the esophagus, stomach, or duodenum. Ultrasonography performed by a specially trained endoscopist is the procedure of choice for diagnosing intramural lesions and for staging tumors of the lung and gastrointestinal tract for possible mediastinal or celiac nodes. Plain, noncontrast imaging of the abdomen and chest can provide valuable diagnostic information in selected esophageal and gastric disorders. A plain film of the abdomen, often described as a flat plate or image of the kidney, ureter, and bladder (KUB), can be useful in the evaluation of intestinal gas or pancreatic calcification. The addition of an upright film of the abdomen and chest is often ordered as part of an obstruction series. Delayed gastric emptying is suggested if the gastric image is pronounced, but a definitive diagnosis will require additional studies. Achalasia should be considered when the chest x-ray shows a widened mediastinum with an air-fluid level and no gastric air bubble. If a perforation is suspected, an obstruction series is urgently needed and is essential to look for mediastinal air or free air under the diaphragm in the upright films. An obstruction series, including neck and chest x-rays, should also be the first step in the evaluation of a patient with suspected foreign body ingestion. In both situations, however, if these films are negative but suspicion is high for a perforation or persistent foreign body, further cross-sectional imaging with a CT scan should be performed before barium or endoscopic studies are performed.
Contrast Imaging Of The Upper Gastrointestinal Tract
Because most disorders of the upper digestive system are epithelial in origin, the most common imaging studies ordered for evaluation of the esophagus or stomach are fluoroscopic x-ray contrast studies or endoscopic studies. The professional fee for each is similar, but the associated technical fees are high for both, vary greatly from ambulatory settings to hospital settings, and should rarely be used to determine which study to choose. Contrast studies have no risk except in a patient with an underlying perforation (see below). Contrast fluoroscopic tests must be performed in a radiology department, so a patient must be able to be transported to that area. The patient must be able to swallow or have a nasogastric tube and should be able to cooperate with the radiologist’s efforts to position the patient. Contrast radiography is in some respects an art, and its accuracy depends considerably on the operator’s skill.
When ordering fluoroscopic radiographic imaging, one must determine which swallowed contrast material is to be used. Barium should always be the contrast agent of choice when there is concern about the risk for communication between the digestive tract and the airway, either with aspiration or by fistula. Use of a water-soluble contrast agent such as gastrografin in such circumstances could lead to sudden pulmonary edema and bronchospasm, with dire complications. In contrast, water-soluble contrast such as gastrografin should always be used first to evaluate the esophagus or stomach for the presence of a perforation. Barium leaking into the chest in esophageal perforation or the abdomen in gastric perforation is very difficult to remove and may later serve as a nidus for infection. Techniques used add air to create a double-contrast image and markedly increase the accuracy of the examination.
Contrast imaging of swallowing disorders is particularly valuable when transfer dysphagia is considered or when a lesion in the neck or high in the esophagus is suspected.
Cross-Sectional Radiographic Imaging Of The Upper Digestive Tract
CT scans are invaluable in the evaluation of unexplained pain, liver lesions, pancreatic lesions, and lesions that extend from or into a luminal organ. This includes thyroid, lymphatic, vascular, or lung lesions impinging on or invading the esophagus. Organs adjacent to the stomach and duodenum can impinge onto these upper gastrointestinal organs or be invaded by disorders originating from them, including the aorta, kidney, pancreas, gallbladder, and liver. CT scanning is also invaluable in the evaluation and staging of most cancers.
MRIs are more accurate in characterizing vascular lesions, including aneurysms, and tumors invading arteries and veins, including epithelial and stromal tumors, for staging of cancers and determining whether they are surgically resectable. MRI can also provide highly accurate images of the biliary tree and pancreatic duct (magnetic resonance cholangiopancreatography [MRCP]).
PET scanning is a functional imaging technique similar to CT scanning that uses various short-lived radioactive tracers, most commonly fluorodeoxyglucose. The radiation exposure is similar to that of CT scans. PET scans are often able to identify metastatic lesions that are missed or incompletely characterized by CT or MRI studies.
All three cross-imaging techniques require transporting the patient to the device and variable levels of patient cooperation. This is not an issue for ambulatory patients, but it becomes a major issue and risk to patients who are severely ill, particularly those in an intensive care unit. The patient must lie quietly and be able to tolerate being moved into the confined space of the circular device. CT scans and MRIs are often of greatest value when viewed with intravenous contrast, which also adds risk. Iodinated contrast agents are used in CT scans to differentiate vascular structures. When used in timed infusions, they can differentiate the rapidity of filling times and differences in appearance with and without contrast and in the venous phase. Iodinated contrast poses a substantial risk, however, of causing renal dysfunction, particularly in patients with underlying renal disease, diabetes, uncontrolled hypertension, or dehydration. Patients are also commonly allergic to these dyes. A history of allergic reaction to shellfish or to previously administered dyes must be obtained because, although the reaction is most commonly manifested by a rash, it can lead to anaphylaxis. Oral contrast material does not carry any of these risks but can only be used in patients who can swallow effectively or who have a nasogastric tube. CT scanning is usually ordered only rarely in a patient’s lifetime, but some patients undergo multiple scans. There is concern that such repeated x-ray exposures increase the risk for neoplasms, particularly in younger patients. Such cancers do not develop for many years, or even decades, so the magnitude of the risk is incompletely understood.
MRI does not carry a risk for renal injury or neoplasm. Gadolinium-based contrast used in MRI enhances the value of the study by improving images of the vascular structures. This contrast can rarely cause a serious skin injury known as nephrogenic systemic fibrosis, particularly in elderly patients or patients with impaired renal function. During the MRI, the patient passes through a ring; this may be problematic for patients who are claustrophobic or morbidly obese. Open MRIs can be or ered for such patients, but with a sacrifice in accuracy.