Contrast radiography made simple: GI tract (Proceedings)
The inherent poor contrast within the abdomen and the fact that soft tissue and fluid can not be differentiated radiographically means that contrast media are required for assessment of luminal surfaces, and therefore wall thicknesses of the gastrointestinal tract. Contrast studies are most commonly performed to identify anatomy not visible on plain radiographs and to evaluate the integrity the hollow viscus organs . They may be used to determine function, as in gastric emptying and small intestinal transit.
Positive contrast media includes barium and iodinated options. The choice may depend on clinical, cost and availability limitations. The most important variable is the clinical situation.Barium is very, very opaque and very inexpensive. We depend on barium contrast media for routine studies of most components of the gastrointestinal tract. Barium is inert, relatively palatable, has no osmotic potential and coats the mucosal of the gastrointestinal tract very well. Barium may be mixed with food for esophageal studies, although this will compromise a subsequent upper GI series. Barium is formulated as a suspension is not osmotically active.
Barium sulphate comes in a variety of centrations, normally classified by "weight-to-volume" (w/v). We usually dilute the concentration to make it 60% w/v for esophageal or 30% w/v for gastric and intestinal studies. Aspiration of barium has morbidity associated more with the volume and concurrent acidic gastric contents, then from any direct effect of the barium suphate. Barium is relatively contraindicated in species with extremely slow transit time (especially reptiles) and with known gastrointestinal tract rupture.
Iodinated contrast media are based on attachment of the iodine molecule to benzene ring compounds. These tri-iodinated monomers or dimmers protect against many adverse reactions of unprotected iodine in the body. Iodinated media are water soluble and therefore osmotically active. Iodinated agents are preferred if endoscopic evaluation of the GI tract is being consider after the GI radiographic contrast study.
High-osmolar iodinated contrast media
These are less expensive than low-osmolar agents. There are contraindications to the high osmolarity and when used as a gastrointestinal agent the contrast material becomes more dilute as it passes along the GI tract. This may be clinical important in dehydrated or neonatal patients. The high osmolarity irritates the GI mucosa and GI transit times are faster with iodinated agents, compared to barium agents. Aspiration of high osmolar agents results in severe pulmonary edema. These agents are diluted by their osmolar effects pulling fluid in from the interstitium into the lung alveoli.
Low-osmolar iodinated contrast media
Both ionic and non-ionic agents are available in this category of ionic agents. These agents result in fewer adverse effect including pulmonary edema if aspirated. Low-osmolar agents are preferred for the GI tract because they are not diluted as they pass through the GI tract.
Survey radiographs provide the basis of our contrast imaging of patients. In the abdomen we gain a tremendous amount of information regarding the gastrointestinal tract; overall dimensions, content and evidence of regional disease. Survey radiographs fall short of providing a definitive diagnosis when our confidence is low, there are ambiguous radiological findings, the radiological findings conflict with the clinical signs or the disease is entirely occult on survey radiographs.
Esophagagraphy is very useful to identify a cause of regurgitation or to rule-out a possible stricture, perforation or foreign body. Other differentials include broncho-esophageal fistula, hiatal hernia and gastroesophageal hernia. Verify location of pulmonary mass in relationship to esophagus. A dose of 1 ml/10 lb body weight of barium suspension per os is usually sufficient. However, extravasation of barium into the mediastinum is a relative contraindication and may make the use of a nonionic iodinated contrast safer. The contrast is administered per os, trying to avoid overdosing while the patient alternatively breathes and swallows. Radiographs should be taken within 5 sec of start of contrast administration. Whilst this study is still "state of the art" for functional pharyngeal disorders, these studies are dynamic and require videofluoroscopy and slow motion frame-by-frame analysis for complete evaluation.