Gastrointestinal Contrast Agents
In performing a CT imaging examination of the gastrointestinal tract, the contrast agent is the important medium. It is use to clearly visualize and differentiate the loops of the bowel from a pathology; like cyst, abscess and neoplasm. Oral contrast is always given for a abdominal and pelvis CT examinations, and for some other reasons the rectal contrast administration may be very useful.
Normally, CT scan contrast medium is classified as a positive contrast because it appears white on the image. In contrary, a negative contrast media, that is also frequently use as a combination of two agents, like the air; it will appears black on the radiograph.
For oral preparation, barium sulfate solutions or iodinated water soluble is use, while for rectal administration it can be air, carbon dioxide, barium sulfate and iodinated water soluble. All these contrast material are utilized in gastrointestinal CT exams, but the ideal contrast agent should provide adequate differentiation of bowel from surrounding structures without creating artifacts.
The image below demonstrates the use of water as a contrast enhancement solution, on a low Hounsfield units (HU) barium solution, a standard barium sulfate solution, and an ionic, iodinated agent to brighten / highlight the gastrointestinal tract.
The image below demonstrates the use of water as a contrast enhancement solution, on a low Hounsfield units (HU) barium solution, a standard barium sulfate solution, and an ionic, iodinated agent to brighten / highlight the gastrointestinal tract.
 |
| These images show the various options in oral contrast agents. Water was used for image (A). VoLumen was used for image (B). Barium sulfate suspension was used for image (C). An ionic, iodinated agent was used for image (D). (In all images the patient was also given an IV contrast agent.) |
Use of Barium Solutions
Unlike the barium suspension used on conventional radiography, it cannot be use in gastrointestinal CT examinations. Because this contrast agents has a such full strength solutions that would cause unacceptable streak artifacts. These conventional barium agents cannot simply be diluted for use in CT examination because of their tendency to settle after ingestion. This tendency leads to irregular opacification of the bowel. Fortunately, there are products available for specific use in gastrointestinal CT studies. The most common positive contrast agents that contains a 1 to 3% of barium sulfate suspension, and these are specially formulated to resist settling.
Commercial Product of Barium and its Preparation
Commercial barium preparation like Readi-Cat (E Z EM), Baro-Cat (Mallinckrodt) may include a number of additive to enhance the mucosal coating properties or to improve the taste for oral ingestion.
There are also a higher dose of oral contrast material to provides greater bowel opacification. The timing and correct dosage are largely dependent on the area to be opacified. For most examinations a minimum of 500mL of dilute barium sulfate is given 45 mins to 1 hr before scanning, this is because to fully fill the stomach and the proximal small bowel.
Some patients who cannot take fluids by mouth, a nasogastric tube can be used and inserted to them, and contrast medium can be introduced through the tube. If vomiting is a problem slowing the rate of administration may be helpful.
Some patients who cannot take fluids by mouth, a nasogastric tube can be used and inserted to them, and contrast medium can be introduced through the tube. If vomiting is a problem slowing the rate of administration may be helpful.
The typical low concentration, low viscosity barium sulfate solutions may not be adequate for an esophageal study. In such cases, high viscosity, low concentration, paste designed contrast is use for this purpose are recommended.
Disadvantage of Positive Contrast Media
One of the disadvantage of positive contrast media is that they make mucosal surface more difficult to evaluate after intravenous administration of contrast medium. Another problem is that the density of positive contrast media may create streak artifacts or impede 3D modeling of the image. To overcome these disadvantages a low HU oral barium sulfate suspension was develop (VoLumen, E Z EM). With just 0.1% barium sulfate, the agent resembles water on CT but provides improved distention (as compared with water), faster transit than positive barium sulfate solutions, and more effective visualization of both the bowel wall and the mucosa. On CT images, VoLumen measures from 15 to 30 HU, a density lower than the wall of the Gastrointestinal tract. For this reason, it is most often considered a negative agent as defined by attenuation compared with the bowel wall, but by some definitions it is called a neutral agent.
Contraindication of Barium Sulfate
Barium sulfate should not be given if perforation of the GI tract is suspected. Barium leaking into the peritoneal cavity is referred to a barium peritonitis. The mortality rate from this complication is significant. It can be prevented by substituting a water soluble iodinated oral contrast agent whenever perforation is suspected. Barium sulfate should not be given if perforation of the gastrointestinal tract is suspected. Barium peritonitis and is associated with a significant mortality rate.
Side Effects / Adverse Reaction
When the barium sulfate is introduced, it is an inert substance that passes through the GI tract basically unchanged. Allergic reaction to oral barium solutions are rare. The severe reaction cases report of the product is approximately 1 in 500,000 cases and fatalities in 1 in 2 million cases. It is likely that these reactions can be attributed to the additives in the suspension like flavorings. Although procedural complications are rare, they include aspiration pneumonitis, barium impaction and extravasation.
Although definitive answer are not available, fewer complications from aspiration appear to occur with barium sulfate than with high osmolality iodinated agents.
Iodinated Contrast Agents
Both High Osmolality Contrast Media (HOCM) and Low-Osmolality Contrast Media (LOMC) are positive agents that can be diluted and administered orally. Because of the unpleasant taste of High-osmolality Contrast Media, flavorings is normally added to the solution. A 2 to 5% solution of a water soluble contrast agent is normally used. Even with these dilute solutions and given orally, iodinated contrast agents usually stimulate intestinal peristalsis. Therefore, patients may experience diarrhea after ingestion of water soluble agents. Dosage are similar to those used with barium sulfate. However, water solution oral contrast material tends to pass through the gastrointestinal tract slightly faster.
Comparison on High-osmolality and Low-osmolality Contrast Media
In most situations, High-osmolality Contrast Media is used for oral administration because it is less expensive than Low-osmolality Contrast Media and provides equivalent gastrointestinal opacification. However, in selective cases Low-osmolality Contrast Media has advantages over High-osmolality Contrast Media that justify its higher price. If aspirated, Low-osmolality causes less pulmonary edema than High-osmolality Contrast Media. Researchers of oral contrast medium in newborns have concluded that Low-osmolality Contrast Media offers a significant reduction in complication compared with barium or High-osmolality Contrast Media. Low-osmolality Contrast Media should be used in infants and young children under the following conditions:
- When the possibility of entry of contrast agent into the lung exist
- When the possibility of leaking of contrast agent from the gastrointestinal tract exists.
Studies of older children revealed an additional advantage. Because the Low-osmolality contrast Media has a neutral taste when diluted, patient cooperation is much greater.
When rectosigmoid abnormal function is suspected, rectal administration of contrast material may be necessary. In these cases, 150 to 200 ml of dilute water soluble contrast agent at about 1-3% can be given by enema.
Comparison of Barium Sulfate and Water Soluble Contrast Media
Barium sulfate and water soluble contrast material may cause comparable bowel opacification. Because of the low concentrations used, neither coats the mucosa significantly. Instead, most visible contrast is simply from the agents filling the bowel. Barium sulfate, in small amounts, tends to cling to the intestinal wall, providing a minimum of visible contrast. In comparison, a small quantity of water soluble oral contrast is usually absorbed by the bowel. Therefore, if a patient is able to drink only a small amount of oral contrast, it is preferable to give them a barium sulfate solution.
Water Negative Contrast Medium
As in place of positive contrast media water is sometimes used. It is use as a negative or neutral contrast agent, water will not obscure mucosal surfaces, or superimposed abdominal vessels on 3D images. However, water transits quite rapidly and distends the bowel poorly. It will not provide sufficient detail if the bowel is not fully distended.
Air and Carbon Dioxide
Room air or carbon dioxide can be used to produce a very high negative contrast on images of the gastrointestinal tract. Negative contrast agents are particularly useful in CT colonography when adequate colonic distended segment of bowel may be mistaken for carcinoma. Room air or carbon dioxide is administered via a small flexible rectal catheter. Room air is delivered using a standard handheld air bulb insufflator. This air bulb can be controlled either by the patient or the CT scan technologist. Carbon dioxide is delivered using an automated insufflator system.
 |
| Automated carbon dioxide insufflation system for CT colonography. Picture courtesy of the Bracco Diagnostics, Inc. |
Both air and automated carbon dioxide insufflator provides reliable colonic distention. However, carbon dioxide has some advantages over room air and that is readily absorbed by the body and is eliminated by respiration. It induces less spastic response of the bowel wall and is therefore better tolerated by most patients. Room air can result in significant post procedure cramping and discomfort for the patient. In addition, many CT technologist prefer the carbon dioxide technique over patient controlled room air administration. The main reason is that more time is required to coach patients to self-insufflate, whereas automated carbon dioxide requires relatively little patient education to achieve similar results.
The antispasmodic medication, glucagon hydrochloride, is sometimes given by IV injection to further improve bowel distention.
