Gastrointestinal Contrast Media

Monday, March 20, 2017

Gastrointestinal Contrast Medium Agents

    When performing a CT imaging study of the gastrointestinal tract, contrast medium is important to distinguish loops of bowel from a pathology like cyst, abscess, or neoplasm. For this reason, oral contrast material is used in most CT scans of the abdomen and pelvis. For some indications, the rectal administration of contrast material is very useful.
    In general, contrast medium is classified as positive if appears white on the image, and negative if it appears black on the image. The most common definition classifies gastrointestinal agents as positive or negative depending on the density of material relative to the walls of gastrointestinal tract. For this example, by this definition water is considered a negative agent, because with an Hounsfield Units (HU) of zero, it is less dense than the wall of the gastrointestinal tract. Less commonly, contrast media is classified in accordance to its Houndsfield Units (HU); agents with positive Hounsfield Unit values are called negative agents, those with negative HU values are called negative agents. Using this definition, water is considered a neutral agent.
    Options available in oral preparations include barium sulfate solutions, or iodinated water soluble agents. Options available for rectal preparations include air carbon dioxide, barium sulfate, or iodinated water soluble solutions. The ideal agent should provide adequate differentiation of bowel from surrounding structures without creating artifacts. The image below demonstrate the use of water, a low Hounsfield units (HU) barium solution, a standard barium sulfate solution, and an ionic, iodinated agent to brighten / highlight the gastrointestinal tract.

gastrointestinal contrast
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

    The barium suspension used on conventional radiography cannot be used in CT examinations. Because this such full strength solutions 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, products are available specifically of use in CT studies. The most commonly used are positive agents that contains a 1 to 3% of barium sulfate suspension and 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 use.
    A higher dose of oral contrast material provides greater bowel opacification. 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 to fill the stamach and proximal small bowel.
In patients who cannot take fluids by mouth, a nasogastric tube may be used and inserted to them. The contrast medium can be introduced through the tube. If vomiting is a problem slowing the rate of administration may help.
    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 for this purpose are recommended.

Disadvantage of Positive Contrast Media

    One of the disadvantage of positive contrast media is that they make mucosal surfaces 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 3-dementional modeling. 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. Hence, 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.

Barium Sulfate 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 intravasation.

    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, 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 increased expense. 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 abnormally is suspected, rectal administration of contrast material may be necessary. In these cases, 150 to 200mL of dilute water soluble contrast agent at about 1-3% can be given by enema.

Comparison of Positive Oral Contrast Media

    Barium sulfate and water soluble contrast material 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. As a negative or neutral contrast agent, water will not obscure mucosal surfaces, or superimposed abdominal vessels on 3-dimentional 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
Automated carbon dioxide insufflation system
for CT colonography. Picture courtesy of the Bracco
Diagnostics, Inc.

    Both air and automated carbon dioxide provide reliable colonic distention. However, carbon dioxide has some advantages over room air in that it 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.

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