Abdominal Computed Tomography Scanning Medication

Updated: Sep 13, 2017
  • Author: Caroline R Taylor, MD; Chief Editor: Mahan Mathur, MD  more...
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Medication

Medication Summary

It is generally accepted that contrast is not indicated in cases of suspected retroperitoneal bleeding or renal stones. Otherwise, in patients who are tolerant, contrast agents are usually used during routine abdominal CT scans to help identify the internal contents of hollow organs, to visualize spaces in the abdomen, and/or to delineate surrounding blood vessels. These agents can be broadly categorized in terms of their route of administration, either through the gastrointestinal tract or intravenously, and further described by their relative density to the surrounding tissues.

Proprietary names of contrast agents used and protocols detailing the specifics of their administration vary from institution to institution. The radiologist reviews the indications for the study and prescribe appropriate contrast with consultation as needed with the referring clinician. It is crucial that the radiologist tailors the use of contrast when dealing with special or high-risk populations (ie, children, uremic patients, patients with documented allergies). The radiologist must know the possible complications of administering contrast and how to treat them as they arise. [21, 22]

Gastrointestinal Contrast Agents

GI contrast agents are most commonly administered orally but may also be given per rectum, bladder, or support tube. Oral contrast agents have the advantage of being quite dilute compared with their intravenous counterparts; as such, the risk of contrast reactions is lower, precluding the need for patient consent. The only major contraindication to their use is in patients who are at risk for aspiration, in whom they can still be administered via an enteric tube.

One point of debate among different radiologists is the length of the interval between initial administration of oral contrast and the beginning of the CT scan. In general, longer wait times allow for increased transit through the gastrointestinal tract, improving the quality of the scan. However, this may not always be clinically feasible in the emergency setting.

Positive GI agents such as barium or iodinated water-soluble contrast are used when distention and opacification of the gastrointestinal lumen is needed. These agents aid in the evaluation of GI tumors, viscus perforation and/or extraluminal fluid collections. Iodinated water soluble contrast agents are advantageous when compared to barium because they cause minimal GI distress, are absorbed from the peritoneum in cases of viscus perforation, and rapidly dissipate from the lungs if aspirated. They have lower overall viscosity than barium, allowing for uniform distribution. In patients with known severe reactions to intravenous iodinated contrast, however, barium should be used instead.

Neutral GI contrast agents are used to distend the bowel without obscuring evaluation of the mucosa. They include water, CO2, and Volumen.

These agents are especially helpful in cases of suspected bowel ischemia, in which mucosal enhancement (or lack thereof) helps point to the diagnosis. Studies have shown that milk may have a promising role as a cost-effective and tolerable alternative. [23]

Contrast agents can also be administered rectally to evaluate for rectosigmoid cancer, perirectal abscesses, and fistulas. Neutral contrast agents such as CO2 help distend the colon when performing virtual colonoscopy.

Water-soluble contrast agents can be instilled into the bladder via catheter during CT cystography for evaluation of suspected bladder neoplasms or in cases of suspected traumatic bladder perforation.

Intravenous Contrast Agents

Iodine-containing water-soluble, nonionic contrast agents dramatically increase the attenuation within blood vessels while within the blood pool; when they are subsequently distributed within extracellular spaces prior to renal excretion, they cause tissue enhancement within normal and abnormal organs. Ischemic or inflammatory changes or neoplastic processes are associated with distinctive patterns of enhancement during various phases of contrast administration, which can be documented on CT during multiphase acquisitions, aiding in the detection of specific lesions, for example in the liver, kidney, or pancreas. However, in many instances, a single-phase acquisition is adequate for diagnostic purposes.

Intravenous contrast agents are generally described in terms of their iodine concentration (ie, Omnipaque-350 has 350 mg of iodine per mL). It should be noted, however, that higher iodine concentrations do not necessarily correlate with improved image quality and usually lead to more toxic side effects. Because of the risk of contrast-induced nephropathy and allergies with intravenous agents (discussed below), all patients must be appropriately counseled and give consent prior to intravenous contrast administration.

Intravenous contrast is cleared via glomerular filtration in the kidneys, so estimation of renal function must be performed with either serum creatinine or glomerular filtration rate prior to administering the agent. The latter has demonstrated to be more accurate in older patients, as it takes into account age, sex, race, and weight in addition to serum creatinine levels.

Patients who are at high risk for contrast-induced nephropathy include those with chronic renal disease, diabetes, congestive heart failure, and multiple myeloma. Also, patients who are taking nephrotoxic medications or those who have received more than 100 mL of intravenous contrast in the previous 24 hours are considered at risk. Since dehydration is the most common risk factor for contrast-induced nephropathy, especially in those predisposed, patients can be pretreated with bicarbonate or intravenous fluids for volume expansion. Other measures include using isoosmolar or nonionic agents, limiting the volume of contrast used, and avoiding other nephrotoxic drugs.

Patients receiving intravenous contrast agents can have idiosyncratic and nonidiosyncratic reactions. For a detailed discussion of these reactions, please refer to Contrast Medium Reactions.

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Diagnostic Imaging Agents

Class Summary

These agents are used for the radiographic visualization of anatomical structures.

Barium Sulfate (Bar-Test, E-Z-Dose, Maxibar, Tagitol V, Readi-Cat, VoLumen)

Diluted to 1%-3% concentration

Provides good visualization of GI mucosa

More resistant to dilution than water-soluble iodinated contrast

Cheap

Omnipaque (Iohexol)

Low-osmolar water-soluble contrast agent with multiple uses, administration routes, and concentrations (ranging from 140-350 mg/mL iodine). It is the only contrast agent that can be administered PO, intraperitoneally, intrathecally. Can also be used intravenous or intra-arterially. Does not cause irritation to mucosa or serosa. Suitable for preoperative or postsurgical patients or those with suspected bowel perforation or obstruction.

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