In diagnostic imaging, radiocontrast agents (also simply contrast agents or contrast materials) are contrast media given to a patient and used to improve the visibility of internal bodily structures in an X-ray image, including computed tomography (CT).
This article does not include other contrast media not based on the transmission of X-rays through the body, such as gadolinium for magnetic resonance imaging, and preparations that circulate microbubbles through the blood for contrast with ultrasonography. Not contrast agents per se, other forms of medical imaging, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), generate images from substances also introduced into the patient's body, but are sources of radiation rather than radiopaque materials to external radiation.
Types and uses
There are two basic types of contrast agents used in X-ray examinations.
One type of contrast agent is based on barium sulfate, an insoluble white powder. This is mixed with water and some additional ingredients to make the contrast agent. As the barium sulfate doesn't dissolve, this type of contrast agent is an opaque white mixture. It is only used in the digestive tract; it is usually swallowed or administered as an enema. After the examination, it leaves the body with the feces.
The other type of contrast agent is based on iodine. This may be bound either in an organic (non-ionic) compound or an ionic compound. Ionic agents were developed first and are still in widespread use depending on the examination they are required for. Ionic agents have a poorer side effect profile. Organic compounds have fewer side effects as they do not dissociate into component molecules. Many of the side effects are due to the hyperosmolar solution being injected. i.e. they deliver more iodine atoms per molecule. The more iodine, the more "dense" the x-ray effect. There are many different molecules. Some examples of organic iodine molecules are iohexol, iodixanol, ioversol. Iodine based contrast media are water soluble and harmless to the body. These contrast agents are sold as clear colorless water solutions, the concentration is usually expressed as mg I/ml. Modern iodinated contrast agents can be used almost anywhere in the body. Most often they are used intravenously, but for various purposes they can also be used intraarterially, intrathecally (the spine) and intraabdominally - just about any body cavity or potential space.
|Ionic||Diatrizoate (Hypaque 50)||Ionic Monomer||300||1550||High Osmolar|
|Ionic||Metrizoate (Isopaque Coronar 370)||Ionic||370||2100||High Osmolar|
|Ionic||Ioxaglate (Hexabrix)||Ionic dimer||320||580||Low Osmolar|
|Non-Ionic||Iopamidol (Isovue 370)||Non-ionic monomer||370||796||Low Osmolar|
|Non-Ionic||Iohexol (Omnipaque 350)||Non-ionic||350||884||Low Osmolar|
|Non-Ionic||Ioxilan (Oxilan)||Non-ionic||Low Osmolar|
|Non-Ionic||Iodixanol (Visipaque 320)||Non-ionic dimer||320||290||Iso Osmolar|
Modern iodinated contrast agents are safe drugs; adverse reactions exist but they are uncommon. The major side effects of radiocontrast are anaphylactoid reactions and contrast-induced nephropathy.
Anaphylactoid reactions occur rarely (Karnegis and Heinz, 1979; Lasser et al, 1987; Greenberger and Patterson, 1988), but can occur in response to injected as well as oral and rectal contrast and even retrograde pyelography. They are similar in presentation to anaphylactic reactions, but are not caused by an IgE-mediated immune response. Patients with a history of contrast reactions, however, are at increased risk of anaphylactoid reactions (Greenberger and Patterson, 1988; Lang et al, 1993). Pretreatment with corticosteroids has been shown to decrease the incidence of adverse reactions (Lasser et al, 1988; Greenberger et al, 1985; Wittbrodt and Spinler, 1994).
Anaphylactoid reactions range from urticaria and itching, to bronchospasm and facial and laryngeal edema. For simple cases of urticaria and itching, Benadryl (diphenhydramine) oral or IV is appropriate. For more severe reactions, including bronchospasm and facial or neck edema, albuterol inhaler, or subcutaneous or IV epinephrine, plus diphenhydramine may be needed. If respiration is compromised, an airway must be established prior to medical management.
Contribution of seafood and other allergies
It must be noted that suspicion of seafood "allergy", often based more on medical myth than fact, is not a sufficient contraindication to the use of iodinated contrast material. A relationship between iodine levels in seafood and seafood allergy is part of medical lore. While iodine levels in seafood are higher than in non-seafood items, the consumption of the latter exceeds that of the former by far and there is no evidence that the iodine content of seafood is related to reactions to seafood. Available data suggests that seafood allergy increases the risk of a contrast-mediated reaction by approximately the same amount as allergies to fruits or those with asthma. In other words, over 85% of patients with seafood allergies will not have an adverse reaction to iodinated contrast. Finally, there is no evidence that adverse skin reactions to iodine-containing topical antiseptics (e.g., Betadine, Povidine) are of any specific relevance to administration of I.V. contrast material.
Acute kidney injury from radiocontrast is called contrast-induced nephropathy. It is defined as either a greater than 25% increase of serum creatinine or an absolute increase in serum creatinine of 0.5 mg/dL.
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