Radiocontrast agent

Radiocontrast agents are a type of medical contrast medium used to improve the visibility of internal bodily structures in an X-ray based imaging techniques such as computed tomography (CT) or radiography (commonly known as X-ray imaging). Radiocontrast agents are typically iodine or barium compounds.

Despite being part of radiology, magnetic resonance imaging (MRI) functions through different principles and thus utilizes different contrast agents. These compounds work by altering the magnetic properties of nearby hydrogen nuclei.

Types and uses
Radiocontrast agents used in X-ray examinations can be grouped based on its use.

Iodinated (intravascular)
Iodine based contrast media are usually classified as ionic or non-ionic. Both types are used most commonly in radiology, due to its relatively harmless interaction with the body and its solubility. It is primarily used to visualize vessels, and changes in tissues on radiography and CT, but can also be used for tests of the urinary tract, uterus and fallopian tubes. It may cause the patient to feel as if he or she has urinated on himself. It also puts a metallic taste in the mouth of the patient.

Modern intravenous contrast agents are typically 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 requirements but may result in additional complications. Organic agents which covalently bind the iodine 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 and 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 (as in diskography of the spine) and intraabdominally – just about any body cavity or potential space.

Iodine contrast agents are used for the following:
 * Angiography (arterial investigations)
 * Venography (venous investigations)
 * VCUG (voiding cystourethrography)
 * HSG (hysterosalpinogram)
 * IVU (intravenous urography)

High osmolar (ionic)
Ionic contrast media have higher osmolarity and more side-effects, such as diatrizoic acid, Telebrix, etc.

Low osmolar (non-ionic)
Non-ionic contrast media, have lower osmolarity and tend to have fewer side-effects, such as iohexol, iopromide, iodixanol, etc.

Barium (gastro-intestinal)
Barium sulfate is mainly used in the imaging of the digestive system. The substance exists as a water insoluble white powder that is made into a slurry with water and administered directly into the gastrointestinal tract.
 * Barium enema (large bowel investigation) and DCBE (double contrast barium enema)
 * Barium swallow (oesophagael investigation)
 * Barium meal (stomach investigation) and double contrast barium meal
 * Barium follow through (stomach and small bowel investigation)
 * CT pneumocolon / virtual colonoscopy

Barium sulfate, an insoluble white powder is typically used for enhancing contrast in the GI tract. Depending on how it is to be administered the compound is mixed with water, thickeners, de-clumping agents, and flavourings  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.

Air
As in the picture on the left where both air and barium are used together (hence the term "double-contrast" barium enema) air can be used as a contrast material because it is less radio-opaque than the tissues it is defining. In the picture it highlights the interior of the colon. An example of a technique using purely air for the contrast medium is an air arthrogram where the injection of air into a joint cavity allows the cartilage covering the ends of the bones to be visualised.

Other
An older type of contrast agent, Thorotrast was based on thorium dioxide, but this was abandoned since it turned out to be carcinogenic.

Adverse effects
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
Anaphylactoid reactions occur rarely,  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.

Pretreatment with corticosteroids has been shown to decrease the incidence of adverse reactions.

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 suggest 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., povidone-iodine) are of any specific relevance to administration of I.V. contrast material.

Contrast-induced nephropathy
Contrast-induced nephropathy is defined as either a greater than 25% increase of serum creatinine or an absolute increase in serum creatinine of 0.5 mg/dL.

Drug interactions
It has been recommended that metformin, an oral antidiabetic agent, be stopped for 48 hours following the intravascular administration of contrast media and that the use of metformin not be resumed until renal function has been shown to be normal. The reasoning is that if the contrast medium causes kidney failure (as happens rarely) and the person continues to take metformin (which is normally excreted by the kidneys), there may be a toxic accumulation of metformin, increasing the risk of lactic acidosis, a dangerous complication.

However, guidelines published by the Royal College of Radiologists suggest this is not as important for patients who receive less than 100 ml of contrast media and have normal renal function. If renal impairment is found before administration of the contrast, metformin should be stopped 48 hours before and after the procedure.