Gourt Home::Gourt :: Radiocontrast
Radiocontrast agents (or simply contrast agents) are compounds used to improve the visibility of internal bodily structures in an X-ray image.
Contrast agents are also used in MRI (Magnetic Resonance Imaging). Although MRI is usually considered a branch of radiology, it is not based on X-rays. MRI contrast agents are usually gadolinium-based, and work not by being radioopaque, but rather by altering the magnetic properties of nearby hydrogen nucleii.
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 less side effects as they do not dissociate into component molecules. Many of the side effects are due to the hyperosmolar solution being injected. ie they deliver more iodine atoms per molecule. They more iodine, the more "dense" they 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 as harmless as possible 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 thay can also be used intraarterially, intrathecally (the spine) and intraabdominally - just about any body cavity or potential space.
An older type of contrast agent, Thorotrast was based on thorium dioxide, but this was abandoned since it turned out to be carcinogenic.
Side-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 (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.
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. Three factors have been associated with an increased risk of contrast-induced nephropathy: preexisting renal insufficiency, preexisting diabetes, and reduced intravascular volume (McCullough, 1997; Scanlon et al, 1999).The osmolality of the contrast agent is believed to be of great importance in contrast-induced nephropathy. Ideally, the contrast agent should be isoosmolar to blood. Modern iodinated contrast agents are non-ionic, the older ionic types caused more adverse effects and are not used much anymore.
To minimize the risk for contrast-induced nephropathy, various actions can be taken if the patient has predisposing conditions. Low-osmolar or iso-osmolar contrast media slould be chosen. The dose of contrast media should be as low as possible, while still being able to perform the necessary examination. Dehydration should be corrected before the examination, this is usually done by administering intravenous fluids.
Some recent studies suggest that N-acetylcysteine protects the kidney from the toxic effects of the contrast agent (Gleeson & Bulugahapitiya 2004). This effect is, in any case, not overwhelming. Some researchers (e.g. Hoffmann et al 2004) even claim that the effect is due to interference with the creatinine laboratory test itself. This is supported by a lack of correllation between creatinine levels and cystatin C levels.
Other pharmacological agents, such as furosemide, mannitol, theophylline, aminophylline, dopamine, and atrial natriuretic peptide have been tried, but have either not had beneficial effects, or had detrimental effects (Solomon et al, 1994; Abizaid et al, 1999).
References
"Radiocontrast"