Radiocontrast
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Radiocontrast agents (also simply contrast agents or contrast materials) 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 nuclei.
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[edit] 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. 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 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 they 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.
| Name | Type | Iodine Content | Osmolality | ||
|---|---|---|---|---|---|
| Ionic | Diatrizoate (Hypaque 50) | Ionic Monomer | 300 | 1550 | High Osmolar |
| Metrizoate (Isopaque Coronar 370) | Ionic | 370 | 2100 | ||
| Ioxaglate (Hexabrix) | Ionic dimer | 320 | 580 | Low Osmolar | |
| Non-Ionic | Iopamidol (Isovue 370) | Non-ionic monomer | 370 | 796 | |
| Iohexol (Omnipaque 350) | Non-ionic | 350 | 884 | ||
| Iopromide | Non-ionic | ||||
| Iodixanol (Visipaque 320) | Non-ionic dimer | 320 | 290 | Iso Osmolar |
[edit] 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.
[edit] 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.
[edit] 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 correlation 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).
[edit] References
- Abizaid AS, Clark CE, Mintz GS, Dosa S, Popma JJ, Pichard AD, Satler LF, Harvey M, Kent KM, Leon MB (1999). "Effects of dopamine and aminophylline on contrast-induced acute renal failure after coronary angioplasty in patients with preexisting renal insufficiency". Am J Cardiol 83 (2): 260-3, A5. PMID 10073832.
- Gleeson TG, Bulugahapitiya S (2004). "Contrast-induced nephropathy". AJR Am J Roentgenol 183 (6): 1673-89. PMID 15547209.
- Greenberger PA, Patterson R, Tapio CM (1985). "Prophylaxis against repeated radiocontrast media reactions in 857 cases. Adverse experience with cimetidine and safety of beta-adrenergic antagonists". Arch Intern Med 145 (12): 2197-200. PMID 2866755.
- Greenberger PA, Patterson R (1988). "Adverse reactions to radiocontrast media". Prog Cardiovasc Dis 31 (3): 239-48. PMID 3055068.
- Hoffmann U, Fischereder M, Kruger B, Drobnik W, Kramer BK (2004). "The value of N-acetylcysteine in the prevention of radiocontrast agent-induced nephropathy seems questionable". J Am Soc Nephrol 15 (2): 407-10. PMID 14747387.
- Karnegis JN, Heinz J (1979). "The risk of diagnostic cardiovascular catheterization". Am Heart J 97 (3): 291-7. PMID 420067.
- Lang DM, Alpern MB, Visintainer PF, Smith ST (1993). "Elevated risk of anaphylactoid reaction from radiographic contrast media is associated with both beta-blocker exposure and cardiovascular disorders". Arch Intern Med 153 (17): 2033-40. PMID 8102844.
- Lasser EC, Berry CC, Talner LB, Santini LC, Lang EK, Gerber FH, Stolberg HO (1987). "Pretreatment with corticosteroids to alleviate reactions to intravenous contrast material". N Engl J Med 317 (14): 845-9. PMID 3627208.
- Lasser EC, Berry CC, Talner LB, Santini LC, Lang EK, Gerber FH, Stolberg HO (1988). "Protective effects of corticosteroids in contrast material anaphylaxis". Invest Radiol 23 Suppl 1: S193-4. PMID 3058630.
- McCullough PA, Wolyn R, Rocher LL, Levin RN, O'Neill WW (1997). "Acute renal failure after coronary intervention: incidence, risk factors, and relationship to mortality". Am J Med 103 (5): 368-75. PMID 9375704.
- Scanlon PJ, Faxon DP, Audet AM, Carabello B, Dehmer GJ, Eagle KA, Legako RD, Leon DF, Murray JA, Nissen SE, Pepine CJ, Watson RM, Ritchie JL, Gibbons RJ, Cheitlin MD, Gardner TJ, Garson A Jr, Russell RO Jr, Ryan TJ, Smith SC Jr (1999). "ACC/AHA guidelines for coronary angiography. A report of the American College of Cardiology/American Heart Association Task Force on practice guidelines (Committee on Coronary Angiography). Developed in collaboration with the Society for Cardiac Angiography and Interventions". J Am Coll Cardiol 33 (6): 1756-824. PMID 10334456.
- Solomon R, Werner C, Mann D, D'Elia J, Silva P (1994). "Effects of saline, mannitol, and furosemide to prevent acute decreases in renal function induced by radiocontrast agents". N Engl J Med 331 (21): 1416-20. PMID 7969280.
- Wittbrodt ET, Spinler SA (1994). "Prevention of anaphylactoid reactions in high-risk patients receiving radiographic contrast media". Ann Pharmacother 28 (2): 236-41. PMID 8173143.
