A medical contrast medium is a substance used to enhance the contrast of
structures or fluids within the body in medical imaging. It is commonly used to
enhance the visibility of blood vessels and the gastrointestinal tract.
Types Several types of contrast media are in
use in medical imaging and they can roughly be classified based on the
imaging modalities where they are used. Although other types exist, most common
contrast agents work based on X-ray attenuation and magnetic resonance
signal enhancement. = X-ray attenuation =
Iodine and barium are the most common types of contrast medium for enhancing
x-ray-based imaging methods. Various sorts of iodinated contrast media exist,
with variations occurring between the osmolarity, viscosity and absolute
iodine content of different media. Non-ionic dimers are favored for their
low osmolarity and toxicity, but have a correspondingly higher price attached to
their use. = MR signal enhancing =
Gadolinium is used in magnetic resonance imaging as a MRI contrast agent. In the
3+ oxidation state the metal has 7 unpaired of electrons. This causes water
around the contrast agent to relax quickly, enhancing the quality of the
MRI scan. = Ultrasound scattering and frequency
shift = Microbubble contrast agents are used to
aid the sonographic examination, specifically echocardiograms, for the
detection of a cardiac shunt. These bubbles are composed of agitated saline
solution, most of which are too large to pass through the lung capillaries.
Therefore, the only ones that reach the left side of the heart pass through an
abnormal connection between the two sides of the heart, a so-called
right-to-left shunt. In addition, pharmaceutically prepared microbubbles
are composed of tiny amounts of nitrogen or perfluorocarbons strengthened and
supported by a protein, lipid, or polymer shell. These are small enough to
pass through the capillaries and are used to increase the contrast in the
left ventricle, improving the visualization of its walls. The drop in
density on the interface between the gas in the bubble and the surrounding liquid
strongly scatters and reflects the ultrasound back to the probe. This
process of backscattering gives the liquid with these bubbles a high signal,
which can be seen in the resulting image.
Adverse effects While modern contrast media are
generally safe to use, medical conditions can be caused by the
administration of various contrast media. Reactions can range from minor to
severe, sometimes resulting in death with death being about 0.9 per 100,000
cases. To better understand the reactions and to efficiently manage
patients at risk, it is useful to classify them. Risk factors for
developing severe reactions include strong allergies, bronchial asthma,
cardiac disease and beta-blocker use. While the previously suspected IL-2
medication is no risk for the acquisition of adverse events.
A common misconception that even exists among healthcare professionals is that
an allergy to contrast media is related to an allergy to seafood because both
share iodine in their construction, implicating iodine as a source. Numerous
studies have shown that although iodine is common in contrast media, iodine is
not the cause of allergic reactions to contrast media and instead the more
likely culprit are the inert ingredients and the patient's past history of having
other strong allergic reactions. One important distinction is that allergic
effects are by definition immunoglobulin E related histamine storms and studies
have shown that contrast media cause no such reaction in vivo thereby refuting
the possibility that contrast media or the iodine in it is likely to be an
allergen. Although it may seem contradictory, the few rare cases of
contrast medium mediated IgE are exceedingly rare compared to all adverse
reactions and when they happen, are often because the patient already has
multiple risk factors that suggest the patient has other allergy related
problems. Historically, contrast media was
sometimes highly dangerous but these dangers were not well-understood during
the development of the early types of contrast media, such as Thorotrast.
See also iodinated contrast
ipodate sodium Radiocontrast
Medical imaging Radiology
References