What do they do?
Contrast agents are chemicals used to enhance medical and diagnostic images such as X-rays, and magnetic resonance imaging (MRI) and computed tomography (CT) scans. They temporarily highlight specific tissues to increase their visibility. Barium sulphate, for example, a commonly used contrast agent, is so dense that it is opaque to X-rays, thereby allowing barium-filled organs to be photographed and seen.
The type of contrast agent depends on the imaging process and the tissue being examined. For example, barium sulphate is used with X-rays to inspect the oesophagus, stomach, rectum and colon whereas iodinated (iodine-based) contrast agents are used with CT scans to enhance visibility of the gallbladder, urinary tract, blood vessels, liver and spleen. Non-ionic, iodine-based contrast agents are also available and should be opted for whenever possible, as they have fewer adverse reactions, although their high cost prevents them from being commonplace (Am Fam Physician, 2002; 66: 1229-34). Currently, the only agents approved for use with MRI are chelates containing the element gadolinium.
Are they effective?
There is no doubt that contrast agents increase the effectiveness of MRI and CT scans. With them, pancreatic and liver cancers have a 90 per cent detection rate while multiple sclerosis has detection rate of 95-99 per cent (Radiology, 1991, 178: 447-51).
Indeed, contrast agents are limited mostly by the limitations of the imaging technology. CT scans don't show the brainstem and cerebellum very well, nor are cancers smaller than 1 cm in diameter likely to be picked up. In contrast, MRI may sometimes reveal too much detail, thus complicating the diagnosis.
The main limitation of contrast agents is that they are likely to dissipate if scanning takes too long a time.
Are they safe?
Not always, and the dangers are often underemphasised. Adverse reactions associated with contrast agents range from the commonly seen mildly uncomfortable symptoms to rarer life-threatening events. The overall frequency of such ill-effects is around 5-8 per cent - with the life-threatening ones comprising about 0.1 per cent (Arch Intern Med, 2001; 161: 15-21).
People with poor kidney function or those taking medication that is potentially damaging to the kidneys are most at risk of a serious adverse reaction to contrast agents. Such individuals are five to 10 times more likely to develop kidney failure than the general population (Am Fam Physician, 2002; 66: 1229-34).
The chances of experiencing a serious adverse reaction increases if you have multiple food allergies, anaemia or low blood pressure, epilepsy, sickle-cell disease, asthma, hayfever, a heart condition or diabetes mellitus (especially if metformin is also being taken). Also, contrast agents of any sort are not recommended for pregnant women, and iodine-based contrast agents should be avoided by those who have thyroid disorders such as hyperthyroidism (Eur Radiol, 2004; 14: 902-7).
Adverse reactions with iodine-based contrast agents include:
* Metallic taste in the mouth, hot sensation of the skin and hives. The symptoms usually pass quickly, but can be uncomfortable.
* Kidney failure. The intravenous administration of contrast material is responsible for 12 per cent of cases of hospital-acquired kidney failure in the US (Ann J Med, 1983; 74: 243-8).
* Anaphylactic reactions, which can occur within minutes of the contrast agents being administered. These include swelling of the throat or other tissues, acute bronchospasm, low blood pressure and severe itching. Such symptoms can arise from the use of as little as 1 mL of contrast (Am Fam Physician, 2002; 66: 1229-34).
* Local tissue damage, such as skin necrosis (death), can result from leakage of contrast fluid from veins.
* Severe lactic acidosis (when lactic acid builds up in the cells and bloodstream, resulting in nausea/vomiting, abdominal pain, difficulty breathing, and severe weakening of arm and leg muscles) has been reported with intravenous contrast agents in diabetics taking metformin (Clin Radiol, 1999; 54: 29-33).
Barium sulphate can cause:
* Delayed reactions, including constipation and flu-like symptoms with fever, chills, nausea/vomiting, abdominal pain and fatigue.
* Hypotension, sepsis (overwhelming infection), disseminated intravascular coagulation (abnormal bloodclotting, tissue damage and abnormal bleeding), and liver problems, from abscesses to deterioration of liver function (Hosp Physician, 2004; March: 26-8).
* Oedema (swelling) and embolism (bloodclots) of the lung (Arch Bronconeumol, 2003; 39: 531-4).
* Suffocation. If barium sulphate is taken orally, there is a small possibility of breathing it in which, in large amounts, can block the airways.
If you must use contrast agents
* Ensure that you drink enough water - 500 mL before taking a contrast agent and 2500 mL in the 24 hours afterwards - to flush the agent from the body.
* Opt for the newer non-ionic, iodine-based contrast agents (in the case of CT scans) and use the lowest dose possible.
* Leave a two- to five-day gap between scans if more than one is required to give the body time to recover.
* Stop taking any drugs that are potentially toxic to the kidneys for at least 24 hours before undergoing the procedure (Am Fam Physician, 2002; 66: 1229-34).
* Stop taking metformin (if you have diabetes) before the procedure and for at least 48 hours afterwards.