Contrast media

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.

[edit] 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.

[edit] X-ray attenuation

Main article: Radiocontrast

Iodine and barium are the most common types of contrast medium for enhancing x-ray based imaging methods.

[edit] Iodine

Example of Iodine based contrast in Cerebral Angiography

Iodine based contrast media such as Urografin or Omnipaque is used most commonly in radiology, due to its relatively harmless interaction with the body and it solubility. It is primarily used to visualise vessels, but can also be used for tests of the urinary tract, uterus and fallopian tubes.

Iodine contrast agents are used for the following:

• Angiography (Arterial Investigations)
• Venography (Venous Investigations)
• VCUG (Voiding Cystourethrography)
• HSG (Hysterosalpinogram)
• IVU (Intravenous Urography)

[edit] Barium

Example of a DCBE

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

[edit] MR signal enhancing

Main article: MRI contrast agent

This would include gadolinium for use in magnetic resonance imaging as a MRI contrast agent. In the 3+ oxidation state the metal and has 7 unpaired f electrons. This causes water around the contrast agent to relax quickly, enhancing the quality of the MRI scan.

[edit] Ultrasonography

Main article: Contrast-enhanced ultrasound

Microbubbles contrast agents are used to to aid the sonographic, specifically echocardiograms, for the detection of a cardiac shunt. The bubbles are composed of tiny amounts of nitrogen or perfluorocarbons strengthen and supported by a protein, lipid, or polymer shell. 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.

[edit] Adverse effects

Although it not very common, medical conditions can be caused by the administration of various contrast media. Reactions can range from minor to severe, sometimes resulting in death.

[edit] Allergy Reactions

Mild (no treatment necessary)

• Hot flush
• Vomiting

Moderate (treatment necessary, but no intensive care)

• Urticaria (hives)
• Slight wheezing (bronchospasm)

Severe (life-threatening, intensive care necessary)

• Respiratory arrest
• Cardiac arrest

Contrast media is never given to a patient unless a doctor is present to assist should an allergic reaction occur. Patients are usually screened before being given contrast, by means of a series of questions. These typically include an allergy history and a history of any asthma and diabetes.

[edit] Toxicity

Nephrotoxicity (toxicity to the kidneys) is a major consideration for clinicians when requesting tests which use an iodine-based contrast media. Patients whose renal function is impaired (usually with a creatinine >120 micromol/litre) should only have contrast media if absolutely necessary. In these circumstances, a special form of contrast media, which is 'kinder' to the kidneys, can be given to prevent contrast-induced nephropathy.

Nephrogenic systemic fibrosis can appear through the administration of gadolinium for MR contrast enhancement. Although rare, it produces serious side-effects that may involve fibrosis of skin, joints, eyes, and internal organs.

[edit] Drug Interactions

It has been recommended[1] 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 <100mls 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.^[1].

[edit] See also

• Medical imaging
• Radiology
• Radiocontrast
• MRI contrast agent

[edit] References

1. ^ Thomsen HS, Morcos SK, and members of the Contrast Media Safety Committee of the European Society of Urogenital Radiology. Contrast media and Metformin. Guidelines to distinguish the risk of lactic acidosis in non-insulin dependent diabetics after administration of contrast media.European Radiology, 1999; 9: 738-740.

• Dorland's Illustrated Medical Dictionary (Twenty-sixth Edition) (ISBN 0-7216-1645-3)