Contrast medium

Overview
In a medical setting, a contrast medium is any substance that is used to enhance the visibility of structures or fluids within the body. An example of this is the use of a radiopaque substance during an x-ray exam to highlight features that would otherwise be less distinguishable from nearby tissue. Another example is the use of air bubbles during an echocardiogram to search for a shunt in the heart.

Summary
The contrast can either be positive or negative. Positive contrast media has a higher attenuation density than the surrounding tissue. This means that the contrast looks more opaque than the surrounding tissue when seen on an x-ray. Negative contrast media has a lower attenuation density than the surrounding tissue. This means that the contrast looks less opaque than the body. Negative contrast is only found as a gas. Positive contrast is a substance with a high atomic number, but is also non-toxic. Contrast can be used to produce images of almost any hollow structure in the body.

Types of Positive Contrast Medium

 * Iodine Based
 * Non Iodine Based
 * Other

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

Non Iodine Based Contrast Media
These often appear in the form of barium sulfate. Barium is mainly used in the imaging of the digestion system. Technetium, specifically isotope 99, is also used.

Other
This would include gadolinium for use in magnetic resonance imaging as a contrast agent.

Types of Negative Contrast
Negative contrast always occurs in a gas, usually as one of the following:


 * Air
 * Carbon dioxide
 * Oxygen

Clinical Applications
Examples of the use of positive contrast medium are as follows:

Iodine Base Contrast Media

 * Angiography (Arterial Investigations)
 * Venography (Venous Investigations)
 * MCUG (Micturating Cysto Urography)
 * HSG (Hysterosalpinogram)
 * IVU (Intravenous Urography)

Non Iodine Based Contrast Madia (Barium Studies)

 * Barium Enema (Large Bowel Investigation)
 * Barium Swallow (Oesophagael Investigation)
 * Barium Meal (Stomach Investigation )
 * Barium Follow Through (Stomach and Small Bowel Investigation)

Other
Gadolinium, or Gad for short, is used as a component of MRI contrast agents, 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.

Negative Contrast Media
Examples of the use of negative contrast medium are as follows:


 * DCBE (Double Contrast Barium Enema)
 * Double Contrast Barium Meal
 * CT Pneumocolon / Virtual Colonoscopy
 * the use of air bubbles to aid the sonographic detection of a cardiac shunt

Allergy Reactions
Although rare, it is possible to be allergic to contrast media. Reactions can range from minor to severe, in the worst case scenario, resulting in death.

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.

Drug Interactions
It has been recommended 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. .

Toxicity

 * See Contrast-induced nephropathy

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) 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.