Contrast Media Used In Imaging Studies
Medical contrast media are substances that are used to enhance the contrast of structures or fluids within the body in medical imaging. Several types of contrast medium are in used in medical imaging and they can be roughly 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.
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)
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 slurry with water and administered directly into the gastrointestinal tract.
Barium contrast agents are used for the following:
- 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
MR signal enhancing
This would include gadolinium for use in magnetic resonance imaging as a MRI contrast agent. In the 3+ oxidation state the metal has 7 unpaired electrons. This causes water around the contrast agent to relax quickly, enhancing the quality of the MRI scan.
Extracellular fluid (ECF) agents include products such as Magnevist, Prohance, and Omniscan. These agents are generally nonionic and a recent report, points out that the development of nonionic contrast agents for MRI has paralleled that for iodinated contrast materials. Ionic chelates are also hyperosmolar and some of their side effects may be attributed to this property. Gadodiamide (Omniscan, Winthrop Pharm.) is a nonionic complex with two-fifths of the osmolality of Gd-DTPA. Gadoteridol (Prohance, Squibb) is the third intravenous contrast agent on the market. It is a low osmolar, nonionic contrast as is Gadodiamide. Indications for use and efficacy are similar to the other agents.
Another class of contrast agents are Ultrasmall Supermagnetic Iron Oxide Particles. Small particles of ferrite used as paramagnetic contrast medium in MR imaging. These agents exhibit strong T1 relaxation properties, and due to susceptibility differences to their surroundings also produce a strongly varying local magnetic field, which enhances T2 relaxation to darken the contrast media-containing structures
Microbubbles contrast agents are used to aid the sonography, 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.
The currently used CM in the x-ray imaging procedure can be grouped, on the basis of their osmolarity, to such which have a low osmolarity (LOCA) and such having a high osmolarity (HOCA). It should be noted that both have an osmolarity which is above that of the blood. Adverse side effects associated with CMs include such which result from the high osmolarity. The introduction of LOCAs, which are the new generation of CMs, was meant to counter some of these side effects. It should be noted that one big disadvantage of LOCAs is their high price-tag (about 5-10 times that of HOCA).
Some physicians suggest that nonionic, low-osmolality agents be used universally because fewer adverse reactions are associated with them. Nonionic agents should be used in patients who are at increased risk of adverse reactions. This group includes patients who had previous contrast reactions, or who have asthma, multiple allergies, or diseases that could be aggravated by contrast materials. Low-osmolar, nonionic agents should be used in patients known to have renal insufficiency. In addition, when a complete history is difficult to obtain and in patients who are concerned about the use of contrast material or are at risk for aspiration, low-osmolality agents should be considered.
Individuals subjected to procedures involving the use of CMs, are exposed to several hazards, depending on the CM used, including:
1. Hyperosmotic Damage:
Typically 100-200 ml of CM are injected into a total plasma volume of 5 liters within a period of several minutes. Cells such as endothelial cells, red and white blood cells, cells within the kidney, etc., are exposed to a hyperosmotic solution, reaching 200-2000 mOsm/kg at the site of injection, as compared to the osmolarity of the blood with its 300 mOsm/kg, giving rise to a hyperosmotic shock which may elicit related damages. In the following description the term “hyperosmotic CM” will refer to any CM having osmolarity higher than the blood osmolarity which is typically 300 mOsm/kg.
2. Iodine Specific Toxicity:
In an x-ray visualization procedure typically 30-40 grams of iodine (included within the contrast media) are injected into the blood within the period of 2-10 minutes. It should be noted that target visualization requires a minimum accumulation of 15-20 mg of iodine/ml in the target tissue and this is the reason that the initial iodine concentration in the CM is relatively high in the range of 300-420 mg iodine/ml.
3. Kidney Damage:
The iodine load to which the kidney is exposed and which it has to secrete is a potential cause for renal damage. It is generally believed today that 12% of all patients which are injected with an x-ray CM, encounter renal complications. A recent study shows that during cardiac catheterization procedure, 9% of low risk and 16% of high risk patients develop renal failure. Various studies made with CMs have shown that exposure of cells to x-ray CM causes the cell damages
Generally, before a contrast procedure is performed an ampule of antihistamine is already available just in case any adverse reaction may occur to the patient.