Contrast Agents for Magnetic Resonance Imaging have been in use for the past 10 years. These agents improve the resolution of MRI images by increasing the brightness in various parts of the body where the agent resides. Most contrast agents that have been approved for human use are extracellular, Gadolinium-based agents that have a relatively short residence time in the vascular system. More recently, intracellular agents have been introduced that have longer residence times and allow extended imaging procedures without the need for repeated injections of the agent.
Contrast Agents for Magnetic Resonance Imaging have been in use for the past 10 years. These agents improve the resolution of MRI images by increasing the brightness in various parts of the body where the agent resides. Most contrast agents that have been approved for human use are extracellular, Gadolinium-based agents that have a relatively short residence time in the vascular system. More recently, intracellular agents have been introduced that have longer residence times and allow extended imaging procedures without the need for repeated injections of the agent.
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. It has a median lethal dose of 34 mmol/kg resulting in a safety ratio of 2-3 times that of Gd-DOTA, and 3-4 times that of Gd-DTPA. No abnormal serum bilirubin levels occur, however elevated serum iron levels occurred with an incidence of 8.2% in one study of 73 patients.The efficacy of this contrast is similar to that 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. (http://www.mritutor.org/mritutor/ivcon.htm)
Another report makes the point that intravascular contrast agents normally remain confined to the intravascular space, compared to Gd-DTPA which distributes throughout the extracellular fluid space. This is a result of intravascular agents having a molecular weight of approximately 70,000 and above, compared to a molecular weight of 590 for Gd-DTPA. There are several advantages of intravascular agents. They can assess perfusion in areas of ischemia and provide information about capillary permeability in areas of reperfusion. They can show the extent of tumor neovascularity and associated permeability changes. Finally, they are useful in studies requiring prolonged imaging. Three types of intravascular contrast agents have been developed: Gd-DTPA labeled albumin, Gd-DTPA labeled dextran, and chromium-labeled red blood cells. (http://www.mritutor.org/mritutor/ivpool.htm)
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. As particulate matter they are taken up by the RES. Very small particles of less than 300 nanometers also remain intravascular for a prolonged period of time and thus can serve as blood pool agents. The agents are also known by the abbreviation SPIOs ("small particle iron oxides" or "superparamagnetic iron oxides") and USPIOs ("ultrasmall particle iron oxides" or "ultrasmall superparamagnetic iron oxides"). At least two types of USPIO will be available on the market as blood pool agents, while SPIOs have been used as darkening contrast agents for liver imaging and for darkening the bowel.