MRI Contrast Agents

“MRI contrast is used for a variety of reasons.  Primarily, it is used to improve the detection of disease; that is, to increase sensitivity and diagnostic confidence, to enhance the ability to differentiate normal and abnormal tissue; and to identify the extent of the disease” (Muroff, 2001).
Contrast agents are chemical substances used in anatomical or functional imaging for the purpose of increasing visual differences between normal and abnormal tissue.  These chemical substances are used to alter relaxation times.  Contrast agents are classified by changes in relaxation times after injection.
There are six main categories of MRI contrast agents: Gastrointestinal, Intravenous, Intravascular (blood pool), Tumor-specific, and Reticuloendothelial contrast agents.

Within the Gastrointestinal contrast agent category, there two subcategories: Positive and Negative contrast agents.
Positive contrast agents cause a reduction in T1 relaxation times.  These agents appear brightly on images.  They may contain Manganese, Iron or Gadolinium as active elements.
Positive contrast agents have three classifications: Paramagnetic agents, short T1-relaxation agents, and a combination of the two.
Paramagnetic contrast agents have magnetic centers which create magnetic fields.  These fields interact with water protons, and have a larger effect on relaxation rates.  They include ferric chloride and gadolinium, which cause T1 and T2 shortening.  In low concentrations, T1 shortening holds the domination of the intensity of the signal.  In high concentrations, T2 shortening causes the signal to decrease.  At mid-level concentration, T1 and T2 shortening show an increase in the signal on T1 weighted images, and decreased the signal on T2 weighted images.
Short T1-relaxation agents include mineral oil and oil emulsions.  In these agents, protons relax faster than protons in water, which results in short T1 time.  In bowels with T1 weighted images, a bright signal is apparent.
Combinations include emulsion oil that contains corn oil and ferric ammonium citrate, and an emusion containing baby formula with ferrous sulfate.  Combination contrast agents distribute evenly through the bowels.
Negative contrast agents will appear largely dark on images.  They are frequently called superparamagnetic iron oxide (SPIO).  They have shorter T1 and T2 relaxation times.
Negative contrast agents have three classifications: diamagnetic agents, superparamagnetic agents, and perfluorochemicals.
One readily available diamagnetic contrast agent is barium sulfate suspension.  It reduces the loss of bowel signals, resulting in improved pancreatic visualization.
Superparamagnetic contrast agents are generally administered orally.  They include magnelite albumin microspheres and superparamagnetic iron oxide.  A large loss of the signal in the stomach and small bowels that give immense visualization of the pancreas and anterior renal margins.  This contrast agent type “accumulates in the reticuloendothelial system of the liver, and darkens healthy liver tissue in T2-weighted images” (mr-tip.com).
Perfluorochemicals are organic compounds that replace protons with fluorine.  They are a special group of negative contrast agents that appear completely dark on images, because they do not contain hydrogen atoms, which are responsible for the signal in MRIs.  In gastrointestinal imaging, the purpose of perfluorochemicals is to give a complete signal absence in the bowels.
Intravenous contrast agents include both ionic and nonionic chelates.  When using paramagnetic metal ions as contrast agents, there is a high level of toxicity in the doses required for imaging.  Chelates reduce the chances of long term toxicity by reducing the toxic levels.
Intravascular contrast agents remain in the blood longer than most other contrast agents.  They are highly useful in diagnosis imaging that may require longer imaging times.
Tumor-Specific contrast agents are targeted to tumors.  There are four main types of tumor-specific agents:
Metalloporphyrins target multiple types of tumors, such as melanomas and lymphomas.
Monoclonal antibodies are for specific tumors such as colon carcinomas.
Ferrioxamine is a paramagnetic agent used for the kidneys and urinary tract.
Nitroxides are also paramagnetic agents, but are not widely used.
Reticuloendothelial contrast agents are used in liver, spleen and lymph node MRIs.  In liver and spleen imaging, specific contrast agents are used that target the reticuloendothelial system of the liver and spleen.  Because of the inability of most imagers to differentiate between normal and abnormal lymph nodes, USPIO has become widely used.  USPIO allows imagers to have the ability to differentiate between lymph nodes.
“In cases where it is difficult to differentiate two types of tissue, because the signal intensity they produce is so similar, the solution is to add a contrast agent to one of them in order to distinguish it from the other tissue” (GE Healthcare, 2007).  MRI contrast agents affect hydrogen atoms and the time they take to return to their original state, thereby increasing the signal intensity differences between the tissue with the contrast agent and the tissue without the contrast agent.  This results in an increase of contrasts on the image.
References

GE Healthcare (accessed January 13, 2007) www.amershamhealth.com/public/medical/mri_3.shtml
Medline Plus (revised March 2000) http://mplus.nlm.nih.gov/medlineplus/druginfo/uspdi/202770.html
MR-Technology (accessed January 13, 2007) www.mr-tip.com
Muroff, Lawrence R. (Aug. 2001) “MRI Contrast: Current Agents and Issues” Applied Radiology Online (vol. 30, No. 8) www.appliedradiology.com
Runge, Val M. (Aug. 2001) “The Safety of MR Contrast Media: A Literature Review” Applied Radiology Online (Vol. 30, No. 8) www.appliedradiology.com

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