\u201cMRI contrast is used for a variety of reasons.\u00a0 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\u201d (Muroff, 2001).
\nContrast agents are chemical substances used in anatomical or functional imaging for the purpose of increasing visual differences between normal and abnormal tissue.\u00a0 These chemical substances are used to alter relaxation times.\u00a0 Contrast agents are classified by changes in relaxation times after injection.
\nThere are six main categories of MRI contrast agents: Gastrointestinal, Intravenous, Intravascular (blood pool), Tumor-specific, and Reticuloendothelial contrast agents.<\/p>\n
Within the Gastrointestinal contrast agent category, there two subcategories: Positive and Negative contrast agents.
\nPositive contrast agents cause a reduction in T1 relaxation times.\u00a0 These agents appear brightly on images.\u00a0 They may contain Manganese, Iron or Gadolinium as active elements.
\nPositive contrast agents have three classifications: Paramagnetic agents, short T1-relaxation agents, and a combination of the two.
\nParamagnetic contrast agents have magnetic centers which create magnetic fields.\u00a0 These fields interact with water protons, and have a larger effect on relaxation rates.\u00a0 They include ferric chloride and gadolinium, which cause T1 and T2 shortening.\u00a0 In low concentrations, T1 shortening holds the domination of the intensity of the signal.\u00a0 In high concentrations, T2 shortening causes the signal to decrease.\u00a0 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.
\nShort T1-relaxation agents include mineral oil and oil emulsions.\u00a0 In these agents, protons relax faster than protons in water, which results in short T1 time.\u00a0 In bowels with T1 weighted images, a bright signal is apparent.
\nCombinations include emulsion oil that contains corn oil and ferric ammonium citrate, and an emusion containing baby formula with ferrous sulfate.\u00a0 Combination contrast agents distribute evenly through the bowels.
\nNegative contrast agents will appear largely dark on images.\u00a0 They are frequently called superparamagnetic iron oxide (SPIO).\u00a0 They have shorter T1 and T2 relaxation times.
\nNegative contrast agents have three classifications: diamagnetic agents, superparamagnetic agents, and perfluorochemicals.
\nOne readily available diamagnetic contrast agent is barium sulfate suspension.\u00a0 It reduces the loss of bowel signals, resulting in improved pancreatic visualization.
\nSuperparamagnetic contrast agents are generally administered orally.\u00a0 They include magnelite albumin microspheres and superparamagnetic iron oxide.\u00a0 A large loss of the signal in the stomach and small bowels that give immense visualization of the pancreas and anterior renal margins.\u00a0 This contrast agent type \u201caccumulates in the reticuloendothelial system of the liver, and darkens healthy liver tissue in T2-weighted images\u201d (mr-tip.com).
\nPerfluorochemicals are organic compounds that replace protons with fluorine.\u00a0 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.\u00a0 In gastrointestinal imaging, the purpose of perfluorochemicals is to give a complete signal absence in the bowels.
\nIntravenous contrast agents include both ionic and nonionic chelates.\u00a0 When using paramagnetic metal ions as contrast agents, there is a high level of toxicity in the doses required for imaging.\u00a0 Chelates reduce the chances of long term toxicity by reducing the toxic levels.
\nIntravascular contrast agents remain in the blood longer than most other contrast agents.\u00a0 They are highly useful in diagnosis imaging that may require longer imaging times.
\nTumor-Specific contrast agents are targeted to tumors.\u00a0 There are four main types of tumor-specific agents:
\nMetalloporphyrins target multiple types of tumors, such as melanomas and lymphomas.
\nMonoclonal antibodies are for specific tumors such as colon carcinomas.
\nFerrioxamine is a paramagnetic agent used for the kidneys and urinary tract.
\nNitroxides are also paramagnetic agents, but are not widely used.
\nReticuloendothelial contrast agents are used in liver, spleen and lymph node MRIs.\u00a0 In liver and spleen imaging, specific contrast agents are used that target the reticuloendothelial system of the liver and spleen.\u00a0 Because of the inability of most imagers to differentiate between normal and abnormal lymph nodes, USPIO has become widely used.\u00a0 USPIO allows imagers to have the ability to differentiate between lymph nodes.
\n\u201cIn 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\u201d (GE Healthcare, 2007).\u00a0 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.\u00a0 This results in an increase of contrasts on the image.
\nReferences<\/p>\n
GE Healthcare (accessed January 13, 2007) www.amershamhealth.com\/public\/medical\/mri_3.shtml
\nMedline Plus (revised March 2000) http:\/\/mplus.nlm.nih.gov\/medlineplus\/druginfo\/uspdi\/202770.html
\nMR-Technology (accessed January 13, 2007) www.mr-tip.com
\nMuroff, Lawrence R. (Aug. 2001) \u201cMRI Contrast: Current Agents and Issues\u201d Applied Radiology Online (vol. 30, No. 8) www.appliedradiology.com
\nRunge, Val M. (Aug. 2001) \u201cThe Safety of MR Contrast Media: A Literature Review\u201d Applied Radiology Online (Vol. 30, No. 8) www.appliedradiology.com<\/p>\n","protected":false},"excerpt":{"rendered":"
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