Contrast Enhanced MRI

In many cases, the differences between the MR signal intensities of normal and diseased tissues are not large enough to produce obvious contrast for definitive diagnosis. Contrast agents are used to enhance the image contrast between normal and diseased tissues to improve diagnostic sensitivity and specificity.19,20 MRI contrast agents are either paramagnetic metal chelates or ultrasmall superparamagnetic iron oxides. These agents interact with surrounding water molecules and increase the relaxation rates (1/T1 and 1/T2) of water protons. The change of relaxation rate is linearly proportional to the concentration of contrast agents, [M]:

where i = 1,2. In Equation 11.1, 1/Ti obs is the observed relaxation rate with a contrast agent, 1/Ti d the relaxation rate without the contrast agent, and rt the relaxivity of the contrast agent. Relaxivity is a measurement of the efficiency of a contrast agent to enhance the relaxation rate of water protons or the efficiency to generate image contrast enhancement. Because the uptake of contrast agents varies between different tissues, MR image contrast is enhanced in the tissue with a high contrast agent concentration compared to that with a low concentration.

MRI contrast agents approved for clinical applications are mainly stable paramagnetic chelates,

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