Despite the excellent inherent tissue contrast in MR images, intravenous contrast medium is often given to highlight abnormal tissue. Chelates of the paramagnetic substance gadolinium are used - they shorten the relaxation times of nearby protons which results in high signal on Tl-weighted images. Gadolinium has similar pharmacokinetics to the iodinated contrast media used in CT - it is distributed throughout the intra- and extravascular spaces, does not cross the intact blood-brain barrier, is hyperosmolar and excreted renally - caution is needed in patients with renal failure. The frequency of adverse reactions is around 2-5%, although most are mild (nausea, urticaria, etc.). Anaphylactoid reactions are rare, but have been reported.
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