|MadSci Network: Medicine|
Dear Margaret: Thanks for your question. Magnetic resonance (MR) contrast agents are unique in radiology because it is not the chemical that is detected, but rather the effect that the chemical has on surrounding molecules that is detected. MR imaging relies on signal originating from hydrogen nuclei in water and fat. These protons have properties known as T1 and T2 relaxation, which vary depend on the immediate surroundings, and affect signal magnitude from the given region. MR imaging uses various magnetic field changes to accentuate the difference in signal magnitudes from different regions. The reason that different anatomic structures can be seen on an MRI is largely due to differences in T1 and T2 between different tissues. Gadolinium (chelated to a carrier molecule) is an intravenously-injected MR contrast agent which shortens the T1 of surrounding protons, making them appear brighter. Because gadolinium normally stays in blood vessels, it has the effect of making vessels, highly vascular tissues, and areas of blood leakage appear brighter. Gadolinium is excreted through the kidneys, with a half-life of 1.25-1.6 hours. Ray Hsu HSURM@MEDICINE.WUSTL.EDU
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