paramagnetism
Paramagnetic materials include oxygen and ions of various metals like Fe (iron), Mg (magnesium), and Gd (gadolinium). These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than 0.1% of that of ferromagnetic materials.
The effect on MRI is an increase in the T1 and T2 relaxation rates (decrease in the T1 and T2 times). The figure illustrates the effect of a paramagnetic material (grey circle) on the magnetic field flux lines (blue).
Gadolinium is used as a MRI contrast agent. At the proper concentration, gadolinium contrast agents cause preferential T1 relaxation enhancement, causing an increase in signal on T1-weighted images. At high concentrations, as is sometimes seen in the urinary bladder, loss of signal is seen instead, a result of the T2 relaxation effects predominating.
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