Paramagnetic materials include oxygen and ions of various metals like Fe, Mg, and Gd. These ions have unpaired electrons, resulting in a positive magnetic susceptibility. The magnitude of this susceptibility is less than one one-thousands of that of ferromagnetic materials. The effect on MRI is increase in the T1 and T2 relaxation rates (decrease in the T1 and T2 times). The figure above illustrates the effect of a paramagnetic material (grey circle) on the magnetic field flux lines (blue). Gd is used as a in MR contrast agents. At the proper concentration, Gd contrast agents cause preferential T1 relaxation enhancement, causing 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 dominating.