These are substances that shorten the relaxation times of T1 and T2, facilitating the diagnosis. They have a high number of unpaired electrons that give the atom a magnetic moment which will align in an external field. For this reason, they are called paramagnetic.
The Gadolinium chelates are the most common contrast agents used in MRI. Gadolinium (Gd) is a metal of the lanthanide family which has 7 unpaired electrons. It is used as a non-specific contrast of the extracellular space and does not accumulate in the blood. Following intravenous administration, the Gd increases the T1 weighted signal of the accumulated hydrogen protons of free water. It indicates a lesion with an increase of the vascular supply or breaks in the blood-brain barrier. It is an indirect agent because the Gd cannot be visualized. A T1 weighted precontrast study must be done to check for the presence of other paramagnetic substances. Iron is the commonest of these and comes from the process of the degradation of hemoglobin.