Gadolinium n. : A supposed rare metallic element, with a characteristic spectrum, found associated with yttrium and other rare metals. Its individuality and properties have not yet been determined.

Example:
Do I have a right to refuse to have Gadolinium during my MRI?

Taxonomy
Gadolinium, Gd, compounds, element, oxides, 06-06, isotopes, contrast agents, Gadolinium MRI, gadolinium oxyorthosilicate, agents, kJ, stable, metal, stable isotopes, Isotopes of gadolinium, GPa, aq.

Defination from the web:
... Gadolinium has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors. ...