Gadolinium is the common name given to a range of componds of the chemical element gadolinium that are used to enhance magnetic resonance images (MRI). These include gadopentetate dimeglumine, gadoteridol and gadodiamide.
In multiple sclerosis, Gadolinium enhancing lesions tend to be associated with an inflammatory response. Inflammatory lesion production is typical of the relapsing-remitting form of the disease and is less often seen in the secondary progressive and the primary progressive form.
Gadolinium (Gd) is the 64th chemical in the periodic table. In its elemental state, Gd is a silvery white metal that tarnishes readily in air or water. It is classified as a lanthanide rare earth element which doesn't actually mean that it is rare and, in fact, Gd is relatively commonly found in the Earth's crust. Gd is strongly ferromagnetic which means it is attracted by magnetic fields in the way that iron is. It is this property of Gd that makes it so useful in MRI.
A common fear about Gadolinium is that it is radioactive - this is not the case. The naturally occuring isotopes of Gd are not radioactive. Gd is radioactively stable and, with an atomic number of 64, lies well below typical radioactive chemical elements such as Uranium at 92 and Radon at 86.
Chelated compounds of Gadolinium are those that are used in MRI scans. Technically, these are not dyes although in lay terms they can be described as acting as them. They work by altering the local magnetic field in MS lesions and thus enhancing the MRI image. They do this by shortening the "echo" (TE) and "repetition" times (TR) thereby shortening the time taken for the spin to realign itself with the magnetic field (T1). That's what neurologists and radiologists mean by Gadolinium T1 weighted lesions.
Gadolinium is chelated (i.e. covalently bound to two non-metals in a ring shape) because that renders it non-toxic but allows it to retain its magnetic properties. It's completely harmless except for in a tiny minority of people who have an allergic reaction to it (about 1 per 20,000 persons). However, Gadolinium and MRI scans in general are counter-indicated in pregnant women.
The use of chelated Gadolinium in MRI
Periodic Table of the Elements - Gadolinium
How Magnetic Resonance Imaging (MRI) Works