Why is Gadolinium so often used as a contrast agent in MRI as opposed to other paramagnetic elements or superparamagnetic compounds?
Well it has 7 unpaired electrons and the compounds are sufficiently stable.
What other compounds do you have in mind?
My advisor works on Mn2+
compounds which could be used as contrast agents, it is interesting alternative givent the fact that Gd is toxic but Im not realy sure why are those compounds not used, because im doing something different, but I think its because the complexes are not stable enough. If you want to learn more, you might want to read read
Manganese(II) Complexes as Potential Contrast Agents for MRI
by Bohuslav Drahos,Ivan Lukes and Éva Tóth
2012 Eur. J. Inorg Chem, 1975-1986