What are contrast agents in MRI and how do they work?

By reading some Wikipedia pages about MRI and MRI Contrast agent, I understand that contrast agents are mediums that reduce the relaxation times of the body tissues thereby increasing the contrast of the image obtained by MRI. My questions:

1. Are contrast agents just used for targeting a special tissue, such as a tumor, or they can be used the increase the contrast of the image of the whole body?

2. I know they decrease the relaxation time of the hydrogen atoms in body, but how do they do that?

Thanks

2) Wikipedia provides simplistic description. The idea is that to relax efficiently, a magnetic nucleus (including a proton from a hydrogen atom) needs to interact efficiently with chaotic microscopically inhomogeneous magnetic field. Such field may be produces by another microscopic magnetic objects. $\ce{Gd^{+3}}$ has seven unpaired electrons, giving the atom the strongest magnetic field of all single atoms available in reasonable chemical conditions.