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

Question

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

Answer 1

1) Contrast agent is used to create a sharp contrast on an otherwise fuzzy image. Consequently, its distribution must have sharp gradient, i.e it must have strong affinity to some type of tissue or physiological liquid. The target may be widely represented in the body, but it still should be a clear target. Or, alternatively, it should be injected into a physiological liquid and have no time to distribute evenly.

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.

Answer 2

(1) Contrast agents are typically injected in the blood stream and stay there during the MRI scan. They accelerate relaxation of hydrogen nuclei making blood easier to see. It can be blood in vessel or in the tissue if the vessel is broken.
(2) Gadolinium causes relaxation of 1H by relaxation mechanism. It was chosen because it has paramagnetic salts with low toxicity.