Wikipedia: Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a method used to visualise the pathologic or physiologic alterations of tissues of living organisms.

First, the spins of the atomic nuclei of the tissue molecules are redistributed in the presence of a powerful [magnetic field]? and then the field is suddenly reduced. The electromagnetic radiation released by these molecules are recorded and the obtained data are processed by a computer. As a result, the examined tissue can be seen with its quite detailed anatomical features. In clinical practice, it is used to discriminate the pathologic tissue from a normal one (e.g. a [brain tumor]?).

The technique most frequently relies on the relaxation properties of magnetically-excited hydrogen nuclei in water. The sample is briefly exposed to a burst of radiofrequency energy, which in the presence of a magnetic field, puts the nuclei in an elevated energy state. As the molecules undergo their normal, microscopic tumbling, they shed this energy to their surroundings, in a process referred to as "relaxation". Molecules free to tumble more rapidly relax more rapidly. Differences in relaxation rates are the basis of MRI images--the water molecules in blood are free to tumble more rapidly, and hence, relax at a different rate than water molecules in other tissues.

Though the behavior of atomic nuclei in the studied sample is central to the technique, the term "nuclear" was deliberately omitted from the techniques name to avoid an irrational avoidance of the technique in the face of worries or concerns born from the association of the word "nuclear" with the technologies used in atomic bombs. Unlike atomic bomb technology, the nuclei relevant to MRI exist and are in place whether the technique is applied or not.

Functional MRI (fMRI) is a technique which combines the spatially-addressable nature of MRI with the spectroscopically-rich information obtainable from NMR. That is to say, MRI allows one to study a particular region within an organism or sample, but gives relatively little information about the chemical or physical nature of that region--it's chief value is in being able to distinguish the properties of that region relative to those of other, surrounding regions. Functional MRI, however, provides a similar wealth of chemical information about that region as would an NMR spectrum of that region.