MRI provides incredibly detailed images of the structure of the brain
, with a resolution of under 1mm. Certain types of MRI can even give information
about regional blood
flow and the metabolic
state of selected brain regions.
MRI (which, for the reason bobort pointed out, older doctors will call NMR) is based on the measurement of the interaction of a magnet and a magnetic field. Consider, as an example, the earth's magnetic field and a compass magnet. At rest, the compass needle points north. If the needle is tapped, however, it will swing back and forth at a frequency directly proportional to the magnetic field strength. The needle will continue to oscillate until overtaken by friction. Since the oscillation frequency is proportional to magnetic field strength, knowledge about the spacial variation of the field could, in principle, be used to detect the location of the needle on the earth's surface (though rather crudely). In MRI, atomic nuclei (principally hydrogen) act as the compass needle, and a strong magnet plays the role of the earth's field. All the atomic nuclei are aligned by the magnetic field, then "tapped" with a brief radio frequency pulse. They then emit energy in an oscillatory fashion as they return to the alignment imposed by the field. By detecting the oscillations, extremely detailed images can be created.
The resolution of MRI depends on the strength of the magnetic field. Most clinical machines have field strengths of 1.5 - 4 Tesla, providing millimeter to quarter-millimeter resolution.
Neuroscience, Sinaur Associates (QP355.2.N487 1997)