Magnetic resonance imaging (MRI)

Such as Positron emission tomography, magnetic resonance imaging gives information about brain function as well as structure. It combines computerized tomography with NMR (nuclear magnetic resonance). Nuclei with odd mass number, for instance, 11H, generate a magnetic field along their spin axis. In the powerful magnetic field of a magnetic resonance imaging scanner, hydrogen nuclei can adopt one of two orientations; with their magnetic fields either parallel or antiparallel to the external field. The equal state has a slightly lower energy and normally a small excess of nuclei will be in this state. This gives rise to a net longitudinal magnetic field parallel to the scanner field.

A cylindrical coil placed around the head broadcasts a radio frequency pulse to a slice of head at right angles to the major scanner area. The rf pulse makes the nuclei wobble around their magnetic axis rather like a spinning top as it slows down with the rate of wobbling in resonance with the pulse frequency. The wobble generates an electric field which is received through the coil, producing a transverse magnetic area at right angles to the scanner area. When the rf pulse is turned off the nuclei return to their original state and the transverse and longitudinal area decay with relaxation times that are characteristic for the nucleus and its chemical environment for instance aqueous or lipid. Generating an MRI image actually needed a further three coils that produce magnetic area gradients in the x, y, and z directions. MRI has a resolution < 1 mm.

Figure:  The principle of nuclear magnetic resonance (NMR). A radiofrequency pulse will excite atomic nuclei, flipping them from the parallel state into the higher energy antiparallel state. Relaxation of the nuclei back into the low energy state generates the magnetic resonance imaging (MRI) signal.