Nuclear magnetic resonance: its clinical application.

Abstract

Nuclear magnetic resonance (NMR) is a phenomenon in which particular atomic nuclei respond to the application of certain magnetic fields by absorbing or emitting electromagnetic radiation, a phenomenon which was first demonstrated by Bloch, Hansen and Packard in 1946. Its medical application was pioneered by Odeblad, a physicist from Sweden, during the l950s and 1960s (Odeblad and Lindstr#{246}m 1955; Odeblad 1966). The first published NMR image was by Lauterbur in 1973 and subsequently groups of physicists in Nottingham and Aberdeen have developed further imaging systems. Live human images followed in the late 1970s and the first clinical images ofthe brain appeared in 1980. At the present time a number ofgroups are involved in the clinical evaluation of NMR imaging in this country and the United States, and also in Germany and Holland. To date clinical experience is still small but the results so far suggest that NMR imaging will play a significant role in clinical diagnosis in the future. Basic principles. Nuclear magnetic resonance has been widely used in analytical chemistry for the last few decades. The nuclei of some atoms behave like spinning magnets and when these atoms are exposed to static magnet fields their magnetisation is preferentially aligned in the direction of the static magnetic fields, producing a net nuclear magnetisation. When these atoms are exposed to an additional magnetic field oscillating at the spin frequency of each nucleus, the nuclear magnetisation can be rotated from its original orientation through any given angle. Following this pertubation the magnetisation returns to its original position, during which time a small electrical signal can be detected in a surrounding coil. The rate at which magnetisation returns to its original position is described by two relaxation times, T1 and �‘2� T1 describes recovery in the direction of the external static field and �‘2 describes recovery perpendicular to this field. Relaxation in the direction of the magnetic field (T1) depends on interaction between protons and the surrounding nuclei and molecules, whereas relaxation in the transverse