Nuclear Magnetic Resonance

Nuclear magnetic resonance, NMR, is a process in which the nuclei of certain atoms absorb energy from a magnetic field that gyrates, or has a direction which rotates about some fixed axis. NMR provides a means of measuring nuclear properties using ordinary electromagnetic fields rather than high-energy particles as in a particle accelerator. Its applications range from nuclear measurements to medical imaging.

NMR arose from theoretical work first published by the physicist I. I. Rabi in 1937. It was applied by Rabi in measurements of the magnetic moment of atomic nuclei. The method was later applied by physicists Louis Alvarez and Felix Bloch in 1940 to measure the magnetic moment of the neutron. Later it was used to measure atomic and molecular structure. Currently NMR has wide application in imaging of internal organs for medical diagnosis.

The process on which NMR is based is essentially one in which the nucleus of an atom is caused to wobble, or precess, like a top. The wobble is maintained and increased, applying a force that varies at the same rate as the wobble itself.

Atomic nuclei possess nuclear spin, the angular momentum of the nucleus, which is due to rotation. Since nuclei contain an electric charge, in the form of protons, their rotation often produces an electric current which creates a magnetic field. Like an electromagnet, therefore, the nucleus has a magnetic moment.

When immersed in a magnetic field, a nucleus will experience a twisting force, or torque, which tends to line the spin axis of the nucleus up with the field, the same effect that causes two bar magnets to stick to each other in opposed directions. Because the nucleus is spinning, however, it will precess like a spinning top or gyroscope.

The energy of the precessing nucleus depends on its orientation in the magnetic field. This energy can be increased by applying a rotating magnetic force to the nucleus. The force must rotate at a frequency known as the Larmor frequency, which is proportional to the applied magnetic field. This gyrating combination of fixed and rotating magnetic fields produces nuclear magnetic resonance.

Since a nucleus is a system having atomic dimensions, quantum mechanical considerations limit its orientation energy in the magnetic field to certain specific values, which differ by multiples of the energy of a photon having the Larmor frequency. This is because the nucleus gains energy by absorbing photons-light quanta-from the rotating magnetic field.

The resonant frequency of a nucleus in NMR depends on three factors: the distribution of mass and charge in the nucleus, and the magnetic field. Thus even if two atoms have identical nuclei, they may have different resonant frequencies if they are located within different external fields. This may be the case, for example, if they occur within different chemical compounds, the motion of the electrons with a molecule will contribute to the total magnetic nuclei.

Applications of NMR are based on its ability to measure nuclear properties of atoms within a sample of material. All NMR applications use three: (1) a strong magnetic field; (2) a radio frequency signal generator to provide a rotating field; and (3) a detector to observe the resonance. The detector is an induction coil which picks up the electric signal from the precessing nuclei.

The magnetic moment of an atomic nucleus is one of the determining factors of the Larmor frequency. Thus, NMR can be used to get information about nuclear magnetic moments.

The Larmor frequency is dependent on the magnetic field at the location of the nucleus, which depends on the influence of nearby atoms. Thus the NMR frequency depends on the chemical structure of the molecules in a sample of material. NMR is therefore a useful tool for chemical analysis.

The largest area of application of NMR is in medical diagnosis. In this area, the technology is usually referred to as magnetic resonance imaging (MRI). The principle of MRI is identical to that of the use of NMR in chemical analysis. Essentially, the different materials in the body resonate at different frequencies depending on their chemical compositions. Position information is obtained by using an external magnetic field which varies with position, so that resonance at a particular frequency with a given substance, such as fatty tissue, will occur only at a particular position or set of positions within the body. The resonant response is then analyzed and displayed using a computer.