Magnetic Resonance

Type of physical science: Classical physics

Field of study: Electromagnetism

Magnetic resonance is one of the most widely used physical effects in existence. The effect arises from the interaction of a component of electromagnetic radiation with the magnetic properties of charged particles. Magnetic resonance plays a central role in a number of everyday events, including the playing of a radio, the viewing of a television program, the use of the microwave for the warming and cooking of food products, and for delicate surgical procedures.

Overview

Magnetic resonance is a condition that exists when the magnetic properties of a charged particle interact with a certain type of electromagnetic radiation. Since magnetic resonance involves the interaction of matter with radiation, the properties of electromagnetic radiation will be described, including how its coupling with a property of matter gives rise to this phenomenon.

All types of waves can be chartacterized by two fundamental properties: a wavelength (λ) and an amplitude (A). The wavelength is the distance between two identical points on successive waves, while the amplitude is the height from the midpoint of the wave to the peak of the wave. A wave can also be described by its frequency, v. The frequency of a wave is the number of waves that pass through a particular point per unit time.

There are different types of waves, such as sound waves and light waves. Of particular interest here are the properties of light waves. Light waves consist of an electric field component and a magnetic field component that have identical wavelengths and frequencies but travel in two mutually perpendicular planes. This type of wave is called an "electromagnetic wave" or is simply referred to as "electromagnetic radiation." Since all electromagnetic radiation travels with a fixed velocity of 3.0 x 10⁸m s^-1 (speed of light, c), an electromagnetic wave can be described alternatively by its frequency v (v = c/λ).

A second property of electromagnetic radiation is that it is made up of small particle-like bundles of energy called photons. This property of radiation allows it to exhibit particle-like behavior. Photons of a given wave will possess energy that is proportional to its frequency (E = hn). Radio waves, television signals, sunlight, and flash light are all forms of radiation that differ only in their frequency.

The particle-wave dual behavior of radiation allows it to interact with matter under certain conditions. For example, consider a situation in which a type of light source is incident upon a piece of material. The piece of matter may interact with this electromagnetic radiation if two basic conditions are met. One, the particles making up the material must be in some type of periodic motion. This periodic motion will then have a characteristic frequency. Second, the frequency of this periodic motion must match the frequency of the electromagnetic wave exactly.

If both conditions are met, then the photons that make up the light may interact with the material.

When such a condition exists, the system is said to be in "resonance."

Applications

Since its theoretical description in the early twentieth century, the magnetic resonance effect has been applied extensively in many areas of chemistry, physics, biology, medicine, and many other fields.

In chemistry, nuclear magnetic resonance and electron spin resonance yield such valuable structural information that they have become two indispensable forms of spectroscopy.

Their combined application to chemical problems can provide excellent details on the three-dimensional arrangement of atoms in space. Both procedures utilize the interaction of radio frequency electromagnetic radiation with the collection of nuclei that are immersed in a strong magnetic field.

In the field of communications, the telephone is a device that has made great technological advances in the past few years. Telephone companies have successfully incorporated the concepts and properties of electromagnetic radiation into their everyday operation. The telephone was originally modeled after the telegraph. According to its initial design, electrical currents were modulated by changes in the air pressure caused by sound waves arising from speech. This process had limitations, since electrical wires were needed in locations where a telephone was to be installed. Yet with today's technology, this is no longer a limitation.

Presently, electromagnetic information can be beamed to an orbiting satellite and bounced back to any receiving station around the world. A station must first meet the resonance condition with the incoming radiation before it is able to receive the signal.

In the medical profession, nuclear magnetic resonance imaging (NMRI) has become a widely used diagnostic tool. The high resolution anatomical information in cross sectional format is more valauble than other imaging techniques. Since no ionizing radiation is used, there is no severe biological risk. Clinical utility of NMRI is most commonly observed in brain and spinal cord images. Reasons include less tissue variation; more symmetry; and less cardiac, respiratory, and peristaltic motion. Lasers (light amplification by the stimulated emission of radiation) have been used extensively as a surgical tool. Lasers possess some unique properties that make them ideally suited for some delicate surgical procedures.

Lasers may be tuned to produce radiation of a single wavelength and therefore of a given energy.

In addition, the light emitted by lasers is very narrow and can be focused on quite small areas.

These properties allow physicians to operate on a particular type of tissue, while minimizing the effects on surrounding areas. Applications include eye, cell, and skin surgery, along with the treatment of certain types of cancers. It is hoped that the laser may some day replace the surgical scalpel, since laser incisions are likely to cause less lateral damage, involve less bleeding, and are more sterile than commonly used scalpels.

In today's world, the warming or cooking of food products is routinely done via an appliance known as a microwave. Microwaves utilize the interaction of electromagnetic radiation with water molecules contained within the food substance to produce the heating process. A microwave generates electromagnetic radiation of a characteristic frequency that is absorbed by water molecules. In turn, the water molecules release this excess energy to the surrounding areas, causing the warming effect.

An everyday example of the resonance condition occurs when a person listens to a radio or views a television program. In order to listen or to view a certain radio or television station, the receiver on the electronic set (that is, radio or television set) must be adjusted to match the frequency of the signal that is being transmitted by the station. When a clear sound or picture is received, the resonance condition has been met.

Context

Magnetic resonance is the basis for the understanding of the properties of the interaction of electromagnetic radiation with matter. Yet, it is often very difficult to describe this concept in a classical manner. A full description of the effect would require the theoretical treatment of electricity, electromagnetism, particle physics, and such quantum concepts as electrons, nuclei, nuclear spins, photons, and the particle-wave behavior of radiation.

The connection between electricity and magnetism was first proposed in the early nineteenth century. Several scientists observed that when electricity (the flow of electrons) was made to flow along a wire, an "invisible field" was generated around the conducting wire.

Michael Faraday, an English chemist and physicist, performed a number of experiments that led to the discoveries relating electricity, magnetism, and light. He found that when electricity is made to travel in a coil, a magnetic field was produced. Through his observations, Faraday concluded that magnetic fields existed in space and that light must also exist in space. It was not until late in the century that scientists accepted Faraday's concept that radiant energy could be transmitted through space in the form of waves.

In 1873, James Clerk Maxwell theoretically demonstrated that light waves were part of a whole group of waves, which he called electromagnetic radiation. These waves consist of an electric and a magnetic field component that vibrate at right angles to each other and travel in the same direction.

In the twentieth century, the quantum mechanical account of light allowed for the description of the interaction of light with matter. According to Max Planck, Albert Einstein, and other scientists, electromagnetic radiation was composed of tiny particles of energy called photons. Each of the photons possesses a fixed amount of energy that depends on the frequency of radiation. When materials are exposed to a certain type of radiation, the material may absorb the radiation only if the energy of the photons is compatible to the energy of the electrons, atoms, molecules, or of the motions of these bodies in the material. If the energies are the same, then the resonance condition has been met and the interaction/absorption of energy can take place.

In magnetic resonance, the magnetic properties of the material must match the magnetic frequency component of radiation exactly for the magnetic resonance effect to occur.

Principal terms

ANGULAR MOMENTUM: an intrinsic property of a rotating particle possessing mass

ELECTROMAGNETIC RADIATION: radiation that is composed of oscillating, wavelike/particle-like waves that travel in two perpendicular planes

MAGNETIC DIPOLE MOMENT: a property of a particle that arises from its possession of angular momentum and an electric charge

MAGNETIC FIELD: a field generated by the circular motion of a charged particle

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY: the study of molecular structures by means of the interaction of radio frequency with a collection of nuclei that have been placed in a strong magnetic field

QUANTUM NUMBER: a number in quantum mechanics that is related to the energy, orientation, momentum, or spin of a particle

Bibliography

Branley, Franklyn M. THE ELECTROMAGNETIC SPECTRUM. New York: Thomas Y. Crowell, 1979. An excellent review on the theory of electromagnetism. Branley presents a brief history of the development of electricity, magnetism, and their applications. Many fine illustrations assist the reader in understanding the material. No mathematical treatment of the subject matter is presented.

Dobbs, Roland. ELECTRICITY AND MAGNETISM. London: Routledge & Kegan Paul, 1986. Dobbs provides a good mixture of formal and informal discussions on electricity and magnetism. The development of both topics was original, simplistic, and to the point. Provides well-placed illustrations and diagrams that provide visual aids to the reader. A superbly written book.

Levin, Edith M. THE PENETRATING BEAM: REFLECTIONS ON L. New York: Richard Rosen Press, 1978. The basic properties of light such as intensity, velocity, and composition are presented. Levin explains the various types of radiant energy very well. She provides some great examples of the interaction of light with objects.

Lynden-Bell, Ruth M., And Robin K. Harris. NUCLEAR MAGNETIC RESONANCE SPECTROS COPY. London: Thomas Nelson and Sons, 1971. A technical approach to the theory and applications of nuclear magnetic resonance. The authors present a detailed study of angular momentum, nuclear spins, magnetic moments, and the action of a magnetic moment in an applied magnetic field.

McLauchlan, K. A. MAGNETIC RESONANCE. London: Oxford University Press, 1972. The author explains the physical origins of the magnetic interactions before presenting the theory. Little is presented in the form of quantum mechanics. A reader with a basic understanding of general physics should not have any problems following the text.