When introduced into a strong magnetic field, they found that these nuclei would take up energy in the radio-frequency range and re-emit this energy afterwards. Because the magnetic field strength and the radio-frequency must match each other, the phenomenon was called nuclear magnetic resonance (NMR), nuclear because it is only the nuclei of the atoms which react; magnetic because it happens in a magnetic field; and resonance because of the direct dependence of field strength and frequency.

The two scientists, Felix Bloch and Edward M. Purcell, were awarded the Nobel Prize in Physics in 1952. In 1991, the Nobel Prize for Chemistry was awarded to Richard R. Ernst of Zurich for his contributions to the field of magnetic resonance spectroscopy.

Nearly twenty-five years after the first description of the magnetic resonance phenomenon, a professor of chemistry at the State University of New York at Stony Brook, Paul C. Lauterbur, developed this method into an imaging technology, firstly aimed at medical applications, but later also at industrial applications. Lauterbur became the father of a new modality which today has a similar impact on medicine as had Roentgen’s invention of x-rays some 100 years ago. He received the Nobel Prize for Medicine and Physiology (together with Peter Mansfield of Nottingham) in 2003.

Magnetic resonance imaging (MRI) is based on the observation that certain nuclei in the periodic system such as hydrogen, phosphorus, and sodium possess magnetic properties. They behave like small bar magnets and align with an outside magnetic field like the needle of a compass.

If radiowaves of a certain frequency, equal to the resonance frequency of the nucleus in question, are transmitted into a sample, they will be lifted to a higher state of energy. Once the radio transmitter is switched off, the nuclei will start to return to the equilibrium state of energy and re-emit radio waves.

As an encoded message, these radiowaves contain information about their physical and chemical environment. By applying a second low gradient magnetic field, one can determine the position of nuclei within a sample and thus create a picture of their distribution or, for instance in medical imaging, create an image of the body of a patient.

Compared to x-rays and radioisotope methods, much less energy is required and no permanent harmful side-effects of MRI have been demonstrated to date. The energy of magnetic resonance imaging and spectroscopy is nine orders of magnitude lower than the one of X-rays or radioisotope techniques.

Magnetic resonance spectroscopy (MRS) is the basic analytic application of the technique. Originally used as a method for studying the composition of chemical compounds in vitro, today there are applications in a wide range of chemical, physical, biological, and medical areas, from the analysis of foodstuffs to the study of the phosphorus metabolism of humans in vivo.

For medical imaging magnetic resonance is a major break-through because regions of the body such as the brain and the spinal cord, but also the musculo-skeletal system, the pelvis, the heart, and the blood vessels can be depicted for the first time without any invasive manipulation in a way only direct inspection during operation or autopsy previously allowed.

The same holds for biological imaging where plants and animals can be imaged non-destructively.

Details on how NMR and MRI function can be found in EMRF's textbook.