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Encyclopedia of Physical Science and Technology EN008M-395 June 29, 2001 15:52

960 Magnetic Resonance in Medicine

Nuclear magnetism Magnetic properties acquired by a commercially until about 1982, it is estimated that by 2000
substance when there is a net alignment of the nuclear there were more than 10,000 magnetic resonance (MR)
spins. scanners in use worldwide and more than 20 million MR
Pixel Individual picture element. The numerical value of scans were being done each year. On the order of 150 mil-
the pixel brightness is used to represent it in the image lion diagnostic MR scans were performed between 1982
display. and the end of the year 2000.
Precession Motion of a rapidly spinning object when it During the 1980s and 1990s, thousands of clinicians
is subjected to a twisting force or torque. The axis scientists and engineers were engaged in intense efforts to
of the spin moves at a constant rate along a conical enhance the capabilities of MR scanners by reducing their
surface. costs, improving image quality, shortening scan times, and
Relaxation time Measure of the time required for an ex- increasing the number of clinically useful applications.
cited system to decay or relax toward its equilibrium These activities have been extremely productive and it
state. Longitudinal relaxation refers to the longitudinal seems likely that they will continue for years to come and
magnetization and is measured by T 1 . Transverse re- the number of scanners in service and the number of scans
laxation refers to the transverse magnetization and is performed each year will continue to increase.
measured by T 2 .
Resonance Enhanced response of a physical system that
occurs when the frequency of an applied driving force I. MAGNETIC RESONANCE AS
is equal or nearly equal to some natural frequency of A PHYSICAL PHENOMENON
the system.
Rotating frame Coordinate system rotating around the A. Current Status in Medicine
direction of the static magnetic field at or near the
Larmor frequency. It is useful for depicting the mo- Although the term nuclear magnetic resonance has been in
tion of the magnetization vector. use in the technical community since before World War II,
Selective excitation Excitation of the spins in a limited some changes in terminology have accompanied the onset
region, usually a thin slice, within a sample to be of the medical applications in the 1980s. Partly to avoid
imaged. any fear on the part of patients that radioactive isotopes
Spectrometer Deviceforresolving,measuring,andchar- might be involved (they are not) and partly to forestall any
acterizing electromagnetic energy produced when mat- confusion with the distinct discipline of nuclear medicine
ter changes between allowed energy levels. Examples (which does utilize radioactive isotopes), the adjective nu-
include X-ray spectrometers, optical spectrometers, in- clear is often dropped. The term NMR is still used in the
frared spectrometers, and NMR spectrometers. more technical discussions of the subject, however, as it
Spin Quantum mechanical property of a nucleus or an will be in this article. Two broad general areas of medical
electron that gives it an intrinsic angular momentum applications of NMR have arisen: one is called magnetic
and magnetic moment. resonance imaging (MRI) and the other is called magnetic
Spin–warp technique Method of using gradient fields resonance spectroscopy (MRS). Almost all MRI studies
to encode position-dependent information into NMR are done using signals from hydrogen nuclei (protons),
signals, then using Fourier transform mathematics to although in it is possible in certain specialized cases to
decode the signal and produce an image. use signals from other nuclei, such as sodium, to con-
Voxel Volume element in the object being imaged. struct MR images. Proton MRI, as the name suggests,
The strength of the signal from a voxel determines utilizes the NMR signal, arising mainly in the protons of
the brightness of the corresponding pixel in the the water-containing and fatty tissues of the body, to pro-
image. duce cross-sectional images of the internal anatomy. MRS
utilizes the fact that the resonant frequency of a given nu-
cleusvariesslightlydependingonwhatchemicalmolecule
NUCLEAR MAGNETIC RESONANCE (NMR) is a it is located within—a phenomenon called the chemical
subtle physical phenomenon that can be used to observe shift. Some MRS studies utilize signals from protons to
and exploit a very weak form of magnetism associated distinguish chemical compounds, such as lactic acid and
with the nuclei of certain chemical elements. Beginning the amino acids, that are present in much lower concen-
in the late 1970s and early 1980s, it became possible to uti- trations in the body than is water. But MRS studies more
lize NMR signals from within human patients as a means commonly utilize nuclei such as 31 P (phosphorus) and
of imaging internal disease states for the purpose of medi- 13 C (carbon) whose density in tissues is also much lower
cal diagnosis. Although MR scanners were not introduced than that of the water protons. In proton spectroscopy it is