Page 237 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GRB/GWT P2: GPJ/GAX QC: GAE/FYD Final Pages
Encyclopedia of Physical Science and Technology EN008M-395 June 29, 2001 15:52

962 Magnetic Resonance in Medicine

a small, dense, positively charged nucleus around which in use, and when the frequency of the oscillation matched
orbit light, negatively charged electrons. The nuclear atom that of the separation of the nuclear magnetic energy lev-
is reminiscent of the solar system, with planets orbiting els a measurable beam deflection occurred. Although the
around the sun. The model has certain defects, which were beam method had important applications to fundamental
removed with the development of quantum theory. Unfor- physics, because it permitted the accurate measurement of
tunately, the quantum theory replaced the previous con- the strength of the magnetic moments of various nuclei, it
cepts of atomic structure with abstruse mathematical con- was still a very long way from permitting the detection of
cepts, and these cannot be depicted in any fashion that is nuclear magnetism in bulk materials.
at once rigorously correct and also visually informative. An unsuccessful attempt to observe magnetic reso-
In other words, quantum theory does not permit pictorial nance directly in a bulk material was made in 1936 by
representations of atoms to be taken literally. Nonetheless, the Dutch physicist C. G. Gorter, but he was thwarted by
the concept of the nuclear atom still provides a convenient technical difficulties. Many advances in radio-frequency
and useful aid, particularly in visualizing magnetic phe- electronics, however, occurred during World War II,
nomena, and we shall make use of it for this reason. particularly as a result of research on radar technology.
Electric currents (i.e., charges in motion) are known to In 1946, separate groups headed by E. M. Purcell at
produce magnetic fields. The nuclear atomic model sug- Harvard and F. Bloch at Stanford detected NMR in bulk
gests one obvious source of electrical current—the nega- materials by use of resonance techniques. This marked
tively charged, orbiting electrons—and, this is, in fact, one the beginning of modern NMR activity. Spectrometers
major source of the magnetic behavior of materials. The were developed that permitted relatively straightforward
other two examples of moving charges within the atom NMR experimentation on test-tube-sized samples of a
are the electron and nuclear spins. large variety of materials. Because the NMR frequency
Much of the experimental information on atomic prop- turns out to be very sensitive to local magnetic fields at
erties came through detailed study of the light emitted the nuclei being studied, it became a major tool in analytic
from excited atoms as the orbital electrons changed states, chemistry for molecular structure determination and for
or orbits. Very careful studies of such spectra with spectro- the identification of unknown compounds. For example,
scopes of the highest possible resolution showed that the by 1955 the NMR spectrometers had revolutionized
spectra of many atoms were split into several components the structural analysis of organic compounds. This
placed very close together. This phenomenon, called the application of small-bore spectrometers to analytical
hyperfine structure, led W. Pauli in 1924 to suggest that chemistry continues unabated to the present time.
atomic nuclei possessed a spinning motion about a cen- At various times several researchers (e.g., J. R. Singer
tral axis. The hyperfine structure was then explained as a in 1959, T. R. Ligon in 1967, and J. A. Jackson in 1968)
weak nuclear magnetic perturbation of the electron ener- reported NMR measurements of one sort or another on
gies. In 1925 S. Goudsmit and G. E. Uhlenbeck made a human tissues. In a 1971 report, which created substantial
similar proposal, suggesting that the electrons also pos- interest, R. Damadian described the use of NMR to detect
sess an intrinsic spinning motion, which provides them cancer in rat tissues. In the years following that report,
with both angular momentum and a magnetic moment. a controversy developed regarding the ability of NMR to
Thus the nuclear atomic model postulates three types of make a specific distinction between benign and malignant
electric current that can act as sources of magnetic forces: tissues. The tissue is not yet entirely settled, although most
the orbital motion of the electrons and the spinning mo- workers would agree that, with present capabilities, NMR
tions of the electrons and the nucleus. instruments operating alone cannot make this distinction.
The spin magnetic moment of nuclei is usually at least Current work on MRI in humans can be traced to the
1000 times smaller than that of electrons, and conse- report of P. C. Lauterbur in 1973 wherein he proposed
quently, the expected strength of the forces produced by the use of gradient fields to permit the incorporation of
nuclear magnetism are extremely small and essentially un- position-dependent information into NMR signals. He
detectable. During the 1930s, I. I. Rabi was able to detect created the name zeugmatography to describe the pro-
and study nuclear magnetic moments of atoms that were cess. Perhaps not surprisingly, the concept has flourished
moving as atomic beams through an ultrahigh vacuum. An while the name has faded from use (although it is in-
array of magnets was used to slightly deflect the beam by cluded in the recently published second edition of the
an amount dependent on the nuclear magnetic moment. Oxford English Dictionary). The first NMR images were
In 1939, Rabi greatly refined the beam deflection method of small capillary tubes filled with water and placed in
and reported the first instance of nuclear magnetic reso- an appropriately modified spectrometer. These were soon
nance. To accomplish this, a weak oscillating magnetic followed by images of human anatomy. The quality of
field was added to that of the permanent magnets already these images rapidly increased, and many modifications