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Magnetic Resonance in Medicine 973

is repeated episodically, as needed, over the life of the D. Radio-Frequency Coils
scanner.
Betweenthegradientcoilsandthepatientislocatedatleast
Homogeneity is normally specified as the maximum de-
one additional coil, which serves as transmitter and/or re-
viation, in parts per million, of the field within a specified
ceiver of rf energy. Imaging techniques require a series of
diameter spherical volume (dsv) centered on the center of
◦
90 and 180 pulses at the Larmor frequency of the pro-
◦
the room-temperature bore. The homogeneity is better, of
tons. These are the B 1 fields used to excite the spin system.
course, for small volumes. In a typical situation the ho-
The coil that delivers these pulses must be designed to han-
mogeneity of a shimmed magnet might be 0.1 ppm over a
dle the high instantaneous voltages and currents that are
10-cm dsv, 10 ppm over a 30-cm dsv, and 40 ppm over a
required. It should also produce a magnetic field that is as
50-cm dsv.
uniform as possible across the desired field of view and
The superconducting property means that such magnets
at right angles to the static field B 0 . To the extent that this
can be operated in the persistent mode. Once the power
B 1 field is not uniform, the pulses produced will produce
supply has increased the current to the point where the
errors in the desired angles of spin flipping.
desired field has been reached (this normally is done over
The task of building a radio-frequency coil large enough
several hours), a superconducting switch is activated, and
to accommodate the human body, producing a uniform
the power supply is disconnected. As long as the windings
B 1 field, and still capable of resonating at frequencies ap-
are kept below the superconducting transition tempera-
proaching 100 MHz has provided some technical difficul-
ture, no further input of energy is required to maintain the
ties. This is because the large size of the coil produces an
field. Modern magnets, operating in the persistent mode,
inherently large inductance, which interacts with the stray
have no trouble meeting a drift specification of less than
self-capacitance of the coil to produce a self-resonance
0.1 ppm/hr. This drift rate is so slow that the magnets can
phenomenon that degrades the coil performance. This
go for months or years without requiring additional en-
problem has been overcome by using designs with capaci-
ergy input. To maintain the coils in the superconducting
tors distributed along the length of the coil. A particularly
state, they are located within a double cryostat. The inner
effective design for whole-body use at high frequencies
chamber contains the coils immersed in liquid helium at
has been the “birdcage” concept developed by C. Hayes
4.2 K. The outer chamber contains liquid nitrogen at 77 K
and his coworkers.
as an intermediate temperature reservoir. Every few weeks
Scanners are usually provided with at least two sizes of
it is necessary to replace these cryogenic liquids as they
cylindrical rf coils. One with a diameter of about 56 cm is
boil off.
large enough to accommodate the entire body. The other,
smaller coil is about 28 cm in diameter and is designed for
C. Gradient Coils head imaging. Generally speaking, the closer a coil is to
the region being imaged, the better SNR it will provide.
In the space between the main magnet coils and the pa-
For high-resolution imaging it has now become common
tient it is necessary, for imaging purposes, to place a set
to use coils, called surface coils, that are designed to fit
of three coils, each of which is designed to produce a
more closely over the region of the body that is to be
specific gradient in B z , the z component of the static field
imaged. Therefore, specialized coils have been developed
B 0 . These coils are respectively the x, y, and z gradient
to image the spine, the neck, the shoulder, and so on.
coils. The currents in these coils are under the control of
the computer, and they can be pulsed on and off in the
proper sequence to aid in manipulating the spin system as
E. Safety Considerations
required by the imaging technique. The fields produced by
the gradient coils are much smaller than that of the main In 1976 the U.S. Congress amended the Food, Drug, and
magnet. The gradient field strengths commonly used in Cosmetic Act of 1938 to apply certain restrictions on the
present-day scanners range from 1 to 5 G/cm, which cor- introduction of new medical devices. In January 1980
responds to 0.0001 to 0.0005 T/cm. the Food and Drug Administration (FDA) responded to
During the imaging session the patient experiences an the congressional action by issuing regulations that ap-
intermittent series of tapping or banging noises. These plied to the manufacturers of new medical devices and
sounds can vary in intensity, from rather soft to practically to researchers working with such devices. The regula-
unbearable, depending on the magnitude of the currents tions, analogous to those applied to the introduction of
involved and on the degree of acoustic damping used. The new drugs, made it necessary to develop data regarding
sounds originate in the gradient coils and result from the the safety and efficacy of new devices prior to seeking
magnetic forces between the pulsed gradient currents and approval for marketing them. The NMR scanners were
the strong, static field. the first major imaging device to be subjected to these