Page 247 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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

972 Magnetic Resonance in Medicine

temperatures near absolute zero can conduct electricity
with absolutely no electrical resistance and, therefore, no
associated heating or energy loss. Electric currents, once
startedinaloopofsuperconductingwire,havebeenshown
to persist for years without decreasing measurably, even
though no source of electrical energy was used to maintain
them. Unfortunately, the original superconducting mate-
rials that were discovered, such as lead and tin, could not
be used to generate strong magnetic fields because as the
current was increased beyond a rather small value the su-
perconducting state was destroyed and the electrical resis-
tance was restored. In the 1960s a new class of supercon-
ducting materials capable of carrying much higher current FIGURE 6 Cutaway drawing of magnet geometry. A four-coil ge-
ometry is used to produce the static field. The inner coil shown
densities was discovered. During the next decade these
is for the transverse gradient field. (Courtesy of GE Medical
materials, particularly in the form of niobium–titanium Systems.)
alloys, became the basis of a new class of high-field mag-
nets. These were used to make NMR spectrometers for
chemical research applications that were capable of gen-
erating much stronger fields than had previously been that are undesired. The first-order expansion terms repre-
available. They were only big enough, however, to con- sent gradients in all three directions, dB z /dx, dB z /dy, and
tain small, test-tube-sized samples. Small-bore, high-field dB z /dz. These gradients and a large number of additional
systems are now available for chemical research that pro- higher order error terms can be eliminated by correctly
vide steady fields in the range of 20–25 T and with NMR placing the proper number of ampere turns at specific
proton frequencies approaching 1000 MHz (1 GHz). locations along the axis. For example, a six-coil design
By the early 1980s the Oxford Instruments Company commonly utilized in clinical magnets can theoretically
of Oxford, England, had produced whole-body supercon- eliminate all the contaminating spherical harmonic terms
ducting magnets capable of reaching 1.5 T. At the present up to the 12th order. The use of these carefully calcu-
time several manufactures build whole-body magnets of lated coil designs greatly increases the volume within the
this type. magnet over which the homogeneity specifications can be
In 1986 a still newer class of superconducting materials met.
was discovered, capable of maintaining their supercon- There are, however, many sources of slight manufactur-
ducting properties to temperatures much higher than the ing errors that prevent the ideal field from being obtained.
previously known materials. These may eventually have For example, the individual coils may be slightly out of
an application in MRI, perhaps by eliminating the need to round, or slightly out of position along the z axis or not
immerse the coils in liquid helium. The current-handling oriented absolutely perpendicular to the z axis. To cor-
capabilities of these new materials, however, are at present rect for these inevitable manufacturing tolerances, each
too weak to permit their use in whole-body magnets. This magnet is equipped with a set of shim coils; up to a
situation may improve after further research. dozen or more independent coils are usually available.
The exceptionally high homogeneity of MRI magnets Each of these coils, wound on a cylindrical coil form
is achieved in two steps: (1) during the basic coil design near the inner surface of the main field coils, has a dif-
and (2) by the use of shim coils during operation. The ferent geometry. The geometry of a given shim coil is
basic approach is to use a set of coils about 1.5 m in di- chosen to produce a field near the magnet center that has
ameter positioned along the z axis of the magnet (Fig. 6). a pattern closely approximating a single spherical har-
The contribution of each coil to the B 0 field is determined monic. By adjusting the current in each shim coil inde-
by its location along the z axis, its radius, and the num- pendently, it is possible to cancel out the residual errors
ber of turns of superconducting wire wound on it mul- associated with each of the lower order harmonics. The
tiplied by the current in the coil. The z component of shim coils carry much less current than the main coil
the resulting magnetic field can be represented as an ex- windings and, therefore, may be either superconducting
pansion about the center of the magnet by using specific or resistive. Sometimes both resistive and superconduct-
mathematical functions, the spherical harmonics. The ze- ing shim coils are provided. The current settings necessary
roth order of this expansion is the perfectly uniform field, for the shim coils to produce the maximum homogeneity
B z = constant, that is the desired field. All other terms in for a given magnet is determined at the time of magnet
the expansion represent contaminating inhomogeneities manufacture in a process called shimming. This process