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

970 Magnetic Resonance in Medicine

2
for water is D = 3.7 × 10 −5 cm /sec. During a time t II. NUCLEAR MAGNETIC
the random-walk motion associated with self-diffusion RESONANCE SCANNERS
will give each molecule an average displacement equal AS MEDICAL INSTRUMENTS
√
to Dt. For t = 10 msec this gives an average displace-
ment of 6 µm and for t = 100 msec, 19 µm. These dis- A. System Considerations
tances are comparable with average cell sizes. This shows
The overall operation of a NMR scanner is controlled by
that during a typical T 2 interval a given water molecule
a computer (Fig. 3). It provides the pulse-timing infor-
has time to encounter much of the intracellular and local
mation for the gradient and radio-frequency transmitter
extracellular environment, and therefore to average out
coils. It also switches on the preamplifier and the receiver
the inhomogeneities in local field implied by the cellular
circuitry during the time when data is being acquired from
heterogeneity.
the nuclear spins. The data are acquired as free induction
decay or, more commonly, spin echo signals. Extensive
mathematical calculations, usually a two-dimensional
G. Chemical Shift
Fourier transformation, are required to convert the FID
As discussed, the electrons orbiting each atom respond in or spin echo data into an image and the calculations are
a diamagnetic fashion when an external field is applied. done by the computer. For permanent magnet systems
As a result, the total magnetic field at the nucleus of an no magnet power supply is required; for superconducting
atom is some what less than the externally applied field. magnet systems the magnet power supply is needed only
The exact value of this reduction depends on the details initially when the magnet is being energized.
of the electron orbits in the given molecule and will be The NMR signal is sufficiently weak that interfering
different in different molecules. Consequently, in a given electrical noise is a major consideration. To minimize
external field and for a given type of nucleus, the nuclear outside electrical interference, some form of electrical
resonant frequency will vary from one type of molecular screening is normally placed around the scanner or the
environment to another. This change in frequency δf is scan room, and electrical filters are used on those circuits
called the chemical shift. It is usually small, and δf /f 0 that could transmit outside noise to the receiver. Figure 4
can be specified in parts per million. These small shifts shows a patient within a screened room being positioned
are easily measured, however, and in MR spectroscopy for scanning. In Fig. 5 the patient is seen within the magnet
they are the basis for discriminating one molecular species bore in the location that permits scanning to be done.
from another. In MRI the chemical shift leads to an image The strength of NMR signal generated by the precess-
artifact that slightly confuses the boundaries between fat ing spins is proportional to the degree of alignment of
and water-containing tissues. Protons in water and fat have the spins [M 0 in Eq. (2)] and to their rate of precession
resonant frequencies differing by 3.5 ppm. (γB 0 ). Since both those quantities are proportional to field

FIGURE 3 Block diagram of an NMR scanner. (Courtesy of GE Medical Systems.)