Biomedical Engineering Reference
In-Depth Information
in the transverse plane, two types of relaxation occur. The
''longitudinal'' relaxation causes the axial magnetization
to be brought back to equilibrium and is given by
T
1
(longitudinal relaxation time). ''Transverse'' relaxation
causes the transverse magnetization to be reduced to zero
and is given by
T
2
(transverse relaxation time). In most
materials
T
2
< T
1
, and
T
1
is usually shorter for liquids
than for solids. On the other hand,
T
2
is generally greater
for liquids than for solids.
2 T. Some functional MRI systems have 3 Tor 4 T main
magnets.
The primary requirements of the main magnet is that
its field be uniform. In most cases, the magnetic field is
not uniform and for this reason ''shim'' coils are fre-
quently employed. The shim coils are a set of coils built
to produce a field that is polarized in the same direction
as the main field of known spatial dependence. There-
fore, if the main magnet's nonuniformity is known, the
shim coils can be set to carry gradients that cancel (using
superposition) the inhomogeneous components of the
main field.
Most magnetic fields are generated using permanent
magnets. These magnets are simple and affordable. Their
fringing fields are also small. Permanent magnets can also
come in different sizes and shapes. In MRI applications
with permanent magnets, the patient is placed between
the two poles of the magnets. However, temperature
drift of permanent magnets is an issue since the RF is
usually not controlled by any type of feedback and would
therefore not remain at the Larmor frequency if the
temperature changes after calibration.
For magnetic fields greater than 0.5 T, superconducting
magnets must be used. The coil windings are made from
an alloy of niobium-titanium and are cooled to a tem-
perature below 12 K using liquid helium (boiling point of
4.2 K). These fields are very strong and homogeneous.
The Larmor frequency depends on the strength of the
magnetic field
B
0
. Strong gradients for imaging tech-
nique may require the help of gradient pulsing. The
duration of these pulses must be of the order of
T
2
to
be effective. Because gradient coils have a natural self-
inductance, they cannot be switched on or off simulta-
neously. Coils that have large inductances can be driven
to steady state quickly at the expense of power by using
other driver circuits to compensate for the power
needed. Since the gradient coils can induce eddy cur-
rents in the rest of the magnet structure, to compensate,
shielding must be used. Furthermore, the desired gra-
dients, which are typically linear, must provide detailed
spatial information about the sample. Gradients for
most modern imaging schemes can be produced in any of
the three spatial directions without physically rotating
the gradient coils.
The sample under test must be irradiated with an RF
field also known as the
B
1
field. This is done to tip the
magnetization away from the equilibrium position and
generate a detectable NMR signal. The RF fields are
produced by a transmitter and an RF coil. The transmitter
determines pulse shape, duration, power, and timing. The
RF coil is responsible for coupling the energy given by
the transmitter to the nuclei. To generate an RF pulse, the
transmitter uses first a frequency synthesizer to achieve
the defined frequency. A waveform generator creates
a user-defined pulse shape, which is then mixed with the
6.1.4 MRI hardware design
The basic components of MRI hardware are shown in
Figure 6.1-11
. The magnet in MRI is responsible for
generating the static magnetic field
B
0
. The gradient coils
create a gradient which is superimposed upon the main
field
B
0
. A strong magnetic field provides a better SNR
and better resolution in both frequency spatial domains.
Most clinical imaging systems have field strengths around
RF Shielded Room
MAGNET
Gradient Coil
RF Coil
PATIENT BED
RF Coil
Gradient Coil
MAGNET
RF
Receiver
Gradient
Amplifier
RF
AMP
DSP
RF Pulse
Gradient
Pulse
Processor
Imaging
Figure 6.1-11 Hardware representation of MRI.
