Biomedical Engineering Reference
In-Depth Information
90
o
180
o
180
o
RF Pulse
t
NMR Signal
t
T
d
T
d
Figure 6.1-12 Lumped capacitor model for the coil in the
MRI resonator.
Figure 6.1-14 Spin echo sequencing.
pure tone and this creates an RF pulse. The pulse or a se-
quence of several pulses is repeated at a repetition rate.
There are basically two types of pulses used in NMR
machines: the hand pulses, which are typically broad, and
rectangular pulses. The other type of pulse is known as
soft pulses, which are often sinc shaped to provide fre-
quency selectivity. In soft pulsing, it is important that the
center frequency is close to the Larmor frequency.
To receive a good signal, RF coils are required to
couple the energy from nuclei in precedence to a
receiver. The RF coil must be responsive to frequencies
within the general band of the Larmor frequency of
interest. In human imaging, the wavelength is generally of
the same order of magnitude as the sample. Often, a coil
design known as the ''birdcage'' is used. This coil com-
bines lumped capacitors with distributed inductance to
form a volume resonator. If the resonance of the coil is
designed well, it should increase the efficiency of the coil
at the operating frequency. The lumped capacitors pro-
vide a method for the resonator to store energy, as shown
in
Figure 6.1-12
.
The coils must generate a very uniform
B
1
magnetic
field. Any inhomogeneity will introduce a distortion in
the images. The requirement of a uniform field places
a burden on the coil design. Therefore, it is desirable to
use a coil that allows quadrature excitation and detection
(two RF coils are needed, one for transmitting and one
for receiving). Quadrature excitation is capable of gen-
erating and receiving circularly polarized fields. To ac-
complish this, the transmittal power and received power
must be split equally into two channels, with one of the
channels having a 90
phase delay. The two channels are
then fed to the two inputs of the RF coil to produce fields
that are perpendicular to each other as shown in
Figure 6.1-13
.
6.1.5 Pulsing and NMR imaging
One ''pulsing'' scheme commonly used today is called
''spin echo'' sequences. This sequence provides a 90
pulse followed by a time delay
T
d
/2 (
T
d
is time delay),
a 180
pulse, and another time delay, as shown in
Figure 6.1-14
. The term
T
r
denotes the length of a frame.
After the 90
pulse, the magnetization lies in the
transverse plane if the pulse of 90
is applied along the
x
axis. It results in a magnetization aligned along the
y
axis
and coherence has been established. The nuclei under
the influence of stronger fields rotate faster and hence
have some net rotation at the mean Larmor frequency.
Phase
splitter
RF coil
TX
RX
Figure 6.1-13 Pulsing RF coils with perpendicular fields.
