Biomedical Engineering Reference
In-Depth Information
g
G
ym1t
p
g
G
ym2t
p
j
(y)
j
(y
2
)
Gym2
j
(y
1
)
Gym1
gradient
control
pulses
0
y
1
y
2
y
FIGURE 16.45
Phase encoding method for the relative
y
-axis for two locations,
y
1
and
y
2
.
gradient
pulse
symbol
G
y
t
gradient
control
pulses
t
t
t
p
FIGURE 16.46
Pulse sequences for several consecutive lines. Note the amplitude of
G
y
pulses changes on
sequential lines. TR is the repetition time between lines. Usually the amplitudes for
G
y
range from a maximum
value,
G
ymax
, through a negative value of the same magnitude.
scanning along
is needed. Typically, 128 or 256 different excitations are utilized, and each
one has to be uniquely linked with a specific
y
position. Pulse and excitation sequences for
several consecutive lines are shown in Figure 16.46.
y
16.3.6 The Magnetic Resonance Imaging Sequence
Almost all the necessary bits and pieces needed for the imaging process have been intro-
duced. Now, step by step, the different controls and detection methods will be reviewed
and assembled into a sequence of events for imaging. The process will be simplified slightly
for clarification. Figure 16.43 shows an
image plane that contains the isocenter, or center
of magnetic field coordinate system, for a head scan. How net magnetization signals from
specific locations in this plane translate into magnetic fields, detected waveforms, and ulti-
mately into an image will be demonstrated.
Once the subject is within the MRI magnet, a series of sequence pulses are sent, as
shown in Figure 16.47. In the figure, two rf pulses, one a
x
-
y
p
/2 pulse and one a
p
pulse,
force the magnetization vectors to precess into the
x
-
y
plane to set up spin echoes, as
described in Section 16.3.4.The
-axis gradient pulses occur over the same intervals
as the rf pulses. They turn on the gradient coils, which, in combination with the frequency
selected for the rf pulses, select the
G
z
z
z
location of the
x
-
y
image plane, as detailed in
Section 16.3.5.
The gradient pulses for the
-axis occur during the precession phase and also over the
interval during which the spin echoes are formed. The field must be on during these
x
