Biomedical Engineering Reference
In-Depth Information
Z
Z
Z
Z
Y
Y
Y
Y
X
X
X
X
Figure 6.1-15 Nuclei under the influence of a magnetic field.
Figure 6.1-16 Nuclei under the influence of a magnetic field.
The nuclei under the influence of a weaker magnetic field
will rotate slowly, one lag behind, and experience a neg-
ative sensed net rotation in the frame, as shown in
Figure 6.1-15
.
If we now apply a 180
pulse, also on the
x
axis, each
individual magnetic moment will be rotated 180
about
the
x
axis (
Figure 6.1-16
). Now the faster nuclei are
behind the mean nuclei and the slower nuclei are ahead.
This means that the faster nuclei eventually will cross the
path of the slower nuclei and coherence is again estab-
lished (
Figure 6.1-16
).
The peak value of the ''echo'' signal is given by
E
0
¼ð
free induction decay
Þ
exp
T
d
T
2
The inversion recovery imaging that uses the spin
echo is shown in
Figure 6.1-17
, where
G
s
is the slice
selection gradient,
G
f
is the required readout gradient,
and
G
f
is the phase-encoding gradient. The sequence
starts with an RF pulse at 180
in conjunction with
a slice selection gradient. The remainder of the se-
quence after a time
T
i
is the same as the spin echo
imaging sequence. The spin echo sequences employ 90
and 180
RF pulses.
The inversion recovery imaging technique can be
performed in conjunction with the 2D Fourier transform
technique. We first define the slice to be imaged out of
a 3D object by applying a steep gradient perpendicular to
this slice. Next it is necessary to phase encode each of the
longitudinal directions of the 2D slice. This will be ac-
complished by applying a gradient pulse along each of
(6.1.24)
from which the value of
T
2
can be obtained.
RF Pulse
Gradient
Pulses
Gs
G
φ
G
f
NMR
Signal
Ti
Te
T
R
Figure 6.1-17 Recovery of imaging.
