Image Processing Reference
In-Depth Information
z
z
y
′
y
′
90-degree
pulse
x
′
x
′
z
z
(c)
(d)
M
0
y
′
180-degree
pulse
y
′
x
′
x
′
z
z
(a)
(b)
y
′
y
′
90-degree
pulse
x
′
x
′
(e)
(f )
FIGURE 1.7
Rotating frame view of a magnetization subjected to an IR sequence.
inverts the magnetization from positive z axis to negative z axis, it is sometimes
referred to as an inversion pulse. During the period TI, magnetization relaxes
exponentially to the equilibrium position at a rate determined by T 1; the mag-
nitude of magnetization along the z axis (M
z
) at a time T I after the 180
°
pulse
is given by
MM
z
=
0
12e
[
−
(
−
TI T
/
1
]
(1.27)
Hence, the magnetization present at the end of the period T I is dependent
on the ratio T I/T 1. The magnetization following relatively short and long T I
periods is shown in Figure 1.7c and Figure 1.7e, respectively. During the period
T I, the magnetization, though not at equilibrium, is completely oriented along
the z axis (i.e., M
x
=
M
y
=
0); hence, an MR signal cannot be observed during
this delay.
Application of a 90
°
pulse causes the magnetization present at that time to
rotate by 90
°
about the x axis. Thus, at the end of the 90
°
pulse, the magnetization
will be directed along the negative y
axis, depending on
the value of TI chosen (Figure 1.7d and Figure 1.7f, respectively). In the subse-
quent time period, the transverse magnetization generated by the 90
′
axis or the positive y
′
pulse allows
a FID to be observed. The initial amplitude of the FID will be proportional to
z
the z component of magnetization changes as a function of the parameter T I for
different T1 values.
°
M , remaining at the end of the delay period TI. In
Figure 1.8
it is shown how
1.7.3
S
PIN
E
CHO
The spin-echo (SE) pulse sequence consists of an initial 90
°
pulse followed by
-echo
sequence. The rotating frame view of an on-resonance magnetization subjected
a 180
°
pulse after a period TE/2 (time of echo), that is a 90
°
-TE/2-180
°
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