Image Processing Reference
In-Depth Information
z
B 0
M
α
y
B l
x
FIGURE 1.4 Trajectory of M after an RF pulse.
based on the characteristics of this function. For example, the envelope function
of the widely used rectangular RF pulse (see Figure 1.5a ) is given by:
B
τ
otherwise
0
≤≤
t
t
τ
τ
/
2
=
1
Bt
()
=
B
(1.14)
1
1
0
where
is the pulse time width. Another frequently used RF pulse is the following
sinc pulse (see Figure 1.5b):
τ
"
Bcf
sin [
π
(
t
τ
/
2
)]
0
≤ ≤
t
τ
Bt
()
=
1
ω
(1.15)
1
0
otherwise
When B 1 is switched off, M continues to precess, describing a cone at some
angle
to B 0 as shown in Figure 1.4. This is called the flip angle (FA); it depends
on the strength of the B 1 field and how long it is applied.
We can adjust the value of the FA by changing the duration
α
or the amplitude
of the envelope function B 1 ( t ). If we turn off the transmitter RF after M has
precessed down into the transverse plane, this is called a
τ
π
/2 (or 90
°
) pulse. A
/2 pulse is commonly used as an excitation pulse, because it elicits the maxi-
mum signal from a sample that is in equilibrium. An RF pulse twice as long, a
π
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