Image Processing Reference
In-Depth Information
90
90
rf
a
b
c
d
g
δ
FIGURE 10.1
The 1-1 SPAMM spin preparation sequence. Labels a, b, c, and d refer to
various time points, shown in
Figure 10.2
.
object, the number of tag planes is limited by the time required for each selective
tagging pulse.
10.2.2
S
M
M
PATIAL
ODULATION
OF
AGNETIZATION
In 1989, Axel and Dougherty developed an efficient nonselective magnetization
preparation pulse sequence called spatial modulation of magnetization (SPAMM)
[13], which produces a cosine modulation of the longitudinal magnetization. The
basic pulse sequence is shown in Figure 10.1.
The first 90
pulse rotates the longitudinal magnetization into the transverse
plane. In the standard reference coordinate system (rotating at the Lamor fre-
quency), the transverse magnetization profile along the x direction at time point
a is shown in Figure 10.2a. The gradient
°
wraps the phase of the magnetization
along the gradient direction (the x direction in Figure 10.2b) by an amount
g
φ
=
γδ
g
⋅
X
, where
X
is spatial position,
γ
is the gyromagnetic ratio and
δ
is the pulse
duration (assuming an ideal gradient pulse). The second 90
pulse rotates the
magnetization back to the longitudinal (z) direction as in Figure 10.2c. After a
period of time in which the transverse component has been removed, due to T2
relaxation or a crusher gradient, the longitudinal magnetization profile (ignoring
T1 relaxation) is shown in Figure 10.2d.
This simple tag sequence is known as a 1-1 sequence, referring to the relative
magnitudes of the two RF pulses. The sequence is typically applied on detection
of the R-wave ECG trigger at end-diastole (ED) and is followed by an imaging
sequence to acquire cine images regularly spaced through the cardiac cycle.
Variations on the imaging sequence include fast low-angle shot (FLASH), blipped
echo-planar imaging (EPI), and steady-state free precession (SSFP). The tag stripe
contrast fades with tissue T1 (about 800 msec for myocardium), and is also
influenced by the imaging procedure (e.g., EPI offers greater tag persistence than
FLASH due to reduced RF excitation).
°
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