Image Processing Reference
In-Depth Information
different field gradient. After the excitation by the first 90
pulse, transversal
magnetization is produced within a slice perpendicular to the z axis in this case.
This magnetization starts to dephase and, after a time TE1/2, the magnetization
of a part of the slice is refocused by the first 180
°
pulse in the same way as in
conventional spin-echo sequences. The spin echo occurring at the time TE1 is not
evaluated, the magnetization dephases again, and the second 180
°
°
pulse is applied
at the time TE1
TE2, where the data
acquisition time starts. In contrast to imaging sequences, in which the spin echo
occurs usually in the center of the data acquisition time, the acquisition of data in
spectroscopy sequences usually starts at the center of the spin echo and lasts for
several 100 msec. This is necessary to be able to analyze small frequency differ-
ences of a few hertz in the chemical shift of the observed molecules.
The use of the double spin echo is necessary for the realization of the desired
spatial selectivity. The steps of volume excitation are depicted in Figure 11.2.
After selective excitation of a slice in a xy plane by the first 90
+
TE2/2 to give an echo at the time TE1
+
°
pulse, the
refocusing effect of the first 180
pulse is restricted to a slice in a xz plane.
Therefore, the first spin echo occurs only for spins within a bar that includes
those spatial positions that are part of both these slices. At all other parts of the
first excited slice, the magnetization at the echo time TE1 is dephased due to the
missing refocusing effect of the 180
°
pulse again affects
a different slice, which is in a yz plane in the example of Figure 11.2. Only those
spins that are located in the crossing volume of the previously selected bar and
°
pulse. The second 180
°
rf
G z
G y
G x
Signal
0
10
20
Time (ms)
30
40
FIGURE 11.2
Selection of a cube with a PRESS sequence. The three RF pulses within
the sequence are marked, and the selected regions after each pulse are shown for a cubic
object.
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