Image Processing Reference
In-Depth Information
1200
TE = 300 ms
TE = 280 ms
TE = 260 ms
TE = 240 ms
TE = 220 ms
TE = 200 ms
TE = 180 ms
TE = 160 ms
TE = 140 ms
TE = 120 ms
TE = 100 ms
TE = 80 ms
TE = 60 ms
TE = 40 ms
TE = 20 ms
1000
800
600
400
200
0
3.5
3
2.5
2
chemical shift (ppm)
FIGURE 11.11
In vitro
spectra of glutamate, acquired at different TE values with a
STEAM sequence.
all possible flip angle values between that in the center of the voxel and the zero
value occur between the center and the borders of the voxel. The signal pattern of
a coupled system might, therefore, be different at different positions of the voxel,
and only the mixture of all these signals can be obtained in SVS. The strong
dependence of the signal pattern on the echo time of the used sequence was
described in detail by Ernst and Hennig (13) and is shown in Figure 11.11 for a
sample with a glutamate solution. The sample was measured with a STEAM
sequence (TR 6 sec, TM 10 msec) using a whole body 3-T system (Siemens Trio)
and with TE values between 20 msec and 300 msec. Three different groups of
signals can be separated, and the frequencies of these groups are unchanged in
all measurements, but especially for the multiplet at 2.1 ppm, the amplitude
variations of the individual peaks lead to an almost complete cancellation of the
signal at echo times larger than 70 msec. In
measurements, additional
superpositions with signals from other metabolites occur, which lead to an even
stronger cancellation of signals. The observation of signals from molecules with
coupled protons often requires, therefore, the use of short echo times in SVS but
the superposition of signals will continue to make difficult the separation of
signals from different metabolites. This problem can be partly overcome if editing
sequences are used or series of sequences with varying measurement parameters
are employed.
So-called homonuclear spectral editing techniques are characterized by addi-
tional frequency selective RF pulses within the sequence. These pulses can be
used as a filter for signals from coupled spins if additional signal subtraction
techniques are used (14). This technique can be combined with PRESS or
STEAM sequences. Another possibility of improved separation of different
in vivo
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