Image Processing Reference
In-Depth Information
already has a negative longitudinal magnetization. Using the three-pulse combi-
nation, a more complete water saturation can be obtained (
Figure 11.10b
).
The optimization of the homogeneity of the local magnetic field within the
volume of interest and the evaluation of the optimal RF pulse amplitudes for the
water suppression are the two adjustment procedures that are usually performed
prior to each single-voxel measurement. In the current generation of MR scanners,
these adjustments are performed automatically and take less time than manual
optimization. However, the time required for the adjustments has to be taken into
account, especially if localized spectroscopy measurements from several positions
should be performed.
11.2.4
C
E
SVS
OUPLING
FFECTS
IN
Most of the phenomena in MRI, including the principles of volume selection in
MRS, can be explained by the simple model of magnetization vectors that are
aligned to the direction of the static magnetic field in the fully relaxed state. These
vectors can be affected by the application of RF pulses and show a precession
within the transverse plane with a frequency that depends on the local magnetic
field. The length of the vectors changes owing to the influence of relaxation pro-
cesses. The amplitude and the shape of the signal peaks of some metabolites in H-
MRS, such as NAA, creatine, and choline, can be completely explained with this
simple model. Other metabolites, however, show a complex signal pattern, which
not only depends on the chemical shift of the examined metabolites, but also on
the interaction of the different protons within these metabolites. If more than one
proton in a larger molecule contributes to the signal and if these protons are not
magnetically equivalent, e.g., the two protons in water, then a coupling between
these protons occurs. The main effect of this is a splitting of the resonance peak.
Instead of only one signal, signals at two or more resonance frequencies are
obtained. The distance between these frequencies is described by the coupling
constant and, unlike the chemical shift, the coupling constant does not depend on
the strength of the static magnetic field but is characteristic for a given molecule.
The amplitude of the signals for each of the resonance frequencies, however, is
strongly dependent on the measurement sequence. The shape of the complex signal
patterns, obtained for molecules such as lactate, glutamate, glutamine, GABA, or
glutathione can be calculated using a product operator formalism based on quantum
mechanics principles (11) if the measurement parameters are exactly known. Most
important for the shape of the signal pattern of a given metabolite is the timing of
the used sequence (TE1 and TE2 in PRESS, and TE and TM in STEAM sequences)
and the used flip angles. For a given combination of measurement parameters, the
expected signal pattern can be calculated (12). In
measurements, the
sequence timing is known, but the exact values of the applied flip angles are often
uncertain, as the actual flip angles of the RF pulses at a selected position are usually
not equal to, but only near the nominal values. Further, it is not possible in volume-
selective spectroscopy to apply pulses with an ideal slice profile. Although the real
flip angle might be nearly the nominal flip angle in the center of a selected voxel,
in vivo
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