Principles of MR Spectroscopy and Chemical Shift Imaging - Advance Image Processing in Magnetic Resonance Imaging - page 373

signals because the chemical shift of these signals are specific and different from
that of water. This separation can fail, however, if contributions of the water signal
get superimposed on the signals from metabolites. Although water has a specific
chemical shift value, a typical water signal from an measurement has a

certain range. The ''foot” of the signal especially can have a considerable width
and can, therefore, get superimposed on the signals of metabolites. The width of
the water signal is usually described by the width of the peak at the half of the
maximum (usually referred to as full width at half maximum, FWHM) and
depends on the range of frequencies within the selected volume element, which
can be strongly reduced by shimming. However, the difference in the signal
intensities between water and the metabolites is in the range of 1000:1 and,
therefore, even with a very good shim, a superposition of signals from the foot
of the water signal on the metabolite signals will occur. This effect can be reduced
if a water suppression technique is used. The simplest technique is the application
of a frequency selective RF pulse prior to the first excitation of the volume
selection part within a sequence (Figure 11.8). This technique was first used for

in vivo

rf

G z

G y

G x

Signal

0 25 50

Time (ms)

75 100

FIGURE 11.8 The implementation of a water suppression pulse within a PRESS

sequence. The additional pulse (denoted by the bold line) is applied with a large pulse
duration and without a gradient prior to the volume selection part (denoted by the dashed
line) of the sequence.