Principles of MR Spectroscopy and Chemical Shift Imaging - Advance Image Processing in Magnetic Resonance Imaging - page 363

of the three available field gradients within the magnet. To select a volume instead
of a slice, three slice selections with different orientations are necessary. The selected
volume (volume of interest) is then built by those spatial points that are part of all
three selected slices, i.e., the intersection of the three selected slices.
The combination of different slice selections can be realized within a single
measurement sequence in such a way that only signals from the selected volume
of interest are obtained (single-shot measurement). Two types of sequences use
this principle: the PRESS sequence (point-resolved spectroscopy (1), with a 90 °

excitation and 2 180 refocusing pulses and the STEAM sequence (stimulated

echo acquisition mode (2), in which 3 90

°

pulses are used. These single-shot
sequences are mostly used in H-MRS, but they require a certain time difference
between the first excitation and the beginning of the data acquisition (echo time).
For measurements with other nuclei, e.g., phosphorus, it might be necessary to
realize short echo times. This is possible if three measurements with different
magnetization preparations are performed, and the obtained signals are combined
in an appropriate way. This measurement technique is called ISIS (3), and is
described at the end of this chapter.

°

11.2.1 PRESS STEAM

AND

The scheme of the PRESS sequence is shown in Figure 11.1. In the upper line,
the three RF pulses can be seen. Each of them is applied simultaneously with a

TE1 TE2

rf

G z

G y

G x

Signal

0 10 20

Time (ms)

30 40

FIGURE 11.1

PRESS sequence with three RF pulses applied simultaneously with field
gradients along the main axes of the magnet. Only the first part of the data acquisition
time is shown.