Principles of MR Spectroscopy and Chemical Shift Imaging - Advance Image Processing in Magnetic Resonance Imaging - page 382

180 180

90

rf

G x

N y

G y

N z

G z

ADC

(a)

(b)

FIGURE 11.14 The scheme of (a) the volume preselected 2-D PRESS CSI sequence and
(b) the size and the position of the preselected volume (depicted as a black rectangle) with
respect to the spectroscopic grid (depicted as a white grid).

the VOI should be always positioned in the center of the spectroscopic grid. An
example of VOI positioning with respect to the spectroscopic grid is shown in
Figure 11.14b.
Spatial encoding of spectra is ensured by the additional phase shifts arising
from application of the phase-encoding gradients. The phase-encoding gradients
can be at different positions within the sequence. However, any signal present
during data sampling and not experiencing this phase shift will not be spatially
encoded and added to all points in k-space with the same phase. Because slice
profiles are never ideal, every 180 pulse produces, apart from the desired

refocusing effect, transverse magnetization also. This magnetization does not
encounter encoding gradients whenever they are applied before the 180

°

pulse.
In this case, the signal from this magnetization will not be encoded, and it will
contribute to the central voxel. Therefore, performing of encoding after the last
pulse in the sequence is preferred. Unfortunately, to achieve shorter echo times
and avoid eddy currents in the measured spectra, phase encoding is often not
realized after the last RF pulse of the sequence. Therefore, proper optimization

°