Principles of MR Spectroscopy and Chemical Shift Imaging - Advance Image Processing in Magnetic Resonance Imaging - page 370

rf

G z

G y

G x

Signal

0 10 20

Time (ms)
(a)

30 40

(b)

FIGURE 11.5 Visualization of the volume excited by the three RF pulses within a

STEAM sequence (three orthogonal slices) in comparison to the selected cube of interest.

application of a second field gradient, which has the opposite dephasing effect.
The position of this second spoiler gradient within the sequence scheme has to be
chosen in such a way that only the desired transverse magnetization within the
region of interest is affected, but not the unwanted magnetization within other
parts of the examined subject. Strong spoiling is especially necessary if small
structures with relatively high signal intensities occur in the examination volume.
In H-MRS examinations of the brain, such a critical structure is the subcutaneous
fat along the skull. The fat layer is partly very thin, but gives a very strong signal
compared to the metabolite signals that should be examined. The adequacy of the
spoiling within a sequence for brain H-MRS can, therefore, be checked by the
strength of unwanted fat signals (6).
The production of unwanted transverse magnetization is not limited to the
excitation pulses of the STEAM sequence. This problem arises also in the PRESS
sequence, which uses refocusing 180

° pulses. The central parts of the slices

selected by these pulses, in which a 180 flip angle is realized, do not experience

any additional magnetization. The transverse magnetization produced by the first
pulse remains, and in other parts of the examined subject, the 180

°

pulse
produces only negative longitudinal magnetization, which does not lead to any
signal. Due to the unavoidable imperfect slice profile of the refocusing pulses,
however, there are zones of 90

°

° excitation at the borders of the slices, selected

by the 180 pulses. The spins within these zones have transverse magnetization

after the occurrence of the 180

°

pulse, and they make undesirable contributions
to the signal if sufficient spoiling is not applied. If this second gradient pulse is
applied prior to the third pulse, it compensates for the spoiling effects of the

°

magnetization from the volume of interest, but not for the unwanted signals

outside this volume ( Figure 11.6 ).