Image Processing Reference
In-Depth Information
the new slice are affected by this refocusing. This crossing volume builds a cube
with an edge length, which is the slice thickness of the three RF pulses. Only in
this cube does a magnetization excited by the 90
°
pulse and fully rephased by
both 180
pulses exist.
The restriction of the acquired signal to this selected volume of interest assumes,
however, that all signals from other parts of the examined subject are cancelled out.
Usually, the homogeneity of the magnetic field in an examined region of the body
is very high and, therefore, the dephasing due to field inhomogeneities of the static
field within the patient is not strong enough to destroy the visible magnetization of
the regions outside of the selected voxel of interest (VOI). To improve the suppres-
sion of signals from outside the VOI, additional field gradients (so-called spoiler
°
gradients) are applied within the PRESS sequence, which are shown in
Figure 11.2
.
The effect of these spoiler gradients vanishes within the VOI due to the effect of
the refocusing 180
pulses, but outside the VOI they intensify the dephasing effect
on the undesirably excited magnetization.
The shape and the orientation of the VOI shown in Figure 11.2 is only an
example of the possibilities. In clinical examinations, the slice thickness of all three
RF pulses can be chosen independently and, also, oblique slices, realized by com-
bining of two or three of the x, y and z gradients, can be selected, resulting in a
tilted cuboid VOI. In the given example, the three selected slices are perpendicular
to each other. This is the most usual situation, but it is also possible to use other
angles between the slices, resulting in a parallelepiped as the shape of the VOI.
Curved and concave surfaces cannot be realized with this measurement technique.
Several modifications of this conventional PRESS technique for special demands
are described in the literature. Some of them are described in Subsection 11.2.4.
The second frequently used sequence for volume-selective spectroscopy is
the STEAM sequence. This technique uses the effect of a stimulated echo occur-
ring after the application of three successive pulses. The most intense signal
strength of a stimulated echo can be obtained if all three pulses are 90
°
°
pulses
(
Figure 11.3
).
The principle of volume selection is the same as with the double spin-echo
sequence in Figure 11.2. The difference is the process of signal production. The
first 90
°
pulse is used to produce transversal magnetization within a selected slice,
identical to the 90
°
pulse within the PRESS sequence (
Figure 11.4a
, Figure 11.4b).
Although most of the spins are in phase immediately after the excitation, they
begin to dephase with time under the influence of local inhomogeneities of the
static magnetic field and applied field gradients (Figure 11.4c). After a time TE/2,
the second 90
°
pulse rotates the dephased magnetization within the xy plane into
the zy plane (
Figure 11.4d
). The axes x and y are here used within a coordinate
system that rotates with the resonance frequency around the z axis. Those com-
ponents of the magnetization vectors that remain in the transverse plane after the
effect of the second RF pulse will experience a dephasing due to inhomogeneities
and spoiler gradients and, after some time, these components will cancel each
other out (Figure 11.4f ). For the longitudinal magnetization, however, no dephas-
ing occurs and, therefore, the z components of the magnetization vectors are
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