Image Processing Reference
In-Depth Information
metabolites is the two-dimensional (2-D)-spectroscopy. Here, the measurement
is repeated several times with continuous change of a selected parameter, e.g.,
the echo time in PRESS or STEAM sequences. Whereas uncoupled spin shows
the expected exponential signal decay with T2 as the time constant, the phase
and amplitude of signals of coupled spins oscillate with a monotonously increas-
ing echo time (15). With an additional Fourier transformation, the oscillating
frequency, which is related to the coupling constant, can be obtained, and the
peak of coupled spins occurs at different positions in a resulting 2-D spectrum
(16). This correlation spectroscopy technique (COSY) can also be combined
with conventional techniques of volume selection. A major disadvantage of this
technique is the long measurement time, which is necessary to acquire data not
only at one, but at many different echo times.
11.3
PRINCIPLES OF CSI
11.3.1
B
P
ASIC
RINCIPLES
As described in the preceding section, SVS provides information about metabolite
composition in the selected volume of interest (voxel). However, in many cases
knowledge of metabolite distribution over a large area in the examined sample
is preferred. Typical examples are neurodegenerative diseases, in which lesions
often have a diffuse character. Spectroscopic imaging (SI), also called CSI, is a
method that encodes chemical shift and spatial distribution of metabolites simul-
taneously (17,18). Spectra from several voxels at different locations, instead of
one, are measured during a single measurement. In this respect, the method
combines features of both conventional MRI and SVS.
Because information about chemical shifts of individual metabolites present in
the signal of each voxel has to be preserved, the classical frequency encoding known
from conventional MR imaging cannot be applied. Instead, phase encoding is used
exclusively in CSI measurement sequences to obtain information on the spatial
distribution of signals.
Similar to MR imaging, depending on how many dimensions spectra are
spatially resolved in, 1-D, 2-D, or 3-D CSI can be distinguished. A scheme of
the simple 1-D spin-echo CSI sequence is shown in
Figure 11.12
. Following the
excitation pulse, a phase-encoding gradient G along the
x
axis is switched on for
the time
τ
. During this time, the precession frequency
ω
of all spins along the
axis
x
is modified according to
ω
()
x
= γ
xG
(11.1)
giving rise to space-dependent phase shifts
φ
(
x
) at the end of the phase encoding
φ
()
x
=−
γ
xG
τ
(11.2)
where
x
is the position of the spins along the
x
axis, assuming the gradient
isocenter at x
=
0.
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