Image Processing Reference
In-Depth Information
180
90
rf
G
x
G
y
N
y
G
z
N
z
ADC
(a)
180
90
rf
G
x
N
x
G
y
N
y
G
z
N
z
ADC
(b)
FIGURE 11.13
The scheme of (a) the standard 2-D CSI and (b) the standard 3-D CSI
spin-echo sequence.
applied with the slice-selective gradient, resulting in exciting a slice (or slab in
case of 3-D CSI). The phase-encoding gradients are then applied in the plane
parallel to the slice in 2-D CSI, with the third gradient (called partition-encoding
gradient) pointing perpendicular to the slice in 3-D CSI.
After the spatial reconstruction, spectra from each element of a matrix, which
is called the spectroscopic grid, are available. In 2-D CSI, the size of the grid
corresponds to the field of view (FOV) of the CSI experiment. In 3-D CSI, several
grids with the same FOV, each corresponding to one partition of the excited slice
thickness, are available. The number of voxels in the spectroscopic grid (and also
the number of partitions) depends on the number of phase-encoding steps per-
formed in the sequence along corresponding directions.
To encode positions of the voxels, the sequence with all combinations of phase-
encoding increments in all directions has to be repeated. For 3-D CSI with N
x
,
N
y
, N
z
steps (voxels) along corresponding direction, and with N
acq
representing
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