Biomedical Engineering Reference
In-Depth Information
T,R,l
S
(a)
(b)
(c)
(d)
(e)
C-TE
FIGURE 7.1
Implementation of registration (STRICTER). The first image obtained is shown on the left
and is shaded black (a). The second image (shaded gray) is obtained at a different position
in the same coordinate system (c). Segmentation (S) removes the skull and scalp from the
first image (a) to give (b). Translation (T) Rotation (R) and sinc Interpolation (I) of voxel
values are used to produce image (d) which is matched with the segmented version of the
first images (b). A Chi-squared (C) Test (TE) is used to asses the accuracy of this match.
Following this first attempt as a match, more accurate values for the translation and rotation
are chosen in order to reduce the value of
2
and the process is repeated. When the optimal
match has been found in this way the second image is Reformatted (R) into the same position
as the first image. The original, unsegmented first image (a) can now be compared with the
reformatted second image (e) which is precisely aligned.
direction head to foot to avoid aliasing. Data matrices of 152
256
114 with
two NEX or 192
256
140 and one NEX were acquired in a volume of 25
25
3
18 cm to produce nominally isotropic phase-encoded resolution of 1.6 mm
or
3
1.3 mm
, respectively.
Two-dimensional multislice images were acquired with a Gaussian slice
profile at a TR of 6140 msec and a TE of 80 msec for 64 slices. Slice thickness
(full width at half maximum) was 4.4 mm with a 50% overlap, yielding a slice
separation of 2.2 mm. A 25 cm field of view with a 125
125 matrix was used
8,11,12
to provide approximately isotropic sampling.
Registration proceeded in the order segmentation, translation, rotation
and interpolation, chi-squared test, and reformatting (STRICTER). The stages
are illustrated in Figure 7.1.
This procedure produces an optimal global match for brain tissues. The
inclusion of a segmentation step, by which the skull and all extracranial tis-
sues are excluded from the process of determining the spatial transformation
from one image to another (the transformation T in Chapter 3), ensures that
the correct global transformation for the brain is found. This is a precautionary
measure ensureing that extraneous tissue changes, which can be substantial
with some disease processes or over long periods of time, do not influence the
