Biomedical Engineering Reference
In-Depth Information
2
is
minimized. This results in weak negative signals that tend to follow high con-
trast boundaries. This can be avoided by modifying the definition of
appear to be partially offset by a very slight positional mismatch when
2
to
include only those voxels for which the postcontrast scan has lower intensity
than the precontrast scan. This ensures that the pattern of enhancement does
not influence the spatial match. Algorithms that employ mutual information
as a voxel similarity measure are also likely to achieve correct alignment
when there is signal change due to contrast enhancement.
7.4.2
Interpretation of Pure Changes in Site, Shape, or Size
Changes in site, shape, or size are manifest as a local shift in position of at
least some part of the brain and its surrounding tissues or fluids. The size and
distribution of these local shifts provide information about the nature of the
overall change (i.e., whether it has been one of site, shape, size, or a combina-
tion of these).
7.4.2.1
Model for Analyzing Shifts
The effects of shifts can be analyzed using a simple model of two tissues
(and
or fluids), one with a higher plateau of signal intensity, the other with a
lower plateau, and a border zone (region of partial volume effects) between
them (Figure 7.5a). (Although Figure 7.5a is shown in one spatial dimension,
the model is used in three dimensions). At the ventricular margin with a typical
T1-weighted pulse sequence, white matter or central gray matter forms a
higher plateau, and CSF a lower one. Likewise, within the brain, white matter
forms a higher plateau and gray matter a lower one. With a heavily T2-
weighted pulse sequence, the central white matter has a low signal intensity
(lower signal plateau) and the CSF has a high signal intensity (higher signal
plateau). The principles outlined later are the same, but because of the rever-
sal in sign of the gray and white matter and CSF, the polarity of changes for
the same shift is reversed.
It is sometimes useful to regard the cortex as part of a composite (or double)
border zone between central white matter and CSF. Shifts can be thought of
as larger, in which one plateau crosses the border zone so that it overlays the
other plateau, and smaller, in which the displacement is less than the width
of the border zone.
Larger (interplateau) shifts produce a high and constant (full scale) central
signal change on difference images (Figures 7.5b, 7.5c). The full scale signal
change on the difference image is locally monophasic. It is positive when the
high signal plateau has shifted into the region of the low signal plateau (e.g.,
Figure 7.2, lateral margin of right lateral ventricle), and vice versa. The inter-
mediate regions on either side of the full scale change correspond to the two
border zones on the source images. The origin of interplateau shifts is usually
clear from the source images. The size of the shift in the direction of the
