Biomedical Engineering Reference
In-Depth Information
consideration is necessary. The key concept is that a difference in signal inten-
sity of a tissue or fluid in the boundary zone not only produces a change in
its own right, but also changes the signal intensity gradient so that the effect
of a shift may be increased or decreased. The detailed change depends on the
pulse sequence being employed. For example, with a T1-weighted sequence, a
lesion with increased T1 would decrease its signal intensity and also decrease
the gradient at its interface with CSF. The decreased signal intensity would pro-
duce a negative signal on the difference image and the effect of any shift in pro-
ducing a change on the difference image would be reduced.
When a lesion is characterized only by an increase or decrease in its sig-
nal intensity relative to the surrounding normal tissue (and has no mass
effect) it may be difficult or impossible to distinguish a change in its signal
intensity from a change in its size. The change in signal intensity usually
changes the signal intensity gradient in the border zone, and this deter-
mines the threshold for detecting the border of the lesion in relation to the
image noise and artifact level. In this situation, a decrease in signal inten-
sity difference between the lesion and its surroundings usually appears to
be accompanied by a decrease in size, but if the signal intensity gradient in
the border zone is maintained the lesion size may appear unchanged. A
''pure'' change in signal intensity may thus be inextricably linked to a
change in size of the lesion.
7.5
Regional and Tissue-Specific Appearances
on Difference Images
In this section, specific anatomical regions are considered in more detail and
a wider range of tissues and fluids is discussed.
7.5.1
The Ventricular System
The boundaries of the ventricles are smooth, continuous, and regular; their
signal intensity gradients are predictable both inplane and over adjacent
slices (Figures 7.12 and 7.13). The border zone for shifts in certain directions
is quite narrow. In this situation, full scale changes due to relatively small
interplateau shifts are readily seen when the shift is parallel to the maximum
signal intensity gradient. Locally monophasic curvilinear changes are typi-
cally seen with smaller border zone shifts.
Changes within the ventricles may be complicated by the presence of the
choroid plexus and other structures which may produce multiphasic
effects. Large shifts may lead to complete overlap of the ventricular system
with the brain, and there may be changes in the shape of the ventricles as
well as size.
