Biomedical Engineering Reference
In-Depth Information
7.4
Interpretation and Registered Subtraction Images
Interpretation of precisely registered difference images involves additional
considerations. Once residual changes due to artifacts and misregistration of
deformed or displaced extracerebral tissues have been discounted, changes
on the difference images can be attributed to (a) change in the intrinsic signal
intensity of one or more tissues or fluids; (b) change in site, shape, or size of one
or more tissues or fluids; or (c) a combination of (a) and (b). These changes can
generate either positive or negative signals on difference images. In order to
understand the type of change present and to locate the anatomical context
in which the change has occurred, one must refer to the source image from
which the difference image has been generated.
7.4.1
Interpretation of Pure Changes in Signal Intensity
When changes in signal intensity of a tissue or tissues occur, individual or
groups of voxels will have different intensity values, which can be detected
provided the change dominates over the level of artifact and noise in the
images. The detection of changes in signal intensity is usually simpler on dif-
ference images than on source images because signals from unchanged tis-
sues and fluids are reduced to a common background level (shown
schematically in Figure 7.3). This can be particularly useful for detecting
signal
intensity
distance
(a) baseline
(b) follow up
(c) difference
FIGURE 7.3
Profiles of the signal intensities of tissue and
or fluid with distance demonstrates the effect of
a local change in signal intensity on the difference image. The follow-up scan (b) shows a
small change in one region. The difference in this region is more clearly shown by subtracting
(a) from (b) to give (c). This shows the local increase in signal intensity.
