Biomedical Engineering Reference
In-Depth Information
(a) baseline
(b) follow up
(c) overlay
(d) difference
[i]
[ii]
[iii]
[iv]
FIGURE 7.8
Effect of shift size. Baseline image profiles (a), successively displaced image profiles (b),
overlays (c) of (b) on (a), and differences (d) for increasing shifts [i]-[iv]. As the shift is increased,
the height of the difference increases to a maximum and the width of this maximum then
increases. The area under the difference curve is directly proportional to the shift difference.
The effect of inplane shifts is shown in Figure 7.9. There is an increase in
signal intensity on the side to which the high signal tissue or fluid has shifted
on the difference image. The difference is maximal at the point of the maxi-
mum signal intensity gradient in the direction of the shift. When the displace-
ment is perpendicular to the local intensity gradient, no signal intensity
change is seen. This effect is seen in the right lateral ventricle in Figure 7.2c,
where a positive change is seen at the lateral margin and a negative change
at the medial margin.
The effect of rotation is to produce differential shifts. These produce
changes on the difference images where there is a radial edge, since this has
a circumferential intensity gradient component that is parallel to the local
shift direction. The displacement increases with radial distance from the cen-
ter of rotation.
The mechanism for signal change produced by throughplane shifts is the
same as for inplane shifts, i.e., changes are produced on difference images
whenever the shift has a component parallel to the local signal intensity gra-
dient. However, the visual appearance of difference signals produced by
throughplane shifts is more heterogeneous than that produced by inplane
shifts (Figure 7.10).
The practical effects of both inplane and throughplane shifts for a transverse
and a coronal slice taken from the same 3D data set are shown in Figures 7.11
and 7.12. The images were produced by shifting the volume data by a single
voxel (approximately 0.97 mm) in each of the principal matrix directions in
turn and then subtracting the original images from the displaced images.
