Biomedical Engineering Reference
In-Depth Information
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FIGURE 7.9
Effect of the shift direction (inplane). Profiles (left) and plans (right) of an object are shown
in the initial position (a) with shift to the right (b) and on the corresponding difference image
(c). Difference image profiles along the lines AA and plans are shown for an upward inplane
displacement (d) and counter-clockwise rotation (e). The changes in signal on the difference
images can be directly related to the signal intensity gradients and shifts. A shift that is
perpendicular to the local signal intensity gradient produces no change on the difference
image (e.g., c, arrow).
Figure 7.11a shows a transverse slice taken from the volume set, and Figure 7.11b
shows the adjacent slice. The difference images in Figures 7.11c-7.11e show
the effect of left-right, postero-anterior, and head-foot shifts, respectively, of
just one voxel. The inplane shifts in Figures 7.11c and 7.11d produce predict-
able results in which edges (border zones) in Figure 7.11a are accentuated
as high or low signal according to the pattern of the local signal intensity
gradients.
The result of the throughplane displacement (Figure 7.11e) is more com-
plex. The origin of individual regions of positive or negative signal difference
can be determined by careful comparison of Figures 7.11a and 7.11b, since
Figure 7.11e is simply Figure 7.11b minus Figure 7.11a. However, while some reg-
ions contain curvilinear changes of the same type seen with inplane shifts, in
many areas the overall effect does not correspond overtly to the anatomic
structures visible in Figure 7.11a or 7.11b. This is because the regions on the
