Biomedical Engineering Reference
In-Depth Information
through
plane
(a)
Object and
position of
transverse
slice
in plane
(b)
Transverse slice
(c)
Difference image:
in plane shift
(d)
Difference image:
through plane
shift
[i]
[ii]
[iii]
FIGURE 7.10
Effects of inplane and throughplane shifts. Three objects with differently oriented boundaries
between regions of high and low signal are shown (a[i]-[iii]). The position of a transverse
slice through each object is also shown (a, dotted lines). The corresponding images are shown
(b[i]-[iii]). For an inplane shift to the right the difference image shows a linear change in
both (c[i]) and (c[ii]) but no change in c(iii). For a throughplane shift a variety of patterns
are seen. There is no change in (d[i]), change along a line in (d[ii]) and a change within the
whole plane in (d[iii]). In the last case there is a steep gradient throughplane which is not
apparent from the image (b[iii]). Inplane shifts produce predictable difference signals where
there are inplane edges (c[i]) and c[ii]). However, throughplane shifts, which reveal the presence
of throughplane intensity gradients, can produce either linear or regional difference patterns
(d[ii-iii]). In this diagram white indicates positive signal, gray is zero, and black is a negative
signal.
difference image where the largest signal intensity changes occur are those
with the maximum
signal intensity gradient. These are just the
places which tend to have the smallest
throughplane
signal intensity gradients
because they arise where tissue boundaries are parallel or nearly parallel to
the slice plane (cf. Figure 7.10[iii]). Small inplane gradients imply a relatively
uniform appearance in the source image with little hint of the throughplane
differences.
A similar effect is seen in Figure 7.12, but this time it is the postero-anterior
shift (Figure 7.12d) which produces throughplane differences from the slice
in Figure 7.12a to that in Figure 7.12b. Note that simply by reformatting to the
coronal plane, the head-foot displacement that was difficult to interpret in
Figure 7.11e is transformed to the anatomically linked pattern in Figure 7.12e.
A throughplane shift in a case of cerebral atrophy is shown in Figure 7.13.
The movement of the upper surface of the corpus callosum upwards into the
region of the pericallosal sulcus and gray matter produces an increase in sig-
nal on the transverse difference image (Figure 7.13c).
inplane
