Biomedical Engineering Reference
In-Depth Information
(a)
baseline
(b)
follow up
(c)
overlay
(d) difference
[iv]
[iii]
[i]
[ii]
FIGURE 7.6
Profiles of two plateaus and a border zone showing the effect of change in signal intensity
gradient size for a constant border zone shift. Initial image profile (a), shifted image profile
(b), overlay (c) of (b) on (a), and subtraction of (a) from (b) is shown in (d) for vertical signal
change (infinite slope) [i] and decreasing slopes [ii], [iii], and [iv]. As the slope is decreased,
the width of the difference profile is increased and the height decreases after image [ii]. This
makes the change less easy to detect against background noise (not shown). The area under
the curves (d) is the same in each case for the constant shift which is illustrated.
area of a parallelogram, base
height). The steep gradient produces the
highest contrast to noise value; eventually as the slope decreases, the change
on the difference image becomes so slight that it merges into the noise of the
image and is not detectable.
7.4.2.3
Effects of Signal Intensity Gradient Direction
At the junction between the higher signal tissue plateau (e.g., white matter)
and the border zone, it is possible to draw an iso-intensity contour (a line of
constant signal intensity and hence zero signal intensity gradient). The local
direction of maximum signal intensity gradient is perpendicular to this
(Figure 7.7). The same applies to the junction between the border zone and the
lower signal intensity plateau. The size of the gradient generally decreases as
the width of the border zone increases, since the two plateaus have a constant
signal intensity difference. An iso-intensity contour divides the image into vox-
els of higher signal intensity on one side and lower signal intensity on the other.
It is convenient to indicate the signal intensity gradient direction with an arrow
showing the direction of
signal intensity across a contour because
then, if the shift has a component parallel to the direction shown, there will be
an
decreasing
in signal on the difference image. If the shift has a parallel compo-
nent, but in the opposite direction, there will be a decrease in signal on the dif-
ference image. A high signal intensity gradient inplane generally corresponds
to a low signal intensity gradient throughplane and vice versa.
increase
