Biomedical Engineering Reference
In-Depth Information
plateau 1
signal
intensity
border zone
(a) baseline
plateau 2
distance
(b) follow up
larger shift
inter-plateau
shift
(c) difference
(d) follow up
smaller shift
border zone
shift
(e) difference
FIGURE 7.5
Plateaus and border zones showing interplateau and border zone shifts. A tissue or fluid
with a higher signal intensity in shown in plateau 1 and one with a lower signal intensity
is shown in plateau 2 (a). This is appropriate for central white matter (plateau 1) and CSF
(plateau 2) using a T1-weighted pulse sequence. There is a border zone (region of partial
volume effects) between them, in which signal intensity changes with distance. A larger,
interplateau shift of the image is shown in (b) and subtraction of (a) from (b) is shown in
the difference profile (c). This has a central full scale change (plateau 1 minus plateau 2)
with two intermediate zones on either side. A smaller border zone shift, which is less than
the border zone in width is shown in (d). This results in a difference profile (e) which is
smaller than (c) in both amplitude and width.
maximum signal intensity gradient is equal to the width of the full scale
region plus a fraction of the width of the border zone region (see later).
Smaller shifts (where higher and lower plateaus do not overlap) primarily
involve border zones. On difference images they produce smaller changes
in signal intensity than the full scale changes seen with interplateau shifts
(Figures 7.5d, 7.5e and Figure 7.2, right hemisphere). The precise effect
depends on the size and direction of the local signal intensity gradient and
the shift.
7.4.2.2
Effect of Signal Intensity Gradient Size
In the limit of a vertical slope (idealized maximum signal intensity gradient)
there is a step function change in signal intensity on the difference image
over the width of the shift. (It is really an interplateau shift; see Figure 7.6.)
With smaller signal intensity gradients (less than vertical slopes) the same
shift results in a rise that is more gradual and to a lower height, but over a
wider area. The area under the difference curve is the same in each case (i.e., the
