Biomedical Engineering Reference
In-Depth Information
baseline
(a)
(b)
follow up
with shift
difference:
(c)
(d)
difference:
difference:
(e)
[i]
[ii]
FIGURE 7.14
Intensity profiles showing effects of shifts on sulci and fissures. Wedge [i] and trough [ii]
profiles are shown in (a). A displacement is shown in (b) and the corresponding differences
in (c). Effects of sulci moving together (d) and apart (e) are also shown.
accurate images of local changes, the matching process can be restricted to
smaller volumes around the area of interest.
7.5.5
Blood Vessels and Venous Sinuses
With the T1-weighted sequence used in most of this study, only minimal
inflow effects were seen in proximal arteries, but changes in signal intensity
were more obvious with angiographic sequences. The signal intensity in
many vessels reflected the T1 of blood which is intermediate between brain
and CSF. The blood vessels may be circular, elliptical, or triangular in cross
section, and subtraction of these following an increase or decrease in size
may produce different effects that can be predicted from the plateau model
(Figure 7.15). There may also be positional shifts of blood vessels.
7.5.6
Meninges
Because the images are matched to the brain, shift of the brain within the
cranial cavity may appear as a shift of the dura and skull into the subarach-
noid space surrounding the brain. This shift then shows up along the
smooth outline of the dura and adjacent skull rather than the convolutions
of the cortex.
7.5.7
Other Extracerebral Tissues and Fluids
The process of registering the brain also aligns tissues and fluids surrounding
it, as long as these are in fixed relation to the brain. If they are not in fixed rela-
tion, differences will be seen on the subtraction images. Such tissues may
undergo changes in their own right, and these need to be interpreted with
