Biomedical Engineering Reference
In-Depth Information
FIGURE 7.18
Contrast enhancement Wegener's disease: T1-weighted spin echo images (SE 720/20) before
(a) and after (b) contrast enhancement; T1-weighted volume images before (c) and after (d)
contrast enhancement, with subtraction image, (d)
(c), shown in (e). The tentorial enhance-
ment is best seen on the registered volume and subtraction images (e) (arrows).
to overt misregistration. The same absolute displacement may ap-
pear as only a slight difference in position of a thick slice, and so
permit valid image interpretation, but when a narrow slice thick-
ness is used, the same displacement may take the region of interest
into a completely different slice and create uncertainty in interpre-
tation. Precise registration enables the patient position to be main-
tained, and allows the advantage to thin slices to be utilized.
(e)
Follow-up studies
. Assessing changes in the degree of enhancement
between examinations may be rendered difficult by misregistration
between examinations. This problem can be overcome by accurate
alignment of images on serial examinations.
The technique may also be used to provide angiography.
In a number of conditions, such as detection of metastases and demonstra-
tion of enhancement in multiple sclerosis (MS) plaques, scans delayed by one
or two hours may be more useful than immediate postinjection scans. Mis-
registration problems are usually increased in this situation, since the patient
is taken out of the scanner and positioned back again later. Results from this
