Biomedical Engineering Reference
In-Depth Information
7.13
Multiple Sclerosis
There is considerable interest in monitoring the progress of multiple sclerosis
(MS) with and without treatment; some cohorts have now been studied for
several years. The expected annual change in lesion burden is about 5 to 10%.
This is of the same order as misregistration errors inherent with conventional
techniques, thus assessing the effectiveness of new therapies such as inter-
feron may be difficult in short term studies.
Twelve patients were studied on two or more occasions. Registration not
only allowed changes to be identified in specific lesions, but also separation of
differences in contrast enhancement from other changes using ''difference of
difference'' images (i.e., difference of post-minus precontrast enhancement
scans at two different times). Changes in unenhanced lesion T1 were much
more readily recognized with the subtraction images, including effects due to
both increased and decreased T1. Obvious changes were seen in the T1 of areas
where no enhancement was observed. Decreased brain size was observed in
patients who had treatment with steroids and others who did not have this
treatment.
Subvoxel registration revealed that many lesions are more complicated
than they appear at first sight, with some undergoing remission and others
in very close proximity undergoing exacerbation with both parallel and dis-
cordant contrast enhancement. Also observed was shortening (as well as
lengthening) of T1 in the acute phase.
For example, a 31-year-old woman with MS was examined with and without
contrast enhancement on two occasions six months apart. Many contrast-
enhancing lesions were either much better seen or only seen on the registered
images. There were changes in lesion signal intensity and the degree of
enhancement, as well as evidence of decreased brain size on the follow-up
scans (Figure 7.22). This may have been due to brain swelling at the time of
the initial examination that had been partly or completely resolved when the
follow-up study was performed. It may also have been due to progressive
atrophy. The patient did not receive steroids. Registration and quantification
may also be used to detect and monitor atrophic change in MS.
21
7.14
Tumors
On the registered difference images, increased tumor size was manifest as
encroachment on the ventricular system and subarachnoid space.
22, 23
Figure
7.23 (a) shows an astrocytoma grade III before treatment. After one month,
the registered difference image (Figure 7.23 (c)) shows tumor expansion in
