Biomedical Engineering Reference
In-Depth Information
FIGURE 7.13
Throughplane shift: Registered transverse T1-weighted RF spoiled images prior to bone
marrow transplantation (a) and eight months later (b). The difference image is shown in (c).
The corresponding registered difference images in the sagittal (d) and coronal (e) plane are
also shown. With the cerebral atrophy the corpus callosum has shifted superiorly and its
superior surface encroaches on the region previously occupied by the pericallosal sulcus
and gray matter giving the high signal in (c) (arrow). The small size of shift is apparent in
(d) and (e) (arrows). Cerebral atrophy has developed and this produces low signal margins
around the ventricular system and the external surface of the brain.
7.4.3
Interpretation of Changes in Signal Intensity Combined
with Changes in Site, Shape, or Size on Difference Images
Changes in signal intensity and shifts frequently occur together. In many
pathologies, the areas of abnormal tissue have increased T1 and T2 and may
also produce mass effects. The increase or decrease in signal intensity may take
it beyond the plateau levels in the model used to describe shifts. As a result,
shifts may produce signal differences which are beyond the ''full scale'' of the
model. Using the T1-weighted 3D volume sequences described here, this
occurred with very short T1 lesions due either to subacute hemorrhage or
contrast enhancement. A lesion involving the higher plateau (e.g., white
matter) that lengthens its T1 decreases the size of the difference between the
two plateaus so the interplateau difference is less than full scale.
When the changes in signal intensity and shifts are at different sites, they
can be treated independently (as with displaced but otherwise normal tissue
around a space-occupying lesion). When they occur together, some further
