Biomedical Engineering Reference
In-Depth Information
FIGURE 7.2
Astrocytoma grade III: Registered contrast-enhanced T1-weighted RF spoiled images
before (a) and four weeks after (b) treatment with Temozolamide. The difference image
[(b)
(a)] is shown (c). The white line around the right lateral ventricle (large arrow) is
largely due to an interplateau shift (c) of the ventricles towards the midline. There are
also increases in signal intensity in (c) due to increased contrast enhancement (arrowed).
Border zone shifts are also seen in the right hemisphere where the tumor is present (small
arrows) (c).
fluids being studied. Second, it was possible to use the precisely registered
images to produce subtraction images for direct visualization of change,
and, third, precise registration provided a basis for accurate serial measure-
ments either directly from the images or through the use of computerized
algorithms.
The overall approach was to examine baseline, registered follow-up, and
subtraction images simultaneously. Collateral information was obtained
from regions apart from those of immediate interest, including tissues
other than the brain. This provided a means of recognizing artifacts and
checking the fidelity of the registration. On each image, validation of the
registration process was assessed by examining regions where there were
boundaries between tissues that produced steep changes in signal inten-
sity and checking that these were reduced to the noise level of the differ-
ence image. The large amount of data acquired and the ability to review
the images in different planes ensured that this could be performed at
many different locations.
Interpretation of accurately aligned baseline and follow-up images follows
established principles. In this study, the sequences used were mainly sensi-
tive to tissue mobile proton density, T1 and T2. Disease processes which
increased mobile proton density T1 and T2 included inflammation, demyeli-
nation, edema, and most tumors. Diseases that tend to decrease proton den-
sity T1 and T2 included late scar formation, calcification, and subacute
hemorrhage. The effect that these changes produce on image signal intensity
can be determined using simple models.
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