Biomedical Engineering Reference
In-Depth Information
technique may be improved by registration. It may also be possible to reduce
the dosage of contrast enhancement if subtraction is used routinely. The need
for triple dose enhancement may also be reduced.
A limitation of the technique in contrast studies is that of increased acqui-
sition times because of the need to produce isotropic volume data sets. Even
slab imaging takes 2-30 s. As a result, the technique is not particularly suited
to the timescale of dynamic studies.
7.11
Pediatrics
In children physiological, pathological, and therapeutic changes of the type
described above may be present.
19
These may be complicated by growth
and development, which may be normal or abnormal. Growth and devel-
opment includes not only increase in size of the brain, but also cortical fold-
ing, myelination, and decrease in brain proton density, T1 and T2. The
surrounding tissues and fluids may also change. Growth of the skull means
that much larger changes are generally seen in children than in adults,
where the skull generally imposes rigid limits on changes in brain size.
Because of the large changes in children, it is often useful to match exami-
nations in successive pairs so that the differences between any two image
sets are minimized (Figure 7.19). The small subarachnoid space over the cor-
tex also makes segmentation more difficult than in adults.
Changes in children may be quite complex, with, for example, the ventri-
cular system showing changes due to expansion and contraction at different
phases of growth (Figure 7.20).
In the late phase of neonatal infarction (one to nine months) increased
growth can be seen atthe margins of the lesion (Figure 7.20). The rapidly pro-
liferating tissue seen in the late phase of infarction did not have the features
FIGURE 7.19
Growth and development in a child aged 10 months (a) and 14 months (b) on registered
T1-weighted images with subtraction images (c) = (b)
(a). The white line around the gyral
tips of the brain in (c) indicates growth of the brain.
