Biomedical Engineering Reference
In-Depth Information
Vessel through
plane
Vessel in
plane
(a)
Baseline images
(b)
Difference:
inplane shift
Difference:
expansion
(c)
Difference:
contraction
(d)
FIGURE 7.15
Views of blood vessels (inplane and throughplane) and surrounded by CSF for a T1-weighted
image are shown in (a). The difference for an inplane shift is shown in (b) and the effects
of expansion (c) and contraction (d) on difference images are also shown.
reference to the brain. Susceptibility effects from air in the nasal sinuses may
impact on brain signal with T2-weighted sequences. The scalp shows changes
in most subtraction images consistent with differences in blood distribution
within it. Changes are seen in the neck due to changes in the relative position
of the head and neck at the craniovertebral junction. Marked contrast enhance-
ment is seen in blood vessels, the nasal mucosa, scalp, and the skin.
7.6
Artifacts and Failed Registration
Difference images are subject to all of the artifacts present on the source
images, and subtraction usually makes these more obvious. Artifacts may be
produced by global motion of the subject as well as more localized motion of
structures such as the eyes and the pharynx. The use of phase encoding in
two directions as required by the 3D acquisition increases the vulnerability of
the sequence-to-motion artifact in different directions. In addition, phase
wrap artifacts, in which signals from outside the defined field of view or at
the edges of the excited slab get aliased to appear within the images, can be
intrusive but are usually easily recognized. For the 2D multislice acquisitions
fully interleaved single sequences were used, with all slices being excited
within each TR. This ensured a coherent data set, even if artifacts were present.
