Neuroimaging Assessment of Cerebral Vasospasm - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 583

to align the nuclear magnetization of hydrogen atoms in the body.
These hydrogen atoms return to their equilibrium state at different
rates, and these differences can be used to construct an image. It can
be used in conjunction with MRA, MRP, and MRS. MRA provides
images of the cerebral arteries with the administration of intravenous
contrast. MRP or perfusion-weighted MRI is used to assess the cere-
bral microvasculature by evaluating the change in contrast over time.
MRS is used to measure the level of different metabolites in the body
tissue, where individual metabolites have characteristic spectra.
MRI provides high quality images of soft tissue. This imaging
modality can also be coupled with intravenous contrast to provide
information about the cerebral vessels and microvasculature. The
soft tissue image quality provided by MRI surpasses that of CT.
However, it is inferior in providing information about the cerebral
vasculature. Other limitations of this modality include increased
cost, length of time required for image acquisition, high suscepti-
bility to motion artifact, and need for standardized protocols.
Several studies have used MRI to evaluate the degree of isch-
emia caused by vasospasm ( 24 ). Jadhav et al. ( 24 ) examined the
parenchymal effects of SAH in a double hemorrhage canine model.
They found that the ischemia persisted in the gray matter for up to
7 days following SAH ( 24 ). Similarly, van den Bergh et al. ( 25 )
found that cerebral ischemia peaked between day 0 and day 2 in
rats, following injection of blood into their cisterna magna. Nosko
et al. ( 26 ) studied the effi cacy of clot removal following SAH in
monkeys. They found that removing the clot within 24 h of SAH
reduced the extent of ischemia caused by vasospasm ( 26 ). The
extent of ischemia was assessed by MRI 7 days after SAH ( 26 ).
The use of MRA to assess vasospasm has been limited. Vatter
et al. ( 17 ) studied the time course of vasospasm in rats using MRI
and MRA. They found that reductions in cerebral blood fl ow and
basilar artery diameter peaked at 5 days after SAH ( 17 ). Compared
to MRA, MRP has been more widely used to study vasospasm in
animal models because it can demonstrate reductions in blood
fl ow. Vatter et al. ( 17, 27 ) utilized MRP in several studies to con-
fi rm the presence of vasospasm between days 3 and 5 after a double
hemorrhage technique in rats. MRS has also been used to confi rm
the presence of metabolites characteristic of ischemic conditions.
Domingo et al. ( 28 ) used MRI and MRS to evaluate the role of
endothelins in the development of vasospasm in rats. They found
that there was an increase in the inorganic phosphate to phospho-
creatinine ratio as well as lactate following endothelin administra-
tion, and this corresponded to diffusion defi cits seen on MRI ( 28 ).
Likewise, Handa et al. ( 20 ) studied the metabolite changes that
occur after SAH in primates. The presence of 50% narrowing on
angiogram corresponded to a signifi cant reduction in N -acetyl-
aspartate, creatinine, and phosphocreatine ratio, as well as an
increase in choline/creatine and phosphocreatine ratios on days 7
and 14 after SAH in monkeys ( 20 ).