Individually, diffusion and perfusion imaging provides valuable
functional information about ischemic brain parenchyma. Diffusion
imaging detects severely ischemic dysfunctional tissue in which the
neurons are either actually or imminently irreversibly damaged. It
provides a measure of neuronal tissue that is irretrievably infarcted
and is not thought to be salvageable with thrombolysis. In contrast,
perfusion imaging detects all tissue affected by arterial hypoperfusion,
including both irreversibly damaged tissue detected by
diffusion-weighted imaging and tissue that is potentially viable if
local blood flow is restored.
By combining the two imaging methods, it is possible to detect the
ischemic penumbra. The penumbra is an area of viable tissue located
around the periphery of infarcted tissue.1 The size of the diffusion
abnormality most likely increases if the abnormality detected on
perfusion imaging is larger than that on diffusion-weighted imaging.[11]
This mismatch between a larger perfusion and smaller diffusion
abnormality is thought to reflect the presence of an ischemic penumbra.
Such a mismatch is more likely to occur if the proximal artery supplying
the affected area is occluded,[20] suggesting that angiographic
appearance may also be an important predictor of outcome.
Cellular ischemic changes are thought to occur more slowly in the
ischemic penumbra, a reflection of a less severe perfusion deficit at
this peripheral site. Combined diffusion-perfusion provides a unique
opportunity to visualize and quantify the ischemic penumbra and to
identify whether salvageable tissue exists for which thrombolysis would
be effective. Thrombolysis aims to restore perfusion to this area before
cellular damage becomes irreversible, thereby salvaging tissue.
Practical Considerations
As novel therapies for the treatment of stroke are developed, they
will likely be more effective the earlier after the insult they are
applied. This factor creates challenges for current medical practice. At
present only 4% of patients present to hospitals nationwide within 3
hours of symptom onset.[25] If MR imaging is to be performed in subjects
presenting with hyperacute ischemic symptoms, it should be both rapid
and widely available to minimize delays in progression to treatment. A
recent study indicated that a rapid imaging protocol is feasible in
subjects with hyperacute stroke.[27] An average MR imaging time of less
than 15 minutes was achieved in 41 subjects, with a sequence protocol
comprising T2-weighted turbo gradient- and spin-echo images and
echo-planar perfusion and diffusion-weighted images. The mean time from
entering the emergency department to beginning MR imaging was 45
minutes. If a rapid hyperacute imaging protocol is to be offered before
acute stroke treatment, possible delays related to clinical evaluation,
transportation, and MR imaging need to be minimized. Such coordination
of effort represents a significant organizational challenge.
Furthermore, both image postprocessing and radiological interpretation
need to be rapid, if imaging results are to influence patient care.
Figure 13. By combining MR angiography with
diffuse-weighted imaging, both the site of arterial occlusion and distal
ischemic damage can be identified. (A) MR angiogram demonstrates that
the site of occlusion is the distal M1 segment of the right middle
cerebral artery. (B) Diffusion-weighted imaging confirms infarction to
the right parietal cortex with (C) an early T2-weighted change.
It is important that any imaging method offered for hyperacute stroke
reliably detect intracerebral hemorrhage and exclude the presence of
this treatment contraindication. Previously, CT was thought to be
superior to MR imaging in detecting hyperacute hemorrhage. A recent
study, however, has confirmed that susceptibility-weighted MR imaging
performed with an echo-planar T2* sequence[14] can reliably detect
hemorrhage.
Finally, MR angiography might be added to a hyperacute imaging
protocol to assess proximal arterial patency (Fig. 13). This concern is
likely to be of particular importance if combined intravenous and
intra-arterial thrombolysis becomes an approved treatment for subjects
with proximal arterial occlusion. In this situation, MR angiography
would render important diagnostic information, which would change
patient management.
Even further in the future, some of these techniques might also be of
use in defining responses to treatment. Where cardiologists rely upon
electrocardiography to detect a reocclusion of coronary vessels and to
consider the possibility of repeated thrombolysis, imaging may allow
such decisions to be made rationally in the future for stroke patients.
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