CT perfusion - occipital core infarct
Right sided mild weakness and visual disturbance.
CT Perfusion performed 3.5 hours after onset of symptoms.
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Plain scan shows subtle hypodensity in the left occipital lobe; this was only seen retrospectively after reviewing the CTA and perfusion images. The CTA shows tapering and occlusion of the left occipital artery (arrows). MTT is raised consistent with ischaemia. The areas of MTT elevation also have corresponding low CBV, corresponding to mainly core infarct rather than penumbra. This is supported by the established parenchymal changes.
MRI performed one month after CT perfusion.
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MRI shows established infarct in the region predicted by the core infarct in the perfusion maps.
1 case question available
In this case, the patient was not thrombolysed due to the mild symptoms only and because the CT perfusion showed a predominance of core infarct rather than penumbra.
Following review of the plain scan and CTA, the CTA source images should be viewed with narrow windows. The microvasculature in the normal perfused brain fills with contrast whereas the infarcted brain does not, which can help in identifying any hypodensities.
The ROI graphic generated from the perfusion scan provides important information about the technical adequacy of the scan. The curves should have an initial plateau, before rising and declining before the end of acquisition (if the patient has poor cardiac output, there will a long delay before the curves rise and the peak may not be reached within the perfusion window). The venous curve should have a delay of 1-2 seconds after the arterial curve and should be higher than the arterial peak. Note that the arterial and venous curves do not return to baseline straight away, but decline to a plateau first, before later returning to baseline (this occurs beyond the scanning time). This is due to recirculation of contrast (ie this is no longer first pass).
The mean transit time (MTT) is elevated in both core and penumbra. This can be attributed to the clot effect causing increased time difference between arterial inflow and venous outflow. Thus, reviewing the MTT map first is often most useful when reading perfusion scans, to decide if there is ischaemia or not. Next to decide between penumbra versus core, the cerebral blood volume (CBV) map is reviewed. In penumbra (non-critical ischaemia), auto-regulation is preserved and the release of nitric oxide causes vasodilatation, resulting in normal or increased CBV. At this stage, no hypodensity is seen on the plain scan (however subtle sulcal effacement can be occasionally seen due to raised CBV). As ischaemia progresses further into core infarct, auto-regulation fails, leading to a fall in CBV. This is reflected on the plain scan by hypodensity.
It is important to review the plain scan and CTA again in the light of the perfusion maps. Correlation between hypodensity and core infarct should be sought, as a confirmatory step. A significant defect on the perfusion maps should lead one to carefully hunt for the site of arterial occlusion on the CTA.
ASPECTS scoring does not apply to posterior circulation infarcts as in this case (although a variant Posterior Circulation ASPECTS (pc-ASPECTS) has been proposed).
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