Reliability of CT perfusion in the evaluation of the ischaemic penumbra

Abstract

CT perfusion (CTP) is part of the initial evaluation of stroke patients, allowing differentiation between infarcted tissue and the ischaemic penumbra and helping in the selection of patients for endovascular treatment. This study assessed the reliability of the qualitative evaluation CTP maps in defining the ischemic penumbra and identified potential pitfalls associated with this technique. We reviewed CTP scans of 45 consecutive patients admitted to our institution with anterior circulation acute ischaemic stroke. Two neuroradiologists performed qualitative evaluations of cerebral blood volume (CBV) and mean transit time (MTT) maps, using 24h follow-up non-contrast CT as surrogate marker for the area of definitive infarct. For each slice analyzed, the area of qualitative alteration in the CBV and MTT maps was classified as either being inferior, equal or superior to the area of infarct on the follow-up CT. Three out of 45 (7%) patients had admission CT CBV abnormalities larger than follow-up lesions; 34/45 (76%) patients had infarct areas smaller than initial MTT prolongation. In the group of patients with no recanalization 12/19 (63%) had infarct areas smaller than initial MTT lesion. CBV abnormality is a reliable marker for an irreversible ischaemic lesion, although rarely it may overestimate the ischaemic "core", possibly due to delay in contrast arrival to the brain. In the majority of patients without recanalization, MTT overestimated final infarct areas, probably because it does not differentiate true "at risk" penumbra from benign oligaemia. Qualitative evaluation of CBV and MTT maps may overestimate the real ischaemic penumbra.

Keywords: CT perfusion; acute stroke; brain infarction; cerebral blood volume; endovascular treatment.

Figure 1

Initial CT-CBV map in a patient with acute left middle cerebral artery occlusion (M1 segment) demonstrates a fronto-parieto-temporal area of CBV reduction, representing the ischaemic core.

Figure 2

24h follow-up CT in the same patient and at the same level as Figure 1 reveals no major parenchymal hypodensity matching the CBV defect, suggesting reversibility of the ischaemic core.

Figure 3

Admission CT-MTT map in a patient with acute occlusion of the top of the left internal carotid artery shows an extensive area of MTT prolongation, representing the total territory of the left middle cerebral artery. The patient was submitted to intra-arterial thrombectomy, which failed to remove the thrombus.

Figure 4

24h follow-up CT in the same patient and at the same level as Figure 3 demonstrates a small hypodense area of acute infarction at the left fronto-opercular region. No other areas of acute hypodensity were found in the remaining territory of the middle cerebral artery.