What defines a large ischemic core on CT perfusion and CT head for mechanical thrombectomy eligibility in stroke patients?

From the Guidelines

A large ischemic core on CT perfusion and CT head for mechanical thrombectomy eligibility in stroke patients is typically defined as an infarct core volume of >70 mL or an Alberta Stroke Program Early CT Score (ASPECTS) of <6.

Key Factors

• The DAWN trial used clinical imaging mismatch, which includes a combination of NIHSS score and imaging findings on CTP or DW-MRI, as eligibility criteria to select patients with large anterior circulation vessel occlusion for mechanical thrombectomy between 6 and 24 hours from last known normal 1.
• The DEFUSE 3 trial used perfusion-core mismatch and maximum core size as imaging criteria to select patients with large anterior circulation occlusion 6 to 16 hours from last seen well for mechanical thrombectomy 1.
• Some studies suggest that patients with large ischemic cores are at an increased risk of reperfusion hemorrhage, but the overall risk is relatively low, and these patients may still benefit from mechanical thrombectomy 1.

Imaging Modalities

• Non-contrast CT (NCCT) can be used to estimate the ischemic core, with the ASPECTS score being a useful tool to quantify the extent of hypoattenuation in middle cerebral artery (MCA) strokes 1.
• CT perfusion (CTP) can assess the risk of ischemic core by calculating cerebral blood volume (CBV), cerebral blood flow, mean transit time, and time to peak enhancement 1.
• Diffusion-weighted MR imaging (DWI-MRI) is considered the gold standard for ischemic core assessment, as it relies on cytotoxic edema to estimate ischemic core 1.

Eligibility Criteria

• The DAWN and DEFUSE 3 trials are the only RCTs showing benefit of mechanical thrombectomy >6 hours from onset, and their eligibility criteria should be strictly adhered to in clinical practice 1.
• Patients with large ischemic cores may still be eligible for mechanical thrombectomy if they meet other eligibility criteria, such as a sizable mismatch between ischemic core and either clinical deficits or area of hypoperfusion 1.

From the Research

Definition of Large Ischemic Core

• A large ischemic core is typically defined as an infarct volume greater than 50-70 mL on CT perfusion or diffusion-weighted imaging 2, 3, 4, 5.
• The specific threshold for defining a large ischemic core may vary depending on the study, with some using a cutoff of 70 mL 4 and others using 90 mL 2.
• CT perfusion thresholds, such as cerebral blood flow (CBF) < 30%, may also be used to estimate the ischemic core volume 2.

Mechanical Thrombectomy Eligibility

• Patients with large ischemic cores may still be eligible for mechanical thrombectomy (MT) if they have a significant amount of penumbral tissue, as identified by perfusion imaging 4, 5.
• The core perfusion mismatch ratio (CPMR) can be used to identify patients with large ischemic cores who may benefit from MT, with a higher CPMR indicating a greater potential benefit 4, 5.
• The treatable upper core limit for MT may be approximately 120 mL in selected patients with large ischemic cores 3.
• Patients older than 76 years with large cores (>90 mL) may be unlikely to benefit from MT 2.

Imaging Modalities

• CT perfusion (CTP) is a widely used imaging modality for assessing ischemic core volume and penumbral tissue in acute ischemic stroke patients 2, 3, 4, 5, 6.
• However, CTP may not always accurately identify the ischemic core, and other imaging modalities such as MRI may be necessary in some cases 6.
• The accuracy and reliability of CTP are debated among vascular neurologists, and it should not be used as the sole criterion for excluding patients from endovascular treatment 6.