Introduction: The aim of the study was to assess the impact of collaterals on the evolution of hypodensity on non-contrast CT (NCCT) in anterior circulation stroke with reperfusion by mechanical thrombectomy (MT).

Methods: Stroke patients with middle cerebral artery occlusion who were reperfused by MT in early and late time windows were retrospectively included. Artificial intelligence (AI)-based software was used to calculate hypodensity volumes at baseline NCCT (V1) and at follow-up NCCT 24 hours after MT (V2), along with the difference between the two volumes (V2-V1) and the follow-up (V2)/baseline (V1) volume ratio (V2/V1). The same software was used to classify collateral status on a 4-point scale, where a score of zero indicated no collaterals and a score of three represented contrast filling of all collaterals. The volumetric values were correlated with the collateral scores.

Results: Collateral scores had a significant negative correlation with V1 (p = 0.035), V2, V2−V1, and V2/V1 (p < 0.001). In cases with a collateral score of 3, V2 was significantly smaller or absent compared to V1; in those with a collateral score of 2, V2 was slightly larger than V1, and in those with scores 1 and 0, V2 was significantly larger than V1. These relationships were observed in both early and late time windows.

Conclusions: Collateral status determined the evolution of baseline hypodensity on NCCT in patients with anterior circulation stroke who had MT reperfusion. Damage can be stable or reversible in patients with good collaterals, while in those with poor collaterals, tissues that initially appear normal will frequently appear necrotic after 24 hours. With good collaterals, the damage is stable or reversible, while with poor collaterals, normal-looking tissue frequently becomes necrotic in follow-up exams. Hence, acute hypodensity represents different states of ischemic brain parenchyma.