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Introduction: Intra- and inter-individual variation in cerebral artery territory volumes is likely due to haemodynamic factors mediated by collaterals. We investigated the association of intracranial atherosclerosis (ICAD) with changes in vascular territory volumes as a potential marker of collateral function. Methods: Patients with symptomatic ICAD (50-99% stenosis) and controls were identified. Territories were mapped using a published CT Perfusion (CTP) post-processing technique where seed points for the MCA, ACA and PCA territories are manually chosen, and the arrival of contrast bolus is tracked until it arrives at the boundaries of adjacent territories. In ICAD patients, the ratio of the intracranial atherosclerosis-affected territory volume to its contralateral counterpart, and ratios of the adjacent territory volumes to their contralateral counterparts, were calculated. Ratios of paired arteries were calculated in the control group. Using the Mann-Whitney U test, territory ratios in ICAD patients were compared to control patients. Results: In 30 patients (15 cases, 15 controls), the ICAD-affected territory ratios (median = 0.79, IQR 0.46 - 0.93) were significantly smaller than the control ratios (median = 0.98, range IQR 0.87 - 1.12), p = 0.004. The adjacent territory ratios (median = 1.30, IQR 1.07 - 1.65) were significantly larger than control ratios, p<0.00001. Conclusion: This study suggests that routinely-obtained CTP-derived vascular territory maps can quantify territory volume derangement in intracranial atherosclerosis. Our findings are consistent with the hypothesis that collateral recruitment from adjacent territories maintains perfusion in ICAD-affected territories. Notably, the range of values seen in ICAD patients was wide, possibly reflecting different collateralisation patterns. This technique may be useful as an objective measure of collaterals in ICAD and to better understand hemodynamics in individual patients.

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