Background Arterial spin labelling (ASL) is an MRI technique for measuring perfusion at the brain tissue level. The aim of the study was to investigate cerebrovascular reactivity (CVR) at brain-tissue level in patients with an internal carotid artery (ICA) occlusion by combining ASL-MRI with a vascular challenge, and determine whether the CVR varies within the perfusion territory of the brain-feeding arteries.

Methods Sixteen patients with a symptomatic ICA occlusion and 16 age-matched healthy control subjects underwent perfusion and perfusion-territory selective ASL-MRI before and after acetazolamide administration. CVR was assessed throughout the brain in the grey matter supplied by the unaffected asymptomatic ICA and the basilar artery.

Results Cerebral blood flow increased (p<0.01) in all perfusion territories after acetazolamide in the patients and controls. In the tissue supplied by the unaffected contralateral ICA, CVR was lower in the tissue supplied by the unaffected contralateral ICA in the patients when compared with the controls (22.8±16.1 vs 54.2±13.1%; mean difference, −31.5%, 95% CI −42.1 to −20.8). Within the perfusion territory of the unaffected ICA, the CVR was lower in the brain tissue on the side of the occluded ICA than on the side of the unaffected ICA (13.5±20.4 vs 26.2±16.0%; paired mean difference −12.5%, 95% CI −20.3 to −4.7).

Conclusion ASL-MRI can assess impaired cerebrovascular reactivity at the brain-tissue level in patients with a symptomatic ICA occlusion. Assessment of CVR with ASL-MRI may help to identify the tissue most at risk for future stroke and as such may guide medical treatment.