Evaluation of haemodynamic impairment and collateral flow in symptomatic carotid artery disease using arterial spin labelling MRI

Introduction: Arterial spin labeling (ASL) is a new MRI technique that provides information on cerebral hemodynamics by consecutive readout of labeled blood captured in arteries or in the microvasculature. The aim of the present study was to analyze the feasibility of ASL-MRI for the evaluation of both intracranial collateral pathways and hemodynamic compromise in patients with symptomatic high-grade stenosis or occlusion of the internal carotid artery (ICA). Therefore, we used a combination of a dynamic ASL-based MR angiography (DynAngio), and of ASL time series optimized for microvascular perfusion analysis.

Methods: 18 patients with symptomatic, unilateral ICA obstruction were investigated using ASL-MRI. We obtained a new dynamic angiography (DynAngio) providing time-resolved images of the inflow of blood into the arterial tree displayed in 36 different phases. As indicator for collateral flow, vessel visibility and time of blood bolus appearance of the proximal MCA and of intracranial collateral vessels were determined. Micro-vascular perfusion (CBF) and bolus arrival time (BAT) was measured in ASL perfusion time series using multiple TI ranging from 400 to 3200 ms. In addition, a time-of-flight MR angiography (TOF-MRA), and dynamic susceptibility perfusion MRI analyzed on time-to peak maps (TTP) were acquired.

Results: Using ASL-MRI, the inflow of labeled blood was demonstrated in good quality in 17/18 patients. DynAngio showed a delayed blood bolus appearance in the MCA ipsilateral to ICA obstruction in 82%. In these, microvascular CBF and BAT measurements showed signs of delayed and reduced perfusion in the MCA territory, corresponding to TTP maps. Anterior collateral pathways were detectable on DynAngio in 65%, collateral flow via posterior communicating artery was visible in 71%. The inflow of blood was earlier to the ipsilateral P1-segment of the PCA in 75% of patients with presence of posterior collateral flow.

Conclusions: ASL-MRI is a fast and useful technique that provides additional functional information in patients with symptomatic ICA disease without contrast agents. Dynamic assessment of intracranial collateral flow is feasible with good correlation to patency of collateral vessels on TOF-MRA. The combination of DynAngio and ASL perfusion is promising since it has the potential to demonstrate a delayed inflow of blood to both macro- and microvascular compartments of the brain circulation and collateral pathways.