4D flow MRI assessment of extracranial-intracranial bypass: qualitative and quantitative evaluation of the hemodynamics

Our aim was to assess the feasibility of using time-resolved 3D phase-contrast (4D flow) MRI to characterize extracranial-intracranial (EC-IC) bypass. We enrolled 32 patients who underwent EC-IC bypass (15 men, 17 women; mean age 66.4 years). In all, 16 underwent radial artery graft (RAG) bypass and 16 underwent superficial temporal artery (STA) bypass. 4D flow MRI, time-of-flight (TOF) magnetic resonance angiography (MRA), and computed tomography angiography (CTA) were performed. Bypass patency, flow direction, and blood flow volume (BFV) of each artery were determined by 4D flow MRI. Arterial diameters were measured by TOF-MRA and CTA. We compared RAG and STA bypasses by evaluating the flow direction and BFV of each artery. We evaluated the correlation between arterial diameters (measured by CTA or MRA) and the BFV and the detectability of flow direction (measured by 4D flow MRI) of each artery. 4D flow MRI confirmed the patency of each bypass artery. Flow direction of the M1 segment of the middle cerebral artery and BFV in the bypass artery differed between RAG and STA groups (p < 0.01). BFV in the bypass slightly correlated with the diameters on CTA (p < 0.05, R (2) = 0.287). Of the 29 arteries in the circle of Willis, nine were not depicted on 4D flow MRI. Cutoff values for arterial diameters on CTA and TOF-MRA for detecting the artery on 4D flow MRI were 2.4 and 1.8 mm, respectively. 4D flow MRI provided unique information for characterizing EC-IC bypasses, although this detectability is limited when addressing small arteries with slow flow.