Beyond the flow: the evolving role of magnetic resonance imaging in arteriovenous shunt evaluation.
Although digital subtraction angiography remains the gold standard for the diagnosis and treatment planning of intracranial arteriovenous shunts (AVS), including arteriovenous malformations and arteriovenous fistulas, non-invasive imaging is increasingly sought for comprehensive evaluation. Magnetic resonance imaging (MRI) plays a crucial role in AVS detection, identification of feeding arteries and draining veins, localization of shunt points, classification of subtypes, assessment of venous reflux and congestion, and evaluation of post-treatment residual or recurrent lesions. Clinical techniques such as time-of-flight MR angiography (MRA), contrast-enhanced time-resolved MRA, susceptibility-weighted imaging, and arterial spin labeling (ASL) are established for these assessments. Recent advances have expanded MRI capabilities: Ultrashort echo time MRA can overcome turbulent flow-related signal loss and susceptibility artifacts, improving visualization of complex nidus architecture; compressed sensing substantially accelerates three-dimensional and four-dimensional (4D)-MRA while maintaining diagnostic quality; ASL-based 4D-MRA provides high-temporal-resolution dynamic evaluation without contrast, with vessel-selective techniques enabling independent assessment of individual vascular territories; and high-resolution vessel wall imaging shows promise for risk stratification. Emerging artificial intelligence applications enable automated AVS segmentation and characterization, with potential to enhance image quality and reduce scan times. This review summarizes current MRI techniques, recent innovations, and future perspectives in the non-invasive assessment of intracranial AVS.