Arterial Spin–Labeled Perfusion Imaging in Acute Ischemic Stroke
Arterial spin labeling (ASL) is an MRI method that enables the measurement of tissue perfusion without the use of exogenous contrast agents by magnetically tagging the water in inflowing blood.1 It was first proposed in the early 1990s and, since then, has been primarily a research technique. Recent advances in pulse sequence design and the more widespread availability of higher-field (ie, 3T) MRI scanners, both of which improve the signal-to-noise ratio (SNR) of the technique dramatically, have resulted in increased adoption in the clinical neuroimaging community. All 3 major MRI vendors now support some form of ASL imaging as a product.
At first glance, it would seem that ASL is a particularly ill-suited technique to image acute ischemic stroke. The ASL signal is roughly proportional to cerebral blood flow (CBF), which is markedly decreased in the core of large vessel ischemic stroke. Furthermore, what CBF does remain is often supplied by circuitous collateral routes, leading to longer arterial arrival times. Because the magnetic label decays with the blood T1 (typically between 1.2 and 1.8 s at clinical magnetic field strengths), the ASL signal will not accurately represent CBF in these regions under normal operating conditions. Finally, there is extensive research and clinical experience with another MRI perfusion method, bolus dynamic susceptibility contrast (DSC), which can be acquired relatively rapidly in the acute setting. Remarkably, despite these limitations, an increasing number of studies are showing that there is a place for ASL in the workup of acute ischemic stroke with MRI.
In this review, I highlight recommended imaging parameters for ASL stroke studies, discuss typical image quality and artifacts, review the recent clinical literature of ASL in stroke (particularly vis a vis the more routine and accepted method of DSC), mention limitations, and finally discuss the future of this promising …