Time-Resolved MR Angiography in Wake-up Stroke: An Innovative Application of a Proven Technique

Abstract

Stroke is a common disorder with high incidence of disability and death. Cerebral ischemia foreven a minute leads to extensive neuronal death with catastrophic results for patients. Treatment strategies focus on minimizing neuronal loss and reviving neurons on the brink of ischemia. Imaging plays a critical role in assessing the cause of acute and transient ischemic strokes and extent of neuroparenchymal involvement (1). Accuracy and speed of diagnosis are important for initiating treatment within the golden hours or the window period to reduce morbidity and mortality. Computed tomography angiography (CTA), digital subtraction angiography (DSA), and magnetic resonance (MR) angiography (MRA) have all been widely used to study vascular conditions leading to strokes. DSA and CTA involve exposure to radiation and require administration of iodinated contrast media that are nephrotoxic (2,3). MRA has replaced conventional DSA in screening for disease in intracranial and neck vessels because of its noninvasive and nonionizing character. Although the resolution of MRA is not as high as that of conventional angiography, it is sufficient to evaluate pathologic conditions in the carotid and vertebral arteries in the neck and arteries of Circle of Willis. MRA makes use of the MR effect in differentiation of blood flow motion and stationary vessel walls and tissues surrounding them. These techniques are not applicable in the case of patients with vascular disease having slow or disturbed flow (2). A wide array of MRA techniques such as time-of-flight MRA (TOF-MRA), phase-contrast MRA (PC-MRA), contrast-enhanced MRA (CE-MRA), and time-resolved (TR) three-dimensional (3D) MRA have beenused for imaging of the neck and intracranial vasculature and to identify steno-occlusive vascular disease. CE-MRA is rapidly evolving asaclinicallyviableanddiagnostically useful toolthat iseasy to implement in a routine stroke protocol, is minimally invasive, and provides high-contrast vascular images. Arterial catheterization and use of large volumes of iodinated contrast can thus be avoided. The technique uses T1 shortening gadolinium contrast, thereby diminishing the severity of flow-induced signal loss. High-resolution images of cerebral vascular vessels are obtained, wherein vascular pathology is demonstrated better than the images acquired using non‐CE-MRA methods. CE-MRA to evaluate status of cervical and intracranial arteries and dynamic susceptibility contrast-perfusion imaging have been used in comprehensive stroke protocols to determine presence of hypoperfused ‘‘at-risk’’ tissue. In the evaluation of steno-occlusive disease, the 3D TOF-MRAtechniqueisrecommendedforarterialevaluation and the two-dimensional (2D) TOF or 2D PC-MRA technique for venous evaluation. For the evaluation of aneurysms and arterio‐venous malformations (AVMs), 3D CE-MRA technique was recommended, especially with dynamic se