Arterial Spin Label Imaging of Acute Ischemic Stroke and Transient Ischemic Attack
Section snippets
Indications for ASL perfusion imaging
Given that bolus PWI has gained some level of acceptance in the clinical arena during the past decade, the question arises as to why one would want to perform a perfusion study using an alternative method, particularly one with less SNR. The recent recognition of the relationship between gadolinium-containing contrast agents and nephrogenic systemic fibrosis has led to both absolute and relative contraindications to such agents at many institutions. In such patients, ASL is the only option for
Previous literature on ASL findings in TIA and stroke
Several early studies documented that ASL imaging could be used to image cerebrovascular disease, such as carotid stenosis and occlusion, as well as acute ischemic stroke. Chalela and colleagues20 reported on 15 patients imaged within the first 24 hours after acute ischemic stroke. Using CASL, they found perfusion changes in the ipsilateral hemisphere consistent with the diffusion and clinical findings. They found that the quantitative CBF levels in the affected hemisphere correlated with
Typical findings on ASL studies: stroke
This section highlights some of the experiences at the author’s institution with a 3D–FSE pcASL method at 1.5 T in patients with acute ischemic stroke and TIA. In addition, whenever possible, images from bolus PWI are included so that readers may contrast the information gained from each modality.
Fig. 3 demonstrates an example of acute ischemic stroke in a 25-year-old woman 6 hours after repair of aortic coarctation. Hypoperfusion is noted within a large region of the left middle cerebral
Typical findings on ASL studies: TIA
Imaging plays an important role in the work-up of TIA, given that patients are asymptomatic at clinical presentation. Although clinical scales, such as the ABCD2 scale,40 have gained acceptance as a way to predict the risk of recurrent stroke, many studies have shown that imaging can improve the predictive ability of these scales.41, 42 Because approximately 30% of TIA patients have diffusion changes, it is not unexpected that such patients also may demonstrate perfusion lesions, either with
Problems with ASL imaging in stroke and TIA
Although it is likely that ASL will have an important role in the diagnosis or monitoring of stroke and TIA, it is important to recognize its limitations. The most important of these is the sensitivity of ASL to delayed transit times. As discussed previously, this is a double-edged sword; the sensitivity allows the recognition of subtle changes based on ATA. In severe cases and in the presence of large artery stenosis and occlusion, however, ASL may overestimate the region of CBF abnormality.
Future of ASL in stroke and TIA
Two newer ASL methods may prove to be useful in the work-up of patients with cerebrovascular disease. Velocity-selective ASL (VS-ASL) is a method in which the blood is labeled in or near the voxel of interest rather than at a distant labeling plane, and this may mitigate the errors and artifacts associated with delayed transit.45 Despite theoretic advantages, this technique has not yet been applied in a systematic fashion to patients with long delay times. Some initial evidence at the author’s
Summary
ASL is a promising technique for evaluating patients with cerebrovascular disease. Recent improvements in pulse sequence design, the more widespread use of 3T neuroimaging, and the recognition of nephrogenic systemic fibrosis have driven renewed clinical interest in this technique. Although prolonged transit times can lead to quantitative errors in ASL CBF maps, the increased sensitivity to minor perfusion alterations may be of clinical value in patients with TIA and minor stroke. Initial
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2021, Magnetic Resonance Imaging Clinics of North AmericaCitation Excerpt :In pathologic conditions that produce slow flow, the use of short PLDs yields a mixed pattern of low and artifactually high signal on ASL. The latter is known as arterial transit artifact, a characteristic feature of ischemia in which labeled signal remains trapped within arteries proximal to or surrounding the ischemic territory.24–26 In these scenarios, long-delay or multidelay ASL can be used to acquire multiple PLDs for calculation of ATT and ATT-corrected CBF maps.27–30
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2018, Journal of NeuroradiologyCitation Excerpt :Anatomic imaging may reveal subtle or no corresponding vascular abnormalities in the setting of transient small vessel vasoconstriction. ASL can identify critical areas of intravascular or cortical slow flow with secondary cerebral hypoperfusion, which if untreated can progress to irreversible infarcts (Fig. 3A–K) [16–22]. Intracranial vascular malformations are dysplastic collections of vessels that can be low-flow: developmental venous anomaly (DVA) and cavernous malformation; or high-flow: arteriovenous malformation (AVM) and arteriovenous fistula (AVF).
The author would like to acknowledge the funding sources that supported this article, including a Scholar Award from the American Society of Neuroradiology’s Neuroradiology Education Research Fund (NERF), NIH R01-NS066506–01, and the Richard S. Lucas Center for Imaging at Stanford University.