Review
Interpretation of fluid-attenuated inversion recovery vascular hyperintensity in stroke

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Abstract

Fluid-attenuation inversion recovery (FLAIR) vascular hyperintensity (FVH) is a common presentation on brain magnetic resonance images of patients with acute ischemic stroke. This sign is known as a sluggish collateral flow. Although FVH represents the large ischemic penumbra and collateral circulation, the clinical significance of FVH has not been established. Varying protocols for FLAIR, treatment differences, and heterogeneity of endpoints across studies have complicated the interpretation of FVH in patients with acute stroke. In this review article, we describe the mechanism of FVH, as well as its association with functional outcome, perfusion-weighted images, and large artery stenosis. In addition, we review the technological variables that affect FVH and discuss the future perspectives.

Introduction

Fluid-attenuation inversion recovery (FLAIR) is a commonly used sequence in routine magnetic resonance imaging (MRI) of the brain. It has a long repetition time (TR) and echo time (TE) with an inversion recovery pulse that effectively nullifies signals from the cerebrospinal fluid (CSF).1 Moreover, FLAIR is an indispensable MRI sequence in for patients with acute stroke as it can be used to estimate the age of stroke lesions.2, 3 According to previous studies, negative findings on FLAIR imaging suggest that a stroke lesion is <6 h old, which can be used as evidence to administer intravenous tissue plasminogen activator (IV-tPA).4, 5, 6, 7

Besides estimating the time of stroke onset, other interesting features of FLAIR have been reported. One of the well-known features is the FLAIR vascular hyperintensity (FVH) sign. In moyamoya disease and stroke, the sluggish collateral flows that are distal to the occlusion site appear hyperintense against the dark CSF background. Recently, the FVH sign has received considerable attention as it can be used as a surrogate marker to predict collateral status and patient outcomes.8, 9, 10 Although numerous studies have investigated FVH signs, the results are contradictory, which complicates the accurate interpretation of FVH signs in stroke. Thus, this article reviews the principles of the FVH sign, a controversial issue, and the clinical significance of FVH.

Section snippets

Mechanism underlying FVH presentation

A slow blood flow is presumed to be the most probable mechanism for FVH, as reported by a previous phantom study and a correlation study with cerebral angiography. A flow phantom analysis demonstrated that FVH signs are visible only when the velocity of the blood-mimicking fluid is <5.72 cm/s (Fig. 1).11 This phenomenon can be explained by the following MRI principles. As FLAIR is a spin echo sequence, blood protons in the setting of normal hemodynamics move out of the imaging slice when a

Can FVH be a surrogate for perfusion-weighted image (PWI)?

Many studies have found consistent associations of FVHs with PWI. Lee et al. reported that more prominent FVHs have larger diffusion-perfusion (prolonged mean transit time) mismatch.8 Although Gawlitza et al. reported contrasting results of predictive prognosis, compared with that reported previously, they reported consistent findings for the association of FVHs with PWI. The degree of FVH sign was significantly correlated with perfusion deficit (prolonged time to peak) and

Prediction of large artery stenosis or occlusion

The association of FVHs with large artery occlusion or stenosis is well known.14, 33 A multi-center study reported that FVHs detected proximal arterial occlusions with a specificity and sensitivity of 0.86 and 0.76, respectively. 27 A subsequent study of patients with transient ischemic attack also showed that those with FVHs were significantly more likely to have severe stenosis or occlusion than those without FVHs.34 Although stenosis or occlusion were rarely examined in asymptomatic

Technological variables affecting FVH

It should be noted that some variables might affect FVHs; therefore, results of FVHs should be interpreted cautiously. A phantom study demonstrated that a flip angle (FA) of refocusing pulse and TE may affect the signal intensity of FVHs. With a higher FA and lower TE, a faster flow can be visualized on FLAIR sequences.11 The FA of refocusing pulse determines the dephasing effect from mixed spin and stimulated echoes, and TE is associated with intravoxel dephasing; thus, the FA and TE might

FVH in the anterior cerebral artery (ACA), posterior cerebral artery (PCA), and basilar artery

FVHs have rarely been investigated in the context of ACA territory infarction. In a previous study, FVHs were found in 26 of 41 patients (63.4%) with acute ACA territory infarction. Their FVH scores were associated with perfusion deficit and perfusion-diffusion mismatch.44 FVHs within PCA have also been less reported than MCA because of a smaller number of patients with infarctions in the PCA territory and the anatomical characteristics of the PCA including a short and tortuous pathway compared

Appearance of “white snake sign” is consistent with FVH

As 3D high-resolution black-blood images are widely used in clinical settings, hyperintense arteries are frequently encountered on gadolinium-enhanced 3D T1-weighted black-blood fast-spin echo imaging of patients with acute stroke. This sign was first mentioned as a “white snake sign” and is also interpreted as a collateral sluggish flow-like FVH.47 A white snake sign may be explained by the combined T1-shortening effect of gadolinium and phenomenon of flow void loss. A recent study revealed

Future perspectives

The clinical significance of FVHs would be enhanced if good and poor collaterals are differentiated. This review perceives that the extent of FVHs can be modified by varying the MRI parameters. Thus, the extent of FVHs should be adjusted in agreement with the region where the perfusion deficit of Tmax is > 6 s. The clinical significance of FVHs would be enhanced if good and poor collaterals are differentiated. This review perceives that the extent of FVHs can be modified by varying the MRI

Conclusion

Although the FVH signs have been studied extensively, its clinical applicability has not yet been determined. However, because FVH can be a surrogate over PWI and complements magnetic resonance angiography, persistent efforts are needed to determine its potential applications. In this paper, we reviewed the mechanism of FVH, controversies associated with its clinical application, variables that might affect FVH, and future perspectives. Generally, FVH sign may indicate favorable functional

Disclosure of interest

The authors declare that they have no competing interest.

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