ReviewInterpretation of fluid-attenuated inversion recovery vascular hyperintensity in stroke
Graphical abstract
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.
References (56)
- et al.
FLAIR can estimate the onset time in acute ischemic stroke patients
J Neurol Sci
(2010) - et al.
Vascular hyperintensity on fluid-attenuated inversion recovery indicates the severity of hypoperfusion in acute stroke
J Stroke Cerebrovasc Dis
(2020) Collaterals in acute stroke: Beyond the clot
Neuroimaging Clin N Am
(2005)- et al.
FLAIR-hyperintense vessel sign, diffusion-perfusion mismatch and infarct growth in acute ischemic stroke without vascular recanalisation therapy
J Neuroradiol
(2014) - et al.
Significance of hyperintense vessels in negative DWI area as a prognostic factor after thrombolysis
J Neurol Sci
(2017) - et al.
Pitfalls of 3D FLAIR brain imaging: A prospective comparison with 2D FLAIR
Acad Radiol
(2012) - et al.
Imaging features and safety and efficacy of endovascular stroke treatment: A meta-analysis of individual patient-level data
Lancet Neurol
(2018) - et al.
MR of the brain using fluid-attenuated inversion recovery (FLAIR) pulse sequences
AJNR Am J Neuroradiol
(1992) - et al.
Wake-up stroke: Clinical characteristics, imaging findings, and treatment option - an update
Front Neurol
(2014) - et al.
Wake-up stroke and stroke of unknown onset: A critical review
Front Neurol
(2014)
Fluid-attenuated inversion recovery (FLAIR) sequences for the assessment of acute stroke: Inter observer and inter technique reproducibility
J Neurol
Negative fluid-attenuated inversion recovery imaging identifies acute ischemic stroke at 3 hours or less
Ann Neurol
MRI-guided thrombolysis for stroke with unknown time of onset
N Engl J Med
Distal hyperintense vessels on FLAIR: an MRI marker for collateral circulation in acute stroke?
Neurology
Prognostic value of hyperintense vessel signals on fluid-attenuated inversion recovery sequences in acute cerebral ischemia
Eur Neurol
Fluid-attenuated inversion recovery images and stroke outcome after thrombolysis
Stroke
Can FLAIR hyperintense vessel (FHV) signs be influenced by varying MR parameters and flow velocities? A flow phantom analysis
Acta Radiol
Time-of-flight effects in MR imaging of flow
Magn Reson Med
Diagnostic and prognostic value of early MR Imaging vessel signs in hyperacute stroke patients imaged <3 hours and treated with recombinant tissue plasminogen activator
AJNR Am J Neuroradiol
Angiography reveals that fluid-attenuated inversion recovery vascular hyperintensities are due to slow flow, not thrombus
AJNR Am J Neuroradiol
Different meaning of vessel signs in acute cerebral infarction
Neurology
Correlation between hyperintense vessels on FLAIR imaging and arterial circulation time on cerebral angiography
Magn Reson Med Sci
Do FLAIR vascular hyperintensities beyond the DWI lesion represent the ischemic penumbra?
AJNR Am J Neuroradiol
Fluid-attenuated inversion recovery vascular hyperintensity: An early predictor of clinical outcome in proximal middle cerebral artery occlusion
Arch Neurol
Distal hyperintense vessels on FLAIR: A prognostic indicator of acute ischemic stroke
Eur Neurol
Hyperintensity of distal vessels on FLAIR is associated with slow progression of the infarction in acute ischemic stroke
Cerebrovasc Dis
Do FLAIR vascular hyperintensities beyond the DWI lesion represent the ischemic penumbra?
Am J Neuroradiol
Hyperintense vessels on FLAIR: Hemodynamic correlates and response to thrombolysis
AJNR Am J Neuroradiol
Cited by (10)
Motor sequela of adult and pediatric stroke: Imminent losses and ultimate gains
2023, Handbook of Clinical NeurologyFLAIR Vascular Hyperintensities as a Surrogate of Collaterals in Acute Stroke: DWI Matters
2023, American Journal of Neuroradiology