Jeremy Douglas Collins1
1Mayo Clinic, Rochester, MN, United States
Synopsis
4D Flow MRI is a volumetric flow technique extending the 2D flow paradigm into both space and time. This method has demonstrated utility in arteriovenous malformations, intracranial atherosclerosis, intracranial venous disease, assessing abnormalities in CSF flow, and identifying high risk flow features in intracranial aneurysms. Additionally 4D flow MRI has identified new insights into potential sources of embolic stroke.
Introduction
4D Flow MRI is a volumetric flow technique with applications in Neuroimaging and embolic stroke evaluation. Technical enhancements in the pulse sequence, notably novel acceleration schemas reaching 10x, access to higher field scanners to improve the velocity-to-noise-ratio, and improved dynamic range through the use of dual velocity encoding has opened doors to Neuroradiology applications. Embolic Source Evaluation
Volumetric flow enables assessment of left atrial flow and quantitation of left atrial appendage stasis, which has been shown to better align with embolic risk than clinical risk scores. 4D flow MRI is unique among imaging methods to quantitatively capture stasis - part of Virchow's triad - to move beyond clinical risk scores and assessment of chamber volume to predict thrombus formation. Additionally, it has been suggested that an earlier form of atrial dysfunction - atriopathy - may precede overt atrial fibrillation, while potentiating risk for LAA thrombus formation through stasis.
In addition, retrograde descending thoracic aortic flow from complex thoracic atheroma beyond the origin of the left subclavian artery has been shown as a potential embolic mechanism in stroke patients. Although healthy control subjects also demonstrate a degree of flow reversal in the descending thoracic aorta in diastole, the extent is increased in patients with stroke. Neuroradiologists should pay attention to the location of descending aortic plaques as a potential etiology for embolic stroke.Intracranial atherosclerosis evaluation
4D Flow MRI can extend the morphologic evaluation of intracranial atherosclerosis by determining impact on blood flow and peak velocities. Peak velocity information can be converted to lesion specific pressure drops to further determine the impact on brain flow accounting for circle of willis variants.Venous disease
4D flow MRI is well suited to assess intracranial venous flow due to the volumetric nature of the acquisition and the known variant intracranial venous anatomy. Unlike arterial stenoses venous stenoses manifest primarily by redirecting flow into collateral channels; hence treatment effects are often seen through flow redistribution back towards a normal state. 4D flow MRI has been studied in multiple sclerosis patient and used to assess the impact of sinus thrombosis on flow redistribution.Arteriovenous Malformations
4D Flow MRI is helpful in characterizing the different arterial feeders in an arteriovenous malformation and is unique in its ability to identify the downstream venous drainage pathways through 3D flow connectivity mapping. This technique is useful in assessing the impact of staged embolization and for guiding subsequent embolization strategies. Through comparing quantitative changes in the lesion over time this method may be useful in identifying early recurrence through changes in venous drainage pathways and recruitment of new arterial feeders.Cerebrospinal fluid flow
Cerebrospinal fluid (CSF) flow patterns are known to be disrupted in a central nervous system pressure disorders as well as anatomic configurations impacting the CSF cisterns. By extending the assessment to a volume of tissue 4D flow MRI allow for a more comprehensive assessment of CSF flow, confirming suspected levels of obstruction and visualizing redirected CSF flow.Intracranial Aneurysms
It is difficult to identify consistent morphologic features that predict aneurysm instability or rupture, to enable aneurysm treatment early in the course of disease. 4D Flow MRI extends the work done with CFD simulations through adding flow features morphologic characteristics to better predict adverse outcomes. It has been shown the more complex flow features and low wall shear stress in particular, portents a higher risk for aneurysm growth. Importantly, unstable aneurysms demonstrated similar flow features to those which subsequently ruptured, indicating that unstable aneurysms progress to rupture.Conclusion
4D Flow MRI is an emerging clinical tool for the neuroradiologist. Scan time is improving but remains a barrier to clinical translation with manufacturer provided pulse sequences. Additionally, post-processing tools are commercially available but are often optimized for specific post-processing tasks and do not yet provide a comprehensive suite for all clinical applications. 4D flow MRI is poised to impact assessment of arterial aneurysms, intracranial atherosclerosis, arteriovenous malformations, and embolic stroke work-up and may be useful in the assessment of venous collateral drainage routes and treatment response.Acknowledgements
Susanne Schnell, PhD and Michael Markl, PhD for their work in this field and for lending slides.References
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