Four-dimensional MRI flow examinations in cerebral and extracerebral vessels - ready for clinical routine?

Abstract

Purpose of review: To evaluate the feasibility of 4-dimensional (4D) flow MRI for the clinical assessment of cerebral and extracerebral vascular hemodynamics in patients with neurovascular disease.

Recent findings: 4D flow MRI has been applied in multiple studies to qualitatively and quantitatively study intracranial aneurysm blood flow for potential risk stratification and to assess treatment efficacy of various neurovascular lesions, including intraaneurysmal and parent artery blood flow after flow diverter stent placement and staged embolizations of arteriovenous malformations and vein of Galen aneurysmal malformations. Recently, the technique has been utilized to characterize age-related changes of normal cerebral hemodynamics in healthy individuals over a broad age range.

Summary: 4D flow MRI is a useful tool for the noninvasive, volumetric and quantitative hemodynamic assessment of neurovascular disease without the need for gadolinium contrast agents. Further improvements are warranted to overcome technical limitations before broader clinical implementation. Current developments, such as advanced acceleration techniques (parallel imaging and compressed sensing) for faster data acquisition, dual or multiple velocity encoding strategies for more accurate arterial and venous flow quantification, ultrahigh-field strengths to achieve higher spatial resolution and streamlined postprocessing workflow for more efficient and standardized flow analysis, are promising advancements in 4D flow MRI.

Figure 1

Intracranial 4D flow workflow example including (A) data acquisition of the time-resolved 3D volume, (B) the data pre-processing such as phase offset correction, anti-aliasing and the phase-contrast MR angiogram calculation in order to segment the vessel wall or to mask the measured velocities within the vessel constraints. Panel (C) illustrates the visualization of blood flow using color-coded streamlines at peak systole and the locations of potential quantification of hemodynamic parameters such as peak velocity, net flow and WSS.

Figure 2

Comparisons of normal cerebral blood flow between a healthy adult volunteer (55 years) and a pediatric volunteer (6 years). Time-integrated 3D pathlines show overall higher cerebral blood flow velocities in the pediatric volunteer (B) compared to the adult volunteer (A). Regional flow measurements at the left middle cerebral artery (LMCA) quantitatively illustrate the differences of LMCA peak velocity (C) and flow rate (D) between the adult and pediatric volunteers.

Figure 3

4D flow MRI of a saccular left C4 segment ICA aneurysm (left) and a fusiform basilar artery aneurysm (right). It could be shown that small saccular aneurysms have significant largest peak velocities and WSS compared to large and giant saccular aneurysms and fusiform aneurysms. Fusiform aneurysms expressed significant lowest peak velocity and WSS along the vessel wall (30).

Figure 4

Illustration of 4D flow MRI for hemodynamic assessment in a patient with cerebral AVM (age 40 years, female, Spetzler-Martin grade = 4). A: 3D phase-contrast MR angiogram (PC-MRA) used for the orientation of 2D analysis planes. B: regional flow characteristics (peak velocity and flow rate) at user-defined vessel locations (examples for the feeding LMCA and the largest draining vein). C: Cumulative flow pathways of the entire cerebral vasculature over one cardiac cycle are depicted using time-integrated 3D pathlines which are color-coded according to local vascular velocity magnitude. D: selective vascular cartography illustrating connectivity and flow contribution of the major feeding arteries and draining vein. LMCA: left middle cerebral artery (green), LPCA: left posterior cerebral artery (yellow), ACA: anterior cerebral artery (pink), DV: draining vein (blue).