Feasibility of in vivo measurement of carotid wall shear rate using spiral fourier velocity encoded MRI

Abstract

Arterial wall shear stress is widely believed to influence the formation and growth of atherosclerotic plaque; however, there is currently no gold standard for its in vivo measurement. The use of phase contrast MRI has proved to be challenging due to partial-volume effects and inadequate signal-to-noise ratio at the high spatial resolutions that are required. This work evaluates the use of spiral Fourier velocity encoded MRI as a rapid method for assessing wall shear rate in the carotid arteries. Wall shear rate is calculated from velocity histograms in voxels spanning the blood/vessel wall interface, using a method Atherosclerosis affects over 18 million Americans. The associated formation, growth, and rupture of intraarterial plaque represent the fundamental process leading to myocardial infarction and thrombotic stroke. Arterial wall shear stress (WSS)-the drag force acting on the endothelium as a result of blood flow-is widely believed to influence the formation and growth of atherosclerotic plaque and may have prognostic value. WSS can be estimated as the product of wall shear rate (WSR) and blood viscosity (µ), where WSR is the radial gradient of blood flow velocity (dv /dr) at the vessel wall. Low WSS (1,2) and highly oscillatory WSS (3) have been linked to the formation and growth of atherosclerotic plaques, and this link has been validated in vitro (4). High WSS has also been hypothesized as a factor responsible for the topography of atherosclerotic lesions (5). Serial and noninvasive WSS measurement would be of value to the in vivo testing of these hypotheses and to better our understanding of the causal relationships between hemodynamics and the process of atherosclerotic plaque formation, growth, and rupture.Direct methods for measuring WSS are highly invasive and/or can only be used in conjunction with in vitro models (6-8). Indirect methods for measuring WSS are based on extrapolation of the measured axial velocity profile near the vessel wall (9), which can be measured by either ultrasound (10,11) or MRI (11-16). The accuracy of such methods is limited by data quality and the spatial resolution of the velocity estimates. Particularly, phase contrast (PC) MRI suffers from partial-volume effects (17) and inadequate signal-to-noise ratio (SNR) at high spatial resolutions (Fig. 1). PC is not currently capable of providing accurate absolute measurements of WSS (18) and has been shown to underestimate blood flow velocities in the carotid bifurcation by 31-39% (19). An alternative approach for estimating WSS is to reconstruct a complex flow field via computational fluid dynamics (CFD) simulation, using vascular geometries and input/output functions derived from noninvasive imaging data (20)(21)(22). This approach is computationally intense and may be sensitive to many assumptions and simplifications that are frequently made about the properties of blood and endothelium. CFD is also limited in modeling wall compliance and is sensitive to boundary conditions (11).In 1995, Frayne and Rutt (23) proposed a no...

Fast and Accurate 4D Flow MRI for Cardiovascular Blood Flow Assessment

“…However, FVE is very time-consuming as one measurement is necessary for every k v . Spiral trajectories have been used to decrease the scan time of FVE measurements used for WSS estimation [96].…”

“…As described in Section 4.6, an intravoxel velocity profile can be estimated from the intravoxel velocity distribution as obtained by a FVE measurement [95][96]. A sampling function defined as was used to sample s(u), simulating a FVE measurement with a velocity resolution of .…”

“…For example, this approach has been used to evaluate the accuracy of WSS estimates from 2D FVE measurements [96], and to evaluate the accuracy of PC-MRI turbulence mapping [65]. Gibbs ringing, velocity aliasing, partial volume artifacts and phase dispersion are also simulated using this approach.…”

Assessment of Carotid Flow Using Magnetic Resonance Imaging and Computational Fluid Dynamics

“…This approach for deriving FVE data from high-resolution velocity maps can be used for many simulation purposes (Carvalho et al, 2010).…”

Rapid Quantitation of Aortic Valve Flow Using Spiral Fourier Velocity Encoded MRI

“…This approach for deriving FVE data from high-resolution velocity maps (Eq. 16) can be used for many simulation purposes (Carvalho et al, 2010). …”