Several well-resolved 4D Flow MRI acquisitions of an idealized rigid flow phantom featuring an aneurysm, a curved channel as well as a bifurcation were performed under pulsatile regime. The resulting hemodynamics were processed to remove MRI artifacts. Subsequently, they were compared with CFD predictions computed on the same flow domain, using an in-house high-order low dissipative flow solver. Results show that reaching a good agreement is not straightforward but requires proper treatments of both techniques. Several sources of discrepancies are highlighted and their impact on the final correlation evaluated. While a very poor correlation (r 2 = 0.63) is found in the entire domain between raw MRI and CFD data, correlation as high as r 2 = 0.97 is found when artifacts are removed by post-processing the MR data and down sampling the CFD results to match the MRI spatial and temporal resolutions.This work demonstrates that, in a well-controlled environment, both PC-MRI and CFD might bring reliable and correlated flow quantities when a proper methodology to reduce the errors is followed. KEYWORDS cardiovascular blood flows, computational fluid dynamics, large eddy simulation, phase contrast magnetic resonance imaging, validation work flow

A bdominal aortic aneurysm (AAA) is a serious and common pathologic abnormality that accompanies aging. Among men older than 65 years, the prevalence of AAA reaches 7.7%, increasing from 5.7% in ages 64-69 years to 8.9% in individuals older than 74 years (1). The high overall mortality from ruptured AAAs makes growth and subsequent rupture risk assessment crucial for AAA management.Since the 1970s, many studies (2) have demonstrated that AAA diameter correlates with rupture rate. Accordingly, AAA maximum diameter, effectively measured by using diagnostic US, has been the primary prognostic variable used to determine patient care (3,4). Current guidelines dictate elective repair to be appropriate at a diameter threshold of 50-55 mm or for AAA exhibiting growth greater than 1 cm per year (5,6).However, whereas some AAAs smaller than 55 mm do not grow more than 1 cm per year (7), other AAAs that are too small to trigger intervention grow rapidly. The sole use of maximal diameter measurement may be insufficient to

A numerical approach is presented to efficiently simulate time-resolved 3D phase-contrast Magnetic resonance Imaging (or 4D Flow MRI) acquisitions under realistic flow conditions. The Navier-Stokes and Bloch equations are simultaneously solved with an Eulerian-Lagrangian formalism. A semi-analytic solution for the Bloch equations as well as a periodic particle seeding strategy are developed to reduce the computational cost. The velocity reconstruction pipeline is first validated by considering a Poiseuille flow configuration. The 4D Flow MRI simulation procedure is then applied to the flow within an in vitro flow phantom typical of the cardiovascular system. The simulated MR velocity images compare favorably to both the flow computed by solving the Navier-Stokes equations and experimental 4D Flow MRI measurements. A practical application is finally presented in which the MRI simulation framework is used to identify the origins of the MRI measurement errors.

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The presence of calcium led to significant underestimation of choline measurements. The influence of calcium is dependent on its concentration and distribution. Quantitative MR spectroscopy of calcified tumours should be interpreted with caution.

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