Analysis of aortic pressure fields from 4D flow MRI in healthy volunteers: Associations with age and left ventricular remodeling

Abstract

Background Aging‐related arterial stiffness is associated with substantial changes in global and local arterial pressures. The subsequent early return of reflected pressure waves leads to an elevated left ventricular (LV) afterload and ultimately to a deleterious concentric LV remodeling. Purpose To compute aortic time‐resolved pressure fields of healthy subjects from 4D flow MRI and to define relevant pressure‐based markers while investigating their relationship with age, LV remodeling, as well as tonometric augmentation index (AIx) and pulse wave velocity (PWV). Study Type Retrospective. Population Forty‐seven healthy subjects (age: 49.5 ± 18 years, 24 women). Field Strength/Sequence 3 T/4D flow MRI. Assessment Spatiotemporal pressure fields were computed by integrating velocity‐derived pressure gradients using Navier–Stokes equations, while assuming zero pressure at the sino‐tubular junction. To quantify aortic pressure spatiotemporal variations, we defined the following markers: 1) volumetric aortic pressure propagation rates ΔP E1/ΔV and ΔP E2/ΔV, representing variations of early and late systolic relative pressure peaks along the aorta, respectively, according to the cumulated aortic volume; 2) ΔA PE1‐PE2 defined in four aortic regions as the absolute difference between early and late systolic relative pressure peaks amplitude. Statistical Tests Linear regression, Wilcoxon rank sum test, Bland–Altman analysis, and intraclass correlation coefficients (ICC). Results Spatiotemporal variations of aortic pressure peaks were moderately to highly reproducible (ICC ≥0.50) and decreased significantly with age, in terms of absolute magnitude: ΔP E1/ΔV (r = 0.70, P < 0.005), ΔP E2/ΔV (r = –0.45, P < 0.005) and ΔA PE1‐PE2 (|r| > 0.39, P < 0.005). ΔP E1/ΔV was associated with LV remodeling (r = 0.53, P < 0.001) and ascending aorta ΔA PE1‐PE2 was associated with AIx (r = –0.59, P < 0.001). Both associations were independent of age and systolic blood pressures. Only weak associations were found between pressure indices and PWV (r ≤ 0.40). Data Conclusion 4D flow MRI relative aortic pressures were consistent with physiological knowledge as demonstrated by their significant volumetric and temporal variations with age and their independent association with LV remodeling and augmentation index. Level of Evidence 2 Technical Efficacy Stage 3 J. Magn. Reson. Imaging 2019;50:982–993.

Quantitative magnetic resonance imaging measures of three‐dimensional aortic morphology in healthy aging and hypertension

“…This ultimately enhances the signal to noise ratio in the resulting aortic images but might limit the usefulness in patients with counterindication to contrast agents. However, one might note that our segmentation technique was shown to be robust to low image quality, as it was able to reliably segment four‐dimensional flow MRI‐derived aortic angiograms, 36,37 which are known to have lower signal‐to‐noise ratio than conventional anatomical images. Our MRI data were acquired from three different sites using different field strengths (1.5 T and 3 T) and sequences (SPGR and SSFP).…”

Pressure and Flow Interplay in Aortic Dilation Using 4D Flow Magnetic Resonance Imaging

“…The method described in this previous work was applied here to aneurysmal patients and age-matched healthy controls and a fine characterization of local pressure changes was proposed to enhance the understanding of vortex appearance and wall shear stress increase in the setting of pathological aortic dilation. Finally, relative pressure has been shown to be related with age and left ventricular geometry in previous study [7].…”

“…Afterwards the final relative pressure Pf is calculated while verifying that the gradient of Pf is as close as possible to the Navier-Stokes pressure gradient ∇P. This is done through solving the following iterative equation [7]:…”

“…where Pf k is the pressure value at the k th iteration, Pj and ∇Pj were relative pressure and pressure gradient of the six orthogonal neighbors, Δrj is the voxel size and  is a scalar set to 0.5 [7]. Finally, 3D+t aortic pressure maps relative to the zero reference in the valve were obtained.…”

“…Despite well-established theoretical associations between pressure gradients and blood flow patterns and their changes with disease, there are no studies in the literature focusing on simultaneous evaluation of pressure gradients and indices quantifying flow disorganization using MRI. Thus, our aims were to: 1) non-invasively estimate local aortic pressure gradients by combining Navier-Stokes equations and 4D flow MRI velocity fields [7] in parallel to aortic flow vorticity and WSS, 2) to evaluate local associations between pressure gradients and flow-derived quantitative indices in a group of patients with a dilated aortic root and a tricuspid aortic valve, paired for age with healthy subjects.…”