Abnormal Flow Dynamics Result in Low Wall Shear Stress and High Oscillatory Shear Index in Abdominal Aortic Dilatation: Initial <i>in vivo</i> Assessment with 4D-flow MRI

Abstract

Purpose: To characterize the non-laminar flow dynamics and resultant decreased wall shear stress (WSS) and high oscillatory shear index (OSI) of the infrarenal abdominal aortic dilatation, cardiac phase-resolved 3D phase-contrast MRI (4D-flow MRI) was performed. Methods: The prospective single-arm study was approved by the Institutional Review Board and included 18 subjects (median 67.5 years) with the dilated infrarenal aorta (median diameter 35 mm). 4D-flow MRI was conducted on a 1.5T MRI system. On 3D streamline images, laminar and non-laminar (i.e., vortex or helical) flow patterns were visually assessed both for the dilated aorta and for the undilated upstream aorta. Cardiac phase-resolved flow velocities, WSS and OSI, were also measured for the dilated aorta and the upstream undilated aorta. Results: Non-laminar flow represented by vortex or helical flow was more frequent and overt in the dilated aorta than in the undilated upstream aorta ( P < 0.0156) with a very good interobserver agreement (weighted kappa: 0.82–1.0). The WSS was lower, and the OSI was higher on the dilated aortic wall compared with the proximal undilated segments. In mid-systole, mean spatially-averaged WSS was 0.20 ± 0.016 Pa for the dilated aorta vs. 0.68 ± 0.071 Pa for undilated upstream aorta ( P < 0.0001), and OSI on the dilated aortic wall was 0.093 ± 0.010 vs. 0.041 ± 0.0089 ( P = 0.013). The maximum values and the amplitudes of the WSS at the dilated aorta were inversely proportional to the ratio of dilated/undilated aortic diameter ( r = −0.694, P = 0.0014). Conclusion: 4D-flow can characterize abnormal non-laminar flow dynamics within the dilated aorta in vivo . The wall of the infrarenal aortic dilatation is continuously and increasingly affected by atherogenic stimuli due to the flow disturbances represented by vortex or helical flow, which is reflected by lower WSS and higher OSI.

WSSNet: Aortic Wall Shear Stress Estimation Using Deep Learning on 4D Flow MRI

“…Time-averaged WSS (TAWSS) and OSI patterns have been suggested as disease risk indicators, such as for atherosclerosis and aortic dilatation ( 1 , 5 , 37 ). Callaghan et al ( 1 ) presented a normal pattern and range of WSS from 4D Flow MRI across a large population.…”

“…Overall, recent studies have suggested that WSS is a potential biomarker for atherosclerosis, aortic dilatation, and aneurysm ( 1 , 5 , 37 ). WSS measurement also has improved risk stratification in patients with carotid and coronary artery disease ( 38 ).…”

Initial experience with intensity distribution analysis of hemodynamic parameters in the thoracic aorta using four-dimensional magnetic resonance imaging

“…It has also been reported that in areas where the aorta is dilated, the generation of laminar flow is disturbed and turbulent flow occurs, resulting in a reduction in WSS; [ 10 ] therefore, WSS is expected to decrease in areas where Vort and EL are high. In this study, the concordance rates between the intensity distribution of WSS and those of the other 2 parameters were low, indicating, albeit indirectly, that such relationships do exist.…”

Utility of 4D Flow MRI in Thoracic Aortic Diseases: A Literature Review of Clinical Applications and Current Evidence

“…A large OSI means that instantaneous WSS vectors fluctuate more compared with WSS in the mainstream direction at the calculated point during one cardiac cycle. 30 Flow studies using in vitro models and computational fluid dynamics (CFD) have shown promising results in calculating consistent WSS in true patient anatomic models. 31,32 Advances in postprocessing software have made WSS estimation using 4D flow MRI easier.…”