Aortic wall shear stress in Marfan syndrome.

The aim of this study was to quantify changes in thoracic aortic wall shear stress (WSS) in asymptomatic patients with Marfan syndrome (MFS) compared with healthy controls. WSS in the thoracic aorta was quantified based on time-resolved 3D phase contrast MRI with three-directional velocity encoding (4D flow MRI, temporal resolution ∼44 ms, spatial resolution ∼2.5 mm) in 24 patients with confirmed MFS (age = 18 ± 12 years) and in 12 older healthy volunteers (age = 25 ± 3 years). Diameters of the thoracic aorta normalized to body surface area were similar for both groups. Peak systolic velocity, absolute WSS, time-averaged WSS, circumferential WSS, peak systolic WSS, and WSS eccentricity were calculated in eight analysis planes distributed along the thoracic aorta. Plane-wise comparison revealed significant differences between MFS patients and volunteers in the proximal ascending aorta for peak systolic velocities (1.11 ± 0.23 m/s vs. 1.34 ± 0.18 m/s, P = 0.004) and circumferential WSS (0.14 ± 0.03 N/m(2) vs. 0.11 ± 0.02 N/m(2), P = 0.007). WSS eccentricity was altered in most of the ascending aorta and proximal arch (P = 0.009-0.020). MFS patients demonstrated segmental differences in peak systolic WSS with a significantly higher WSS at the inner curvature in the proximal ascending aorta and at the anterior part in the more distal ascending aorta (P < 0.01). These findings indicate differences in WSS associated with MFS despite similar aortic dimensions compared to controls.

Assessment of flow instabilities in the healthy aorta using flow-sensitive MRI.

PURPOSE: To assess blood flow velocities and spatial distribution of aortic Reynolds numbers in vivo using flow-sensitive magnetic resonance imaging (MRI) and probe for flow instabilities along the aorta based on an empirical model for physiological pulsatile blood flow. MATERIALS AND METHODS: Thirty young healthy volunteers were examined by flow-sensitive MRI at eight imaging planes distributed along the thoracic aorta. Flow, Womersley, Strouhal, Reynolds, and critical Reynolds numbers were calculated and used to assess the presence of flow instabilities. RESULTS: The average peak Reynolds number was higher in the ascending (≈4500) and descending aorta (≈4200) than in the aortic arch (≈3400). According to the calculated critical Reynolds numbers, flow instabilities were prominent in the ascending (14/30 volunteers) and descending aorta (22/30 volunteers) but not in the aortic arch (3/30 volunteers). A significant difference (P < 0.05) in supracritical peak Reynolds numbers was observed between genders. The supracritical Reynolds number, indicating flow instabilities, significantly correlated (P < 0.05) with body weight (r = 0.34), aortic diameter (r = 0.48), and cardiac output (r = 0.53). CONCLUSION: Flow-sensitive MRI was used to indirectly assess the presence of flow instabilities in vivo. The results in volunteers indicate the presence of flow instabilities in the young healthy aorta with a higher prevalence for men than women.