4D flow MRI aortic cross-sectional pressure changes and their associations with flow patterns in health and aneurysm.

BACKGROUND: Ascending thoracic aortic aneurysm (ATAA) is a silent and threatening dilation of the ascending aorta (AscAo). Maximal aortic diameter which is currently used for ATAA patients management and surgery planning has been shown to inadequately characterize risk of dissection in a large proportion of patients. Our aim was to propose a comprehensive quantitative evaluation of aortic morphology and pressure-flow-wall associations from 4D flow MRI data in healthy aging and in patients with ATAA.

METHODS: We studied 17 ATAA patients (64.7±14.3 years, 5 females) along with 17 age- and sex-matched healthy controls (59.7±13.3 years, 5 females) and 13 younger healthy subjects (33.5±11.1 years, 4 females). All subjects underwent an MRI exam including 4D flow and 3D anatomical images of the aorta. This latter dataset was used for aortic morphology measurements including AscAo maximal diameter (iDMAX ) and volume, indexed to body surface area. 4D flow MRI data were used to estimate: 1) cross-sectional local AscAo spatial (∆PS ) and temporal (∆PT ) pressure changes as well as the distance (∆DPS ) and time duration (∆TPT ) between local pressure peaks, 2) AscAo maximal wall shear stress (WSSMAX ) at peak systole, 3) AscAo flow vorticity amplitude (VMAX ), duration (VFWHM ) and eccentricity (VECC ).

RESULTS: Consistency of flow and pressure indices was demonstrated through their significant associations with AscAo iDMAX (WSSMAX : r=-0.49, p<0.001; VECC :r=-0.29, p=0.045; VFWHM :r=0.48, p<0.001; ∆DPS :r=0.37, p=0.010; ∆TPT :r=-0.52, p<0.001) and indexed volume (WSSMAX :r=-0.63, VECC :r=-0.51, VFWHM :r=0.53, ∆DPS :r=0.54, ∆TPT :r=-0.63, p<0.001 for all). Intra-AscAo cross-sectional pressure difference, ∆PS , was significantly and positively associated with both VMAX (r=0.55, p=0.002) and WSSMAX (r=0.59, p<0.001) in the 30 healthy subjects (48.3±18.0 years). Associations remained significant after adjustment for iDMAX , age and systolic blood pressure. Superimposition of ATAA patients to normal aging trends between ∆PS and WSSMAX as well as VMAX allowed identifying patients with substantially high pressure differences concomitant with AscAo dilation.

CONCLUSION: Local variations in pressures within ascending aortic cross-sections derived from 4D flow MRI were associated with flow changes, as quantified by vorticity, and with stress exerted by blood on the aortic wall, as quantified by wall shear stress. Such flow-wall and pressure interactions might help for the identification of at-risk patients.