Quantitative Analysis of Aortic Hemodynamics

Maps of aortic hemodynamics were derived based on home-built analysis tools (MatLab; MathWorks, Natick, MA) similar to a recently reported workflow.^{25 (link)} The 4D flow velocity data were regridded to isotropic 1 mm³ voxels using spline interpolation. A 3D aortic centerline was calculated and orthogonal planes were automatically placed every millimeter.²⁶ Each voxel was matched to the nearest plane (ie, having center point at shortest 3D distance) to determine directional flow along the centerline, ie, forward (AAo to DAo) and reverse (ie, DAo to AAo); see Fig. 1f,g. For each voxel inside the TL and FL (DAD patients) or entire aorta (controls), net forward flow (FF) and reverse flow (RF) were calculated as the sum over the cardiac cycle.
The velocity magnitude was determined for each voxel at each cardiac time-frame, ie, v(t). Voxelwise flow stasis was calculated as the percentage of cardiac timeframes with v(t) < 0.10 m/s. In addition, voxelwise kinetic energy (KE) was determined by:
$$KE=\frac{1}{2}\cdot \rho \cdot dV\cdot v{\left(t\right)}^2$$
with ρ the blood density assumed as 1060 kg/m³ and dV the unit voxel volume (ie, 1 mm³)^{27 ,28 (link)} and summed over the cardiac cycle.
To correct for errors in plane orientation at the beginning and end of the centerline, the first and last four orthogonal planes along the centerline were not included in the directional flow quantifications. To reduce noise, a 3D median 3-by-3-by-3 filter was applied to all voxelwise parameters.
To provide an intuitive visualization of the spatial distribution of hemodynamic parameters across the TL and FL (patients) or entire aorta (controls), anatomic maps for FF, RF, stasis, and KE were calculated. The 3D voxelwise data for each parameter was collapsed into an average intensity projection, ie, average of all voxels in the segmented TL or FL (patients) or aorta (controls) along the projection direction. For quantitative regional analysis, the aorta was separated into five regions of interest (ROIs): 1) AAo (aortic root to brachiocephalic artery); 2) aortic arch (brachiocephalic artery to left subclavian artery); 3) proximal-DAo (left subclavian artery to vertical DAo); 4) mid-DAo (vertical DAo to half the distance to the celiac trunk); and 5) distal-DAo (distal edge of ROI 4 to the celiac trunk) (Fig. 1h). For each ROI, the mean FF, RF, KE, and flow stasis were quantified. For quantification in the FL, only ROIs containing at least 4000 voxels (=4 mL) were included to ensure a sizeable flow region for quantification.