Magnetic resonance measurement of turbulent kinetic energy for the estimation of irreversible pressure loss in aortic stenosis

Abstract

Objectives: The authors sought to measure the turbulent kinetic energy (TKE) in the ascending aorta of patients with aortic stenosis and to assess its relationship to irreversible pressure loss.

Background: Irreversible pressure loss caused by energy dissipation in post-stenotic flow is an important determinant of the hemodynamic significance of aortic stenosis. The simplified Bernoulli equation used to estimate pressure gradients often misclassifies the ventricular overload caused by aortic stenosis. The current gold standard for estimation of irreversible pressure loss is catheterization, but this method is rarely used due to its invasiveness. Post-stenotic pressure loss is largely caused by dissipation of turbulent kinetic energy into heat. Recent developments in magnetic resonance flow imaging permit noninvasive estimation of TKE.

Methods: The study was approved by the local ethics review board and all subjects gave written informed consent. Three-dimensional cine magnetic resonance flow imaging was used to measure TKE in 18 subjects (4 normal volunteers, 14 patients with aortic stenosis with and without dilation). For each subject, the peak total TKE in the ascending aorta was compared with a pressure loss index. The pressure loss index was based on a previously validated theory relating pressure loss to measures obtainable by echocardiography.

Results: The total TKE did not appear to be related to global flow patterns visualized based on magnetic resonance-measured velocity fields. The TKE was significantly higher in patients with aortic stenosis than in normal volunteers (p < 0.001). The peak total TKE in the ascending aorta was strongly correlated to index pressure loss (R(2) = 0.91).

Conclusions: Peak total TKE in the ascending aorta correlated strongly with irreversible pressure loss estimated by a well-established method. Direct measurement of TKE by magnetic resonance flow imaging may, with further validation, be used to estimate irreversible pressure loss in aortic stenosis.

Figure 1. Total TKE in the Ascending Aorta Over Time

Plots of the total turbulent kinetic energy (TKE_total) in the ascending aorta over time (time after R-peak) for normal volunteers (dotted lines) and patients with aortic stenosis (solid lines). Subjects are ordered according to peak total TKE along the second horizontal axis. The peak total TKE in the aortic stenosis patients was significantly higher than in the normal volunteers, p < 0.001.

Figure 2. Visualization of Flow Patterns and TKE

Visualization of flow patterns and turbulent kinetic energy (TKE) in 1 normal volunteer and 3 patients with aortic stenosis. For each subject, volume renderings of TKE (red to yellow color scale) at the time point of peak total TKE have been combined with streamline visualization of the instantaneous velocity field at the time of peak velocity (blue color scale). Color settings were the same in all subjects.

Figure 3. Total TKE Versus Pressure Loss

Total turbulent kinetic energy (TKE_total) in the ascending aorta plotted against index pressure loss (iPL). The total TKE was obtained by integrating the TKE per unit volume across the entire ascending aorta. The iPL was calculated based on formulas validated by Garcia et al. (12,13). Total TKE was strongly correlated with iPL (R² = 0.91).