Assessment of hemodynamic responses to exercise in aortic coarctation using MRI-ergometry in combination with computational fluid dynamics

Figures

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  Figure 1. (Upper panel) Visual illustration of the study design. Cardiac MRI of 20 participants with aortic coarctation was acquired during rest and moderate exercise. An absolute increase in heart rate of 50 bpm was targeted during exercise. Using the MRI images, the participant-specific aortic geometry was reconstructed. Using computational fluid dynamics, the transstenotic pressure gradient was calculated during rest and exercise. (Lower panel) Visualization of the image data used for segmentation. The participant-specific anatomy of the aorta was segmented from 3D SSFP (steady-state free precession) images (A,C). If the respective participant was treated using a stent, additional image information from black blood MR sequences (D) was used to improve the segmentation of the stented region of the aorta. An example from only 3D SSFP images is shown in panel (B), whereas a combined segmentation of a previously stented patient is shown in panel (E).

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  Figure 2. Illustration of all measurements and calculations reported in this study. The patient-specific volume flow rate was measured using 4D VEC MRI (4D velocity-encoded MRI, abbreviated 4D-QF) at the sinotubular junction as well as the descending aorta. Those measurements as well as the patient-specific anatomy (grey vessel shape) were used as boundary conditions for the numerical simulations which were used to calculate all other parameters. Secondary flow degree (SFD) and normalized flow displacement (NFD) were calculated at the mid-section of the ascending aorta and the descending aorta. The surface-average of calculated wall shear stresses (WSS) was evaluated for the ascending aorta. The pressure gradient was calculated using the pressure upstream (P1) and downstream of the stenosis (P2), where the pressure recovery due to deceleration of the blood flow was maximal (right panel).

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  Figure 3. Overview of the relative static pressure distribution that was calculated for all patients at rest and during exercise using computational fluid dynamics. As the individual pressure gradients varied, the upper scale of the colour bar (pmax) is given next to each patient.

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  Figure 4. Transstenotic pressure gradients (upper left), left ventricular stroke volume index (upper right), heart rate (lower left) and cardiac index (lower right) at rest and during exercise. Individual cases where the stroke volume decreased during exercise are highlighted using stroked, red lines in all panels.