Hemodynamic effects of entry and exit tear size in aortic dissection evaluated with in vitro magnetic resonance imaging and fluid-structure interaction simulation

Abstract

Understanding the complex interplay between morphologic and hemodynamic features in aortic dissection is critical for risk stratification and for the development of individualized therapy. This work evaluates the effects of entry and exit tear size on the hemodynamics in type B aortic dissection by comparing fluid-structure interaction (FSI) simulations with in vitro 4D-flow magnetic resonance imaging (MRI). A baseline patient-specific 3D-printed model and two variants with modified tear size (smaller entry tear, smaller exit tear) were embedded into a flow- and pressure-controlled setup to perform MRI as well as 12-point catheter-based pressure measurements. The same models defined the wall and fluid domains for FSI simulations, for which boundary conditions were matched with measured data. Results showed exceptionally well matched complex flow patterns between 4D-flow MRI and FSI simulations. Compared to the baseline model, false lumen flow volume decreased with either a smaller entry tear (- 17.8 and - 18.5%, for FSI simulation and 4D-flow MRI, respectively) or smaller exit tear (- 16.0 and - 17.3%). True to false lumen pressure difference (initially 11.0 and 7.9 mmHg, for FSI simulation and catheter-based pressure measurements, respectively) increased with a smaller entry tear (28.9 and 14.6 mmHg), and became negative with a smaller exit tear (- 20.6 and - 13.2 mmHg). This work establishes quantitative and qualitative effects of entry or exit tear size on hemodynamics in aortic dissection, with particularly notable impact observed on FL pressurization. FSI simulations demonstrate acceptable qualitative and quantitative agreement with flow imaging, supporting its deployment in clinical studies.

Figure 1

Pipeline showing the key methodological steps: (1) image-based patient-specific model generation from CTA data; (2) compliant 3D-printing; (3) experiments (MRI + catheter-based pressure mapping) with in vitro setup; (4) FSI simulations with boundary conditions informed by measured data (dashed lines); and (5) data analysis, i.e. quantitative and qualitative comparison of hemodynamics between measured and simulated data. CTA: computed tomography angiogram, Q(t): 2D-PC measured net flow, $P_{\mathit{sys}}$: systolic pressure, $P_{\mathit{dias}}$: diastolic pressure, $P_{\mathit{MAP}}$: mean arterial pressure, $E_{y,t}$: tangent Young’s modulus wall, ${\rho }_s$: density wall, ${\rho }_f$: density fluid, ${\mu }_f$: dynamic viscosity fluid, RCR: Windkessel components, ETS: external tissue support, TL: true lumen, FL: false lumen.

Figure 2

TBAD models. (a) Original digital wall model with definition of 18 landmarks that were defined for tuning and analysis purposes. Note that ${\mathit{DAo}}_{\mathit{prox}}$ cuts through the entry tear, while ${\mathit{DAo}}_{\mathit{dist}}$ is positioned just below the exit tear. Inset images show 2D-cine MRI frames for cross-sections along the dissected region. Close-up view of entry and exit tear regions for the (b) original model ${\text{TBAD}}_{\mathit{OR}}$, (c) smaller entry tear model ${\text{TBAD}}_{\mathit{EN}}$, and (d) smaller exit tear model ${\text{TBAD}}_{\mathit{EX}}$.

Figure 3

(a) Streamlines at peak systole (t= 200 ms) rendered from 4D-flow MRI (top) and FSI-simulated (bottom) data. Each model exhibits unique local flow characteristics that are in agreement between techniques. Key observations include: (i) increased flow velocities through the entry tear region, particularly in ${\text{TBAD}}_{\mathit{EN}}$ , and local helical flow in the proximal TL and FL in the vicinity of the entry tear (blue box, close-up view in (b)); (ii) increased TL flow velocity for the modified ${\text{TBAD}}_{\mathit{EN}}$ and ${\text{TBAD}}_{\mathit{EX}}$  models (arrows); (iii) flow jet through small size exit tear in ${\text{TBAD}}_{\mathit{EX}}$ , with recirculating TL flow distal to the exit tear (orange box, close-up view in (c)). Graphics created using ParaView (v5.7, https://www.paraview.org).

Figure 4

Flow and pressure dynamics in the arch region. Velocity vectors in the aortic arch at (a) peak systole (t= 200 ms) (b) and mid-diastole (t= 600 ms) based on 4D-flow MRI data and FSI simulations. During systole, flow patterns are in agreement between both modalities and for each model, but velocities are higher in FSI simulations. Mid-diastole renderings reveal a secondary push through the entry tear, that is most pronounced in ${\text{TBAD}}_{\mathit{EN}}$ . For full-cycle animations refer to Supplementary videos 1–5. (c) Absolute pressure at the lumen boundary (t = 250 ms) from the FSI simulations. ${\text{TBAD}}_{\mathit{EN}}$ shows a local pressure difference of $\approx 35$ mmHg in the FL impingement zone. Graphic created using ParaView (v5.7, https://www.paraview.org).

Figure 5

(a) Flow rates over the cardiac cycle along the dissected descending aorta based on 4D-flow MRI (green), FSI simulations (blue), and 2D-PC MRI (black, dashed). (b) Cross-sectional area over the cardiac cycle based on 2D-cine MRI (light blue) and FSI simulations (purple). Relative area change was defined as absolute area divided by area in the first cycle frame. For landmark label definition see Fig. 2a.

Figure 6

(a) Relative pressure (i.e. normalized to peak pressure at inlet) drops along the aortic centerline including inlet), outlet), as well five landmarks in both the TL and FL. (b) Maximum cross-sectional area measurements at the identical landmarks used for pressure mapping, except for AAo, which was chosen over ’inlet’, since inlet is fixed both in the 3D-printed model and in the simulation setup. Dashed bars denote area values obtained from the STL model file, squares denote size of entry tear, and triangles denote size of exit tear. For landmark label definition see Fig. 2a.

Figure 7

Inter-luminal pressure difference $\mathrm{\Delta }{P}_{TL-FL}$ between TL and FL according to pressure transducer measurements (top) and FSI simulations (bottom). $\mathrm{\Delta }{P}_{TL-FL}$ increased when the entry tear was made smaller (${\text{TBAD}}_{\mathit{EN}}$ ), and decreased—including negative pressure difference at distal landmarks—when exit tear made smaller (${\text{TBAD}}_{\mathit{EX}}$ ).