Patient-Specific Flow Descriptors and Normalized wall index in Peripheral Artery Disease: a Preliminary Study

Abstract

Background and aims: MRI-based hemodynamics have been applied to study the relationship between time-averaged wall shear stresses (TAWSS), oscillatory shear index (OSI) and atherosclerotic lesions in the coronary arteries, carotid artery, and human aorta. However, the role of TAWSS and OSI are poorly understood in lower extremity arteries. The aim of this work was to investigate the feasibility of hemodynamic assessment of the superficial femoral artery (SFA) in patients with peripheral artery disease (PAD) and we hypothesized that there is an association between TAWSS and OSI, respectively, and atherosclerotic burden expressed as the normalized wall index (NWI).

Methods: Six cases of 3D vascular geometries of the SFA and related inlet/outlet flow conditions were extracted from patient-specific MRI data including baseline, 12 and 24 months. Blood flow simulations were performed to compute flow descriptors, including TAWSS and OSI, and NWI.

Results: NWI was correlated positively with TAWSS (correlation coefficient: r = 0.592; p < 0.05). NWI was correlated negatively with OSI (correlation coefficient: r = -0.310, p < 0.01). Spatially averaged TAWSS and average NWI increased significantly between baseline and 24-months, whereas OSI decreased over 2-years.

Conclusions: In this pilot study with a limited sample size, TAWSS was positively associated with NWI, a measure of plaque burden, whereas OSI showed an inverse relationship. However, our findings need to be verified in a larger prospective study. MRI-based study of hemodynamics is feasible in the superficial femoral artery.

Figure 1

I) MR image at the arbitrary location of the superficial femoral artery (SFA) II) Vascular geometry reconstruction and meshing; III) simulated blood flow and corresponding flow field.

Figure 2

(case-1, 2 and 3) show the plot of time-averaged wall shear stress (TAWSS in Pa) and oscillatory shear index (OSI, dimensionless) over a full cardiac cycle, respectively in a superficial femoral artery (SFA). Letters A and P represent the anterior and posterior view of the artery with co-registered regions at baseline (case-1: pre-intervention) and post-intervention 12 & 24-months (case-2 and 3 respectively). Color bars in the right panel represent a range of TAWSS and OSI values, respectively.

Figure 3

(case-4, 5 and 6) show the plot of time-averaged wall shear stress (TAWSS in Pa) and oscillatory shear index (OSI, dimensionless) over a full cardiac cycle, respectively in a superficial femoral artery (SFA). Letters A and P represent the anterior and posterior view of the artery with co-registered regions at baseline (case-4: pre-intervention) and post-intervention 12 & 24-months (case-5 and 6 respectively). Color bars in the right panel represent a range of TAWSS and OSI values, respectively.

Figure 4

(A: case-1, 2 and 3) Attached MR images represent one of the locations of the region of interest R₁, R_2, R₃ at baseline (pre-intervention) and post-intervention 12 & 24-months. The White arrow indicates the location of the superficial femoral artery. Similarly, (B: case-4, 5 and 6) attached MR images represent one of the slices of a region of interest R₁, R_2, R₃ at 0, 12 & 24-months.

Figure 5

(case-1, 2 and 3) shows the plot of normalized wall index (NWI) for co-registered MRI slices in the artery at 0, 12 and 24-months. R₁, R_2, and R₃ are a region of interest.

Figure 6

(case-4, 5 and 6) shows the plot of normalized wall index (NWI) for co-registered MRI slices in the artery at baseline, 12-months, and 24-months. R₁, R_2, and R₃ denote a region of interests.

Figure 7

Shows the plot of (A) the plot of average normalized wall index (NWI), (B) the spatial average of TAWSS [Pa]; (C) the spatial average of OSI for all six cases specifically at the region of interest R₁, R_2, and R₃. Shown error bars are standard error, and ** indicates significance from baseline (P-value < 0.01) and *** indicates significance from baseline (P-value < 0.001). For this plot, a sample size of MRI slices was 90. Non-parametric Kruskal-Wallis test and Dunn’s multiple comparison tests were performed to assess significance.