Extending arterial stiffness assessment along the circumference using beam-steered ARFI and wave-tracking: A proof-of-principle study in phantoms and ex vivo

Figures

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  Fig. 1. Schematic overview of a) the phantom mold and rod used to produce the b) vessel-mimicking polyvinyl alcohol cryogel phantoms (only the homogeneous concentric phantom is shown for visualization purposes) with c) an overview of the phantom transverse cross-sections (as similar over the entire phantom length) with their corresponding dimensions.

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  Fig. 2. Data acquisition and data processing. (a) Measurement setup allowing vessel rotation and elongation; (b) definition of wave-tracking ellipses within the vessel wall every 0.5 mm over the wall thickness; (c) subdivision of the wall circumference in 60⁰-segments (only −20° to −80° and 20–80° segments are shown for visualization purposes) with (d) manual tracking of the wave propagation in spatiotemporal maps for the − 20⁰ to − 80⁰ segment (1) and the 20–80° segment (2) per tracking ellipse (top) with a visualization of the angular velocities over all ellipses in the corresponding segments (bottom, only one ellipse is shown for visualization purposes); (e) construction of 360⁰ angular velocity maps for individual top and bottom wall pushes or supersonic pushes based on a combination of 0° and beam-steered acquisitions (top row), based on 0° acquisitions only (middle row), or reference map based on the −20° to −80° and 20° to 80° top segments of the supersonic push (bottom row).

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  Fig. 3. Reference 360⁰ velocity maps (1st row) and example of 360⁰ angular wave velocity maps for half-half phantoms and plaque phantoms per compounding method (2nd to 5th row). All color maps are scaled to 7.5 m/s divided by the center ellipse radius. Vang = angular velocity; Hom = homogeneous; Ecc. = eccentric.

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  Fig. 4. Percentage of rotational maps with a visible wave as seen from the push location averaged over all phantom rotations for all approaches for all phantoms.

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  Fig. 5. Median (solid line) and 25th and 75th percentile (dashed lines) relative error (in %) of the 0°-only and the compounded 0⁰ and beam-steered single-push approach (top row) and of the 0⁰-only and the compounded 0⁰ and beam-steered supersonic push approach (bottom row) compared to the gold-standard angular velocities as seen from the push location over all phantoms over all trajectories (1st column), for the trajectories on the luminal side of the central trajectory (2nd column), the central trajectory itself (in case of an even number two center trajectories, 3rd column), and the remaining outer trajectories (4th column).

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  Fig. 6. Overview of the ex vivo sample with (a) a picture of the specimen in the longitudinal direction and (b) a picture of the transverse vessel cross-section at the location of wave velocity estimations with atherosclerosis along the entire vessel wall; (c) Masson’s Trichome Staining with collagen in the atherosclerotic regions and surrounding smooth vessel cells of the arterial wall; (d) CD68 staining coloring macrophages in the atherosclerotic regions; (e) SM-1 staining coloring actine in the vessel wall myofibroblasts and smooth muscle cell; (f) Hematoxylin & Eosin staining with collagen in the plaque region and smooth muscle cells in the surrounding vessel wall; (g) zoomed region of the top wall; (h) zoomed region of the bottom wall; B-mode ultrasound images of the sample in (i) longitudinal and (j) transverse direction; 360⁰ angular velocity maps of the single-push (k) 0⁰ only and (l) compounded 0⁰ and beam-steered acquisitions; 360⁰ angular velocity maps of the supersonic push (m) 0⁰ only acquisitions (n) compounded 0⁰ and beam-steered acquisitions; (o) the reference 360⁰ angular velocity map; (p) the percentage of rotational maps with a visible wave as seen from the push…

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  Example of stiffness estimation for the soft (left column) and stiff half (right column) of the thick half-half phantom, both when located at the top of the phantom (i.e. 0⁰ (left) and 180⁰ phantom rotation (right)) with a top wall push with B-mode image of the vessel (1st row); selection of the region-of-interest (green rectangle) and tracking line in the middle of the vessel wall (black, dotted line) for particle velocity estimation in the upper wall (2nd row); the corresponding spatiotemporal maps with wave-tracking for the wave traveling to the left and the right (3rd row); and dispersion curve fitting (4th row, only the dispersion curve of the wave traveling to the left is shown for visualization purposes), The black circles correspond to the measured phase velocity per frequency, the red lines to the dispersion curve following the theoretical leaky Lamb-wave model fit from the calculated cut-off frequency to fit the curve as visualized using the white dashed line. Settings for the theoretical model for the soft wall were: wall thickness 2.3 mm, inner radius 3.7 mm, stiffness 125 kPa, cut-off frequency 547 kPa; for the stiff wall: wall thickness 2.4 mm, inner radius 3.6 mm,…

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  360⁰ angular wave velocity maps based on 60⁰ segments (20–80° and −20° to −80°) of the 0° supersonic push acquisitions without correction for the length of the tracking ellipses within the vessel wall. Shape factors, i.e. ratio inner radius and wall thickness, per phantom wall are stated below the velocity maps.