PMC2846097_01
The transducer tracks a specific underlying radiofrequency to determine the positional changes of specific regions within a vessel.
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F1
High-resolution ultrasound with phase-sensitive speckle-tracking was applied to accurately track the motion of structures within regions of interest (ROIs) using the specific signal signature contained in the underlying radiofrequency (RF) signal along the beam at the frequency of the transducer (Fig. 1).
(a) The deformations of the regions of interest are measured along the axis of the beam (arrows). As the vessel changes shape, changes in the longitudinal direction are measured by the transducer. Undilated and dilated vessels are displayed on left and right, respectively. (b) Positive strain occurs at the sides of the vessel as the region of interest lengthens along the beam (Deltay > 0), and negative strain is apparent at the top and bottom of the vessel with shortening of the region of interest along the beam (Deltay < 0). In the computer-simulated and ultrasound strain maps, regions of positive and negative strain are depicted, respectively, as lighter and darker regions.
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F2
Therefore, as shown in Figure 2, the deformations of the ROIs are measured along the axis of the beam (arrows).
(a) B-scan image of a vein wall. (b) A computer simulation of a vein strain map developed using Abaqus (Simulia, Providence, RI) finite element analysis software. The solid inner circle represents the lumen of the vein and the outer circle represents the vein wall. (c) A venous strain-map obtained by ultrasound from subject. (d) Longitudinal lengthening and shortening of regions of interest at the sides and top/bottom of the wall of the vessel are shown as positive (light regions) and negative (dark regions) strains, respectively.
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F3
To assess the validity of the ultrasound strain measurements, we generated a computer simulation of a venous strain map (Fig. 3b) using a finite element analysis (FEA) program, Abaqus 6.4 (Simulia, Lighter regions in the strain map (Fig. 3c) are under positive strain (lengthening along the ultrasound beam), whereas darker regions are under negative strain (shortening along the beam).
The stress-strain relationship in bovine venous tissue, calculated by a microelastometer, was used as the Young's modulus of the vein wall in the computer simulation.
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The vein wall measurements were approximately equal to values obtained by other investigators. The Young's modulus of the vein was non-linear (Fig. 4) and the Young's modulus of the surrounding tissue was estimated to be 80 kPa.