Large isotropic organs, whose smallest dimension is much larger than the shear wavelength (order of magnitude of millimetres), can be considered of infinite extent and the so-called group velocity of the shear wave can be used to estimate the shear modulus. The shear wave speed is physically related to the shear modulus, which can therefore be derived and used as a measure of the tissue stiffness. The SWE technique is based on generating shear waves by means of highly-focused ultrasound beams and then measuring their propagation speed in the tissue. Shear wave elastography (SWE) is an ultrasound-based technique for quantitative and local estimation of tissue stiffness introduced in the late 1990s (Sarvazyan et al 1998) and since then has been applied to many clinical areas, such as detection of breast cancer (Chang et al 2011), characterization of liver fibrosis (Zheng et al 2015) and stratification of follicular-patterned thyroid nodules (Samir et al 2015). Several research methods based on arterial motion tracking in ultrasound imaging have the potential to overcome these problems and measure arterial stiffness accurately and locally (Teixeira et al 2016). These are mostly due to the fact that PWV measurements are performed over a very long segment of the arterial tree, which can introduce large bias errors, up to 30%, because of inexact knowledge of the true length of the arterial segment (Nichols et al 2011, Davies et al 2012). This technique is at the moment the most simple and reproducible method (Laurent et al 2016), but suffers from several limitations. Quantitative, non-invasive assessment of arterial mechanical properties could therefore be highly beneficial in the clinics for both early diagnosis of arteriosclerosis and follow up of treatment.Ĭurrent commercially-available methods aim at measuring global arterial stiffness by detecting the pulse wave velocity (PWV) between two arterial sites. Notably, increase in arterial stiffness has proven to be an independent predictor for many cardiovascular diseases (Laurent 2006, Hamilton et al 2007, Palatini et al 2011, Scuteri et al 2014), which are the leading cause of death in the world (WHO 2011). As arteries become stiffer, the arterial compliance throughout the cardiac cycle decreases, increasing the work on the heart to pump blood throughout the vascular tree (O'Rourke 2007, Maksuti et al 2016a). Therefore, wall thickness should correctly be measured in arterial SWE applications for accurate mechanical properties estimation.Ĭhanges in arterial mechanical properties strongly affect cardiovascular function and blood pressure levels (Hamilton et al 2007, Chirinos et al 2012, Palatini et al 2011). An underestimation of 0.1–0.2 mm in wall thickness introduces an error 4–9 kPa in hollow cylinders with shear modulus of 21–26 kPa. Wall thickness had a larger effect than diameter on the dispersion curves, which did not have major effects above 400 Hz. The phase velocity curves obtained from experiments and simulations were compared in the frequency range 200–1000 Hz and showed good agreement ( R 2 = 0.80 ± 0.07 for plates and R 2 = 0.82 ± 0.04 for hollow cylinders). In addition, simulations in hollow cylinders with wall thickness difficult to achieve in phantoms were performed ( h = 0.5–1.3 mm, D = 5–8 mm). In this study the influence of geometry on the estimated mechanical properties of plates ( h = 0.5–3 mm) and hollow cylinders ( h = 1, 2 and 3 mm, D = 6 mm) was assessed by experiments in phantoms and by finite element method simulations. Nevertheless, the effect of arterial geometry in SWE has not yet been systematically investigated. Arterial wall thickness ( h) and inner diameter ( D) vary with age and pathology and may influence the shear wave propagation. Arterial shear wave elastography (SWE) and wave velocity dispersion analysis have previously been applied to measure arterial stiffness. Quantitative, non-invasive and local measurements of arterial mechanical properties could be highly beneficial for early diagnosis of cardiovascular disease and follow up of treatment.
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