The cardiovascular and cerebrovascular diseases are still ranked amongst the main mortality causes in the world. However, it is known that the onset and progression of these diseases are associated with the alteration of the biomechanical properties of tissues. Therefore, some of the indicators describing this mechanical behavior, such as stress and strain might play the important roles in identifying the processes of arterial pathologies. For example, TS, an indictor based on Laplace’s law, gives a relation between the inflation pressure, wall thickness and inside diameter . However, in vivo, TS of arterial wall is significantly connected to its viscoelasticity. In present study, the SWER and SWDR, being the parameters of viscoelasticity were lower in the older subjects than in the younger ones. We also observed that the PTS and MTS, describing the carotid peak and mean TS, showed less difference between two age groups, with only right carotid PTS in the older subjects showing lower values than the younger ones. Subsequently, the TSs, including PTS and MTS, were positively connected with SWER, while were not related to SWDR. This suggested that the arterial elasticity contributed its mechanical behavior rather than viscidity.
Carotid structure was closely related to its mechanical properties. CIMT is a noninvasive ultrasound measurement and is strongly associated with cardiovascular and cerebrovascular events . CIMT is a strong independent predictor of remolding and atherosclerosis. In this study, the CIMT was measured using ultrasonic radio-frequency technique. Based on the clear visualization of the two-dimensional vascular structure, it can receive complete RF signal, and detect the CIMT in six cardiac cycles in real-time, which is up to 10 µm. This technology is considered as a reliable and feasible method of clinical evaluation of arterial structure and function . In this study, the CIMT increased in the subjects who were ≥50 years old. This suggested that the carotid arteries remolding associated with increasing age. CIMT measurements respectively made on the left and right carotid arteries could represent separate phenotypes because their patters of associations with risk factors are different. For example, on the left carotid artery, the CIMT is thicker and shows stronger associations with blood lipid, glucose and lower estrogens; while on the right carotid artery, the CIMT is thinner and significantly related tohemodynamics, such as hypertension and heart rate. These results suggest that the weights of risk factors are different on left and right carotid arteries . The CCID is also an important indicator of remolding and atherosclerosis . In this study, the CCID was enlarged in the older ones, while CCAD, i.e. the difference of CCID between diastole and systole was decreased. These results also exhibited that carotid artery occurred remodeling with age.
Carallo et al.  had demonstrated that the circumferential wall tension (WT) significantly increased with age . Conversely, the present results showed the TS did not parallel the increase in CIMT and CCID. Only the right carotid PTS decreased with age, and no marked difference was observed in the parameters of TS, including bilateral MTS and left PTS. Various reasons for participating in that: (1) Mechanical models of artery, such as WT and TS, derived from Laplace’s law can be used to relate the arterial inner radius (r) and internal pressure (P).. The WT was defined as P × r . The WT model assumes a very thin wall, and then handles the pressure, which does not take into account wall thickness. The TS, being a corrected Laplace model, was P ×r/CIMT. TS could reflect the tensile response in circumference . (2) The mechanical stretch can induce structural changes in the arterial wall, including VSMC hyperplasia and hypertrophy, as well as increased deposition of ECM collagen and elastin and result in arterial remodeling [19, 20]. On the other hand, the arterial remodeling could act on its mechanical properties . (3) The arterial tissue is viscoelasticity and show non-linear.
Alterations in the structural and mechanical properties of arteries are thought to be crucial for early demonstration of the atherosclerotic changes, which could serve as the indicators for future atherosclerotic diseases . However, the arteries are of viscoelastic properties and exhibit the nonlinear stress-stain relations . Several new non-invasive techniques have been used to study arterial elasticity, such as dimensional speckle-tracking imaging , ultrasonic radiofrequency tracking  and shear wave elastography [26,27]. However, in vivo, it is difficult to study the arterial viscidity due to its complex temporal changing behavior. Shear wave elasticity imaging may noninvasively evaluate the properties of soft tissues based on a group shear wave speed assuming that tissue is elastic; however, soft tissues are known to be viscoelastic, meaning the shear wave speed is dependent on the wave’s frequency content. Over the last years, there has been significant innovation in the area of describing the viscoelastic properties of soft tissue by the frequency-dependent: shear wave dispersion (SWD, the change in speed with frequency). In this work, the viscoelastic properties of carotid artery were evaluated by SWD. The SWER and SWDR decreased with age. In addition, the TSs were positively connected with SWER, while were not related to SWDR. This suggested that the arterial elasticity contributed its mechanical behavior rather than viscidity. The vascular smooth muscle cells, extracellular matrix proteins collagen and elastin play a crucial role in the viscoelastic properties, i.e. their spatial organization and interaction dominate the macroscopic non-linear vessel properties [29,30]. Higher vascular stiffness is typically found in older subjects because the elastic lamellae decreases with age, while the connective tissue and collagen fibers increase . The mechanical characteristics of arteries were related to local pathologies of the arterial system, while wall viscosity change reflects a more general influence of age and diseases .
Despite the applied approach tried to use state-of-the-art methods, several limitations should be mentioned. A limitation in the present study was the small sample size. In addition, the curved abdominal transducer was used to evaluate the carotid viscoelasticity, while transducer of linear array could provide better images and measurements. The arterial viscoelasticity in the patients with cardiovascular or cerebrovascular diseases merited further investigation.