The cardiovascular and cerebrovascular diseases are still the number 1 cause of death globally. However, it is known that the onset and progression of these diseases can be predicted by the biomechanical features of large arteries, such as carotid arteries. Therefore, some of the predictors, such as mechanical stress and strain are very important for identifying the potential risk over the processes of arterial pathologies. TS, a parameter derived from certain measurements by using Laplace’s law, is associated with the inflation pressure, wall thickness and inner diameter . In addition, TS of arterial wall is significantly related to the viscoelasticity of arteries. Our study shows that SWER and SWDR, the two parameters that represent the viscoelasticity of arteries were lower in the subjects older than 50 years that those of the younger subjects. We also found that no significant difference of PTS and MTS exist between the two groups, but the PTS of the right carotid arteries in the group with older ages was lower than the younger ones. The PTS and MTS, were positively corelated with SWER, but no correlation was found between PTS and SWDR, which implys that the arterial elasticity may have some implication in the change of tensile stress.
The geometry changes of the carotid artery are closely related to the mechanical properties. CIMT, which is usually measured by noninvasive ultrasound, is strongly associated with cardiovascular and cerebrovascular events and used for evaluating progression: regression of atherosclerosis and for predicting arterial remolding and subsequent clinical complications [13,14]. We measured the CIMT using radiofrequency-based ultrasound technique. The two-dimensional vascular structure can be clearly visualized, which allows the real-time measurement of CIMT within six cardiac cycles, with a resolution of 10 µm. This technology a reliable method for the clinical evaluation of arterial structure and function . Our study shows that the CIMT increased in the subjects older than 50 years, suggesting that the carotid arterial remolding has some relationship with ages. CIMT measurements respectively made on the left and right carotid arteries could represent separate phenotypes because their patterns 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 [16, 17]. 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.
The study by Carallo et al.  demonstrated that the circumferential wall tension (WT) of carotid arteries significantly increased with age. Conversely, the present results showed the TS did not parallel the increase in CIMT and CCID. The PTS of right carotid arteries is lower in the group with older ages, and no remarkable difference was found for the parameters of 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 [20, 21]. On the other hand, the arterial remodeling could act on its mechanical properties [22,23]. (3) The arterial tissue is viscoelasticity and show non-linear.
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 [27–29]. 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 [31,32]. 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 .
There are limitations for this study. We included a small sample size of 45 subjects in this study. In addition, the curved abdominal transducer was used to evaluate the carotid viscoelasticity, while transducer of linear array could provide better images and measurements. We only include the subjects with healthy carotid arteries. In the future, we will explore the value of using this technique to characterize the tensile stress features of the carotid arteries with pathology, such as atherosclerosis.