★ The in vivo regional stress–strain relationship in the healthy human carotid wall was established noninvasively. ★ The carotid wall constituents were characterized by two separate models. ★ The stiffness of the carotid artery increased with strain during the systolic phase of cardiac cycle. ★ The tunica media was not significantly stiffer (P > 0.05) than the tunica adventitia. ★ The collagen fibers were found to be five times higher in stiffness than the elastin lamellae.
Arterial stiffness is well accepted as a reliable indicator of arterial disease. Increase in carotid arterial stiffness has been associated with carotid arterial disease, e.g., atherosclerotic plaque, thrombosis, stenosis, etc. Several methods for carotid arterial stiffness assessment have been proposed. In this study, in vivo noninvasive assessment using applanation tonometry and an ultrasound-based motion estimation technique was applied in seven healthy volunteers (age 28 ± 3.6 years old) to determine pressure and wall displacement in the left common carotid artery (CCA), respectively. The carotid pressure was obtained using a calibration method by assuming that the mean and diastolic blood pressures remained constant throughout the arterial tree. The regional carotid arterial wall displacement was estimated using a 1D cross-correlation technique on the ultrasound radio frequency (RF) signals acquired at a frame rate of 505–1010 Hz. Young's moduli were estimated under two different assumptions: (i) a linear elastic two-parallel spring model and (ii) a two-dimensional, nonlinear, hyperelastic model. The circumferential stress (σθ) and strain (εθ) relationship was then established in humans in vivo. A slope change in the circumferential stress–strain curve was observed and defined as the transition point. The Young's moduli of the elastic lamellae (E1), elastin–collagen fibers (E2) and collagen fibers (E3) and the incremental Young's moduli before (Symbol) and after the transition point (Symbol) were determined from the first and second approach, respectively, to describe the contribution of the complex mechanical interaction of the different arterial wall constituents. The average moduli E1, E2 and E3 from seven healthy volunteers were found to be equal to 0.15 ± 0.04, 0.89 ± 0.27 and 0.75 ± 0.29 MPa, respectively. The average moduli Symbol and Symbol of the intact wall (both the tunica adventitia and tunica media layers) were found to be equal to 0.16 ± 0.04 MPa and 0.90 ± 0.25 MPa, respectively. The average moduli Symbol and Symbol of the tunica adventitia were found to be equal to 0.18 ± 0.05 MPa and 0.84 ± 0.22 MPa, respectively. The average moduli Symbol and Symbol of the tunica media were found to be equal to 0.19 ± 0.05 MPa and 0.90 ± 0.25 MPa, respectively. The stiffness of the carotid artery increased with strain during the systolic phase. In conclusion, the feasibility of measuring the regional stress–strain relationship and stiffness of the normal human carotid artery was demonstrated noninvasively in vivo.