In vivo viscoelasticity estimation of myocardium

Though myocardial viscoelasticity is essential in the evaluation of heart diastolic properties, it has never been noninvasively measured in vivo. By the ultrasonic measurement of the myocardial motion, we have already found that some pulsive waves are spontaneously excited by aortic-valve closure (AVC) at end-systole (T₀). In this study, a sparse sector scan at a sufficiently high frame rate clearly reveals wave propagation along the heart wall. The propagation time of the wave along the heart wall is very small, namely, several milliseconds, and cannot be measured by conventional equipment. From the measured phase velocity, we estimate the myocardial viscoelasticity in vivo. In in vivo experiments applied to 6 healthy subjects, 3 patients with hypertrophic cardiomyopathy (HCM), and 3 patients with aortic stenosis (AS), the propagation of the pulsive wave was clearly visible in all subjects. For the frequency component up to 90 Hz, the typical propagation speed is about several m/s and rapidly decreased around the time of AVC. For the healthy subject, the typical value of elasticity was about 24-30 kPa and did not change around the time of AVC. The typical transient values of viscosity decreased rapidly from 400 Pa·s to 70 Pa·s around the time of AVC. The measured shear elasticity and viscosity in this study are comparable to those obtained for the human tissues using audio frequency in in vitro experiments reported in the literature. The method cannot be easily applied to these patients because there were inhomogeneities in the phase velocities due to the diseased myocardium. By applying the measurement of the phase velocity to each of 5 layers set in the heart wall, the phase velocity in the middle layer was lower than those in the LV- and RV-sides of the heart wall, which will corresponds to the change in myocardial fiber orientations. This method offers potential for in vivo imaging of the spatial distribution of the passive mechanical properties of the myocardium, which cannot be obtained by conventional echocardiography, CT, or MRI.