Strain and strain rate generated by shear wave elastography in ex vivo porcine aortas

Abstract

In order to generate trackable shear waves in soft tissues, transmitted pulses in shear wave elastography (SWE) are longer than conventional clinical ultrasound pulses. Nevertheless, they typically obey mechanical and thermal regulatory limits. In arterial applications, specific safety concerns may arise, as acoustic radiation (ARF)-induced stresses and strain rates could potentially affect the arterial wall. The aim of this study was to assess ARF-induced strain and strain rates in ex vivo arteries. A porcine aorta (diameters 8.5 mm, wall thickness 1.2 mm) was pressurized by a saline-filled water column at 60 and 120 mmHg. A Verasonics V1 system and a L7-4 transducer were used to generate the ARF in the middle of the anterior wall (F-number = 1, push length = [100, 200, 300] μs) and to perform plane-wave imaging (10 kHz). Cumulative axial displacement was estimated using 2D auto-correlation. The axial strain rate was calculated as the time-derivative of the axial strain, obtained by spatial linear regression of the displacement inside the anterior wall. The ex vivo peak strain and strain rate were compared with peak strain and strain rate values induced by the blood pressure changes in two healthy individuals and two patients with coronary artery disease at rest and measured by a dedicated in house speckle tracking algorithm. ARF-induced ex vivo peak strains were in the range 0.3–1% and strain rates in the range 6–23 s−1. Peak values were more affected by longer push duration than pressurization level. In vivo physiological peak strain was 33% and strain rate was 2 s−1. ARF-induced strain rates in vivo are likely to be lower than those assessed in this ex vivo setup due to ultrasound attenuation and the effect of surrounding tissue. Therefore, the results of the performed study suggest that SWE could be used in a safe manner for arterial applications even though specific effects of high strain rates are to be explored.