Capriotti, Margherita

Abstract Objective . Arterial dispersion ultrasound vibrometry (ADUV) relies on the use of guided waves in arterial geometries for shear wave elastography measurements. Both the generation of waves through the use of acoustic radiation force (ARF) and the techniques employed to infer the speed of the resulting wave motion affect the spectral content and accuracy of the measurement. In particular, the effects of the shape and location of the ARF beam in ADUV have not been widely studied. In this work, we investigated how such variations of the ARF beam affect the induced motion and the measurements in the dispersive modes that are excited. Approach. The study includes an experimental evaluation on an arterial phantom and an in vivo validation of the observed trends, observing the two walls of the waveguide, simultaneously, when subjected to variations in the ARF beam extension ( F / N ) and focus location. Main results. Relying on the theory of guided waves in cylindrical shells, the shape of the beam controls the selection and nature of the induced modes, while the location affects the measured dispersion curves (i.e. variation of phase velocity with frequency or wavenumber, multiple modes) across the waveguide walls. Significance. This investigation is important to understand the spectral content variations in ADUV measurements and to maximize inversion accuracy by tuning the ARF beam settings in clinical applications.

Shear wave elastography (SWE) is an established ultrasonic imaging technique, that provides quantitative measurements of tissue mechanical properties. This technique relies on the combination of acoustic radiation force push beams to generate propagating shear waves and ultrafast imaging, which allows high two-dimensional imaging frame rates to capture the induced motion. Utilizing coherent plane wave compounding (PWC) can improve signal-to-noise ratio (SNR) of acquired data. While the image quality improves, the pulse repetition frequency (PRF) is reduced, reaching limiting bandwidths to perform SWE. To address these challenges, a method for making SWE measurements with high PRF and SNR is presented. The idea follows the Time Aligned Sequential Tracking method, proposed to allow SWE with enhanced SNR in conventional clinical ultrasound scanners. The time alignment process is applied here to the full field-of-view, acquired by using plane waves transmitted at different angles. In this way, the full PRF and SNR gains are obtained. Results are shown from experiments in tissue-mimicking phantoms, where motion and extracted shear wave velocity are compared for the traditional and proposed data processing schemes. Preliminary results on arterial phantoms are also discussed, to emphasize the beneficial effects of the method in dispersive geometries.