Optical validation of acoustic displacement underestimation in ARFI ultrasound using a clinical imaging system

Acoustic radiation force impulse (ARFI) imaging is an elastography technique that uses ultrasonic pulses to both displace and track tissue motion. Previous modeling studies have shown that ARFI displacements are susceptible to underestimation due to lateral and elevational shearing that occurs within the tracking resolution cell. In this study, optical tracking was utilized to experimentally measure the displacement underestimation achieved by acoustic tracking using a clinical ultrasound system. Two optically translucent phantoms of varying stiffness with embedded microspheres were created, and ARFI excitation pulses with F/1.5 or F/3 lateral focal configurations were transmitted from a standard linear array to induce phantom motion. Displacements were tracked using confocal optical and acoustic methods, with an acoustic tracking focal configuration of F/1.5 laterally. Acoustic displacement underestimation was observed for both excitation focal configurations; the maximum underestimation error was 51% of the optically measured displacement for the F/1.5 excitation pulse. This work experimentally demonstrates limitations of ARFI implemented on a clinical scanner using a standard linear array and sets up a framework for future displacement tracking validation studies.