Combined optical and ultrasound-based tracking of an acoustic radiation force-induced excitation

Acoustic radiation force (ARF) has become a common excitation mechanism in elasticity imaging; however, the high acoustic intensities and subsequent generation of harmonics hamper the effectiveness of using conventional radio frequency (RF) tracking to investigate the dynamics of tissues and catheterbased transducers, especially during the excitation. Thus, in trying to gain a better understanding of their response to ARF excitations, a more robust tracking option could prove useful. We propose a combined optical-ultrasound tracking method, where the dynamic response resulting from an ARF-induced excitation in soft tissue and on an unbounded catheter is tracked during and after insonification. Both impulsive and harmonic (i.e. amplitude-modulated) excitations were investigated. The displacement estimates obtained from the optical method were then compared to those obtained from the more conventional ultrasound-based method. For the impulsive excitation case (pulse length ≥1.3 ms), the mean absolute percent difference between the peak displacements estimated by the two methods was 4.0%. For the harmonic excitation case, the mean absolute percent difference between amplitude estimations obtained from the two techniques was 7.4% at lower frequencies (50-200 Hz) and 29.1% at 500 Hz. The relatively large disparity between the displacement estimates at higher frequencies is thought to be a result of the finite length of the tracking marker, which was assumed to move as an infinitesimal point. Given the good agreement seen for the harmonic case at lower frequencies and the impulsive case at longer pulse lengths, this technique could be insightful in future investigations of tissue/transducer response to ARF-induced excitations in controlled, experimental settings.