Optical tracking of superficial dynamics from an acoustic radiation force-induced excitation

Acoustic radiation force (ARF) has become a common excitation mechanism in elasticity imaging. The high acoustic intensities and subsequent generation of harmonics, however, hamper the effectiveness of using conventional radiofrequency (rf) tracking to investigate the dynamics of tissues and catheter-based transducers, especially during the excitation. Optical tracking, on the other hand, does not utilize acoustic echo and thus is not affected by ARF-generated interference. Additionally, it is able to track equally well in two dimensions, something that rf tracking is unable to do. Despite the inherent near-field scattering that will likely preclude optical (i.e., visible spectrum) tracking from supplanting current ultrasound-based methods in a clinical setting, it could offer valuable new tools in the pursuit of a better understanding of ARF-induced dynamic responses. We utilized an optically-based method to track the dynamic response resulting from an ARF-induced excitation on the surface of a tissue-mimicking phantom and on an unbounded catheter. These tracking data were then compared to tracking data obtained from the conventional rf tracking method. Both impulsive and harmonic (i.e., amplitude-modulated) excitations were investigated. In general, there was good agreement between the conventional (i.e., ultrasound-based) and optically-based tracking methods. Disparities between displacement estimates from the two tracking methods is thought to be a result of the finite length of the tracking marker, which was assumed to move as an infinitesimal point, and aberration of reflected light due to surface waves. Given the reasonable agreement seen for the harmonic and impulsive excitation cases, an optical tracking method could be insightful in future investigations of tissue/transducer response to ARF-induced excitations in controlled experimental settings.