Viscoelastic property measurement in thin tissue constructs using ultrasound

A variety of imaging methods employing the principle of acoustic radiation force (ARF) have been proposed recently. It is now well accepted that ARF-based methods produce significant improvement in contrast compared to coventional ultrasound. However, interpretation of the ARF-induced displacements is rendered difficult in the presence of nearby boundary conditions, e.g. skin imaging. We present a dual-element ultrasound transducer system for generating and tracking of localized tissue displacements in thin tissue constructs on hard substrates. The system is comprised of a highly focused 5-MHz ARF transducer and a confocal 25-MHz PVDF imaging transducer. The ARF transducer produces a sharp focus with 200 μm diameter and 1 mm depth of field. This allows for the generation of measurable displacements in tissue samples on hard substrates with thickness values down to 500 μm. The ARF transducer is driven by an arbitrary waveform generator for modulating the 5-MHZ ARF carrier. Impulse-like and longer duration sine-modulated ARF pulses are possible with intermittent M-mode data acquisition for displacement tracking. Spatio-temporal maps of tissue displacements in response to a variety of modulated ARF beams are produced in tissue-mimicking elastography phantoms on a hard substrate. The frequency response was measured for phantoms with different stiffness and thickness values. The frequency response exhibits resonant behavior determined by the stiffness and the thickness of the samples. We have also used the extended Kalman filter (EKF) for tracking the apparent stiffness and viscosity of samples subjected to sinusoidaly-modulated ARF. Finally, C-mode imaging results of a soft phantom with a harder inclusion are shown to demonstrate the potential for significant contrast enhancement with ARF-based methods.