Acoustic radiation force biorheology

Abstract

Acoustic radiation force techniques are being developed to image rheological properties of engineered tissues and cell cultures. An embedded sphere couples an acoustic source field to the medium, while induced deformations are imaged dynamically via Doppler methods. We conducted three experiments. First, we modeled the response to a step force as a second-order differential equation. Fitting Doppler data to the model, we estimated shear modulus (G) and coefficient of viscosity (ν) for a broad range of tissue-like hydrogels. Measuring a 2% hydrogel over 8d, we estimated G=450–800 Pa, ν=0.26–0.4 Pa<th>s, in close agreement with measurements using a cone-plate viscometer. In the second experiment, we applied harmonic acoustic radiation force stimuli and measured the complex modulus of the gels over a very broad bandwidth. The advantage obtained is significantly higher SNR. Finally, spatial variations in gel rheological properties were mapped from shear waves radiating from the harmonically-driven embedded sphere. The wavelength of the shear wave indicates the shear modulus. These three methods describe spatiotemporal variations in scaffolds designed for tissue engineering and cancer cell cultures, all without physically touching the samples. Acoustic radiation force rheometry is being developed as a tool for basic biological research.