ABSTRACT
We experimentally observe the shear and secondary compression waves inside soft porous water-saturated melamine foams by high-frame-rate ultrasound imaging. Both wave speeds are supported by the weak frame of the foam. The first and second compression waves show opposite polarity, as predicted by Biot theory. Our experiments have direct implications for medical imaging: melamine foams exhibit a similar microstructure as lung tissue. In the future, combined shear wave and slow compression wave imaging might provide new means of distinguishing malignant and healthy pulmonary tissue.
ABSTRACT
In most elastography experiments, shear waves are generated using a single source on the surface with a shaker, or in the bulk with radiation pressure of ultrasound. However, emitting controlled shear waves from multiple sources is a good way to improve the signal to-noise-ratio for shear-wave elastography. The experiments are conducted using six shakers with independent driving electronics in gelatin-graphite to mimic the tissue. Based on time reversal, our approach shows the feasibility of controlling shear-wave field in space with multiple focal spots at chosen locations, and in time with a chosen delay between each focusing. Improved by 10 dB compared to the use of a single source, the signal-to-noise ratio demonstrates that time-reversal as an adaptive filter is a good method to deliver maximum energy vibrations toward deep regions. Furthermore, this adaptive approach allows controlled vibrations to be delivered through bone conduction: a shear-wave focal spot is experimentally observed in a soft brain tissue-mimicking phantom using the multiple sources array applied to a skull model.
