Remote generation of shear wave in soft tissue by pulsed radiation pressure

Abstract

Remote generation of shear waves in tissues by a focused ultrasonic beam is the basis of a new acoustic method of medical diagnostics, namely, shear wave elasticity imaging [A. P. Sarvazyan, U.S. Patent No. 5,606,971, 1997]. The feasibility of SWEI was demonstrated recently in experiments with optical and NMR detection of ultrasonically induced shear waves in tissue phantoms. In the present study an SWEI system with ultrasonic detection of shear waves was designed and tested. The shear wave was excited inside an inhomogeneous tissue phantom using radiation force generated by the reflection of ultrasound from internal inhomogeneity. A focusing transducer of 6 cm in diameter and with 6-cm focal length was used. The carrier frequency was 2 MHz, and the intensity was varied in range from 10–30 W/cm2. The rectangular envelope was 0.2 ms in duration. Induced shear motion was evaluated using a 10-μs-long 3-MHz ultrasonic pulse by measuring the time delay for the pulse backscattered by moving inhomogeneities in the tissue. Maximum displacement in the propagating shear wave was over 10 μm, which agrees with theoretical predictions.