Optical observation of shear waves excited by focused ultrasound in a tissue-mimicking phantom

Acoustic radiation force imaging is a new approach to characterize mechanical properties of soft tissues. This method provides to the physician a virtual "finger" to probe the elasticity of internal regions. The tissue response to the impulse stress caused by the localized radiation force can be described by the elastodynamic Green function: the remotely induced displacements are mainly created by shear waves. This work describes how an optical interferometric method can be used to study both the displacements induced by the radiation force at the focal point and the axial shear displacement along the radial direction around the focal point of the transducer. A tissue-mimicking phantom including a 1 mum thick metallized sheet of Mylar is placed in a water tank. While the transient shear wave propagates, this reflective target moves with the tissue. The central plane of the phantom was positioned between the transducer and the laser beam, perpendicular to both ultrasonic and laser beams. The laser beam propagates in the optically transparent phantom and is reflected back by the Mylar. A 1 MHz transducer focalizes at the optical focal point. By moving the transducer and continuously measuring the displacement at the same location with the laser probe, we can reconstruct the temporal and spatial behavior of axial shear displacement along the radial direction around the focal point of the transducer. From these displacement measurements, attenuation and celerity distributions of this shear wave are extracted. By using this configuration (for pressures in the MPa range), displacements in the gin range can be measured with a noise level of 2 nm whereas the ultrasonic methods usually used to measure these tissue displacements have a noise level of 1 mum.