Viscoelastic imaging using acoustic impedance microscope and its application to biological tissue

A new method of viscoelastic micro-imaging for soft biological tissue has been proposed. A target is placed on a plastic substrate. Ultrasonic beam, which is focused on the target, is transmitted and the reflection is received by the same transducer. Shear wave can be produced as the result of oblique incidence to the substrate. By adjusting the distance between the transducer and target either pressure or shear wave can be focused onto the target. It is easy to identify and extract each mode of the reflection in time domain, because pulse wave is transmitted. The reflection is once interpreted into characteristic acoustic impedance of the target, making use of the acoustic parameters of the substrate and reference material. Precise sound field analysis is required to create the calibration curve, because the incidence is not totally perpendicular to the substrate. The acoustic impedance for pressure wave is directly converted into bulk modulus. The (apparent) acoustic impedance for shear wave is interpreted into shear modulus and viscosity, making use of its frequency dependency. A 2-D profile of bulk modulus, shear modulus and viscosity may be obtained by mechanically scanning the transducer. As for a preliminary experiment, cancerous liver of rat was observed using acoustic impedance microscope with a frequency range of roughly 30-100 MHz. Viscosity profile showed a good correlation with stained image by optical microscope.