Transient elastography in biological tissues

Elastography is used in different ways to characterize soft tissues. Ophir uses static elastography to estimate strains in the tissue after a quasi‐static compression. Strains can also be measured by sonoelasticity using mechanically forced low‐frequency vibrations and the ultrasonic pulsed Doppler method (Parker and Sato). These techniques are subjected to bias due to unknown boundary conditions. In this article a technique called transient elastography is presented which is not sensible to boundary conditions. It uses a low‐frequency pulsed vibration (∼100 Hz) and a cross‐correlation technique to measure displacements on the order of 1 μm. This technique was first developed for one‐dimensional measurements on the axis of the ultrasonic transducer. It is now used with an array of 64 transducers to get time‐dependent two‐dimensional displacements at a rate of 2000 frames per second. Movies of the shear wave propagation through homogeneous and inhomogeneous phantoms and biological tissues have been obtained. It will be discussed how to inverse near‐field data in order to recover the medium shear viscosity and elasticity fields.