Monitoring local temperature changes in soft tissues by time-reversal elastography

Elastography is a potential tool to evaluate local elasticity changes produced by thermal therapy ultrasound. The protein denaturation of tissue cells produces an irreversible increase in local stiffness. Thus elastographic methods are able to evaluate where and if this critical point has been reached. In previous works we have employed 1D transient elastography to monitor local changes in the shear elastic modulus in bovine skeletal muscle. In this work we include time-reversal elastography (TRE) experiments to evaluate temperature-induced changes created by a heat source whose position is known. TRE is currently a developing method in our laboratory. It is based on the time inversion of the low-frequency field recorded by speckle tracking methods. The low-frequency particle velocity field on the direction of the ultrasound beam is measured at the volume of the soft solid by the transient elastography technique. The time refocusing field of these mechanical waves is computed using spatio-temporal correlations equivalent to time-reversal experiments. Due to temperature increase, the refocusing time at each position is shifted. Since this is a cumulative effect with depth, the spatial derivation of the time shift is a direct visualization of heat-induced changes in tissue elasticity. The method has the advantages of time-reversal experiments: it is independent of boundary conditions, is applicable in non-homogeneous media and is robust as regard to the low-frequency source shape and position. Thus it overcomes some limitations of 1D transient elastography.