Empirical mode decomposition and tissue harmonic imaging

Empirical mode decomposition (EMD) is a relatively new technique used in the analysis of nonlinear and nonstationary time series. Previous signal-processing methods used for medical ultrasound have been based on the assumption of a linear time-invariant system. More recently, the technique of tissue harmonic imaging (THI) has become prevalent. This technique relies on the nonlinear propagation of the sound wave through the medium to disperse the signal energy into the harmonic frequencies of the transmitted signal. In this paper, results are presented from using EMD to process received ultrasound echo signals that have passed through nonlinear media. The Hilbert spectrum is used to demonstrate an interpretation of the physical process underlying THI that is based on the concept of intrawave frequency modulation, rather then the spreading of signal energy into harmonic frequencies. The technique of EMD is shown to be able to produce superior results to the bandpass filtering method of THI, even when the band width of the transducer was such that the second harmonic would be suppressed.