Subharmonic threshold for chirp excitations of high frequency contrast agents

The subharmonic response of ultrasound contrast agents facilitates imaging for high frequency (>20 MHz) applications in terms differentiation from scattered tissue responses and the reduction signal loss due to attenuation. The subharmonic response has specific pressure threshold. Chirp excitation have been shown to enhance the subharmonic response though the underlying mechanisms is not completely understood. The widely used frequency domain pressure threshold criteria based on ratio of the spectral amplitudes subharmonic and fundamental has limitations with respect to nonstationary and nonlinear signals. We propose a more robust criteria based on the Intrinsic Mode Functions (IMF) of the Empirical Mode Decomposition (EMD). A comparison is highlighted with signals from two types of ultrasound polymer contrast agent data for mono-frequency and chirp excitations.