Higher order nonlinear ultrasonic imaging

The processing of second harmonic echoes from both biological tissue and contrast agents has generated new diagnostic methods in medical ultrasound. The work presented here demonstrates the extraction of higher order nonlinearities. The underlying idea is to model the nonlinear wave propagation or reflection from a contrast bubble by a polynomial expansion of some basis waveform. When this model is excited by a number of transmit pulses which only differ in their amplitude and phase then the coefficients of this polynomial model can be extracted through least squares inversion. The coefficients correspond to the individual nonlinear components. An important feature of the method is the evaluation of nonlinear components whose spectra are folded back into the transmission band. All odd order nonlinearities can create such echo components. The reception of these components eliminates the high bandwidth requirements encountered in second harmonic imaging. Higher-order even harmonics may also be detected by taking advantage of the harmonic fold-back process. Folded frequency components will be centered around DC and at two times the transmit frequency (2 f0). This still requires a bandwidth sufficient to detect signals at 2 f0 but eliminates the reception at higher multiples of f0.