A Simulation Study on the Second Harmonic Amplitude Generated with Plane Wave and Focused Wave Transmission

There is growing interest in evaluating the strength of harmonic components generated by the nonlinear propagation of Plane Waves (PW). Hydrophone measurements in water recently showed that the harmonics amplitudes reached by PW are generally higher than those achieved by Focused Waves (FW) at comparable (low) MI. On the other hand, simulations in tissue at MI=1,9 showed that the second harmonic amplitude is 2 to 16 dB lower for PW than for FW. To explain this apparent discrepancy, this paper compares via a simulation study the second harmonic amplitudes generated by PW and FW in different media over a wide range of MI levels (0.05 to 1.2). The results show that the second harmonic amplitude generated with PW can be significantly higher than for FW (up to 9 dB and 2,5 dB in water and blood, respectively), and that increasing absorption levels tend to favour FW over PW.