Harmonics Amplitude in Plane and Focused Waves - a Comparative Study at Equal Mechanical Index

Although there is increasing interest for the use of plane waves (PWs) in high-frame-rate fundamental and harmonic imaging, no much experimental data is available about their behavior in terms of nonlinear propagation. In this paper, hydrophone measurements of PW propagation in water are presented and compared to the results obtained for focused waves (FWs) at various levels of peak negative pressure (PNP). Preliminary experiments were based on a linear array probe with 4 and 6 MHz as transmitted center frequency. The results show that the harmonics amplitudes reached by PW are generally higher than those achieved with FW at comparable PNP. At MI=0.2 the second harmonic turns out to be 8 dB higher in PW than in FW in the focal region (25 mm depth), and 20 dB higher at 40 mm depth.