The overall goal of the project is to develop enhanced shock scattering histotripsy, a novel technique of achieving histotripsy, by applying a series of pseudo-monopolar peak positive pulses following an initial pseudo-monopolar peak negative pulse. We aim to generate multiple continuous secondary bubble clouds by shock scattering from the peak positive pulses. With this new technique, we could potentially overcome the challenges of pre-focal cavitation by minimizing negative pressures as well as generating clean and energetic secondary bubble clouds for performing aberration correction.
The pseudo-monopolar ultrasound pulses are generated based on the concept of frequency compounding. A frequency compounding transducer has been designed and built. The transducer consists of 113 individual piezoelectric elements with various resonant frequencies (250 kHz, 500 kHz, 750 kHz, 1 MHz, 1.5 MHz, 2 MHz, and 3 MHz). The principle of how frequency compounding works is that pseudo-monopolar peak positive pulses are generated by aligning the principal peak positive pressures of individual frequency components temporally so that they add constructively, and destructive interference occurs outside the peak-positive-overlapped temporal window. After inverting the polarity of the excitation signals, pseudo-monopolar peak negative pulses are generated in a similar way by aligning principal peak negative pressures.
Figure 1. Fully assembled frequency compounding transducer.
Figure 2. Representative temporal focal waveforms with a principal peak negative pressure of individual frequency components, shown in (a1)-(a7). A frequency-compounded pseudo-monopolar peak negative pulse shown in (b).
Figure 3. Representative temporal focal waveforms with a principal peak positive pressure of individual frequency components, shown in (a1)-(a7). A frequency-compounded pseudo-monopolar peak positive pulse shown in (b).
 A. D. Maxwell, T. Y. Wang, C. A. Cain, J. B. Fowlkes, O. A. Sapozhnikov, M. R. Bailey, and Z. Xu, “Cavitation clouds created by shock scattering from bubbles during histotripsy,” J. Acoust. Soc. Am., vol. 130, no. 4, pp. 1888–1898, Oct. 2011.
 K.-W. Lin, T. L. Hall, R. J. McGough, Z. Xu, and C. A. Cain, “Synthesis of monopolar ultrasound pulses for therapy: The frequency-compounding transducer,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 61, no. 7, pp. 1123–1136, Jul. 2014.