Monopolar Acoustic Pulses in Histotripsy and Other Applications

Abstract

Monopolar acoustic pulses decouple the compressional (positive) and rarefactional (negative) half-cycles within acoustic bursts and could be crucial for many applications. In this work, a frequency compounding transducer was designed and built to generate pseudo-monopolar peak positive pulses and peak negative pulses. The transducer consisted of 113 individual piezoelectric elements with 7 various resonant frequencies. Focal waveforms of both peak positive pulses and peak negative pulses were measured. Different pulsing sequences were then designed and applied for studying several aspects of histotripsy. First off, the use of pseudo-monopolar pulses with variable, controllable delays could achieve a new technique called “enhanced shock scattering histotripsy”. The shock scattering process in normal shock scattering histotripsy might not be optimal because it involves a complex interaction between positive and negative phases within an acoustic pulse to initiate a robust cavitation bubble cloud. With enhanced shock scattering histotripsy, we aimed to generate cavitation bubble clouds by shock scattering with mostly peak positive pulses. Observations of bubble clouds generated by this technique were achieved by using high-speed photography. For example, 16 successive bubble clouds were generated by 16 peak positive pulses following an initial peak negative pulse. The feasibility of the technique was tested by generating a precise lesion in a red-blood-cell phantom. Additional efforts were made to investigate the cavitation thresholds at pressure-release interfaces by applying pseudo-monopolar peak positive pulses with various pressure levels. Different interface models were explored. Threshold curves showed that the thresholds at interfaces were less than 20 MPa negative, which was lower than the intrinsic threshold in free water. They also varied with spatial locations for certain materials. Another potential application of high amplitude monopolar pulses is ultrasonic neural stimulation. Preliminary work was done where we hypothesized the generation of de-modulated low frequency currents from simultaneous ultrasound and high frequency, oscillating magnetic fields. Varying the two frequencies by a few kHz could produce a de-modulated, difference-frequency current similar to that generated by Transcranial Magnetic Stimulation. The pressure field generated by a 500 kHz ultrasound transducer and the resultant current density magnitude distribution in the presence of a magnetic field were simulated. Experimentally, with same conditions, currents of 0.34 μA/cm^2 at 4 kHz and 0.39 μA/cm^2 at 3 kHz were detected, which matched the simulation results.