The histotripsy spectrum: differences and similarities in techniques and instrumentation.

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Dual-Mode 1-D Linear Ultrasound Array for Image-Guided Drug Delivery Enhancement Without Ultrasound Contrast Agents.

Pulsed high-intensity focused ultrasound (pHIFU) uses nonlinearly distorted millisecond-long ultrasound pulses of moderate intensity to induce inertial cavitation in tissue without administration of contrast agents. The resulting mechanical disruption permeabilizes the tissue and enhances the diffusion of systemically administered drugs. This is especially beneficial for tissues with poor perfusion such as pancreatic tumors. Here, we characterize the performance of a dual-mode ultrasound array designed for image-guided pHIFU therapies in producing inertial cavitation and ultrasound imaging. The 64-element linear array (1.071 MHz, an aperture of 14.8×51.2 mm, and a pitch of 0.8 mm) with an elevational focal length of 50 mm was driven by the Verasonics V-1 ultrasound system with extended burst option. The attainable focal pressures and electronic steering range in linear and nonlinear operating regimes (relevant to pHIFU treatments) were characterized through hydrophone measurements, acoustic holography, and numerical simulations. The steering range at ±10% from the nominal focal pressure was found to be ±6 mm axially and ±11 mm azimuthally. Focal waveforms with shock fronts of up to 45 MPa and peak negative pressures up to 9 MPa were achieved at focusing distances of 38-75 mm from the array. Cavitation behaviors induced by isolated 1-ms pHIFU pulses in optically transparent agarose gel phantoms were observed by high-speed photography across a range of excitation amplitudes and focal distances. For all focusing configurations, the appearance of sparse, stationary cavitation bubbles occurred at the same P- threshold of 2 MPa. As the output level increased, a qualitative change in cavitation behavior occurred, to pairs and sets of proliferating bubbles. The pressure P- at which this transition was observed corresponded to substantial nonlinear distortion and shock formation in the focal region and was thus dependent on the focal distance of the beam ranging within 3-4 MPa for azimuthal F -numbers of 0.74-1.5. The array was capable of B-mode imaging at 1.5 MHz of centimeter-sized targets in phantoms and in vivo pig tissues at depths of 3-7 cm, relevant to pHIFU applications in abdominal targets.

Towards acoustic particle velocity sensors in air using entrained balloons: Measurements and modeling.

In this article, the feasibility of using balloons for the measurement of acoustic particle velocity in air is investigated by exploring the behavior of an elastic balloon in air as it vibrates in response to an incident acoustic wave. This is motivated by the frequent use of neutrally buoyant spheres as underwater inertial particle velocity sensors. The results of experiments performed in an anechoic chamber are presented, in which a pair of laser Doppler vibrometers simultaneously captured the velocities of the front and back surfaces of a Mylar balloon in an acoustic field. From phase measurements, the motion is described in terms of contributions from odd-order vibration modes (including bulk translation) and even-order vibration modes. The measured entrainment factors for the balloon are seen to be in good agreement with a physical model based on the scattering from an entrained rigid sphere. This demonstrates the feasibility of using entrained balloons for direct measurement of acoustic particle velocity in air.

Thévenin acoustics.

Thévenin's theorem is commonly used in the analysis of acoustic transducers to provide a simplified representation of a transducer or its environment. The method may be extended to the analyses of other acoustic systems, without limitation to systems that have been reduced to analogous circuit models, and is particularly convenient in the analysis of acoustic scattering when the scattering object is mobile. In this paper, the method is illustrated through an alternative derivation of the well-known "mass law" for transmission through a partition, and is also applied to the case of acoustic scattering from a rigid, mobile cylinder of arbitrary size in an ideal plane progressive wave. Differences between the conventional solution approach for such problems and the Thévenin-inspired method are discussed, along with the potential benefits of taking such an approach for the simplification of other problems in physical acoustics.