Acoustic Methods for Increasing the Cavitation Initiation Pressure Threshold

Abstract

Histotripsy is a tissue ablation method that utilizes focused, high-amplitude ultrasound to generate a cavitation bubble cloud that mechanically fractionates tissue. Effective histotripsy depends on the initiation, control, and maintenance of cavitation bubble clouds in the targeted area. In this study, we hypothesized that a low-pressure acoustic pulse sequence applied before and/or during histotripsy therapy would increase the cavitation initiation pressure threshold and the growth of cavitation bubble clouds. This technique could shrink or "sharpen" the focal zone during histotripsy to produce more precise and well-defined lesions with minimal collateral damage. It may also be a way to actively protect the soft tissue from cavitation damage during lithotripsy by increasing the pressure threshold for bubble cloud initiation. We applied these low-amplitude acoustic pulse sequences before and during histotripsy treatments with the pulse repetition frequency of 1 and 100 Hz, in three different mediums: water, tissue phantom agarose gel, and bovine liver in vitro. Acoustic backscatter signals and optical imaging were used to detect and monitor the initiation, maintenance, and growth of the resulting cavitation bubble cloud. The results demonstrated that the use of low-amplitude acoustic pulse sequences could increase the cavitation pressure amplitude threshold by 20% in the targeted area.

Bubble Cloud Behavior and Ablation Capacity for Histotripsy Generated from Intrinsic or Artificial Cavitation Nuclei

“…The respective values of 40 and 3 nm for the NCs and intrinsic nuclei are inferred based on the maximum radii and collapse times achieved experimentally. Our assumption of a 3-nm intrinsic nucleus was further supported by our previous studies modeling intrinsic threshold histotripsy cavitation (Vlaisavljevich et al 2015b;Mancia et al 2017;Alavi Tamaddoni et al 2018;Mancia et al 2019;Wilson et al 2019).…”

Optically-generated focused ultrasound for noninvasive brain stimulation with ultrahigh precision

“…With our laser input used in the in vivo experiment, the estimated transcranial peak-to-peak pressure 6.0 MPa delivered to the mouse brain was below the level of 40 MPa, at which no tissue lesion was reported 39 . The transcranial peak negative pressure of OFUS was estimated to be 2.7 MPa, which is below the threshold of bubble cloud generation in soft tissue (25–30 MPa) 40 . An MI of 0.5 is obtained for OFUS stimulation based on acoustic attenuation coefficient for brain tissue of 0.91 dB/(cm × MHz).…”

A Conformable Ultrasound Patch for Cavitation‐Enhanced Transdermal Cosmeceutical Delivery

“…The standard deviation in the ultra‐subharmonic signal magnitude starts increasing significantly when the applied voltage is 50 V and above, indicating sporadic appearance of ultra‐subharmonic peaks. Given the stochastic nature of inertial cavitation, its dependence on existing bubble nuclei populations, [ 61 ] and short lifetime of inertial bubbles, [ 62 ] we see large variations in the measured ultra‐subharmonic signal even over a long sampling duration (averaged over 32 frames, corresponding to a total sampling period of 1.6 ms (see Experimental Section)). However, since the total “on‐time” for each ultrasound pulse for the final system (≈50 ms) is far greater than this sampling period, there is a much greater likelihood of obtaining inertial cavitation action for a significant portion of the application time.…”