Visualizing the histotripsy process: Bubble cloud-cancer cell interactions in a tissue-mimicking environment

Abstract

Histotripsy is a non-invasive ultrasonic ablation method that uses cavitation to mechanically fractionate tissue into acellular debris. With a sufficient number of pulses, histotripsy can completely fractionate tissue into a liquid-appearing homogenate with no cellular structures. The location, shape, and size of lesion formation closely match those of the cavitation cloud. Previous work has led to the hypothesis that the rapid expansion and collapse of histotripsy bubbles fractionate tissue by inducing large stress and strain on the tissue structures immediately adjacent to the bubbles. In this work, the histotripsy bulk tissue fractionation process is visualized at the cellular level for the first time using a custom-built 2 MHz transducer incorporated into a microscope stage. A layer of breast cancer cells were cultured within an optically transparent fibrin-based gel phantom to mimic cells inside a three dimensional extracellular matrix. To test the hypothesis, the cellular response to single and multiple histotripsy pulses was investigated using high speed optical imaging. Bubbles were always generated in the extracellular space, and significant cell displacement/deformation was observed for cells directly adjacent to the bubble during both bubble expansion and collapse. The largest displacements were observed during collapse for cells immediately adjacent to the bubble, with cells moving more than 150-300 μm in less than 100 μs. Cells often underwent multiple large deformations (>150% strain) over multiple pulses, resulting in the bisection of cells multiple times before complete rupture. To provide theoretical support to the experimental observations, a numerical simulation was conducted using a single bubble model, which showed that histotripsy exerts the largest strains and cell displacements in the regions immediately adjacent to the bubble. The experimental and simulation results support our hypothesis, which helps to explain the formation of the sharp lesions formed in histotripsy therapy localized to the regions directly exposed to the bubbles.

Comparative study of the dynamics of laser and acoustically generated bubbles in viscoelastic media

“…The dynamics of a single isolated, spherical, gas-filled bubble in both water and in homogeneous viscoelastic media (representative of the agarose gels) are numerically simulated using an approach similar to previous studies of histotripsy cavitation [25,26,[46][47][48]. In the present study, only acoustically nucleated bubbles are modeled.…”

“…Two sets of simulations are carried out using the specified material properties: one where the gel viscosity is taken to be equal to that of water's [25,47,51], µ w , and another where it is taken to be the prescribed viscosity for agarose, µ a (Table II). With the material properties specified, we then calibrate the bubble's unknown initial size by comparing with the experimental data.…”

“…The pressure amplitude, p A = −24 MPa, and frequency, f = 1 MHz (ω = 2πf ), correspond to experimental measurements. The time delay, δ = 5 µs, and fitting parameter, n = 3.7, were chosen as in previous studies [25,26,46,47]. Here, it is hypothesized that the largestamplitude cycle of the waveform gives rise to the bubble growth.…”

Effects off-number on the histotripsy intrinsic threshold and cavitation bubble cloud behavior

“…Histotripsy depends on the initiation and maintenance of a cavitation bubble cloud to produce mechanical tissue fractionation (Parsons et al , 2006a; Xu et al , 2005; Xu et al , 2004). With sufficiently high pressure and dose, histotripsy can completely fractionate soft tissues into an acellular liquid homogenate by inducing very large strains to the adjacent tissue structures (Parsons et al , 2006a; Vlaisavljevich et al , 2014a; Xu et al , 2005; Vlaisavljevich et al , 2016d). Histotripsy is currently being studied for many clinical applications where noninvasive tissue removal is desired including benign prostatic hyperplasia (Hempel et al , 2011; Roberts et al , 2014), kidney stones (Duryea et al , 2011), deep vein thrombosis (Maxwell et al , 2011a; Zhang et al , 2015), congenital heart disease(Owens et al , 2011; Xu et al , 2010), and cancer (Styn et al , 2010; Vlaisavljevich et al , 2016c; Vlaisavljevich et al , 2013b).…”

For Whom the Bubble Grows: Physical Principles of Bubble Nucleation and Dynamics in Histotripsy Ultrasound Therapy

“…Histotripsy is a form of therapeutic ultrasound that produces mechanical disintegration of tissues that can be utilized to complement current ablative techniques or as a stand-alone modality (Parsons et al 2006;Roberts et al 2006;Xu et al 2004). The delivery of high-amplitude, short-duration histotripsy pulses repeatedly strains tissue at the subcellular level (Parsons et al 2006;Roberts et al 2006;Vlaisavljevich et al 2016d). Through application of a sufficient number of pulses, a target tissue structure can be homogenized precisely within the focal region.…”