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.

Robotically-Assisted Sonic Therapy for Renal Ablation in a Live Porcine Model: Initial Preclinical Results

“…The use of thermal ablation can be limited for tumors involving the renal hilum, as mechanical and thermal trauma can result in urine leaks, bleeding, urothelial strictures, and loss of renal function (2–4). Histotripsy, based on cavitation, depends on the rapid expansion and collapse of a bubble cloud to disrupt targeted tissue via focused ultrasound in a purely mechanical method of tissue destruction (5). Previous studies with this technology have demonstrated tissue-selective properties, suggesting that large vessels and collecting system structures may be spared, while tumors and normal parenchyma are lysed (6,7).…”

“…RAST is an emerging ablation technology based on histotripsy: a noninvasive and nonthermal focused ultrasound technique that uses short-duration (<20 μs), low-duty-cycle (<1%) ultrasound pulses at high peak negative pressures (>10 MPa). The result is mechanical tissue disruption at the cellular level (5,9). Microbubbles produced by cavitation are visible with conventional B-mode ultrasound in real time throughout the treatment (10,11).…”

A 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.…”

“…A more detailed exploration of the simulation parameter space in relation to the present work will be presented in a future study. 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 = 2π f ), 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 largest-amplitude cycle of the waveform gives rise to the bubble growth.…”