Large abscesses are walled-off collections of pus and bacteria that often do not respond to antibiotic therapy. Standard of care involves percutaneous placement of indwelling catheter(s) for drainage, a long and uncomfortable process with high rehospitalization rates. The long-term goal of this work is to develop therapeutic ultrasound approaches to eradicate bacteria within abscesses as a noninvasive therapeutic alternative. Inertial cavitation induced by short pulses of focused ultrasound (histotripsy) is known to generate lethal mechanical damage in bacteria. Prior studies with Escherichia coli (E. coli) in suspension demonstrated that bactericidal effects increase with increasing peak negative amplitude, treatment time, and duty cycle. The current study investigated correlates of bactericidal activity with histotripsy cavitation cloud size. Histotripsy was applied to E. coli suspensions in 10-mL sample vials at 810 kHz, 1.2 MHz, or 3.25 MHz for 40 min. The cavitation activity in the sample vials was separately observed with high-speed photography. The cavitation cloud area was quantified from those images. A linear relationship was observed between bacterial inactivation and cavitation cloud size ( R² = 0.98 ), regardless of the acoustic parameters (specifically frequency, pulse duration, and power) used to produce the cloud.

Pulsed high-intensity focused ultrasound (pHIFU) has the capability to induce de novo cavitation bubbles, offering potential applications for enhancing drug delivery and modulating tissue microenvironments. However, imaging and monitoring these cavitation bubbles during the treatment presents a challenge due to their transient nature immediately following pHIFU pulses. A planewave bubble Doppler technique demonstrated its potential, yet this Doppler technique used conventional clutter filter that was originally designed for blood flow imaging. Our recent study introduced a new approach employing dynamic mode decomposition (DMD) to address this in an ex vivo setting. This study demonstrates the feasibility of the application of DMD for in vivo Doppler monitoring of the cavitation bubbles in porcine liver and identifies the candidate monitoring metrics for pHIFU treatment. We propose a fully automated bubble mode identification method using k-means clustering and an image contrast-based algorithm, leading to the generation of DMD-filtered bubble images and corresponding Doppler power maps after each HIFU pulse. These power Doppler maps are then correlated with the extent of tissue damage determined by histological analysis. The results indicate that DMD-enhanced power Doppler map can effectively visualize the bubble distribution with high contrast, and the Doppler power level correlates with the severity of tissue damage by cavitation. Further, the temporal characteristics of the bubble modes, specifically the decay rates derived from DMD, provide information of the bubble dissolution rate, which are correlated with tissue damage level—slower rates imply more severe tissue damage.

Bacterial loads can be effectively reduced using cavitation-mediated focused ultrasound, or histotripsy. In this study, gram-negative bacteria (Escherichia coli) in suspension were used as model bacteria to evaluate the effectiveness of two regimens of histotripsy treatments: cavitation histotripsy (CH) and boiling histotripsy (BH).

The results of this study suggest that both CH and BH can be used to inactivate E. coli in suspension, with the optimal regimen depending on the attainable peak negative focal pressure at the target.

Pulsed high intensity focused ultrasound (pHIFU) is a non-invasive method that allows to permeabilize pancreatic tumors through inertial cavitation and thereby increase the concentration of systemically administered drug. In this study the tolerability of weekly pHIFU-aided administrations of gemcitabine (gem) and their influence on tumor progression and immune microenvironment were investigated in genetically engineered KrasLSL.G12D/ƥ; p53R172H/ƥ; PdxCretg/ƥ (KPC) mouse model of spontaneously occurring pancreatic tumors. KPC mice were enrolled in the study when the tumor size reached 4–6 mm and treated once a week with either ultrasound-guided pHIFU (1.5 MHz transducer, 1 ms pulses, 1% duty cycle, peak negative pressure 16.5 MPa) followed by administration of gem (n = 9), gem only (n = 5) or no treatment (n = 8). Tumor progression was followed by ultrasound imaging until the study endpoint (tumor size reaching 1 cm), whereupon the excised tumors were analyzed by histology, immunohistochemistry (IHC) and gene expression profiling (Nanostring PanCancer Immune Profiling panel). The pHIFU + gem treatments were well tolerated; the pHIFU-treated region of the tumor turned hypoechoic immediately following treatment in all mice, and this effect persisted throughout the observation period (2–5 weeks) and corresponded to areas of cell death, according to histology and IHC. Enhanced labeling by Granzyme-B was observed within and adjacent to the pHIFU treated area, but not in the non-treated tumor tissue; no difference in CD8 + staining was observed between the treatment groups. Gene expression analysis showed that the pHIFU + gem combination treatment lead to significant downregulation of 162 genes related to immunosuppression, tumorigenesis, and chemoresistance vs gem only treatment.

One of the challenges of transcutaneous high-intensity focused ultrasound (HIFU) therapies, especially ones relying heavily on shock formation, such as boiling histotripsy (BH), is the loss of focusing from aberration induced by the heterogeneities of the body wall. Here, a methodology to execute aberration correction in vivo is proposed. A custom BH system consisting of a 1.5-MHz phased array of 256 elements connected to a Verasonics V1 system is used in pulse/echo mode on a porcine model under general anesthesia. Estimation of the time shifts needed to correct for aberration in the liver and kidney is done by maximizing the value of the coherence factor on the acquired backscattered signals. As this process requires multiple pulse/echo sequences on a moving target to converge to a solution, tracking is also implemented to ensure that the same target is used between each iteration. The method was validated by comparing the acoustic power needed to generate a boiling bubble at one target with aberration correction and at another target within a 5-mm radius without aberration correction. Results show that the aberration correction effectively lowers the acoustic power required to reach boiling by up to 45%, confirming that it indeed restored formation of the nonlinear shock front at the focus.

Ultrasound and microbubbles are useful for both diagnostic imaging and targeted drug delivery, making them ideal conduits for theranostic interventions. Recent reports have indicated the preclinical success of microbubble cavitation for enhancement of chemotherapy in abdominal tumors; however, there have been limited studies and variable efficacy in clinical implementation of this technique. This is likely because in contrast to the high pressures and long cycle lengths seen in successful preclinical work, current clinical implementation of microbubble cavitation for drug delivery generally involves low acoustic pressures and short cycle lengths to fit within clinical guidelines. To translate the preclinical parameter space to clinical adoption, a relevant safety study in a healthy large animal is required. Therefore, the purpose of this work was to evaluate the safety of ultrasound cavitation treatment (USCTx) in a healthy porcine model using a modified Philips EPIQ with S5-1 as the focused source. We performed USCTx on eight healthy pigs and monitored health over the course of 1 wk. We then performed an acute study of USCTx to evaluate immediate tissue damage. Contrast-enhanced ultrasound exams were performed before and after each treatment to investigate perfusion changes within the treated areas, and blood and urine were evaluated for liver damage biomarkers. We illustrate, through quantitative analysis of contrast-enhanced ultrasound data, blood and urine analyses and histology, that this technique and the parameter space considered are safe within the time frame evaluated. With its safety confirmed using a clinical-grade ultrasound scanner and contrast agent, USCTx could be easily translated into clinical trials for improvement of chemotherapy delivery. This represents the first safety study assessing the bio-effects of microbubble cavitation from relevant ultrasound parameters in a large animal model.