Ultrasound-guided noninvasive pancreas ablation using histotripsy: feasibility study in an in vivo porcine model

Evaluation of targeting accuracy of cone beam CT guided histotripsy in an in vivo porcine model

PURPOSE

The application of histotripsy, an emerging noninvasive, non-ionizing, and non-thermal tumor treatment, is currently limited by the inherent limitations of diagnostic ultrasound as the sole targeting modality. This study evaluates the feasibility and accuracy of cone beam computed tomography (CBCT) guidance for histotripsy treatments in an in vivo porcine model.

MATERIALS AND METHODS

Histotripsy treatments were performed in the liver of seven healthy swine under the guidance of a C-arm CBCT system that was calibrated to the robotic arm of the histotripsy system. For each treatment, pseudotumors (small histotripsy treatments of 15 mm) were created using conventional US guidance to serve as targets for subsequent CBCT guided treatments. A pretreatment CBCT with intravenous contrast was acquired for each swine and the center of the pseudotumor was selected as the target. The robotic arm automatically aligned the transducer to the selected target location. Ultrasound based aberration offset correction was performed when possible, and a 25 mm diameter treatment was performed. A post-treatment CBCT with intravenous contrast was then acquired to evaluate coverage, treatment size, and distance between the pseudotumor target and actual treatment zone center.

RESULTS

Treatments were technically successful and pseudotumors were completely covered in all seven treatments (7/7). The average treatment diameter was 39.3 ± 4.2 mm. The center-to-center distance between pseudotumor and actual treatments was 3.8 ± 1.3 mm.

CONCLUSION

CBCT provides accurate targeting for histotripsy treatment in vivo. While future work is required to assess safety and efficacy in the presence of obstructions, the proposed approach could supplement ultrasound imaging for targeting.

Safety and efficacy of histotripsy delivery through overlying gas-filled small bowel in an ex vivo swine model

PURPOSE

To evaluate the safety and efficacy of performing histotripsy through overlying gas-filled bowel in an ex vivo swine model.

METHODS

An ex vivo model was created to simulate histotripsy treatment of solid organs through gas-filled bowel. Spherical 2.5 cm histotripsy treatments were performed in agar phantoms for each of five treatment groups: 1) control with no overlying bowel (n = 6), 2) bowel 0 cm above phantom (n = 6), 3) bowel 1 cm above phantom (n = 6), 4) bowel 2 cm above phantom (n = 6), and 5) bowel 0 cm above the phantom with increased treatment amplitude (n = 6). Bowel was inspected for gross and microscopic damage, and treatment zones were measured. A ray-tracing simulation estimated the percentage of therapeutic beam path blockage by bowel in each scenario.

RESULTS

All histotripsy treatments through partial blockage were successful (24/24). No visible or microscopic damage was observed to intervening bowel. Partial blockage resulted in a small increase in treatment volume compared to controls (p = 0.002 and p = 0.036 for groups with bowel 0 cm above the phantom, p > 0.3 for bowel 1 cm and 2 cm above the phantom). Gas-filled bowel was estimated to have blocked 49.6%, 35.0%, and 27.3% of the therapeutic beam at 0, 1, and 2 cm, respectively.

CONCLUSION

Histotripsy has the potential to be applied through partial gas blockage of the therapeutic beam path, as shown by this ex vivo small bowel model. Further work in an in vivo survival model appears indicated.

Investigation of Optimum Production Conditions and the Stability of β-Cyclodextrin-Perfluorocarbon Nanocone Clusters for Histotripsy Applications

Nanocone clusters (NCCs) have been developed as clusters with inclusion complexes of FDA-approved β-cyclodextrin (βCD) and perfluorocarbons (PFC) (i.e., perfluoropentane (PFP) and perfluorohexane (PFH)) and have shown promise in nanoparticle-mediated histotripsy (NMH) applications owing to their lowered cavitation threshold, ease of production, and fluorocarbon quantification. However, there is still a lack of information on the best conditions of the synthesis of NCCs as a product that can have a maximum determinable fluorocarbon content and maintain the stability of the NCC during synthesis and when used as histotripsy agents or exposed to physiological conditions. These concerns about the stability of the clusters and the best possible formulation are investigated in the current work. The cluster formation potential was tested taking into consideration the nature of both PFCs and βCD by employing different synthesis conditions in terms of solution and environmental parameters such as concentration of solvent, stoichiometry between βCD and PFCs, temperature, pH, solvent type, etc. The best route of synthesis was then translated into various batch sizes and investigated in terms of the PFC loading and yield. These studies revealed that preparing NCCs in double-distilled water in an ice bath at the optimized solution concentration gave the highest yields and optimal PFC loading, as determined from gas chromatography. Furthermore, the stability of the clusters with different stoichiometries was scrutinized in varying concentrations, mechanical disruption times, pH levels, and temperature conditions, showing effects on each cluster's particle size in dynamic light scattering, visualized in transmission electron microscopy, and cavitation behavior in agarose gel tissue phantoms. These studies revealed stable clusters for all formulations, with PFH-containing NCCs emerging to be the most stable in terms of their cluster size and bubble formation potential in histotripsy. Finally, the shelf life of these clusters was investigated using DLS, which revealed a stable cluster. In conclusion, NCCs have shown high stability in terms of both synthesis, which can be replicated in gram-level production, and the cluster itself, which can be exposed to harsher conditions and still form stable bubbles in histotripsy.