In vivo photoacoustics and high frequency ultrasound imaging of mechanical high intensity focused ultrasound (HIFU) ablation

Noninvasive Monitoring of Tissue Temperature Changes Induced by Focused Ultrasound Exposure using Sparse Expression of Ultrasonic Radio Frequency Echo Signals

Background

Noninvasive therapies such as focused ultrasound were developed to be used for cancer therapies, vessel bleeding, and drug delivery. The main purpose of focused ultrasound therapy is to affect regions of interest (ROI) of tissues without any injuries to surrounding tissues. In this regard, an appropriate monitoring method is required to control the treatment.

Methods

This study is aimed to develop a noninvasive monitoring technique of focused ultrasound (US) treatment using sparse representation of US radio frequency (RF) echo signals. To this end, reasonable results in temperature change estimation in the tissue under focused US radiation were obtained by utilizing algorithms related to sparse optimization as orthogonal matching pursuit (OMP) and accompanying Shannon's entropy. Consequently, ex vivo tissue experimental tests yielded two datasets, including low-intensity focused US (LIFU) and high-intensity focused US (HIFU) data. The proposed processing method analyzed the ultrasonic RF echo signal and expressed it as a sparse signal and calculated the entropy of each frame.

Results

The results indicated that the suggested approach could noninvasively estimate temperature changes between 37°C and 47°C during LIFU therapy. In addition, it represented temperature changes during HIFU ablation at various powers, ranging from 10 to 130 W. The normalized mean square error of the proposed method is 0.28, approximately 2.15 on previous related methods.

Conclusion

These results demonstrated that this novel proposed approach, including the combination of sparsity and Shanoon's entropy, is more feasible and effective in temperature change estimation than its predecessors.

Real-time assessment of high-intensity focused ultrasound heating and cavitation with hybrid optoacoustic ultrasound imaging

High-intensity focused ultrasound (HIFU) enables localized ablation of biological tissues by capitalizing on the synergistic effects of heating and cavitation. Monitoring of those effects is essential for improving the efficacy and safety of HIFU interventions. Herein, we suggest a hybrid optoacoustic-ultrasound (OPUS) approach for real-time assessment of heating and cavitation processes while providing an essential anatomical reference for accurate localization of the HIFU-induced lesion. Both effects could clearly be observed by exploiting the temperature dependence of optoacoustic (OA) signals and the strong contrast of gas bubbles in pulse-echo ultrasound (US) images. The differences in temperature increase and its rate, as recorded with a thermal camera for different HIFU pressures, evinced the onset of cavitation at the expected pressure threshold. The estimated temperatures based on OA signal variations were also within 10-20 % agreement with the camera readings for temperatures below the coagulation threshold (∼50 °C). Experiments performed in excised tissues as well as in a post-mortem mouse demonstrate that both heating and cavitation effects can be effectively visualized and tracked using the OPUS approach. The good sensitivity of the suggested method for HIFU monitoring purposes was manifested by a significant increase in contrast-to-noise ratio within the ablated region by > 10 dB and > 5 dB for the OA and US images, respectively. The hybrid OPUS-based monitoring approach offers the ease of handheld operation thus can readily be implemented in a bedside setting to benefit several types of HIFU treatments used in the clinics.

Photoacoustic imaging for characterization of radiofrequency ablated cardiac tissues

Photoacoustic (PA) imaging is an emerging technique being explored for various clinical applications. PA imaging offers a portable, inexpensive, stand-alone modality for evaluating optical contrast agents. PA signals are well-correlated with tissue physical parameters and can quantify various physiological variables (e.g., oxygenation of hemoglobin). Moreover, radiofrequency (RF) ablation is a promising treatment for certain cardiac arrhythmias. Assessment of RF-ablated lesions is of clinical importance. The purpose of this study is to elaborate the PA imaging to characterize RF-ablated cardiac tissues. Specifically, we describe the application of PA imaging to identify, characterize, and quantify cardiac RF lesions, highlighting the fundamental principles and unique benefits of this optical imaging technique. Potential future clinical application of PA imaging that reveals additional information about structural damage in RF-treated cardiac tissue are also anticipated.

Efficient photoacoustic imaging using indocyanine green (ICG) loaded functionalized mesoporous silica nanoparticles

Photoacoustic (PA) imaging is gaining momentum due to its greater depth of field, low background, and 3D imaging capabilities. However, traditional PA imaging agents (e.g. dyes, quantum dots, etc.) are usually unstable in plasma and bind to serum proteins, and thus cleared rapidly. Because of this, the nanoparticle encapsulation of PA imaging agents is becoming increasingly popular. Therefore, the rational design of carrier nanoparticles for this purpose is necessary for strong imaging signal intensity, high biosafety, and precise targeting. Herein, we systematically evaluate the influence of the chemical and physical surface functionalization of mesoporous silica nanoparticles (MSNs) on the photo-stability, loading, release, and photoacoustic (PA) signal strength of the FDA approved small molecule contrast agent, indocyanine green (ICG). Chemical functionalization involved the modification of MSNs with silanes having amine (NH2) or phosphonate (PO3) terminal groups, whereas physical modifications were performed by capping the ICG loaded MSNs with lipid bilayer (LB) or layer-by-layer (LBL) polyelectrolyte coatings. The NH2-MSNs display the highest ICG mass loading capacity (16.5 wt%) with a limited release of ICG (5%) in PBS over 48 h, while PO3-MSNs only loaded ICG around 3.5 wt%. The physically modified MSNs (i.e. LBMSNs and LBLMSNs) were vacuum loaded resulting in approximately 9 wt% loading and less than 10% ICG release in 48 h. Pure ICG was highly photo-unstable and showed 20% reduction in photoluminescence (PL) within 3 h of exposure to 800 nm, while the ICG loaded onto functionalized MSNs did not photo-degrade. Among the tested formulations, NH2-MSNs and LBLMSNs presented 4-fold in vitro PA signal intensity enhancement at a 200 μg mL-1 equivalent ICG dose. Similar to the in vitro PA imaging, NH2-MSNs and LBLMSNs performed the best when subcutaneously injected into mouse cadavers with 1.29- and 1.43-fold PA signal enhancement in comparison to the pure ICG, respectively.

Listening to tissues with new light: recent technological advances in photoacoustic imaging

Photoacoustic tomography (PAT), or optoacoustic tomography, has achieved remarkable progress in the past decade, benefiting from the joint developments in optics, acoustics, chemistry, computing and mathematics. Unlike pure optical or ultrasound imaging, PAT can provide unique optical absorption contrast as well as widely scalable spatial resolution, penetration depth and imaging speed. Moreover, PAT has inherent sensitivity to tissue's functional, molecular, and metabolic state. With these merits, PAT has been applied in a wide range of life science disciplines, and has enabled biomedical research unattainable by other imaging methods. This Review article aims at introducing state-of-the-art PAT technologies and their representative applications. The focus is on recent technological breakthroughs in structural, functional, molecular PAT, including super-resolution imaging, real-time small-animal whole-body imaging, and high-sensitivity functional/molecular imaging. We also discuss the remaining challenges in PAT and envisioned opportunities.

Small PLGA nanocapsules Co-encapsulating copper sulfide nanodots and fluorocarbon compound for photoacoustic imaging-guided HIFU synergistic therapy

High intensity focused ultrasound (HIFU), as a promising and minimally invasive therapeutic modality against various solid tumors, has received considerable attention in the biomedical field. However, both the accuracy and efficacy of this technique are currently unsatisfactory. Herein, a nanometer-sized organic/inorganic hybrid enhancement agent for photoacoustic imaging (PAI)-guided HIFU therapy was designed and fabricated by concurrently encapsulating both Cu_2−xS nanodots (NDs) and perfluorooctyl bromide (PFOB) into a poly(lactic-co-glycolic acid) PLGA nanocapsule (denoted CPPNs). These nanocapsules assumed a unique core/satellite/shell sandwich structure, and combined the merits of small and uniform particle size (about 120 nm), favorable biosafety, and multifunctional theranostic ability into one system. The high performance of Cu_2−xS NDs in the absorption and conversion of near infrared laser confers high PAI contrast capability to the CPPNs, by which the location of the CPPNs within a tumor can be monitored successfully under PAI. Furthermore, our in vitro and in vivo results confirmed that the encapsulated PFOB in CPPNs increased the cavitation effect and thus enhanced the ablation efficacy under HIFU exposure. CPPNs show great potential as an efficient and powerful theranostic agent for future PAI-guided HIFU synergistic therapy.

A nanometer-sized inor-ganic/organic hybrid enhancement agent is constructed for photoacoustic imaging-guided high intensity focused ultrasound therapy.