The dance of the nanobubbles: detecting acoustic backscatter from sub-micron bubbles using ultra-high frequency acoustic microscopy

Decorrelation Time Mapping as an Analysis Tool for Nanobubble-Based Contrast Enhanced Ultrasound Imaging

Nanobubbles (NBs; ~100-500 nm diameter) are preclinical ultrasound (US) contrast agents that expand applications of contrast enhanced US (CEUS). Due to their sub-micron size, high particle density, and deformable shell, NBs in pathological states of heightened vascular permeability (e.g. in tumors) extravasate, enabling applications not possible with microbubbles (~1000-10,000 nm diameter). A method that can separate intravascular versus extravascular NB signal is needed as an imaging biomarker for improved tumor detection. We present a demonstration of decorrelation time (DT) mapping for enhanced tumor NB-CEUS imaging. In vitro models validated the sensitivity of DT to agent motion. Prostate cancer mouse models validated in vivo imaging potential and sensitivity to cancerous tissue. Our findings show that DT is inversely related to NB motion, offering enhanced detail of NB dynamics in tumors, and highlighting the heterogeneity of the tumor environment. Average DT was high in tumor regions (~9 s) compared to surrounding normal tissue (~1 s) with higher sensitivity to tumor tissue compared to other mapping techniques. Molecular NB targeting to tumors further extended DT (11 s) over non-targeted NBs (6 s), demonstrating sensitivity to NB adherence. From DT mapping of in vivo NB dynamics we demonstrate the heterogeneity of tumor tissue while quantifying extravascular NB kinetics and delineating intra-tumoral vasculature. This new NB-CEUS-based biomarker can be powerful in molecular US imaging, with improved sensitivity and specificity to diseased tissue and potential for use as an estimator of vascular permeability and the enhanced permeability and retention (EPR) effect in tumors.

Assessing Tumor Microenvironment Characteristics and Stratifying EPR with a Nanobubble Companion Nanoparticle via Contrast-Enhanced Ultrasound Imaging

The tumor microenvironment is characterized by dysfunctional endothelial cells, resulting in heightened vascular permeability. Many nanoparticle-based drug delivery systems attempt to use this enhanced permeability combined with impaired lymphatic drainage (a concept known as the 'enhanced permeability and retention effect' or EPR effect) as the primary strategy for drug delivery, but this has not proven to be as clinically effective as anticipated. The specific mechanisms behind the inconsistent clinical outcomes of nanotherapeutics have not been clearly articulated, and the field has been hampered by a lack of accessible tools to study EPR-associated phenomena in clinically relevant scenarios. While medical imaging has tremendous potential to contribute to this area, it has not been broadly explored. This work examines, for the first time, the use of multiparametric dynamic contrast-enhanced ultrasound (CEUS) with a novel nanoscale contrast agent to examine tumor microenvironment characteristics noninvasively and in real-time. We demonstrate that CEUS imaging can: (1) evaluate tumor microenvironment features and (2) be used to help predict the distribution of doxorubicin-loaded liposomes in the tumor parenchyma. CEUS using nanobubbles (NBs) was carried out in two tumor types of high (LS174T) and low (U87) vascular permeability, and time-intensity curve (TIC) parameters were evaluated in both models prior to injection of doxorubicin liposomes. Consistently, LS174T tumors showed significantly different TIC parameters, including area under the rising curve (2.7x), time to peak intensity (1.9x) and decorrelation time (DT, 1.9x) compared to U87 tumors. Importantly, the DT parameter successfully predicted tumoral nanoparticle distribution (r = 0.86 ± 0.13). Ultimately, substantial differences in NB-CEUS generated parameters between LS174T and U87 tumors suggest that this method may be useful in determining tumor vascular permeability and could be used as a biomarker for identifying tumor characteristics and predicting sensitivity to nanoparticle-based therapies. These findings could ultimately be applied to predicting treatment efficacy and to evaluating EPR in other diseases with pathologically permeable vasculature.

Radiation combined with ultrasound and microbubbles: A potential novel strategy for cancer treatment

Cancer is one of the leading causes of death worldwide. Several emerging technologies are helping to battle cancer. Cancer therapies have been effective at killing cancer cells, but a large portion of patients still die to this disease every year. As such, more aggressive treatments of primary cancers are employed and have been shown to be capable of saving a greater number of lives. Recent research advances the field of cancer therapy by employing the use of physical methods to alter tumor biology. It uses microbubbles to enhance radiation effect by damaging tumor vasculature followed by tumor cell death. The technique can specifically target tumor volumes by conforming ultrasound fields capable of microbubbles stimulation and localizing it to avoid vascular damage in surrounding tissues. Thus, this new application of ultrasound-stimulated microbubbles (USMB) can be utilized as a novel approach to cancer therapy by inducing vascular disruption resulting in tumor cell death. Using USMB alongside radiation has showed to augment the anti-vascular effect of radiation, resulting in enhanced tumor response. Recent work with nanobubbles has shown vascular permeation into intracellular space, extending the use of this new treatment method to potentially further improve the therapeutic effect of the ultrasound-based therapy. The significant enhancement of localized tumor cell kill means that radiation-based treatments can be made more potent with lower doses of radiation. This technique can manifest a greater impact on radiation oncology practice by increasing treatment effectiveness significantly while reducing normal tissue toxicity. This review article summarizes the past and recent advances in USMB enhancement of radiation treatments. The review mainly focuses on preclinical findings but also highlights some clinical findings that use USMB as a therapeutic modality in cancer therapy.

Detection of clot formation & lysis In-Vitro using high frequency photoacoustic imaging & frequency analysis

Clotting is a physiological process that prevents blood loss after injury. An imbalance in clotting factors can lead to lethal consequences such as exsanguination or inappropriate thrombosis. Clinical methods to monitor clotting and fibrinolysis typically measure the viscoelasticity of whole blood or optical density of plasma over time. Though these methods provide insights into clotting and fibrinolysis, they require milliliters of blood which can worsen anemia or only provide partial information. To overcome these limitations, a high-frequency photoacoustic (HFPA) imaging system was developed to detect clotting and lysis in blood. Clotting was initiated in vitro in reconstituted blood using thrombin and lysed with urokinase plasminogen activator. Frequency spectra measured using HFPA signals (10-40 MHz) between non-clotted blood and clotted blood differed markedly, allowing tracking of clot initiation and lysis in volumes of blood as low as 25 µL/test. HFPA imaging shows potential as a point-of-care examination of coagulation and fibrinolysis.

Ultrasound-mediated nano drug delivery for treating cancer: Fundamental physics to future directions

The application of biocompatible nanocarriers in medicine has provided several benefits over conventional treatment methods. However, achieving high treatment efficacy and deep penetration of nanocarriers in tumor tissue is still challenging. To address this, stimuli-responsive nano-sized drug delivery systems (DDSs) are an active area of investigation in delivering anticancer drugs. While ultrasound is mainly used for diagnostic purposes, it can also be applied to affect cellular function and the delivery/release of anticancer drugs. Therapeutic ultrasound (TUS) has shown potential as both a stand-alone anticancer treatment and a method to induce targeted drug release from nanocarrier systems. TUS approaches have been used to overcome various physiological obstacles, including endothelial barriers, the tumor microenvironment (TME), and immunological hurdles. Combining nanomedicine and ultrasound as a smart DDS can increase in situ drug delivery and improve access to impermeable tissues. Furthermore, smart DDSs can perform targeted drug release in response to distinctive TMEs, external triggers, or dual/multi-stimulus. This results in enhanced treatment efficacy and reduced damage to surrounding healthy tissue or organs at risk. Integrating DDSs and ultrasound is still in its early stages. More research and clinical trials are required to fully understand ultrasound's underlying physical mechanisms and interactions with various types of nanocarriers and different types of cells and tissues. In the present review, ultrasound-mediated nano-sized DDS, specifically focused on cancer treatment, is presented and discussed. Ultrasound interaction with nanoparticles (NPs), drug release mechanisms, and various types of ultrasound-sensitive NPs are examined. Additionally, in vitro, in vivo, and clinical applications of TUS are reviewed in light of the critical challenges that need to be considered to advance TUS toward an efficient, secure, straightforward, and accessible cancer treatment. This study also presents effective TUS parameters and safety considerations for this treatment modality and gives recommendations about system design and operation. Finally, future perspectives are considered, and different TUS approaches are examined and discussed in detail. This review investigates drug release and delivery through ultrasound-mediated nano-sized cancer treatment, both pre-clinically and clinically.

The Influence of Nanobubble Size and Stability on Ultrasound Enhanced Drug Delivery

Lipid-shelled nanobubbles (NBs) are emerging as potential dual diagnostic and therapeutic agents. Similar to their micron-scale counterparts, microbubbles (1-10 μm), they can act as ultrasound contrast agents as well as locally enhance therapeutic uptake. Recently, it has been shown that the reduced size of NBs (<1 μm) promotes increased uptake and accumulation in tumor interstitial space, which can enhance their diagnostic and therapeutic performance. However, accurate characterization of NB size and concentration is challenging and may limit their translation into clinical use. Their submicron nature limits accuracy of conventional microscopy techniques, while common light scattering techniques fail to distinguish between subpopulations present in NB samples (i.e., bubbles and liposomes). Due to the difficulty in the characterization of NBs, relatively little is known about the influence of size on their therapeutic performance. In this study, we describe a novel method of using a commercially available nanoparticle tracking analysis system, to distinguish between NBs and liposomes based on their differing optical properties. We used this technique to characterize three NB populations of varying size, isolated via centrifugation, and subsequently used this to assess their potential for enhancing localized delivery. Confocal fluorescence microscopy and image analysis were used to quantify the ultrasound enhanced uptake of fluorescent dextran into live colorectal cancer cells. Our results showed that the amount of localized uptake did not follow the expected trends, in which larger NB populations out-perform smaller NBs, at matched concentration. To understand this observed behavior, the stability of each NB population was assessed. It was found that dilution of the NB samples from their stock concentration influences their stability, and it is hypothesized that both the total free lipid and interbubble distance play a role in NB lifetime, in agreement with previously proposed theories and models.

Biophysical properties of hydrogels for mimicking tumor extracellular matrix

The extracellular matrix (ECM) is an essential component of the tumor microenvironment. It plays a critical role in regulating cell-cell and cell-matrix interactions. However, there is lack of systematic and comparative studies on different widely-used ECM mimicking hydrogels and their properties, making the selection of suitable hydrogels for mimicking different in vivo conditions quite random. This study systematically evaluates the biophysical attributes of three widely used natural hydrogels (Matrigel, collagen gel and agarose gel) including complex modulus, loss tangent, diffusive permeability and pore size. A new and facile method was developed combining Critical Point Drying, Scanning Electron Microscopy imaging and a MATLAB image processing program (CSM method) for the characterization of hydrogel microstructures. This CSM method allows accurate measurement of the hydrogel pore size down to nanometer resolution. Furthermore, a microfluidic device was implemented to measure the hydrogel permeability (P_d) as a function of particle size and gel concentration. Among the three gels, collagen gel has the lowest complex modulus, medium pore size, and the highest loss tangent. Agarose gel exhibits the highest complex modulus, the lowest loss tangent and the smallest pore size. Collagen gel and Matrigel produced complex moduli close to that estimated for cancer ECM. The P_d of these hydrogels decreases significantly with the increase of particle size. By assessing different hydrogels' biophysical characteristics, this study provides valuable insights for tailoring their properties for various three-dimensional cancer models.