Localized blood-brain barrier opening in infiltrating gliomas with MRI-guided acoustic emissions-controlled focused ultrasound.

Pharmacological treatment of gliomas and other brain-infiltrating tumors remains challenging due to limited delivery of most therapeutics across the blood-brain barrier (BBB). Transcranial MRI-guided focused ultrasound (FUS), an emerging technology for noninvasive brain treatments, enables transient opening of the BBB through acoustic activation of circulating microbubbles. Here, we evaluate the safety and utility of transcranial microbubble-enhanced FUS (MB-FUS) for spatially targeted BBB opening in patients with infiltrating gliomas. In this Phase 0 clinical trial (NCT03322813), we conducted comparative and quantitative analyses of FUS exposures (sonications) and their effects on gliomas using MRI, histopathology, microbubble acoustic emissions (harmonic dose [HD]), and fluorescence-guided surgery metrics. Contrast-enhanced MRI and histopathology indicated safe and reproducible BBB opening in all patients. These observations occurred using a power cycling closed feedback loop controller, with the power varying by nearly an order of magnitude on average. This range underscores the need for monitoring and titrating the exposure on a patient-by-patient basis. We found a positive correlation between microbubble acoustic emissions (HD) and MR-evident BBB opening (P = 0.07) and associated interstitial changes (P < 0.01), demonstrating the unique capability to titrate the MB-FUS effects in gliomas. Importantly, we identified a 2.2-fold increase of fluorescein accumulation in MB-FUS-treated compared to untreated nonenhancing tumor tissues (P < 0.01) while accounting for vascular density. Collectively, this study demonstrates the capabilities of MB-FUS for safe, localized, controlled BBB opening and highlights the potential of this technology to improve the surgical and pharmacologic treatment of brain tumors.

The roles of thermal and mechanical stress in focused ultrasound-mediated immunomodulation and immunotherapy for central nervous system tumors.

BACKGROUND: Focused ultrasound (FUS) is an emerging technology, offering the capability of tuning and prescribing thermal and mechanical treatments within the brain. While early works in utilizing this technology have mainly focused on maximizing the delivery of therapeutics across the blood-brain barrier (BBB), the potential therapeutic impact of FUS-induced controlled thermal and mechanical stress to modulate anti-tumor immunity is becoming increasingly recognized. OBJECTIVE: To better understand the roles of FUS-mediated thermal and mechanical stress in promoting anti-tumor immunity in central nervous system tumors, we performed a comprehensive literature review on focused ultrasound-mediated immunomodulation and immunotherapy in brain tumors. METHODS: First, we summarize the current clinical experience with immunotherapy. Then, we discuss the unique and distinct immunomodulatory effects of the FUS-mediated thermal and mechanical stress in the brain tumor-immune microenvironment. Finally, we highlight recent findings that indicate that its combination with immune adjuvants can promote robust responses in brain tumors. RESULTS: Along with the rapid advancement of FUS technologies into recent clinical trials, this technology through mild-hyperthermia, thermal ablation, mechanical perturbation mediated by microbubbles, and histotripsy each inducing distinct vascular and immunological effects, is offering the unique opportunity to improve immunotherapeutic trafficking and convert immunologically "cold" tumors into immunologically "hot" ones that are prone to generate prolonged anti-tumor immune responses. CONCLUSIONS: While FUS technology is clearly accelerating concepts for new immunotherapeutic combinations, additional parallel efforts to detail rational therapeutic strategies supported by rigorous preclinical studies are still in need to leverage potential synergies of this technology with immune adjuvants. This work will accelerate the discovery and clinical implementation of new effective FUS immunotherapeutic combinations for brain tumor patients.

Mechanisms of enhanced drug delivery in brain metastases with focused ultrasound-induced blood-tumor barrier disruption.

Blood-brain/blood-tumor barriers (BBB and BTB) and interstitial transport may constitute major obstacles to the transport of therapeutics in brain tumors. In this study, we examined the impact of focused ultrasound (FUS) in combination with microbubbles on the transport of two relevant chemotherapy-based anticancer agents in breast cancer brain metastases at cellular resolution: doxorubicin, a nontargeted chemotherapeutic, and ado-trastuzumab emtansine (T-DM1), an antibody-drug conjugate. Using an orthotopic xenograft model of HER2-positive breast cancer brain metastasis and quantitative microscopy, we demonstrate significant increases in the extravasation of both agents (sevenfold and twofold for doxorubicin and T-DM1, respectively), and we provide evidence of increased drug penetration (>100 vs. <20 µm and 42 ± 7 vs. 12 ± 4 µm for doxorubicin and T-DM1, respectively) after the application of FUS compared with control (non-FUS). Integration of experimental data with physiologically based pharmacokinetic (PBPK) modeling of drug transport reveals that FUS in combination with microbubbles alleviates vascular barriers and enhances interstitial convective transport via an increase in hydraulic conductivity. Experimental data demonstrate that FUS in combination with microbubbles enhances significantly the endothelial cell uptake of the small chemotherapeutic agent. Quantification with PBPK modeling reveals an increase in transmembrane transport by more than two orders of magnitude. PBPK modeling indicates a selective increase in transvascular transport of doxorubicin through small vessel wall pores with a narrow range of sizes (diameter, 10-50 nm). Our work provides a quantitative framework for the optimization of FUS-drug combinations to maximize intratumoral drug delivery and facilitate the development of strategies to treat brain metastases.

Acoustic source localization with the angular spectrum approach in continuously stratified media.

The angular spectrum approach (ASA)-a frequency domain method to calculate the acoustic field-enables highly efficient passive source localization and modeling forward propagation in homogeneous media. If the medium is continuously stratified, a first-order analytical solution may be obtained for the field at arbitrary depth. Simulations show that the proposed stratified ASA solution enables accurate source localization as compared to the uncorrected ASA (error from 1.2 ± 0.3 to 0.49 ± 0.3 wavelengths) at scalings relevant to biomedical, underwater, and atmospheric acoustic applications, and requiring milliseconds on nonspecialized hardware. The results suggest the proposed correction enables efficient and accurate localization in stratified environments.

Controlled Drug Release and Chemotherapy Response in a Novel Acoustofluidic 3D Tumor Platform.

Overcoming transport barriers to delivery of therapeutic agents in tumors remains a major challenge. Focused ultrasound (FUS), in combination with modern nanomedicine drug formulations, offers the ability to maximize drug transport to tumor tissue while minimizing toxicity to normal tissue. This potential remains unfulfilled due to the limitations of current approaches in accurately assessing and quantifying how FUS modulates drug transport in solid tumors. A novel acoustofluidic platform is developed by integrating a physiologically relevant 3D microfluidic device and a FUS system with a closed-loop controller to study drug transport and assess the response of cancer cells to chemotherapy in real time using live cell microscopy. FUS-induced heating triggers local release of the chemotherapeutic agent doxorubicin from a liposomal carrier and results in higher cellular drug uptake in the FUS focal region. This differential drug uptake induces locally confined DNA damage and glioblastoma cell death in the 3D environment. The capabilities of acoustofluidics for accurate control of drug release and monitoring of localized cell response are demonstrated in a 3D in vitro tumor mode. This has important implications for developing novel strategies to deliver therapeutic agents directly to the tumor tissue while sparing healthy tissue.

CMUT as a Transmitter for Microbubble-Assisted Blood-Brain Barrier Opening.

Focused ultrasound (FUS) combined with microbubbles (MBs) has emerged as a promising strategy for transiently opening the blood-brain barrier (BBB) to enhance drug permeability in the brain. Current FUS systems for BBB opening use piezoelectric transducers as transmitters and receivers. While capacitive micromachined ultrasonic transducers (CMUTs) have been suggested as a FUS receiver alternative due to their broad bandwidth, their capabilities as transmitters have not been investigated. This is mainly due to the intrinsic nonlinear behavior of CMUTs which complicates the detection of MB generated harmonic signals and their low pressure output at FUS frequencies. Various methods have been proposed to mitigate CMUT nonlinearity; however, these approaches have primarily targeted contrast enhanced ultrasound imaging. In this study, we propose the use of polyphase modulation (PM) technique to isolate MB emissions when CMUTs are employed as transmitters for BBB opening. Our calculations for a human scale FUS system with multiple CMUT transmitters show that 10 kPa peak negative pressure at 150 mm focal distance will be sufficient for MB excitation for BBB opening. Experimental findings indicate that this pressure level can be easily generated at 400-800 kHz using a readily available CMUT. Furthermore, more than 50 dB suppression of the fundamental harmonic signal is obtained in free field and transcranial hydrophone measurements by processing receive signals in response to phase modulated transmit waveforms. In-vitro validation of PM is also conducted using Definity MB flowing through a tube phantom. MB-filled tube phantoms show adequate nonlinear signal isolation and SNR for MB harmonic detection. Together our findings indicate that PM can effectively mitigate CMUT harmonic generation, thereby creating new opportunities for wideband transmission and receive operation for BBB opening in clinical and preclinical applications.

A non-exothermic cell-embedding tissue-mimicking material for studies of ultrasound-induced hyperthermia and drug release.

PURPOSE: The present study aims to create and characterise a cell-embedding tissue-mimicking material (TMM) that has thermal and acoustic properties similar to liver tissue, in order to enable study and optimisation of protocols for ultrasound-induced hyperthermia and drug delivery. MATERIALS AND METHODS: An agarose-based, cell-embedding TMM was iteratively developed and characterised. The acoustic properties (attenuation coefficient, speed of sound and cavitation threshold) and thermal response of the material were compared with those of fresh degassed liver tissue over a range of acoustic pressures and frequencies. A luminescence intensity assay was used to evaluate viability of HuH-7 cells in the material. The efficacy of ultrasound-mediated chemotherapeutic treatment in the material was tested by localised activation of low temperature thermally sensitive liposomes. Drug activation was measured by fluorescence microscopy. RESULTS: Similar acoustic properties (attenuation coefficient, speed of sound) to liver tissue were achieved over the therapeutically relevant frequency range of 1-4 MHz and similar thermal response was achieved for acoustic pressures up to 4.8 MPa peak to peak (ppk) at 1.1 MHz. Above 4.8 MPa ppk cavitation enhanced heating occurred in the TMM. Drug release from low-temperature-sensitive liposomes was achieved with 4.4 MPa ppk 6-s exposures at 1.1 MHz and cell compatibility of the material was confirmed. CONCLUSIONS: A platform for in vitro work for activation of thermally sensitive liposomes using high intensity focused ultrasound (HIFU)-induced hyperthermia was established. The TMM presents similar acoustic properties and thermal response to liver tissue over a broad range of ultrasound exposure conditions.

Experimental Demonstration of Trans-Skull Volumetric Passive Acoustic Mapping With the Heterogeneous Angular Spectrum Approach.

Real-time, 3-D, passive acoustic mapping (PAM) of microbubble dynamics during transcranial focused ultrasound (FUS) is essential for optimal treatment outcomes. The angular spectrum approach (ASA) potentially offers a very efficient method to perform PAM, as it can reconstruct specific frequency bands pertinent to microbubble dynamics and may be extended to correct aberrations caused by the skull. Here, we experimentally assess the abilities of heterogeneous ASA (HASA) to perform trans-skull PAM. Our experimental investigations demonstrate that the 3-D PAMs of a known 1-MHz source, constructed with HASA through an ex vivo human skull segment, reduced both the localization error (from 4.7 ± 2.3 to 2.3 ± 1.6 mm) and the number, size, and energy of spurious lobes caused by aberration, with the modest additional computational expense. While further improvements in the localization errors are expected with arrays with denser elements and larger aperture, our analysis revealed that experimental constraints associated with the array pitch and aperture (here, 1.8 mm and 2.5 cm, respectively) can be ameliorated by interpolation and peak finding techniques. Beyond the array characteristics, our analysis also indicated that errors in the registration (translation and rotation of ±5 mm and ±5°, respectively) of the skull segment to the array can lead to peak localization errors of the order of a few wavelengths. Interestingly, errors in the spatially dependent speed of sound in the skull (±20%) caused only subwavelength errors in the reconstructions, suggesting that registration is the most important determinant of point source localization accuracy. Collectively, our findings show that HASA can address source localization problems through the skull efficiently and accurately under realistic conditions, thereby creating unique opportunities for imaging and controlling the microbubble dynamics in the brain.

Spatially targeted brain cancer immunotherapy with closed-loop controlled focused ultrasound and immune checkpoint blockade.

Despite the challenges in treating glioblastomas (GBMs) with immune adjuvants, increasing evidence suggests that targeting the immune cells within the tumor microenvironment (TME) can lead to improved responses. Here, we present a closed-loop controlled, microbubble-enhanced focused ultrasound (MB-FUS) system and test its abilities to safely and effectively treat GBMs using immune checkpoint blockade. The proposed system can fine-tune the exposure settings to promote MB acoustic emission-dependent expression of the proinflammatory marker ICAM-1 and delivery of anti-PD1 in a mouse model of GBM. In addition to enhanced interaction of proinflammatory macrophages within the PD1-expressing TME and significant improvement in survival (P < 0.05), the combined treatment induced long-lived memory T cell formation within the brain that supported tumor rejection in rechallenge experiments. Collectively, our findings demonstrate the ability of MB-FUS to augment the therapeutic impact of immune checkpoint blockade in GBMs and reinforce the notion of spatially tumor-targeted (loco-regional) brain cancer immunotherapy.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound.

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.

Morphological Reconstruction Improves Microvessel Mapping in Super-Resolution Ultrasound.

Generation of super-resolution (SR) ultrasound (US) images, created from the successive localization of individual microbubbles in the circulation, has enabled the visualization of microvascular structure and flow at a level of detail that was not possible previously. Despite rapid progress, tradeoffs between spatial and temporal resolution may challenge the translation of this promising technology to the clinic. To temper these tradeoffs, we propose a method based on morphological image reconstruction. This method can extract from ultrafast contrast-enhanced US (CEUS) images hundreds of microbubble peaks per image (312-by-180 pixels) with intensity values varying by an order of magnitude. Specifically, it offers a fourfold increase in the number of peaks detected per frame, requires on the order of 100 ms for processing, and is robust to additive electronic noise (down to 3.6-dB CNR in CEUS images). By integrating this method to an SR framework, we demonstrate a sixfold improvement in spatial resolution, when compared with CEUS, in imaging chicken embryo microvessels. This method that is computationally efficient and, thus, scalable to large data sets may augment the abilities of SR-US in imaging microvascular structure and function.

Closed-loop trans-skull ultrasound hyperthermia leads to improved drug delivery from thermosensitive drugs and promotes changes in vascular transport dynamics in brain tumors.

Effective drug delivery in brain tumors remains a major challenge in oncology. Although local hyperthermia and stimuli-responsive delivery systems, such as thermosensitive liposomes, represent promising strategies to locally enhance drug delivery in solid tumors and improve outcomes, their application in intracranial malignancies remains unexplored. We hypothesized that the combined abilities of closed-loop trans-skull Magnetic Resonance Imaging guided Focused Ultrasound (MRgFUS) hyperthermia with those of thermosensitive drugs can alleviate challenges in drug delivery and improve survival in gliomas. Methods: To conduct our investigations, we first designed a closed loop MR-guided Focused Ultrasound (MRgFUS) system for localized trans-skull hyperthermia (ΔT < 0.5 °C) in rodents and established safety thresholds in healthy mice. To assess the abilities of the developed system and proposed therapeutic strategy for FUS-triggered chemotherapy release we employed thermosensitive liposomal Dox (TSL-Dox) and tested it in two different glioma tumor models (F98 in rats and GL261 in mice). To quantify Dox delivery and changes in the transvascular transport dynamics in the tumor microenvironment we combined fluorescent microscopy, dynamic contrast enhanced MRI (DCE-MRI), and physiologically based pharmacokinetic (PBPK) modeling. Lastly, to assess the therapeutic efficacy of the system and of the proposed therapeutic strategy we performed a survival study in the GL261 glioma bearing mice. Results: The developed closed-loop trans-skull MRgFUS-hyperthermia system that operated at 1.7 MHz, a frequency that maximized the brain (FUS-focus) to skull temperature ratio in mice, was able to attain and maintain the desired focal temperature within a narrow range. Histological evidence (H&E and Nissl) suggests that focal temperature at 41.5 ± 0.5 °C for 10 min is below the threshold for tissue damage. Quantitative analysis of doxorubicin delivery from TSLs with MRgFUS-hyperthermia demonstrated 3.5-fold improvement in cellular uptake in GL261 glioma mouse tumors (p < 0.001) and 5-fold increase in delivery in F98 glioma rat tumors (p < 0.05), as compared to controls (TSL-Dox-only). Moreover, PBPK modeling of drug transport that was calibrated using the experimental data indicated that thermal stress could lead to significant improvement in the transvascular transport (2.3-fold increase in the vessel diffusion coefficient; P < 0.001), in addition to promoting targeted Dox release. Prospective experimental investigations with DCE-MRI during FUS-hyperthermia, supported these findings and provided evidence that moderate thermal stress (≈41 °C for up to 10 min) can promote acute changes in the vascular transport dynamics in the brain tumor microenvironment (Ktrans value for control vs. FUS was 0.0097 and 0.0148 min-1, respectively; p = 0.026). Crucially, survival analysis demonstrated significant improvement in the survival in the TSL-Dox-FUS group as compared to TSL-Dox-only group (p < 0.05), providing supporting evidence on the therapeutic potential of the proposed strategy. Conclusions: Our investigations demonstrated that spatially controlled thermal stress can be attained and sustained in the mouse brain, using a trans-skull closed-loop MRgFUS system, and used to promote the effective delivery of chemotherapy in gliomas from thermosensitive drugs. This system also allowed us to conduct mechanistic investigations that resulted in the refinement of our understanding on the role of thermal stress in augmenting mass and drug transport in brain tumors. Overall, our study established a new paradigm for effective drug delivery in brain tumors based on closed-loop ultrasound-mediated thermal stress and thermosensitive drugs.

Single-cell analysis reveals effective siRNA delivery in brain tumors with microbubble-enhanced ultrasound and cationic nanoparticles.

RNA-based therapies offer unique advantages for treating brain tumors. However, tumor penetrance and uptake are hampered by RNA therapeutic size, charge, and need to be "packaged" in large carriers to improve bioavailability. Here, we have examined delivery of siRNA, packaged in 50-nm cationic lipid-polymer hybrid nanoparticles (LPHs:siRNA), combined with microbubble-enhanced focused ultrasound (MB-FUS) in pediatric and adult preclinical brain tumor models. Using single-cell image analysis, we show that MB-FUS in combination with LPHs:siRNA leads to more than 10-fold improvement in siRNA delivery into brain tumor microenvironments of the two models. MB-FUS delivery of Smoothened (SMO) targeting siRNAs reduces SMO protein production and markedly increases tumor cell death in the SMO-activated medulloblastoma model. Moreover, our analysis reveals that MB-FUS and nanoparticle properties can be optimized to maximize delivery in the brain tumor microenvironment, thereby serving as a platform for developing next-generation tunable delivery systems for RNA-based therapy in brain tumors.

Cavitation-enhanced delivery of macromolecules into an obstructed vessel.

Poor drug penetration through tumor tissue has emerged as a fundamental obstacle to cancer therapy. The aim of this study was to examine the ability of cavitation instigated by high-intensity focused ultrasound (HIFU) to increase convective transport of a model therapeutic in an in vitro tumor model. Cavitation activity was quantified by analyzing passively recorded acoustic emissions, and mass transfer was quantified using post-treatment image analysis of the distribution of a dye-labeled macromolecule. The strong correlation between cavitation activity and drug delivery suggests the potential for non-invasive treatment and monitoring.

Heterogeneous Angular Spectrum Method for Trans-Skull Imaging and Focusing.

Ultrasound, alone or in concert with circulating microbubble contrast agents, has emerged as a promising modality for therapy and imaging of brain diseases. While this has become possible due to advancements in aberration correction methods, a range of applications, including adaptive focusing and tracking of the microbubble dynamics through the human skull, may benefit from even more computationally efficient methods to account for skull aberrations. Here, we derive a general method for the angular spectrum approach (ASA) in a heterogeneous medium, based on a numerical marching scheme to approximate the full implicit solution. We then demonstrate its functionality with simulations for (human) skull-related aberration correction and trans-skull passive acoustic mapping. Our simulations show that the general solution provides accurate trans-skull focusing as compared to the uncorrected case (error in focal point location of 1.0 ± 0.4 mm vs 2.2 ± 0.7 mm) for clinically relevant frequencies (0.25-1.5MHz), apertures (50-100 mm), and targets, with peak focal pressures approximately 30 ± 17% of the free field case, with the effects of skull attenuation and amplitude shading included. In the case of source localization, our method leads to an average of 75% error reduction (from 2.9 ± 1.8 mm to 0.7 ± 0.5 mm) and 40-60% increase in peak intensity, evaluated over the range of frequencies (0.4-1.2 MHz), apertures (50-100 mm), and point source locations (40 mm by 50 mm grid) as compared to the homogeneous medium ASA. Overall, total computation times for both focusing and point source localization of the order milliseconds (166 ± 37 ms, compared with 44 ± 4 ms for the homogeneous ASA formulation) can be attained with this approach. Collectively our findings indicate that the proposed phase correction method based on the ASA could provide a computationally efficient and accurate method for trans-skull transmit focusing and imaging of point scatterers, potentially opening new possibilities for treatment and diagnosis of brain diseases.

The blood-brain barrier and blood-tumour barrier in brain tumours and metastases.

For a blood-borne cancer therapeutic agent to be effective, it must cross the blood vessel wall to reach cancer cells in adequate quantities, and it must overcome the resistance conferred by the local microenvironment around cancer cells. The brain microenvironment can thwart the effectiveness of drugs against primary brain tumours as well as brain metastases. In this Review, we highlight the cellular and molecular components of the blood-brain barrier (BBB), a specialized neurovascular unit evolved to maintain brain homeostasis. Tumours are known to compromise the integrity of the BBB, resulting in a vasculature known as the blood-tumour barrier (BTB), which is highly heterogeneous and characterized by numerous distinct features, including non-uniform permeability and active efflux of molecules. We discuss the challenges posed by the BBB and BTB for drug delivery, how multiple cell types dictate BBB function and the role of the BTB in disease progression and treatment. Finally, we highlight emerging molecular, cellular and physical strategies to improve drug delivery across the BBB and BTB and discuss their impact on improving conventional as well as emerging treatments, such as immune checkpoint inhibitors and engineered T cells. A deeper understanding of the BBB and BTB through the application of single-cell sequencing and imaging techniques, and the development of biomarkers of BBB integrity along with systems biology approaches, should enable new personalized treatment strategies for primary brain malignancies and brain metastases.

A CMOS active pixel sensor system for laboratory- based x-ray diffraction studies of biological tissue.

X-ray diffraction studies give material-specific information about biological tissue. Ideally, a large area, low noise, wide dynamic range digital x-ray detector is required for laboratory-based x-ray diffraction studies. The goal of this work is to introduce a novel imaging technology, the CMOS active pixel sensor (APS) that has the potential to fulfil all these requirements, and demonstrate its feasibility for coherent scatter imaging. A prototype CMOS APS has been included in an x-ray diffraction demonstration system. An industrial x-ray source with appropriate beam filtration is used to perform angle dispersive x-ray diffraction (ADXRD). Optimization of the experimental set-up is detailed including collimator options and detector operating parameters. Scatter signatures are measured for 11 different materials, covering three medical applications: breast cancer diagnosis, kidney stone identification and bone mineral density calculations. Scatter signatures are also recorded for three mixed samples of known composition. Results are verified using two independent models for predicting the APS scatter signature: (1) a linear systems model of the APS and (2) a linear superposition integral combining known monochromatic scatter signatures with the input polychromatic spectrum used in this case. Cross validation of experimental, modelled and literature results proves that APS are able to record biologically relevant scatter signatures. Coherent scatter signatures are sensitive to multiple materials present in a sample and provide a means to quantify composition. In the future, production of a bespoke APS imager for x-ray diffraction studies could enable simultaneous collection of the transmitted beam and scattered radiation in a laboratory-based coherent scatter system, making clinical transfer of the technique attainable.

Closed Loop Spatial and Temporal Control of Cavitation Activity with Passive Acoustic Mapping.

Ultrasonically actuated microbubble oscillations hold great promise for minimally invasive therapeutic interventions. While several preclinical studies have demonstrated the potential of this technology, real-time methods to control the amplitude and type of microbubble oscillations (stable vs inertial acoustic cavitation) and ensure that cavitation occurs within the targeted region are needed for their successful translation to the clinic. In this paper, we propose a real-time nonlinear state controller that uses specific frequency bands of the microbubble acoustic emissions (harmonic, ultra-harmonic, etc.) to control cavitation activity (observer states). To attain both spatial and temporal control of cavitation activity with high signal to noise ratio, we implement a controller using fast frequency-selective passive acoustic mapping (PAM) based on the angular spectrum approach. The controller includes safety states based on the recorded broadband signal level and is able to reduce sensing inaccuracies with the inclusion of multiple frequency bands. In its simplest implementation the controller uses the peak intensity of the passive acoustic maps, reconstructed using the 3rd harmonic (4.896 × 0.019 MHz) of the excitation frequency. Our results show that the proposed real-time nonlinear state controller based on PAM is able to reach the targeted level of observer state (harmonic emissions) in less than 6 seconds and remain within 10 % of tolerance for the duration of the experiment (45 seconds). Similar response was observed using the acoustic emissions from single element passive cavitation detection, albeit with higher susceptibility to background noise and lack of spatial information. Importantly, the proposed PAM-based controller was able to control cavitation activity with spatial selectivity when cavitation existed simultaneously in multiple regions. The robustness of the controller is demonstrated using a range of controller parameters, multiple observer states concurrently (harmonic, ultra-harmonic, and broadband), noise levels (°6 to 12 dB SNR), and bubble concentrations (0.3 to 180 × 103 bubbles per microliter). More research in this direction under preclinical and clinical conditions is warranted.

Passive Acoustic Mapping with the Angular Spectrum Method.

In the present proof of principle study, we evaluated the homogenous angular spectrum method for passive acoustic mapping (AS-PAM) of microbubble oscillations using simulated and experimental data. In the simulated data we assessed the ability of AS-PAM to form 3D maps of a single and multiple point sources. Then, in the two dimensional limit, we compared the 2D maps from AS-PAM with alternative frequency and time domain passive acoustic mapping (FD- and TD-PAM) approaches. Finally, we assessed the ability of AS-PAM to visualize microbubble activity in vivo with data obtained during 8 different experiments of FUS-induced blood-brain barrier disruption in 3 nonhuman primates, using a clinical MR-guided FUS system. Our in silico results demonstrate AS-PAM can be used to perform 3D passive acoustic mapping. 2D AS-PAM as compared to FD- PAM and TD-PAM is 10 and 200 times faster respectively and has similar sensitivity, resolution, and localization accuracy, even when the noise was 10-fold higher than the signal. In-vivo, the AS-PAM reconstructions of emissions at frequency bands pertinent to the different types of microbubble oscillations were also found to be more sensitive than TD-PAM. AS-PAM of harmonic-only components predicted safe blood-brain barrier disruption, whereas AS-PAM of broadband emissions correctly identified MR-evident tissue damage. The disparity (3.2 mm) in the location of the cavitation activity between the three methods was within their resolution limits. These data clearly demonstrate that AS-PAM is a sensitive and fast approach for PAM, thus providing a clinically relevant method to guide therapeutic ultrasound procedures.

Emerging strategies for delivering antiangiogenic therapies to primary and metastatic brain tumors.

Five-year survival rates have not increased appreciably for patients with primary and metastatic brain tumors. Nearly 17,000 patients die from primary brain tumors, whereas approximately 200,000 cases are diagnosed with brain metastasis every year in the US alone. At the same time, with improved control of systemic disease, the incidence of brain metastasis is increasing. Thus, novel approaches for improving the treatment outcome for these uniformly fatal diseases are needed urgently. In the review, we summarize the challenges in the treatment of these diseases using antiangiogenic therapies alone or in combination with radio-, chemo- and immuno-therapies. We also discuss the emerging strategies to improve the treatment outcome using both pharmacological approaches to normalize the tumor microenvironment and physical approaches (e.g., focused ultrasound) to modulate the blood-tumor-barrier, along with limitations of each approach. Finally, we offer some new avenues of future research.