Development of a spherically focused phased array transducer for ultrasonic image-guided hyperthermia

Spatiotemporal Controllable Sono-Nanovaccines Driven by Free-Field Based Whole-Body Ultrasound for Personalized Cancer Therapy

Therapeutic cancer vaccines fail to produce satisfactory outcomes against solid tumors since vaccine-induced anti-tumor immunity is significantly hampered by immunosuppression. Generating an in situ cancer vaccine targeting immunological cold tumor microenvironment (TME) appears attractive. Here, a type of free-field based whole-body ultrasound (US)-driven nanovaccines are constructed, named G5-CHC-R, by conjugating the sonosensitizer, Chenghai chlorin (CHC) and the immunomodulator, resiquimod (R848) on top of a super small-sized dendrimeric nanoscaffold. Once entering tumors, R848 can be cleaved from a hypoxia-sensitive linker, thus modifying the TME via converting macrophage phenotypes. The animals bearing orthotopic pancreatic cancer with intestinal metastasis and breast cancer with lung metastasis are treated with G5-CHC-R under a free-field based whole-body US system. Benefit from the deep penetration capacity and highly spatiotemporal selectiveness, G5-CHC-R triggered by US represented a superior alternative for noninvasive irradiation of deep-seated tumors and magnification of local immune responses via driving mass release of tumor antigens and "cold-warm-hot" three-state transformation of TME. In addition to irradiating primary tumors, a robust adaptive anti-tumor immunity is potentiated, leading to successful induction of systemic tumor suppression. The sono-nanovaccines with good biocompatibility posed wide applicability to a broad spectrum of tumors, revealing immeasurable potential for translational research in oncology.

Sub-aperture ultrafast volumetric ultrasound imaging for fully sampled dual-mode matrix array

Fully sampled dual-mode matrix array ultrasound transducer is capable of performing imaging and therapeutic ultrasound in three dimensions (3D). It is a promising tool for many clinical applications because of its precise multi-focus therapy with imaging guidance by itself. Our team previously designed a 256-element fully sampled dual-mode matrix array transducer, while its imaging quality needs to be further improved. In this work, we propose a high-contrast sub-aperture volumetric imaging strategy to improve the imaging quality of the dual-mode matrix array. We first analyzed the effect of various parameters of sub-aperture imaging on the imaging quality by Field II. Based on the optimized parameters, we compared the resolution and signal to noise ratio (SNR) of sub-aperture imaging with those of full aperture imaging on phantoms and rabbit brain. The experimental results showed the proposed sub-aperture imaging method could obtain a comparable resolution to full aperture imaging. Moreover, the average intensity of noise signal near the wire phantom decreased by about 5 dB and the SNR of tissue phantom image increased by 8 %. The proposed sub-aperture imaging method also enabled clearer and more accurate imaging of the rabbit brain. The obtained results indicate the proposed sub-aperture imaging is a promising method for practical use of a fully sampled dual-mode matrix array for volumetric ultrasound imaging.

Sonogenetic control of multiplexed genome regulation and base editing

Manipulating gene expression in the host genome with high precision is crucial for controlling cellular function and behavior. Here, we present a precise, non-invasive, and tunable strategy for controlling the expression of multiple endogenous genes both in vitro and in vivo, utilizing ultrasound as the stimulus. By engineering a hyper-efficient dCas12a and effector under a heat shock promoter, we demonstrate a system that can be inducibly activated through thermal energy produced by ultrasound absorption. This system allows versatile thermal induction of gene activation or base editing across cell types, including primary T cells, and enables multiplexed gene activation using a single guide RNA array. In mouse models, localized temperature elevation guided by high-intensity focused ultrasound effectively triggers reporter gene expression in implanted cells. Our work underscores the potential of ultrasound as a clinically viable approach to enhance cell and gene-based therapies via precision genome and epigenome engineering.

PET imaging of focused-ultrasound enhanced delivery of AAVs into the murine brain

Rationale: Despite recent advances in the use of adeno-associated viruses (AAVs) as potential vehicles for genetic intervention of central and peripheral nervous system-associated disorders, gene therapy for the treatment of neuropathology in adults has not been approved to date. The currently FDA-approved AAV-vector based gene therapies rely on naturally occurring serotypes, such as AAV2 or AAV9, which display limited or no transport across the blood-brain barrier (BBB) if systemically administered. Recently developed engineered AAV variants have shown broad brain transduction and reduced off-target liver toxicity in non-human primates (NHPs). However, these vectors lack spatial selectivity for targeted gene delivery, a potentially critical limitation for delivering therapeutic doses in defined areas of the brain. The use of microbubbles, in conjunction with focused ultrasound (FUS), can enhance regional brain AAV transduction, but methods to assess transduction in vivo are needed. Methods: In a murine model, we combined positron emission tomography (PET) and optical imaging of reporter gene payloads to non-invasively assess the spatial distribution and transduction efficiency of systemically administered AAV9 after FUS and microbubble treatment. Capsid and reporter probe accumulation are reported as percent injected dose per cubic centimeter (%ID/cc) for in vivo PET quantification, whereas results for ex vivo assays are reported as percent injected dose per gram (%ID/g). Results: In a study spanning accumulation and transduction, mean AAV9 accumulation within the brain was 0.29 %ID/cc without FUS, whereas in the insonified region of interest of FUS-treated mice, the spatial mean and maximum reached ~2.3 %ID/cc and 4.3 %ID/cc, respectively. Transgene expression assessed in vivo by PET reporter gene imaging employing the pyruvate kinase M2 (PKM2)/[18F]DASA-10 reporter system increased up to 10-fold in the FUS-treated regions, as compared to mice receiving AAVs without FUS. Systemic injection of AAV9 packaging the EF1A-PKM2 transgene followed by FUS in one hemisphere resulted in 1) an average 102-fold increase in PKM2 mRNA concentration compared to mice treated with AAVs only and 2) a 12.5-fold increase in the insonified compared to the contralateral hemisphere of FUS-treated mice. Conclusion: Combining microbubbles with US-guided treatment facilitated a multi-hour BBB disruption and stable AAV transduction in targeted areas of the murine brain. This unique platform has the potential to provide insight and aid in the translation of AAV-based therapies for the treatment of neuropathologies.

Image-guided focused ultrasound-mediated molecular delivery to breast cancer in an animal model

Tumors become inoperable due to their size or location, making neoadjuvant chemotherapy the primary treatment. However, target tissue accumulation of anticancer agents is limited by the physical barriers of the tumor microenvironment. Low-intensity focused ultrasound (FUS) in combination with microbubble (MB) contrast agents can increase microvascular permeability and improve drug delivery to the target tissue after systemic administration. The goal of this research was to investigate image-guided FUS-mediated molecular delivery in volume space. Three-dimensional (3-D) FUS therapy functionality was implemented on a programmable ultrasound scanner (Vantage 256, Verasonics Inc.) equipped with a linear array for image guidance and a 128-element therapy transducer (HIFUPlex-06, Sonic Concepts). FUS treatment was performed on breast cancer-bearing female mice (N= 25). Animals were randomly divided into three groups, namely, 3-D FUS therapy, two-dimensional (2-D) FUS therapy, or sham (control) therapy. Immediately prior to the application of FUS therapy, animals received a slow bolus injection of MBs (Definity, Lantheus Medical Imaging Inc.) and near-infrared dye (IR-780, surrogate drug) for optical reporting and quantification of molecular delivery. Dye accumulation was monitored viain vivooptical imaging at 0, 1, 24, and 48 h (Pearl Trilogy, LI-COR). Following the 48 h time point, animals were humanely euthanized and tumors excised forex vivoanalyzes. Optical imaging results revealed that 3-D FUS therapy improved delivery of the IR-780 dye by 66.4% and 168.1% at 48 h compared to 2-D FUS (p= 0.18) and sham (p= 0.047) therapeutic strategies, respectively.Ex vivoanalysis revealed similar trends. Overall, 3-D FUS therapy can improve accumulation of a surrogate drug throughout the entire target tumor burden after systemic administration.

Sonodynamic Therapy With Concentric Ultrasound Imaging Array for Precision Theranostics for Atherosclerotic Plaque

Atherosclerotic cardiovascular disease is a major cause of human disability and mortality. Our previous study demonstrated the safety and efficacy of sonodynamic therapy (SDT) on atherosclerotic plaques. However, traditional single-element therapeutic transducer has single acoustic field, and positioning therapeutic and imaging transducers in the same position is difficult during ultrasound imaging-guided SDT. Continuously changing the position of transducers to intervene lesions in different positions is required, increasing the difficulty of treatment. Thus, an SDT device with precise theranostics is required. Therefore, we designed and fabricated a "concentric ultrasound transducer for theranostics" (CUST-T), comprising a central 8-MHz linear array transducer for ultrasound imaging, and a peripheral 1-MHz hollow two-dimensional (2-D) planar array transducer for generating phased-array focused ultrasound (PAFUS). The CUST-T exhibited high imaging resolution at a distance of up to 20 mm from the transducer and could generate a personalized complex PAFUS acoustic field to match various lesions. In vitro biomedical results showed that PAFUS-SDT induced RAW264.7-derived foam cell apoptosis leading to a targeting field apoptotic rate 4.36-6.24 times that of the nontargeting field and the significant apoptotic region was consistent with the PAFUS acoustic field. In vivo, PAFUS-SDT guided by ultrasound imaging significantly increased the lumen area ( ) and collagen level ( ), whereas the wall thickness ( ) and lipid content ( ) of rabbit femoral artery were reduced. In conclusion, CUST-T provided image guidance sufficient for accurate SDT for atherosclerotic plaques in peripheral arteries and could be applied in clinical practice.

Development of an ultrasound guided focused ultrasound system for 3D volumetric low energy nanodroplet-mediated histotripsy

Low pressure histotripsy is likely to facilitate current treatments that require extremely high pressures. An ultrasound guided focused ultrasound system was designed to accommodate a rotating imaging transducer within a low frequency therapeutic transducer that operates at a center frequency of 105 kHz. The implementation of this integrated system provides real-time therapeutic and volumetric imaging functions, that are used here for low-cost, low-energy 3D volumetric ultrasound histotripsy using nanodroplets. A two-step approach for low pressure histotripsy is implemented with this dual-array. Vaporization of nanodroplets into gaseous microbubbles was performed via the 1D rotating imaging probe. The therapeutic transducer is then used to detonate the vaporized nanodroplets and trigger potent mechanical effects in the surrounding tissue. Rotating the imaging transducer creates a circular vaporized nanodroplet shape which generates a round lesion upon detonation. This contrasts with the elongated lesion formed when using a standard 1D imaging transducer for nanodroplet activation. Optimization experiments show that maximal nanodroplet activation can be achieved with a 2-cycle excitation pulse at a center frequency of 3.5 MHz, and a peak negative pressure of 3.4 MPa (a mechanical index of 1.84). Vaporized nanodroplet detonation was achieved by applying a low frequency treatment at a center frequency of 105 kHz and mechanical index of 0.9. In ex-vivo samples, the rotated nanodroplet activation method yielded the largest lesion area, with a mean of 4.7 ± 0.5 mm², and a rounded shape. In comparison, standard fixed transducer nanodroplet activation resulted in an average lesion area of 2.6 ± 0.4 mm², and an elongated shape. This hybrid system enables to achieve volumetric low energy histotripsy, and thus facilitates the creation of precise, large-volume mechanical lesions in tissues, while reducing the pressure threshold required for standard histotripsy by over an order of magnitude.

Guiding and monitoring focused ultrasound mediated blood-brain barrier opening in rats using power Doppler imaging and passive acoustic mapping

The blood-brain barrier (BBB) prevents harmful toxins from entering brain but can also inhibit therapeutic molecules designed to treat neurodegenerative diseases. Focused ultrasound (FUS) combined with microbubbles can enhance permeability of BBB and is often performed under MRI guidance. We present an all-ultrasound system capable of targeting desired regions to open BBB with millimeter-scale accuracy in two dimensions based on Doppler images. We registered imaging coordinates to FUS coordinates with target registration error of 0.6 ± 0.3 mm and used the system to target microbubbles flowing in cellulose tube in two in vitro scenarios (agarose-embedded and through a rat skull), while receiving echoes on imaging transducer. We created passive acoustic maps from received echoes and found error between intended location in imaging plane and location of pixel with maximum intensity after passive acoustic maps reconstruction to be within 2 mm in 5/6 cases. We validated ultrasound-guided procedure in three in vivo rat brains by delivering MRI contrast agent to cortical regions of rat brains after BBB opening. Landmark-based registration of vascular maps created with MRI and Doppler ultrasound revealed BBB opening inside the intended focus with targeting accuracy within 1.5 mm. Combined use of power Doppler imaging with passive acoustic mapping demonstrates an ultrasound-based solution to guide focused ultrasound with high precision in rodents.

A theranostic 3D ultrasound imaging system for high resolution image-guided therapy

Microbubble contrast agents are a diagnostic tool with broad clinical impact and an increasing number of indications. Many therapeutic applications have also been identified. Yet, technologies for ultrasound guidance of microbubble-mediated therapy are limited. In particular, arrays that are capable of implementing and imaging microbubble-based therapy in three dimensions in real-time are lacking. We propose a system to perform and monitor microbubble-based therapy, capable of volumetric imaging over a large field-of-view. To propel the promise of the theranostic treatment strategies forward, we have designed and tested a unique array and system for 3D ultrasound guidance of microbubble-based therapeutic protocols based on the frequency, temporal and spatial requirements. Methods: Four 256-channel plane wave scanners (Verasonics, Inc, WA, USA) were combined to control a 1024-element planar array with 1.3 and 2.5 MHz therapeutic and imaging transmissions, respectively. A transducer aperture of ~40×15 mm was selected and Field II was applied to evaluate the point spread function. In vitro experiments were performed on commercial and custom phantoms to assess the spatial resolution, image contrast and microbubble-enhanced imaging capabilities. Results: We found that a 2D array configuration with 64 elements separated by λ-pitch in azimuth and 16 elements separated by 1.5λ-pitch in elevation ensured the required flexibility. This design, of 41.6 mm × 16 mm, thus provided both an extended field-of-view, up to 11 cm x 6 cm at 10 cm depth and steering of ±18° in azimuth and ±12° in elevation. At a depth of 16 cm, we achieved a volume imaging rate of 60 Hz, with a contrast ratio and resolution, respectively, of 19 dB, 0.8 mm at 3 cm and 20 dB and 2.1 mm at 12.5 cm. Conclusion: A single 2D array for both imaging and therapeutics, integrated with a 1024 channel scanner can guide microbubble-based therapy in volumetric regions of interest.

Synergies between therapeutic ultrasound, gene therapy and immunotherapy in cancer treatment

Due to the ease of use and excellent safety profile, ultrasound is a promising technique for both diagnosis and site-specific therapy. Ultrasound-based techniques have been developed to enhance the pharmacokinetics and efficacy of therapeutic agents in cancer treatment. In particular, transfection with exogenous nucleic acids has the potential to stimulate an immune response in the tumor microenvironment. Ultrasound-mediated gene transfection is a growing field, and recent work has incorporated this technique into cancer immunotherapy. Compared with other gene transfection methods, ultrasound-mediated gene transfection has a unique opportunity to augment the intracellular uptake of nucleic acids while safely and stably modulating the expression of immunostimulatory cytokines. The development and commercialization of therapeutic ultrasound systems further enhance the potential translation. In this Review, we introduce the underlying mechanisms and ongoing preclinical studies of ultrasound-based techniques in gene transfection for cancer immunotherapy. Furthermore, we expand on aspects of therapeutic ultrasound that impact gene therapy and immunotherapy, including tumor debulking, enhancing cytokines and chemokines and altering nanoparticle pharmacokinetics as these effects of ultrasound cannot be fully dissected from targeted gene therapy. We finally explore the outlook for this rapidly developing field.

Design and Challenges of Sonodynamic Therapy System for Cancer Theranostics: From Equipment to Sensitizers

As a novel noninvasive therapeutic modality combining low-intensity ultrasound and sonosensitizers, sonodynamic therapy (SDT) is promising for clinical translation due to its high tissue-penetrating capability to treat deeper lesions intractable by photodynamic therapy (PDT), which suffers from the major limitation of low tissue penetration depth of light. The effectiveness and feasibility of SDT are regarded to rely on not only the development of stable and flexible SDT apparatus, but also the screening of sonosensitizers with good specificity and safety. To give an outlook of the development of SDT equipment, the key technologies are discussed according to five aspects including ultrasonic dose settings, sonosensitizer screening, tumor positioning, temperature monitoring, and reactive oxygen species (ROS) detection. In addition, some state-of-the-art SDT multifunctional equipment integrating diagnosis and treatment for accurate SDT are introduced. Further, an overview of the development of sonosensitizers is provided from small molecular sensitizers to nano/microenhanced sensitizers. Several types of nanomaterial-augmented SDT are in discussion, including porphyrin-based nanomaterials, porphyrin-like nanomaterials, inorganic nanomaterials, and organic-inorganic hybrid nanomaterials with different strategies to improve SDT therapeutic efficacy. There is no doubt that the rapid development and clinical translation of sonodynamic therapy will be promoted by advanced equipment, smart nanomaterial-based sonosensitizer, and multidisciplinary collaboration.

A Dual-mode 2D Matrix Array for Ultrasound Image-guided Noninvasive Therapy

Focused ultrasound (FUS) lacks reliable real-time image guidance, which hinders the development of non-invasive ultrasound treatment in many important clinical applications. A dual-mode ultrasound array, capable of both imaging and therapy offers a new and reliable strategy for image-guided ultrasound therapy applications. The strategy has the advantages of real-time use, low cost, portability and inherent registration between imaging and therapeutic coordinate systems. In this work, a dual-mode two-dimensional (2D) matrix array with 1 MHz center frequency and 256 elements for ultrasound image-guided non-invasive therapy is reported. The array can provide three-dimensional (3D) volumetric ultrasound imaging and 3D focus control. Ultrasound imaging and therapeutic applications for the brain of small animals demonstrated the multi-functional capability of the dual-mode 2D matrix array. A method of rat brain positioning based on ultrasound imaging was proposed and verified. Transcranial ultrasound image-guided bloodbrain barrier (BBB) opening of multiple-targets was achieved in vivo, using the proposed dual-mode 2D array. The obtained results indicate that the dual-mode 2D matrix array is a promising method for practical use in ultrasound image-guided non-invasive therapy applications.

Development of thermosensitive resiquimod-loaded liposomes for enhanced cancer immunotherapy

Resiquimod (R848) is a toll-like receptor 7 and 8 (TLR7/8) agonist with potent antitumor and immunostimulatory activity. However, systemic delivery of R848 is poorly tolerated because of its poor solubility in water and systemic immune activation. In order to address these limitations, we developed an intravenously-injectable formulation with R848 using thermosensitive liposomes (TSLs) as a delivery vehicle. R848 was remotely loaded into TSLs composed of DPPC: DSPC: DSPE-PEG2K (85:10:5, mol%) with 100 mM FeSO₄ as the trapping agent inside. The final R848 to lipid ratio of the optimized R848-loaded TSLs (R848-TSLs) was 0.09 (w/w), 10-fold higher than the previously-reported values. R848-TSLs released 80% of R848 within 5 min at 42 °C. These TSLs were then combined with αPD-1, an immune checkpoint inhibitor, and ultrasound-mediated hyperthermia in a neu deletion (NDL) mouse mammary carcinoma model (Her2⁺, ER/PR negative). Combined with αPD-1, local injection of R848-TSLs showed superior efficacy with complete NDL tumor regression in both treated and abscopal sites achieved in 8 of 11 tumor bearing mice over 100 days. Immunohistochemistry confirmed enhanced CD8⁺ T cell infiltration and accumulation by R848-TSLs. Systemic delivery of R848-TSLs, combined with local hyperthermia and αPD-1, inhibited tumor growth and extended median survival from 28 days (non-treatment control) to 94 days. Upon re-challenge with reinjection of tumor cells, none of the previously cured mice developed tumors, as compared with 100% of age-matched control mice. The dose of R848 (10 μg for intra-tumoral injection or 6 mg/kg for intravenous injection delivered up to 4 times) was well-tolerated without weight loss or organ hypertrophy. In summary, we developed R848-TSLs that can be administered locally or systematically, resulting in tumor regression and enhanced survival when combined with αPD-1 in mouse models of breast cancer.

Tumor-specific delivery of gemcitabine with activatable liposomes

Gemcitabine delivery to pancreatic ductal adenocarcinoma is limited by poor pharmacokinetics, dense fibrosis and hypo-vascularization. Activatable liposomes, with drug release resulting from local heating, enhance serum stability and circulation, and the released drug retains the ability to diffuse within the tumor. A limitation of liposomal gemcitabine has been the low loading efficiency. To address this limitation, we used the superior solubilizing potential of copper (II) gluconate to form a complex with gemcitabine at copper:gemcitabine (1:4). Thermosensitive liposomes composed of DPPC:DSPC:DSPE-PEG2k (80:15:5, mole%) then reached 12 wt% loading, 4-fold greater than previously reported values. Cryo transmission electron microscopy confirmed the presence of a liquid crystalline gemcitabine‑copper mixture. The optimized gemcitabine liposomes released 60% and 80% of the gemcitabine within 1 and 5 min, respectively, at 42 °C. Liposomal encapsulation resulted in a circulation half-life of ~2 h in vivo (compared to reported circulation of 16 min for free gemcitabine in mice), and free drug was not detected within the plasma. The resulting gemcitabine liposomes were efficacious against both murine breast cancer and pancreatic cancer in vitro. Three repeated treatments of activatable gemcitabine liposomes plus ultrasound hyperthermia regressed or eliminated tumors in the neu deletion model of murine breast cancer with limited toxicity, enhancing survival when compared to treatment with gemcitabine alone. With 5% of the free gemcitabine dose (5 rather than 100 mg/kg), tumor growth was suppressed to the same degree as gemcitabine. Additionally, in a more aggressive tumor model of murine pancreatic cancer, liposomal gemcitabine combined with local hyperthermia induced cell death and regions of apoptosis and necrosis.

Combining activatable nanodelivery with immunotherapy in a murine breast cancer model

A successful chemotherapy-immunotherapy solid-tumor protocol should accomplish the following goals: debulk large tumors, release tumor antigen for cross-presentation and cross-priming, release cancer-suppressive cytokines and enhance anti-tumor immune cell populations. Thermally-activated drug delivery particles have the potential to synergize with immunotherapeutics to accomplish these goals; activation can release chemotherapy within bulky solid tumors and can enhance response when combined with immunotherapy. We set out to determine whether a single protocol, combining locally-activated chemotherapy and agonist immunotherapy, could accomplish these goals and yield a potentially translational therapy. For effective delivery of free doxorubicin to tumors with minimal toxicity, we stabilized doxorubicin with copper in temperature-sensitive liposomes that rapidly release free drug in the vasculature of cancer lesions upon exposure to ultrasound-mediated hyperthermia. We found that in vitro exposure of tumor cells to hyperthermia and doxorubicin resulted in immunogenic cell death and the local release of type I interferons across murine cancer cell lines. Following intravenous injection, local activation of the liposomes within a single tumor released doxorubicin and enhanced cross-presentation of a model antigen at distant tumor sites. While a variety of protocols achieved a complete response in >50% of treated mice, the complete response rate was greatest (90%) when 1 week of immunotherapy priming preceded a single activatable chemotherapeutic administration. While repeated chemotherapeutic delivery reduced local viable tumor, the complete response rate and a subset of tumor immune cells were also reduced. Taken together, the results suggest that activatable chemotherapy can enhance adjuvant immunotherapy; however, in a murine model the systemic adaptive immune response was greatest with a single administration of chemotherapy.

Nonviral ultrasound-mediated gene delivery in small and large animal models

Ultrasound-mediated gene delivery (sonoporation) is a minimally invasive, nonviral and clinically translatable method of gene therapy. This method offers a favorable safety profile over that of viral vectors and is less invasive as compared with other physical gene delivery approaches (e.g., electroporation). We have previously used sonoporation to overexpress transgenes in different skeletal tissues in order to induce tissue regeneration. Here, we provide a protocol that could easily be adapted to address various other targets of tissue regeneration or additional applications, such as cancer and neurodegenerative diseases. This protocol describes how to prepare, conduct and optimize ultrasound-mediated gene delivery in both a murine and a porcine animal model. The protocol includes the preparation of a microbubble-DNA mix and in vivo sonoporation under ultrasound imaging. Ultrasound-mediated gene delivery can be accomplished within 10 min. After DNA delivery, animals can be followed to monitor gene expression, protein secretion and other transgene-specific outcomes, including tissue regeneration. This procedure can be accomplished by a competent graduate student or technician with prior experience in ultrasound imaging or in performing in vivo procedures.

Fast, Low-Frequency Plane-Wave Imaging for Ultrasound Contrast Imaging

Plane-wave ultrasound contrast imaging offers a faster, less destructive means for imaging microbubbles compared with traditional ultrasound imaging. Even though many of the most acoustically responsive microbubbles have resonant frequencies in the lower-megahertz range, higher frequencies (>3 MHz) have typically been employed to achieve high spatial resolution. In this work we implement and optimize low-frequency (1.5-4 MHz) plane-wave pulse inversion imaging on a commercial, phased-array imaging transducer in vitro and illustrate its use in vivo by imaging a mouse xenograft model. We found that the 1.8-MHz contrast signal was about four times that acquired at 3.1 MHz on matched probes and nine times greater than echoes received on a higher-frequency probe. Low-frequency imaging was also much more resilient to motion. In vivo, we could identify sub-millimeter vasculature inside a xenograft tumor model and easily assess microbubble half-life. Our results indicate that low-frequency imaging can provide better signal-to-noise because it generates stronger non-linear responses. Combined with high-speed plane-wave imaging, this method could open the door to super-resolution imaging at depth, while high power pulses could be used for image-guided therapeutics.

Acoustically targeted chemogenetics for the non-invasive control of neural circuits

Neurological and psychiatric disorders are often characterized by dysfunctional neural circuits in specific regions of the brain. Existing treatment strategies, including the use of drugs and implantable brain stimulators, aim to modulate the activity of these circuits. However, they are not cell-type-specific, lack spatial targeting or require invasive procedures. Here, we report a cell-type-specific and non-invasive approach based on acoustically targeted chemogenetics that enables the modulation of neural circuits with spatiotemporal specificity. The approach uses ultrasound waves to transiently open the blood-brain barrier and transduce neurons at specific locations in the brain with virally encoded engineered G-protein-coupled receptors. The engineered neurons subsequently respond to systemically administered designer compounds to activate or inhibit their activity. In a mouse model of memory formation, the approach can modify and subsequently activate or inhibit excitatory neurons within the hippocampus, with selective control over individual brain regions. This technology overcomes some of the key limitations associated with conventional brain therapies.

Imaging beyond ultrasonically-impenetrable objects

Ultrasound images are severely degraded by the presence of obstacles such as bones and air gaps along the beam path. This paper describes a method for imaging structures that are distal to obstacles that are otherwise impenetrable to ultrasound. The method uses an optically-inspired holographic algorithm to beam-shape the emitted ultrasound field in order to bypass the obstacle and place the beam focus beyond the obstruction. The resulting performance depends on the transducer aperture, the size and position of the obstacle, and the position of the target. Improvement compared to standard ultrasound imaging is significant for obstacles for which the width is larger than one fourth of the transducer aperture and the depth is within a few centimeters of the transducer. For such cases, the improvement in focal intensity at the location of the target reaches 30-fold, and the improvement in peak-to-side-lobe ratio reaches 3-fold. The method can be implemented in conventional ultrasound systems, and the entire process can be performed in real time. This method has applications in the fields of cancer detection, abdominal imaging, imaging of vertebral structure and ultrasound tomography. Here, its effectiveness is demonstrated using wire targets, tissue mimicking phantoms and an ex vivo biological sample.

Acoustical structured illumination for super-resolution ultrasound imaging

Structured illumination microscopy is an optical method to increase the spatial resolution of wide-field fluorescence imaging beyond the diffraction limit by applying a spatially structured illumination light. Here, we extend this concept to facilitate super-resolution ultrasound imaging by manipulating the transmitted sound field to encode the high spatial frequencies into the observed image through aliasing. Post processing is applied to precisely shift the spectral components to their proper positions in k-space and effectively double the spatial resolution of the reconstructed image compared to one-way focusing. The method has broad application, including the detection of small lesions for early cancer diagnosis, improving the detection of the borders of organs and tumors, and enhancing visualization of vascular features. The method can be implemented with conventional ultrasound systems, without the need for additional components. The resulting image enhancement is demonstrated with both test objects and ex vivo rat metacarpals and phalanges.

Beamforming for hyperthermia treatment by training a weighted network of an ultrasonic array

We propose a beamforming algorithm based on waveform diversity for hyperthermia treatment of breast cancer using an ultrasonic array. The introduced array has a structure with a network connecting the feeding nodes and the array elements, and the objective of the algorithm is to train the weight matrix of the network to minimize the difference between the generated beam pattern and the ideal one. The training procedure of the algorithm, which is inspired by the idea of machine learning, comprises three parts: forward calculation, comparison, and backward calculation. The forward calculation maps the weight matrix to the beam pattern, and in the comparison step, the generated beam pattern is modified based on the error, and finally, the backward calculation maps the modified beam pattern to a refined weight matrix which performs better than the original one. An optimal weight matrix is obtained by iterative training. The effectiveness of the algorithm is demonstrated by using numerical simulations.