Combined ultrasound and MR imaging to guide focused ultrasound therapies in the brain

The road ahead to successful BBB opening and drug-delivery with focused ultrasound

This review delves into the innovative technology of Blood-Brain Barrier (BBB) opening with low-intensity focused ultrasound in combination with microbubbles (LIFU-MB), a promising therapeutic modality aimed at enhancing drug delivery to the central nervous system (CNS). The BBB's selective permeability, while crucial for neuroprotection, significantly hampers the efficacy of pharmacological treatments for CNS disorders. LIFU-MB emerges as a non-invasive and localized method to transiently increase BBB permeability, facilitating the delivery of therapeutic molecules. Here, we review the procedural stages of LIFU-MB interventions, including planning and preparation, sonication, evaluation, and delivery, highlighting the technological diversity and methodological challenges encountered in current clinical applications. With an emphasis on safety and efficacy, we discuss the crucial aspects of ultrasound delivery, microbubble administration, acoustic feedback monitoring and assessment of BBB permeability. Finally, we explore the critical choices for effective BBB opening with LIFU-MB, focusing on selecting therapeutic agents, optimizing delivery methods, and timing for delivery. Overcoming existing barriers to integrate this technology into clinical practice could potentially revolutionize CNS drug delivery and treatment paradigms in the near future.

State of the art on microbubble cavitation monitoring and feedback control for blood-brain-barrier opening using focused ultrasound

Focused ultrasound (FUS) is a non-invasive and highly promising method for targeted and reversible blood-brain barrier permeabilization. Numerous preclinical studies aim to optimize the localized delivery of drugs using this method in rodents and non-human primates. Several clinical trials have been initiated to treat various brain diseases in humans using simultaneous BBB permeabilization and drug injection. This review presents the state of the art ofin vitroandin vivocavitation control algorithms for BBB permeabilization using microbubbles (MB) and FUS. Firstly, we describe the different cavitation states, their physical significance in terms of MB behavior and their translation into the spectral composition of the backscattered signal. Next, we report the different indexes calculated and used during the ultrasonic monitoring of cavitation. Finally, the differentin vitroandin vivocavitation control strategies described in the literature are presented and compared.

External Hardware and Sensors, for Improved MRI

Complex engineered systems are often equipped with suites of sensors and ancillary devices that monitor their performance and maintenance needs. MRI scanners are no different in this regard. Some of the ancillary devices available to support MRI equipment, the ones of particular interest here, have the distinction of actually participating in the image acquisition process itself. Most commonly, such devices are used to monitor physiological motion or variations in the scanner's imaging fields, allowing the imaging and/or reconstruction process to adapt as imaging conditions change. "Classic" examples include electrocardiography (ECG) leads and respiratory bellows to monitor cardiac and respiratory motion, which have been standard equipment in scan rooms since the early days of MRI. Since then, many additional sensors and devices have been proposed to support MRI acquisitions. The main physical properties that they measure may be primarily "mechanical" (eg acceleration, speed, and torque), "acoustic" (sound and ultrasound), "optical" (light and infrared), or "electromagnetic" in nature. A review of these ancillary devices, as currently available in clinical and research settings, is presented here. In our opinion, these devices are not in competition with each other: as long as they provide useful and unique information, do not interfere with each other and are not prohibitively cumbersome to use, they might find their proper place in future suites of sensors. In time, MRI acquisitions will likely include a plurality of complementary signals. A little like the microbiome that provides genetic diversity to organisms, these devices can provide signal diversity to MRI acquisitions and enrich measurements. Machine-learning (ML) algorithms are well suited at combining diverse input signals toward coherent outputs, and they could make use of all such information toward improved MRI capabilities. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 1.

Passive acoustic mapping with absolute time-of-flight information and delay-multiply-sum beamforming

BACKGROUND

Passive acoustic mapping (PAM) is showing increasing application potential in monitoring ultrasound therapy by spatially resolving cavitation activity. PAM with the relative time-of-flight information leads to poor axial resolution when implemented with ultrasound diagnostic transducers. Through utilizing the absolute time-of-flight information preserved by the transmit-receive synchronization and applying the common delay-sum (DS) beamforming algorithm, PAM axial resolution can be greatly improved in the short-pulse excitation scenario, as with active ultrasound imaging. However, PAM with the absolute time-of-flight information (referred as AtPAM) suffers from low imaging resolution and weak interference suppression when the DS algorithm is applied.

PURPOSE

This study aims to propose an enhanced AtPAM algorithm based on delay-multiply-sum (DMS) beamforming, to address the shortcomings of the DS-based AtPAM algorithm.

METHODS

In DMS beamforming, the element signals delayed by the absolute time delays are first processed with a signed square-root operation and then multiplied in pairs and finally summed, the resulting beamformed output is further band-pass filtered. The performances of DS- and DMS-based AtPAMs are compared by experiments, in which an ultrasound diagnostic transducer (a linear array) is employed to passively sense the wire signals generated by an unfocused ultrasound transducer and the cavitation signals generated by a focused therapeutic ultrasound transducer in a flow phantom. The AtPAM image quality is assessed by main-lobe width (MLW), intensity valley value (IVV), area of pixels (AOP), signal-to-interference ratio (SIR), and signal-to-noise ratio (SNR).

RESULTS

The single-wire experimental results show that compared to the DS algorithm, the DMS algorithm leads to an enhanced AtPAM image with a decreased transverse MLW of 0.15 mm and an improved SIR and SNR of 31.50 and 18.77 dB. For the four-wire images, the transverse (axial) IVV is decreased by 18.37 dB (13.11 dB) and the SIR (the SNR) is increased by 26.13 dB (18.47 dB) when using the DMS algorithm. The cavitation activity is better highlighted by DMS-based AtPAM, which decreases the AOP by 0.81 mm² (-10-dB level) and 4.43 mm² (-20-dB level) and increases the SIR and SNR by 20.14 and 10.48 dB respectively. The pixel distributions of AtPAM images of both wires and cavitation activity also indicate a better suppression of the DMS algorithm in sidelobe and noise.

CONCLUSIONS

The experimental results illustrate that the DMS algorithm can improve the image quality of AtPAM compared to the DS algorithm. DMS-based AtPAM is beneficial for detecting cavitation activity during short-pulse ultrasound exposure with high resolution, and further for monitoring short-pulse ultrasound therapy.

A PVDF Receiver for Acoustic Monitoring of Microbubble-Mediated Ultrasound Brain Therapy

The real-time monitoring of spectral characteristics of microbubble (MB) acoustic emissions permits the prediction of increases in blood-brain barrier (BBB) permeability and of tissue damage in MB-mediated focused ultrasound (FUS) brain therapy. Single-element passive cavitation detectors provide limited spatial information regarding MB activity, greatly affecting the performance of acoustic control. However, an array of receivers can be used to spatially map cavitation events and thus improve treatment control. The spectral content of the acoustic emissions provides additional information that can be correlated with the bio-effects, and wideband receivers can thus provide the most complete spectral information. Here, we develop a miniature polyvinylidene fluoride (PVDF thickness = 110 μm, active area = 1.2 mm2) broadband receiver for the acoustic monitoring of MBs. The receiver has superior sensitivity (2.36-3.87 V/MPa) to those of a commercial fibre-optic hydrophone in the low megahertz frequency range (0.51-5.4 MHz). The receiver also has a wide -6 dB acceptance angle (54 degrees at 1.1 MHz and 13 degrees at 5.4 MHz) and the ability to detect subharmonic and higher harmonic MB emissions in phantoms. The overall acoustic performance of this low-cost receiver indicates its suitability for the eventual use within an array for MB monitoring and mapping in preclinical studies.

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.

Noninvasive ultrasonic induction of cerebrospinal fluid flow enhances intrathecal drug delivery

Intrathecal drug delivery is routinely used in the treatment and prophylaxis of varied central nervous system conditions, as doing so allows drugs to directly bypass the blood-brain barrier. However, the utility of this route of administration is limited by poor brain and spinal cord parenchymal drug uptake from the cerebrospinal fluid. We demonstrate that a simple noninvasive transcranial ultrasound protocol can significantly increase influx of cerebrospinal fluid into the perivascular spaces of the brain, to enhance the uptake of intrathecally administered drugs. Specifically, we administered small (~1 kDa) and large (~155 kDa) molecule agents into the cisterna magna of rats and then applied low, diagnostic-intensity focused ultrasound in a scanning protocol throughout the brain. Using real-time magnetic resonance imaging and ex vivo histologic analyses, we observed significantly increased uptake of small molecule agents into the brain parenchyma, and of both small and large molecule agents into the perivascular space from the cerebrospinal fluid. Notably, there was no evidence of brain parenchymal damage following this intervention. The low intensity and noninvasive approach of transcranial ultrasound in this protocol underscores the ready path to clinical translation of this technique. In this manner, this protocol can be used to directly bypass the blood-brain barrier for whole-brain delivery of a variety of agents. Additionally, this technique can potentially be used as a means to probe the causal role of the glymphatic system in the variety of disease and physiologic processes to which it has been correlated.

From Focused Ultrasound Tumor Ablation to Brain Blood Barrier Opening for High Grade Glioma: A Systematic Review

BACKGROUND

Focused Ultrasound (FUS) is gaining a therapeutic role in neuro-oncology considering its novelty and non-invasiveness. Multiple pre-clinical studies show the efficacy of FUS mediated ablation and Blood-Brain Barrier (BBB) opening in high-grade glioma (HGG), but there is still poor evidence in humans, mainly aimed towards assessing FUS safety.

METHODS

With this systematic review our aim is, firstly, to summarize how FUS is proposed for human HGG treatment. Secondly, we focus on future perspectives and new therapeutic options. Using PRISMA 2020 guidelines, we reviewed case series and trials with description of patient characteristics, pre- and post-operative treatments and FUS outcomes. We considered nine case series (five about tumor ablation and four about BBB opening) with FUS-treated HGG patients between 1991 and 2021.

RESULTS

Sixty-eight patients were considered in total, mostly males (67.6%), with a mean age of 50.5 ± 15.3 years old. Major complication rates were found in the tumor ablation group (26.1%). FUS has been rarely applied for direct tumoral ablation in human HGG patients with controversial results, but at the best of current studies, FUS-mediated BBB opening is showing good results with very low complication rates, paving the way for a new reliable technique to improve local chemotherapy delivery and antitumoral immune response.

CONCLUSIONS

FUS can become a complementary technique to surgical resection and standard radiochemotherapy in recurrent HGG. Ongoing trials could provide in the near future more data on FUS-mediated BBB opening impact on progression-free survival, overall survival and potential drug-delivery capacities.

Time-Resolved Passive Cavitation Mapping Using the Transient Angular Spectrum Approach

Passive cavitation mapping (PCM), which generates images using bubble acoustic emission signals, has been increasingly used for monitoring and guiding focused ultrasound surgery (FUS). PCM can be used as an adjunct to magnetic resonance imaging to provide crucial information on the safety and efficacy of FUS. The most widely used algorithm for PCM is delay-and-sum (DAS). One of the major limitations of DAS is its suboptimal computational efficiency. Although frequency-domain DAS can partially resolve this issue, such an algorithm is not suitable for imaging the evolution of bubble activity in real time and for cases in which cavitation events occur asynchronously. This study investigates a transient angular spectrum (AS) approach for PCM. The working principle of this approach is to backpropagate the received signal to the domain of interest and reconstruct the spatial-temporal wavefield encoded with the bubble location and collapse time. The transient AS approach is validated using an in silico model and water bath experiments. It is found that the transient AS approach yields similar results to DAS, but it is one order of magnitude faster. The results obtained by this study suggest that the transient AS approach is promising for fast and accurate PCM.

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.

Use of the Cross-Spectral Density Matrix for Enhanced Passive Ultrasound Imaging of Cavitation

Passive ultrasound imaging is of great interest for cavitation monitoring. Spatiotemporal monitoring of cavitation bubbles in therapeutic applications is possible using an ultrasound imaging probe to passively receive the acoustic signals from the bubbles. Fourier-domain (FD) beamformers have been proposed to process the signals received into maps of the spatial localization of cavitation activity, with reduced computing times with respect to the time-domain approach, and to take advantage of frequency selectivity for cavitation regime characterization. The approaches proposed have been mainly nonadaptive, and these have suffered from low resolution and contrast, due to the many reconstruction artifacts. Inspired by the array-processing literature and in the context of passive ultrasound imaging of cavitation, we propose here a robust estimation of the second-order statistics of data through spatial covariance matrices in the FD or cross-spectral density matrices (CSMs). The benefits of such formalism are illustrated using advanced reconstruction algorithms, such as the robust Capon beamformer, the Pisarenko class beamformer, and the multiple signal classification approach. Through both simulations and experiments in a water tank, we demonstrate that enhanced localization of cavitation activity (i.e., improved resolution and contrast with respect to nonadaptive approaches) is compatible with the rapid and frequency-selective approaches of the FD. Robust estimation of the CSM and the derived adaptive beamformers paves the way to the development of powerful passive ultrasound imaging tools.

Therapeutic Agent Delivery Across the Blood-Brain Barrier Using Focused Ultrasound

Specialized features of vasculature in the central nervous system greatly limit therapeutic treatment options for many neuropathologies. Focused ultrasound, in combination with circulating microbubbles, can be used to transiently and noninvasively increase cerebrovascular permeability with a high level of spatial precision. For minutes to hours following sonication, drugs can be administered systemically to extravasate in the targeted brain regions and exert a therapeutic effect, after which permeability returns to baseline levels. With the wide range of therapeutic agents that can be delivered using this approach and the growing clinical need, focused ultrasound and microbubble (FUS+MB) exposure in the brain has entered human testing to assess safety. This review outlines the use of FUS+MB-mediated cerebrovascular permeability enhancement as a drug delivery technique, details several technical and biological considerations of this approach, summarizes results from the clinical trials conducted to date, and discusses the future direction of the field.

Improved therapeutic antibody delivery to xenograft tumors using cavitation nucleated by gas-entrapping nanoparticles

Aims: Testing ultrasound-mediated cavitation for enhanced delivery of the therapeutic antibody cetuximab to tumors in a mouse model. Methods: Tumors with strong EGF receptor expression were grown bilaterally. Cetuximab was coadministered intravenously with cavitation nuclei, consisting of either the ultrasound contrast agent Sonovue or gas-stabilizing nanoscale SonoTran Particles. One of the two tumors was exposed to focused ultrasound. Passive acoustic mapping localized and monitored cavitation activity. Both tumors were then excised and cetuximab concentration was quantified. Results: Cavitation increased tumoral cetuximab concentration. When nucleated by Sonovue, a 2.1-fold increase (95% CI 1.3- to 3.4-fold) was measured, whereas SonoTran Particles gave a 3.6-fold increase (95% CI 2.3- to 5.8-fold). Conclusions: Ultrasound-mediated cavitation, especially when nucleated by nanoscale gas-entrapping particles, can noninvasively increase site-specific delivery of therapeutic antibodies to solid tumors.

Focused Ultrasound for Noninvasive, Focal Pharmacologic Neurointervention

A long-standing goal of translational neuroscience is the ability to noninvasively deliver therapeutic agents to specific brain regions with high spatiotemporal resolution. Focused ultrasound (FUS) is an emerging technology that can noninvasively deliver energy up the order of 1 kW/cm² with millimeter and millisecond resolution to any point in the human brain with Food and Drug Administration-approved hardware. Although FUS is clinically utilized primarily for focal ablation in conditions such as essential tremor, recent breakthroughs have enabled the use of FUS for drug delivery at lower intensities (i.e., tens of watts per square centimeter) without ablation of the tissue. In this review, we present strategies for image-guided FUS-mediated pharmacologic neurointerventions. First, we discuss blood–brain barrier opening to deliver therapeutic agents of a variety of sizes to the central nervous system. We then describe the use of ultrasound-sensitive nanoparticles to noninvasively deliver small molecules to millimeter-sized structures including superficial cortical regions and deep gray matter regions within the brain without the need for blood–brain barrier opening. We also consider the safety and potential complications of these techniques, with attention to temporal acuity. Finally, we close with a discussion of different methods for mapping the ultrasound field within the brain and describe future avenues of research in ultrasound-targeted drug therapies.

A new safety index based on intrapulse monitoring of ultra-harmonic cavitation during ultrasound-induced blood-brain barrier opening procedures

Ultrasound-induced blood-brain barrier (BBB) opening using microbubbles is a promising technique for local delivery of therapeutic molecules into the brain. The real-time control of the ultrasound dose delivered through the skull is necessary as the range of pressure for efficient and safe BBB opening is very narrow. Passive cavitation detection (PCD) is a method proposed to monitor the microbubble activity during ultrasound exposure. However, there is still no consensus on a reliable safety indicator able to predict potential damage in the brain. Current approaches for the control of the beam intensity based on PCD employ a full-pulse analysis and may suffer from a lack of sensitivity and poor reaction time. To overcome these limitations, we propose an intra-pulse analysis to monitor the evolution of the frequency content during ultrasound bursts. We hypothesized that the destabilization of microbubbles exposed to a critical level of ultrasound would result in the instantaneous generation of subharmonic and ultra-harmonic components. This specific signature was exploited to define a new sensitive indicator of the safety of the ultrasound protocol. The approach was validated in vivo in rats and non-human primates using a retrospective analysis. Our results demonstrate that intra-pulse monitoring was able to exhibit a sudden appearance of ultra-harmonics during the ultrasound excitation pulse. The repeated detection of such a signature within the excitation pulse was highly correlated with the occurrence of side effects such as hemorrhage and edema. Keeping the acoustic pressure at levels where no such sign of microbubble destabilization occurred resulted in safe BBB openings, as shown by MR images and gross pathology. This new indicator should be more sensitive than conventional full-pulse analysis and can be used to distinguish between potentially harmful and safe ultrasound conditions in the brain with very short reaction time.

Ultrafast three-dimensional microbubble imaging in vivo predicts tissue damage volume distributions during nonthermal brain ablation

Transcranial magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) thermal ablation is under clinical investigation for non-invasive neurosurgery, though its use is restricted to central brain targets due primarily to skull heating effects. The combination of FUS and contrast agent microbubbles greatly reduces the ultrasound exposure levels needed to ablate brain tissue and may help facilitate the use of transcranial FUS ablation throughout the brain. However, sources of variability exist during microbubble-mediated FUS procedures that necessitate the continued development of systems and methods for online treatment monitoring and control, to ensure that excessive and/or off-target bioeffects are not induced from the exposures. Methods: Megahertz-rate three-dimensional (3D) microbubble imaging in vivo was performed during nonthermal ablation in rabbit brain using a clinical-scale prototype transmit/receive hemispherical phased array system. Results:In-vivo volumetric acoustic imaging over microsecond timescales uncovered spatiotemporal microbubble dynamics hidden by conventional whole-burst temporal averaging. Sonication-aggregate ultrafast 3D source field intensity data were predictive of microbubble-mediated tissue damage volume distributions measured post-treatment using MRI and confirmed via histopathology. Temporal under-sampling of acoustic emissions, which is common practice in the field, was found to impede performance and highlighted the importance of capturing adequate data for treatment monitoring and control purposes. Conclusion: The predictive capability of ultrafast 3D microbubble imaging, reported here for the first time, will enable future microbubble-mediated FUS treatments with unparalleled precision and accuracy, and will accelerate the clinical translation of nonthermal tissue ablation procedures both in the brain and throughout the body.

Real-Time Transcranial Histotripsy Treatment Localization and Mapping Using Acoustic Cavitation Emission Feedback

Cavitation events generated during histotripsy therapy generate large acoustic cavitation emission (ACE) signals that can be detected through the skull. This article investigates the feasibility of using these ACE signals, acquired using the elements of a 500-kHz, 256-element hemispherical histotripsy transducer as receivers, to localize and map the cavitation activity in real time through the human skullcap during transcranial histotripsy therapy. The locations of the generated cavitation events predicted using the ACE feedback signals in this study were found to be accurate to within <1.5 mm of the centers of masses detected by optical imaging and found to lie to within the measured volumes of the generated cavitation events in >~80 % of cases. Localization results were observed to be biased in the prefocal direction of the histotripsy array and toward its transverse origin but were only weakly affected by focal steering location. The choice of skullcap and treatment pulse repetition frequency (PRF) were both observed to affect the accuracy of the localization results in the low PRF regime (1-10 Hz), but the localization accuracy was seen to stabilize at higher PRFs (≥10 Hz). Tests of the localization algorithm in vitro, for treatment delivered to a bovine brain sample mounted within the skullcap, revealed good agreement between the ACE feedback-generated treatment map and the morphological characteristics of the treated volume of the brain sample. Localization during experiments was achieved in real time for pulses delivered at rates up to 70 Hz, but benchmark tests indicate that the localization algorithm is scalable, indicating that higher rates are possible with more powerful hardware. The results of this article demonstrate the feasibility of using ACE feedback signals to localize and map transcranially generated cavitation events during histotripsy. Such capability has the potential to greatly simplify transcranial histotripsy treatments, as it may provide a non-MRI-based method for monitoring and localizing transcranial histotripsy treatments in real time.

Ultrasound-Responsive Cavitation Nuclei for Therapy and Drug Delivery

Therapeutic ultrasound strategies that harness the mechanical activity of cavitation nuclei for beneficial tissue bio-effects are actively under development. The mechanical oscillations of circulating microbubbles, the most widely investigated cavitation nuclei, which may also encapsulate or shield a therapeutic agent in the bloodstream, trigger and promote localized uptake. Oscillating microbubbles can create stresses either on nearby tissue or in surrounding fluid to enhance drug penetration and efficacy in the brain, spinal cord, vasculature, immune system, biofilm or tumors. This review summarizes recent investigations that have elucidated interactions of ultrasound and cavitation nuclei with cells, the treatment of tumors, immunotherapy, the blood-brain and blood-spinal cord barriers, sonothrombolysis, cardiovascular drug delivery and sonobactericide. In particular, an overview of salient ultrasound features, drug delivery vehicles, therapeutic transport routes and pre-clinical and clinical studies is provided. Successful implementation of ultrasound and cavitation nuclei-mediated drug delivery has the potential to change the way drugs are administered systemically, resulting in more effective therapeutics and less-invasive treatments.

Spatiotemporal Assessment of the Cellular Safety of Cavitation-Based Therapies by Passive Acoustic Mapping

Many useful therapeutic bio-effects can be generated using ultrasound-induced cavitation. However, cavitation is also capable of causing unwanted cellular and vascular damage, which should be monitored to ensure treatment safety. In this work, the unique opportunity provided by passive acoustic mapping (PAM) to quantify cavitation dose across an entire volume of interest during therapy is utilised to provide setup-independent measures of spatially localised cavitation dose. This spatiotemporally quantifiable cavitation dose is then related to the level of cellular damage generated. The cavitation-mediated destruction of equine red blood cells mixed with one of two types of cavitation nuclei at a variety of concentrations is investigated. The blood is placed within a 0.5-MHz ultrasound field and exposed to a range of peak rarefactional pressures up to 2 MPa, with 50 to 50,000 cycle pulses maintaining a 5% duty cycle. Two co-planar linear arrays at 90° to each other are used to generate 400-µm-resolution frequency domain robust capon beamforming PAM maps, which are then used to generate estimates of cavitation dose. A relationship between this cavitation dose and the levels of haemolysis generated was found which was comparable regardless of the applied acoustic pressure, pulse length, cavitation agent type or concentration used. PAM was then used to monitor cellular damage in multiple locations within a tissue phantom simultaneously, with the damage-cavitation dose relationship being similar for the two experimental models tested. These results lay the groundwork for this method to be applied to other measures of safety, allowing for improved ultrasound monitoring of cavitation-based therapies.

Preliminary experience with a transcranial magnetic resonance-guided focused ultrasound surgery system integrated with a 1.5-T MRI unit in a series of patients with essential tremor and Parkinson's disease

OBJECTIVE Transcranial magnetic resonance-guided focused ultrasound surgery (tcMRgFUS) is one of the emerging noninvasive technologies for the treatment of neurological disorders such as essential tremor (ET), idiopathic asymmetrical tremor-dominant Parkinson's disease (PD), and neuropathic pain. In this clinical series the authors present the preliminary results achieved with the world's first tcMRgFUS system integrated with a 1.5-T MRI unit. METHODS The authors describe the results of tcMRgFUS in a sample of patients with ET and with PD who underwent the procedure during the period from January 2015 to September 2017. A monolateral ventralis intermedius nucleus (VIM) thalamic ablation was performed in both ET and PD patients. In all the tcMRgFUS treatments, a 1.5-T MRI scanner was used for both planning and monitoring the procedure. RESULTS During the study period, a total of 26 patients underwent tcMRgFUS thalamic ablation for different movement disorders. Among these patients, 18 were diagnosed with ET and 4 were affected by PD. All patients with PD were treated using tcMRgFUS thalamic ablation and all completed the procedure. Among the 18 patients with ET, 13 successfully underwent tcMRgFUS, 4 aborted the procedure during ultrasound delivery, and 1 did not undergo the tcMRgFUS procedure after stereotactic frame placement. Two patients with ET were not included in the results because of the short follow-up duration at the time of this study. A monolateral VIM thalamic ablation in both ET and PD patients was performed. All the enrolled patients were evaluated before the treatment and 2 days after, with a clinical control of the treatment effectiveness using the graphic items of the Fahn-Tolosa-Marin tremor rating scale. A global reevaluation was performed 3 months (17/22 patients) and 6 months (11/22 patients) after the treatment; the reevaluation consisted of clinical questionnaires, neurological tests, and video recordings of the tests. All the ET and PD treated patients who completed the procedure showed an immediate amelioration of tremor severity, with no intra- or posttreatment severe permanent side effects. CONCLUSIONS Although this study reports on a small number of patients with a short follow-up duration, the tcMRgFUS procedure using a 1.5-T MRI unit resulted in a safe and effective treatment option for motor symptoms in patients with ET and PD. To the best of the authors' knowledge, this is the first clinical series in which thalamotomy was performed using tcMRgFUS integrated with a 1.5-T magnet.

Delay multiply and sum beamforming method applied to enhance linear-array passive acoustic mapping of ultrasound cavitation

PURPOSE

Passive acoustic mapping (PAM) has been proposed as a means of monitoring ultrasound therapy, particularly nonthermal cavitation-mediated applications. In PAM, the most common beamforming algorithm is a delay, sum, and integrate (DSAI) approach. However, using DSAI leads to low-quality images for the case where a narrow-aperture receiving array such as a standard B-mode linear array is used. This study aims to propose an enhanced linear-array PAM algorithm based on delay, multiply, sum, and integrate (DMSAI).

METHODS

In the proposed algorithm, before summation, the delayed signals are combinatorially coupled and multiplied, which means that the beamformed output of the proposed algorithm is the spatial coherence of received acoustic emissions. We tested the performance of the proposed DMSAI using both simulated and experimental data and compared it with DSAI. The reconstructed cavitation images were evaluated quantitatively by using source location errors between the two algorithms, full width at half maximum (FWHM), size of point spread function (A₅₀ area), signal-to-noise ratio (SNR), and computational time.

RESULTS

The results of simulations and experiments for single cavitation source show that, by introducing DMSAI, the FWHM and the A₅₀ area are reduced and the SNR is improved compared with those obtained by DSAI. The simulation results for two symmetric or nonsymmetric cavitation sources and multiple cavitation sources show that DMSAI can significantly reduce the A₅₀ area and improve the SNR, therefore improving the detectability of multiple cavitation sources.

CONCLUSIONS

The results indicate that the proposed DMSAI algorithm outperforms the conventionally used DSAI algorithm. This work may have the potential of providing an appropriate method for ultrasound therapy monitoring.

Microstructural changes of the dentato-rubro-thalamic tract after transcranial MR guided focused ultrasound ablation of the posteroventral VIM in essential tremor

Essential tremor is the most common movement disorder in adults. In patients who are not responsive to medical treatment, functional neurosurgery and, more recently, transcranial MR-guided focused ultrasound thalamotomy are considered effective therapeutic approaches. However, the structural brain changes following a thalamotomy that mediates the clinical improvement are still unclear. In here diffusion weighted images were acquired in a cohort of 24 essential tremor patients before and 3 months after unilateral transcranial MR-guided focused ultrasound thalamotomy targeting at the posteroventral part of the VIM. Microstructural changes along the DRTT were quantified by means of probabilistic tractography, and later related to the clinical improvement of the patients at 3-months and at 1-year after the intervention. In addition the changes along two neighboring tracts, that is, the corticospinal tract and the medial lemniscus, were assessed, as well as the relation between these changes and the presence of side effects. Thalamic lesions produced local and distant alterations along the trajectory of the DRTT, and each correlated with clinical improvement. Regarding side effects, gait imbalance after thalamotomy was associated with greater impact on the DRTT, whereas the presence of paresthesias was significantly related to a higher overlap between the lesion and the medial lemniscus. This work represents the largest series describing the microstructural changes following transcranial MR-guided focused ultrasound thalamotomy in essential tremor. These results suggest that clinical benefits are specific for the impact on the cerebello-thalamo-cortical pathway, thus reaffirming the potential of tractography to aid thalamotomy targeting.

Feedback control of microbubble cavitation for ultrasound-mediated blood-brain barrier disruption in non-human primates under magnetic resonance guidance

Focused ultrasound (FUS) in combination with microbubbles is capable of noninvasive, site-targeted delivery of drugs through the blood-brain barrier (BBB). Although acoustic parameters are reproducible in small animals, their control remains challenging in primates due to skull heterogeneity. This study describes a 7-T magnetic resonance (MR)-guided FUS system designed for BBB disruption in non-human primates (NHP) with a robust feedback control based on passive cavitation detection (PCD). Contrast enhanced T₁-weighted MR images confirmed the BBB opening in NHP sonicated during 2 min with 500-kHz frequency, pulse length of 10 ms, and pulse repetition frequency of 5 Hz. The safe acoustic pressure range from 185 ± 22 kPa to 266 ± 4 kPa in one representative case was estimated from combining data from the acoustic beam profile with the BBB opening and hemorrhage profiles obtained from MR images. A maximum amount of MR contrast agent at focus was observed at 30 min after sonication with a relative contrast enhancement of 67% ± 15% (in comparison to that found in muscles). The feedback control based on PCD using relative spectra was shown to be robust, allowing comparisons across animals and experimental sessions. Finally, we also demonstrated that PCD can test acoustic coupling conditions, which improves the efficacy and safety of ultrasound transmission into the brain.

Passive cavitation mapping using dual apodization with cross-correlation in ultrasound therapy monitoring

Recently, passive acoustic mapping (PAM) has been successfully applied for dynamic monitoring of ultrasound therapy by beamforming acoustic emissions of cavitation activity during ultrasound exposure. The most widely used PAM algorithm in the literature is time exposure acoustics (TEA), which is a standard delay, sum, and integrate algorithm. However, it results in large point spread function (PSF) and serious imaging artifacts for the case where a narrow-aperture receiving array such as a standard B-mode linear array is used, therefore degrading the quality of cavitation image. To address these challenges, in this paper, we proposed a novel PAM algorithm namely dual apodization with cross-correlation (DAX)-based TEA, in which DAX was originally used as a reconstruction algorithm in medical ultrasound imaging. In the proposed algorithm, two sets of signals were beamformed by two receive apodization functions with alternating elements enabled, and the cross-correlation coefficient of the two signals served as a weighting factor that would be multiplied to the sum of the two signals. The performance of the proposed algorithm was tested on simulated channel data obtained using a multi-bubble model, and experiments were also performed in an in vitro vessel phantom with flowing microbubbles as cavitation nuclei. The reconstructed cavitation images were evaluated quantitatively using established quality metrics including full width at half maximum (FWHM), A_-6dB area, and signal-to-noise ratio (SNR). The results suggested that the proposed algorithm significantly outperformed the conventionally used TEA algorithm. This work may have the potential of providing a useful tool for highly accurate localization of cavitation activity during ultrasound therapy.

Advances in acoustic monitoring and control of focused ultrasound-mediated increases in blood-brain barrier permeability

Transcranial focused ultrasound (FUS) combined with intravenously circulating microbubbles can transiently and selectively increase blood-brain barrier permeability to enable targeted drug delivery to the central nervous system, and is a technique that has the potential to revolutionize the way neurological diseases are managed in medical practice. Clinical testing of this approach is currently underway in patients with brain tumors, early Alzheimer's disease, and amyotrophic lateral sclerosis. A major challenge that needs to be addressed in order for widespread clinical adoption of FUS-mediated blood-brain barrier permeabilization to occur is the development of systems and methods for real-time treatment monitoring and control, to ensure that safe and effective acoustic exposure levels are maintained throughout the procedures. This review gives a basic overview of the oscillation dynamics, acoustic emissions, and biological effects associated with ultrasound-stimulated microbubbles in vivo, and provides a summary of recent advances in acoustic-based strategies for detecting, controlling, and mapping microbubble activity in the brain. Further development of next-generation clinical FUS brain devices tailored towards microbubble-mediated applications is warranted and required for translation of this potentially disruptive technology into routine clinical practice.

Cavitation dose painting for focused ultrasound-induced blood-brain barrier disruption

Focused ultrasound combined with microbubble for blood-brain barrier disruption (FUS-BBBD) is a promising technique for noninvasive and localized brain drug delivery. This study demonstrates that passive cavitation imaging (PCI) is capable of predicting the location and concentration of nanoclusters delivered by FUS-BBBD. During FUS-BBBD treatment of mice, the acoustic emissions from FUS-activated microbubbles were passively detected by an ultrasound imaging system and processed offline using a frequency-domain PCI algorithm. After the FUS treatment, radiolabeled gold nanoclusters, ⁶⁴Cu-AuNCs, were intravenously injected into the mice and imaged by positron emission tomography/computed tomography (PET/CT). The centers of the stable cavitation dose (SCD) maps obtained by PCI and the corresponding centers of the ⁶⁴Cu-AuNCs concentration maps obtained by PET coincided within 0.3 ± 0.4 mm and 1.6 ± 1.1 mm in the transverse and axial directions of the FUS beam, respectively. The SCD maps were found to be linearly correlated with the ⁶⁴Cu-AuNCs concentration maps on a pixel-by-pixel level. These findings suggest that SCD maps can spatially "paint" the delivered nanocluster concentration, a technique that we named as cavitation dose painting. This PCI-based cavitation dose painting technique in combination with FUS-BBBD opens new horizons in spatially targeted and modulated brain drug delivery.

Closed-loop cavitation control for focused ultrasound-mediated blood-brain barrier opening by long-circulating microbubbles

Focused ultrasound (FUS) exposure in the presence of microbubbles (MBs) has been successfully used in the delivery of various sizes of therapeutic molecules across the blood-brain barrier (BBB). While acoustic pressure is correlated with the BBB opening size, real-time control of BBB opening to avoid vascular and neural damage is still a challenge. This arises mainly from the variability of FUS-MB interactions due to the variations of animal-specific metabolic environment and specific experimental setup. In this study, we demonstrate a closed-loop cavitation control framework to induce BBB opening for delivering large therapeutic molecules without causing macro tissue damages. To this end, we performed in mice long-term (5 min) cavitation monitoring facilitated by using long-circulating MBs. Monitoring the long-term temporal kinetics of the MBs under varying level of FUS pressure allowed to identify in situ, animal specific activity regimes forming pressure-dependent activity bands. This enables to determine the boundaries of each activity band (i.e. steady oscillation, transition, inertial cavitation) independent from the physical and physiological dynamics of the experiment. However, such a calibration approach is time consuming and to speed up characterization of the in situ, animal specific FUS-MB dynamics, we tested a novel method called 'pre-calibration' that closely reproduces the results of long-term monitoring but with a much shorter duration. Once the activity bands are determined from the pre-calibration method, an operation band can be selected around the desired cavitation dose. To drive cavitation in the selected operation band, we developed an adaptive, closed-loop controller that updates the acoustic pressure between each sonication based on measured cavitation dose. Finally, we quantitatively assessed the safety of different activity bands and validated the proposed methods and controller framework. The proposed framework serves to optimize the FUS pressure instantly to maintain the targeted cavitation level while improving safety control.

Theranostic Strategy of Focused Ultrasound Induced Blood-Brain Barrier Opening for CNS Disease Treatment

Focused Ultrasound (FUS) in combination with gaseous microbubbles has emerged as a potential new means of effective drug delivery to the brain. Recent research has shown that, under burst-type energy exposure with the presence of microbubbles, this modality can transiently permeate the blood-brain barrier (BBB). The bioavailability of therapeutic agents is site-specifically augmented only in the zone where the FUS energy is targeted. The non-invasiveness of this approach makes FUS-induced BBB opening a novel and attractive means to perform localized CNS therapeutic agent delivery. Over the past decade, FUS-BBB opening has been preclinically confirmed to successfully enhance CNS penetration of therapeutic agents including chemotherapeutic agents, therapeutic peptides, monoclonal antibodies, and nanoparticles. Recently, a number of clinical human trials have begun to explore clinical utility. This review article, explores this technology through its physical mechanisms, summarizes the existing preclinical findings (including current medical device designs and technical approaches), and summarizes current ongoing clinical trials.

Passive Acoustic Mapping Using Data-Adaptive Beamforming Based on Higher Order Statistics

Sources of nonlinear acoustic emissions, particularly those associated with cavitation activity, play a key role in the safety and efficacy of current and emerging therapeutic ultrasound applications, such as oncological drug delivery, blood-brain barrier opening, and histotripsy. Passive acoustic mapping (PAM) is the first technique to enable real-time and non-invasive imaging of cavitation activity during therapeutic ultrasound exposure, through the recording and passive beamforming of broadband acoustic emissions using an array of ultrasound detectors. Initial limitations in PAM spatial resolution led to the adoption of optimal data-adaptive beamforming algorithms, such as the robust capon beamformer (RCB), that provide improved interference suppression and calibration error mitigation compared to non-adaptive beamformers. However, such approaches are restricted by the assumption that the recorded signals have a Gaussian distribution. To overcome this limitation and further improve the source resolvability of PAM, we propose a new beamforming approach termed robust beamforming by linear programming (RLPB). Along with the variance, this optimization-based method uses higher-order-statistics of the recorded signals, making no prior assumption on the statistical distribution of the acoustic signals. The RLPB is found via numerical simulations to improve resolvability over time exposure acoustics and RCB. In vitro experimentation yielded improved resolvability with respect to the source-to-array distance on the order of 22% axially and 13% transversely relative to RCB, whilst successfully accounting for array calibration errors. The improved resolution and decreased dependence on accurate calibration of RLPB is expected to facilitate the clinical translation of PAM for diagnostic, including super-resolution, and therapeutic ultrasound applications.

Simultaneous Passive Acoustic Mapping and Magnetic Resonance Thermometry for Monitoring of Cavitation-Enhanced Tumor Ablation in Rabbits Using Focused Ultrasound and Phase-Shift Nanoemulsions

Thermal ablation of solid tumors via focused ultrasound (FUS) is a non-invasive image-guided alternative to conventional surgical resection. However, the usefulness of the technique is limited in vascularized organs because of convection of heat, resulting in long sonication times and unpredictable thermal lesion formation. Acoustic cavitation has been found to enhance heating but requires use of exogenous nuclei and sufficient acoustic monitoring. In this study, we employed phase-shift nanoemulsions (PSNEs) to promote cavitation and incorporated passive acoustic mapping (PAM) alongside conventional magnetic resonance imaging (MRI) thermometry within the bore of a clinical MRI scanner. Simultaneous PAM and MRI thermometry were performed in an in vivo rabbit tumor model, with and without PSNE to promote cavitation. Vaporization and cavitation of the nanoemulsion could be detected using PAM, which led to accelerated heating, monitored with MRI thermometry. The maximum heating assessed from MRI was well correlated with the integrated acoustic emissions, illustrating cavitation-enhanced heating. Examination of tissue revealed thermal lesions that were larger in the presence of PSNE, in agreement with the thermometry data. Using fixed exposure conditions over 94 sonications in multiple animals revealed an increase in the mean amplitude of acoustic emissions and resulting temperature rise, but with significant variability between sonications, further illustrating the need for real-time monitoring. The results indicate the utility of combined PAM and MRI for monitoring of tumor ablation and provide further evidence for the ability of PSNEs to promote cavitation-enhanced lesioning.

Broadband Ultrasonic Attenuation Estimation and Compensation With Passive Acoustic Mapping

Several active and passive techniques have been developed to detect, localize, and quantify cavitation activity during therapeutic ultrasound procedures. Much of the prior cavitation monitoring research has been conducted using lossless in vitro systems or small animal models in which path attenuation effects were minimal. However, the performance of these techniques may be substantially degraded by attenuation between the internal therapeutic target and the external monitoring system. As a further step toward clinical application of passive acoustic mapping (PAM), this paper presents methods for attenuation estimation and compensation based on broadband cavitation data measured with a linear ultrasound array. Soft tissue phantom experiment results are used to illustrate: 1) the impact of realistic attenuation on PAM; 2) the possibility of estimating attenuation from cavitation data; 3) cavitation source energy estimation following attenuation compensation; and 4) the impact of sound speed uncertainty on PAM-related processing. Cavitation-based estimates of attenuation were within 1.5%-6.2% of the values found from conventional through-transmission measurements. Tissue phantom attenuation reduced the PAM energy estimate by an order of magnitude, but array data compensation using the cavitation-based attenuation spectrum enabled recovery of the PAM energy estimate to within 2.9%-5.9% of the values computed in the absence of the phantom. Sound speed uncertainties were found to modestly impact attenuation-compensated PAM energies, inducing errors no larger than 28% for a 40-m/s path-averaged speed error. Together, the results indicate the potential to significantly enhance the quantitative capabilities of PAM for ensuring therapeutic safety and efficacy.

Design and Implementation of a Transmit/Receive Ultrasound Phased Array for Brain Applications

Focused ultrasound phased array systems have attracted increased attention for brain therapy applications. However, such systems currently lack a direct and real-time method to intraoperatively monitor ultrasound pressure distribution for securing treatment. This study proposes a dual-mode ultrasound phased array system design to support transmit/receive operations for concurrent ultrasound exposure and backscattered focal beam reconstruction through a spherically focused ultrasound array. A 256-channel ultrasound transmission system was used to transmit focused ultrasonic energy (full 256 channels), with an extended implementation of multiple-channel receiving function (up to 64 channels) using the same 256-channel ultrasound array. A coherent backscatter-received beam formation algorithm was implemented to map the point spread function (PSF) and focal beam distribution under a free-field/transcranial environment setup, with the backscattering generated from a strong scatterer (a point reflector or a microbubble-perfused tube) or a weakly scattered tissue-mimicking graphite phantom. Our results showed that PSF and focal beam can be successfully reconstructed and visualized in free-field conditions and can also be transcranially reconstructed following skull-induced aberration correction. In vivo experiments were conducted to demonstrate its capability to preoperatively and semiquantitatively map a focal beam to guide blood-brain barrier opening. The proposed system may have potential for real-time guidance of ultrasound brain intervention, and may facilitate the design of a dual-mode ultrasound phased array for brain therapeutic applications.

Frequency-sum beamforming for passive cavitation imaging

Beamforming includes a variety of spatial filtering techniques that may be used for determining sound source locations from near-field sensor array recordings. For this scenario, beamforming resolution depends on the acoustic frequency, array geometry, and target location. Random scattering in the medium between the source and the array may degrade beamforming resolution with higher frequencies being more susceptible to degradation. The performance of frequency-sum (FS) beamforming for reducing such sensitivity to mild scattering while increasing resolution is reported here. FS beamforming was used with a data-dependent [minimum variance (MV)] or data-independent (delay-and-sum, DAS) weight vector to produce higher frequency information from lower frequency signal components via a quadratic product of complex signal amplitudes. The current findings and comparisons are based on simulations and passive cavitation imaging experiments using 3 MHz and 6 MHz emissions recorded by a 128-element linear array. FS beamforming results are compared to conventional DAS and MV beamforming using four metrics: point spread function (PSF) size, axial and lateral contrast, and computation time. FS beamforming produces a smaller PSF than conventional DAS beamforming with less computation time than MV beamforming in free space and mild scattering environments. However, it may fail when multiple unknown sound sources are present.

Modulation of Brain Function and Behavior by Focused Ultrasound

PURPOSE OF REVIEW

The past decade has seen rapid growth in the application of focused ultrasound (FUS) as a tool for basic neuroscience research and potential treatment of brain disorders. Here, we review recent developments in our understanding of how FUS can alter brain activity, perception and behavior when applied to the central nervous system, either alone or in combination with circulating agents.

RECENT FINDINGS

Focused ultrasound in the central nervous system can directly excite or inhibit neuronal activity, as well as affect perception and behavior. Combining FUS with intravenous microbubbles to open the blood-brain barrier also affects neural activity and behavior, and the effects may be more sustained than FUS alone. Opening the BBB also allows delivery of drugs that do not cross the intact BBB including viral vectors for gene delivery.

SUMMARY

While further research is needed to elucidate the biophysical mechanisms, focused ultrasound, alone or in combination with other factors, is rapidly maturing as an effective technology for altering brain activity. Future challenges include refining control over targeting specificity, the volume of affected tissue, cell-type specificity (excitatory or inhibitory), and the duration of neural and behavioral effects.

Gas-Stabilizing Gold Nanocones for Acoustically Mediated Drug Delivery

The efficient penetration of drugs into tumors is a major challenge that remains unmet. Reported herein is a strategy to promote extravasation and enhanced penetration using inertial cavitation initiated by focused ultrasound and cone-shaped gold nanoparticles that entrap gas nanobubbles. The cones are capable of initiating inertial cavitation under pressures and frequencies achievable with existing clinical ultrasound systems and of promoting extravasation and delivery of a model large therapeutic molecule in an in vitro tissue mimicking flow phantom, achieving penetration depths in excess of 2 mm. Ease of functionalization and intrinsic imaging capabilities provide gold with significant advantages as a material for biomedical applications. The cones show neither cytotoxicity in Michigan Cancer Foundation (MCF)-7 cells nor hemolytic activity in human blood at clinically relevant concentrations and are found to be colloidally stable for at least 5 d at 37 °C and several months at 4 °C.

Efficient Blood-Brain Barrier Opening in Primates with Neuronavigation-Guided Ultrasound and Real-Time Acoustic Mapping

Brain diseases including neurological disorders and tumors remain under treated due to the challenge to access the brain, and blood-brain barrier (BBB) restricting drug delivery which, also profoundly limits the development of pharmacological treatment. Focused ultrasound (FUS) with microbubbles is the sole method to open the BBB noninvasively, locally, and transiently and facilitate drug delivery, while translation to the clinic is challenging due to long procedure, targeting limitations, or invasiveness of current systems. In order to provide rapid, flexible yet precise applications, we have designed a noninvasive FUS and monitoring system with the protocol tested in monkeys (from in silico preplanning and simulation, real-time targeting and acoustic mapping, to post-treatment assessment). With a short procedure (30 min) similar to current clinical imaging duration or radiation therapy, the achieved targeting (both cerebral cortex and subcortical structures) and monitoring accuracy was close to the predicted 2-mm lower limit. This system would enable rapid clinical transcranial FUS applications outside of the MRI system without a stereotactic frame, thereby benefiting patients especially in the elderly population.

Three-dimensional transcranial microbubble imaging for guiding volumetric ultrasound-mediated blood-brain barrier opening

Focused ultrasound (FUS)-mediated blood-brain barrier (BBB) opening recently entered clinical testing for targeted drug delivery to the brain. Sources of variability exist in the current procedures, motivating the development of real-time monitoring and control techniques to improve treatment safety and efficacy. Here we used three-dimensional (3D) transcranial microbubble imaging to calibrate FUS exposure levels for volumetric BBB opening.

Methods: Using a sparse hemispherical transmit/receive ultrasound phased array, pulsed ultrasound was focused transcranially into the thalamus of rabbits during microbubble infusion and multi-channel 3D beamforming was performed online with receiver signals captured at the subharmonic frequency. Pressures were increased pulse-by-pulse until subharmonic activity was detected on acoustic imaging (p_sub), and tissue volumes surrounding the calibration point were exposed at 50-100%p_sub via rapid electronic beam steering.

Results: Spatially-coherent subharmonic microbubble activity was successfully reconstructed transcranially in vivo during calibration sonications. Multi-point exposures induced volumetric regions of elevated BBB permeability assessed via contrast-enhanced magnetic resonance imaging (MRI). At exposure levels ≥75%p_sub, MRI and histological examination occasionally revealed tissue damage, whereas sonications at 50%p_sub were performed safely. Substantial intra-grid variability of FUS-induced bioeffects was observed via MRI, prompting future development of multi-point calibration schemes for improved treatment consistency. Receiver array sparsity and sensor configuration had substantial impacts on subharmonic detection sensitivity, and are factors that should be considered when designing next-generation clinical FUS brain therapy systems.

Conclusion: Our findings suggest that 3D subharmonic imaging can be used to calibrate exposure levels for safe FUS-induced volumetric BBB opening, and should be explored further as a method for cavitation-mediated treatment guidance.

A dual-mode hemispherical sparse array for 3D passive acoustic mapping and skull localization within a clinical MRI guided focused ultrasound device

Previous work has demonstrated that passive acoustic imaging may be used alongside MRI for monitoring of focused ultrasound therapy. However, past implementations have generally made use of either linear arrays originally designed for diagnostic imaging or custom narrowband arrays specific to in-house therapeutic transducer designs, neither of which is fully compatible with clinical MR-guided focused ultrasound (MRgFUS) devices. Here we have designed an array which is suitable for use within an FDA-approved MR-guided transcranial focused ultrasound device, within the bore of a 3 Tesla clinical MRI scanner. The array is constructed from 5  ×  0.4 mm piezoceramic disc elements arranged in pseudorandom fashion on a low-profile laser-cut acrylic frame designed to fit between the therapeutic elements of a 230 kHz InSightec ExAblate 4000 transducer. By exploiting thickness and radial resonance modes of the piezo discs the array is capable of both B-mode imaging at 5 MHz for skull localization, as well as passive reception at the second harmonic of the therapy array for detection of cavitation and 3D passive acoustic imaging. In active mode, the array was able to perform B-mode imaging of a human skull, showing the outer skull surface with good qualitative agreement with MR imaging. Extension to 3D showed the array was able to locate the skull within  ±2 mm/2° of reference points derived from MRI, which could potentially allow registration of a patient to the therapy system without the expense of real-time MRI. In passive mode, the array was able to resolve a point source in 3D within a  ±10 mm region about each axis from the focus, detect cavitation (SNR ~ 12 dB) at burst lengths from 10 cycles to continuous wave, and produce 3D acoustic maps in a flow phantom. Finally, the array was used to detect and map cavitation associated with microbubble activity in the brain in nonhuman primates.

Power cavitation-guided blood-brain barrier opening with focused ultrasound and microbubbles

Image-guided monitoring of microbubble-based focused ultrasound (FUS) therapies relies on the accurate localization of FUS-stimulated microbubble activity (i.e. acoustic cavitation). Passive cavitation imaging with ultrasound arrays can achieve this, but with insufficient spatial resolution. In this study, we address this limitation and perform high-resolution monitoring of acoustic cavitation-mediated blood-brain barrier (BBB) opening with a new technique called power cavitation imaging. By synchronizing the FUS transmit and passive receive acquisition, high-resolution passive cavitation imaging was achieved by using delay and sum beamforming with absolute time delays. Since the axial image resolution is now dependent on the duration of the received acoustic cavitation emission, short pulses of FUS were used to limit its duration. Image sets were acquired at high-frame rates for calculation of power cavitation images analogous to power Doppler imaging. Power cavitation imaging displays the mean intensity of acoustic cavitation over time and was correlated with areas of acoustic cavitation-induced BBB opening. Power cavitation-guided BBB opening with FUS could constitute a standalone system that may not require MRI guidance during the procedure. The same technique can be used for other acoustic cavitation-based FUS therapies, for both safety and guidance.

Passive acoustic mapping of cavitation using eigenspace-based robust Capon beamformer in ultrasound therapy

Pulse-echo imaging technique can only play a role when high intensity focused ultrasound (HIFU) is turned off due to the interference between the primary HIFU signal and the transmission pulse. Passive acoustic mapping (PAM) has been proposed as a tool for true real-time monitoring of HIFU therapy. However, the most-used PAM algorithm based on time exposure acoustic (TEA) limits the quality of cavitation image. Recently, robust Capon beamformer (RCB) has been used in PAM to provide improved resolution and reduced artifacts over TEA-based PAM, but the presented results have not been satisfactory. In the present study, we applied an eigenspace-based RCB (EISRCB) method to further improve the PAM image quality. The optimal weighting vector of the proposed method was found by projecting the RCB weighting vector onto the desired vector subspace constructed from the eigenstructure of the covariance matrix. The performance of the proposed PAM was validated by both simulations and in vitro histotripsy experiments. The results suggested that the proposed PAM significantly outperformed the conventionally used TEA and RCB-based PAM. The comparison results between pulse-echo images of the residual bubbles and cavitation images showed the potential of our proposed PAM in accurate localization of cavitation activity during HIFU therapy.

3D-printed adaptive acoustic lens as a disruptive technology for transcranial ultrasound therapy using single-element transducers

The development of multi-element arrays for better control of the shape of ultrasonic beams has opened the way for focusing through highly aberrating media, such as the human skull. As a result, the use of brain therapy with transcranial-focused ultrasound has rapidly grown. Although effective, such technology is expensive. We propose a disruptive, low-cost approach that consists of focusing a 1 MHz ultrasound beam through a human skull with a single-element transducer coupled with a tailored silicone acoustic lens cast in a 3D-printed mold and designed using computed tomography-based numerical acoustic simulation. We demonstrate on N  =  3 human skulls that adding lens-based aberration correction to a single-element transducer increases the deposited energy on the target 10 fold.

Toward a Cognitive Neural Prosthesis Using Focused Ultrasound

Non-invasive brain stimulation using focused ultrasound has many potential applications as a research and clinical tool, including its incorporation as either an extracorporeal or implantable neural prosthetic. To this end, we investigated the effect of focused ultrasound (FUS) combined with systemically administered microbubbles on visual-motor decision-making behavior in monkeys. We applied FUS to the putamen in one hemisphere to open the blood-brain barrier (BBB), and then tested behavioral performance 3–4 h later. On days when the monkeys were treated with FUS, their decisions were faster and more accurate than days without sonication. The performance improvement suggested both a shift in the decision criterion and an enhancement of the use of sensory evidence in the decision process. FUS also interacted with the effect of a low dose of haloperidol. The findings indicate that a two-minute application of FUS can have a sustained impact on performance of complex cognitive tasks, and may increase the efficacy of psychoactive medications. The results lend further support to the idea that the dorsal striatum plays an integral role in evidence- and reward-based decision-making, and provide motivation for incorporating FUS into cognitive neural prosthetic devices.

Closed-loop control of targeted ultrasound drug delivery across the blood-brain/tumor barriers in a rat glioma model

Cavitation-facilitated microbubble-mediated focused ultrasound therapy is a promising method of drug delivery across the blood-brain barrier (BBB) for treating many neurological disorders. Unlike ultrasound thermal therapies, during which magnetic resonance thermometry can serve as a reliable treatment control modality, real-time control of modulated BBB disruption with undetectable vascular damage remains a challenge. Here a closed-loop cavitation controlling paradigm that sustains stable cavitation while suppressing inertial cavitation behavior was designed and validated using a dual-transducer system operating at the clinically relevant ultrasound frequency of 274.3 kHz. Tests in the normal brain and in the F98 glioma model in vivo demonstrated that this controller enables reliable and damage-free delivery of a predetermined amount of the chemotherapeutic drug (liposomal doxorubicin) into the brain. The maximum concentration level of delivered doxorubicin exceeded levels previously shown (using uncontrolled sonication) to induce tumor regression and improve survival in rat glioma. These results confirmed the ability of the controller to modulate the drug delivery dosage within a therapeutically effective range, while improving safety control. It can be readily implemented clinically and potentially applied to other cavitation-enhanced ultrasound therapies.

Hybrid ultrasound-MR guided HIFU treatment method with 3D motion compensation

PURPOSE

Treatments using high-intensity focused ultrasound (HIFU) in the abdominal region remain challenging as a result of respiratory organ motion. A novel method is described here to achieve 3D motion-compensated ultrasound (US) MR-guided HIFU therapy using simultaneous ultrasound and MRI.

METHODS

A truly hybrid US-MR-guided HIFU method was used to plan and control the treatment. Two-dimensional ultrasound was used in real time to enable tracking of the motion in the coronal plane, whereas an MR pencil-beam navigator was used to detect anterior-posterior motion. Prospective motion compensation of proton resonance frequency shift (PRFS) thermometry and HIFU electronic beam steering were achieved.

RESULTS

The 3D prospective motion-corrected PRFS temperature maps showed reduced intrascan ghosting artifacts, a high signal-to-noise ratio, and low geometric distortion. The k-space data yielded a consistent temperature-dependent PRFS effect, matching the gold standard thermometry within approximately 1°C. The maximum in-plane temperature elevation ex vivo was improved by a factor of 2. Baseline thermometry acquired in volunteers indicated reduction of residual motion, together with an accuracy/precision of near-harmonic referenceless PRFS thermometry on the order of 0.5/1.0°C.

CONCLUSIONS

Hybrid US-MR-guided HIFU ablation with 3D motion compensation was demonstrated ex vivo together with a stable referenceless PRFS thermometry baseline in healthy volunteer liver acquisitions. Magn Reson Med 79:2511-2523, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

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.

Combined passive acoustic mapping and magnetic resonance thermometry for monitoring phase-shift nanoemulsion enhanced focused ultrasound therapy

Focused ultrasound (FUS) has the potential to enable precise, image-guided noninvasive surgery for the treatment of cancer in which tumors are identified and destroyed in a single integrated procedure. However, success of the method in highly vascular organs has been limited due to heat losses to perfusion, requiring development of techniques to locally enhance energy absorption and heating. In addition, FUS procedures are conventionally monitored using MRI, which provides excellent anatomical images and can map temperature, but is not capable of capturing the full gamut of available data such as the acoustic emissions generated during this inherently acoustically-driven procedure. Here, we employed phase-shift nanoemulsions (PSNE) embedded in tissue phantoms to promote cavitation and hence temperature rise induced by FUS. In addition, we incorporated passive acoustic mapping (PAM) alongside simultaneous MR thermometry in order to visualize both acoustic emissions and temperature rise, within the bore of a full scale clinical MRI scanner. Focal cavitation of PSNE could be resolved using PAM and resulted in accelerated heating and increased the maximum elevated temperature measured via MR thermometry compared to experiments without nanoemulsions. Over time, the simultaneously acquired acoustic and temperature maps show translation of the focus of activity towards the FUS transducer, and the magnitude of the increase in cavitation and focal shift both increased with nanoemulsion concentration. PAM results were well correlated with MRI thermometry and demonstrated greater sensitivity, with the ability to detect cavitation before enhanced heating was observed. The results suggest that PSNE could be beneficial for enhancement of thermal focused ultrasound therapies and that PAM could be a critical tool for monitoring this process.

Multi-focal HIFU reduces cavitation in mild-hyperthermia

Background

Mild-hyperthermia therapy (40–45 °C) with high-intensity focused ultrasound (HIFU) is a technique being considered in a number of different treatments such as thermally activated drug delivery, immune-stimulation, and as a chemotherapy adjuvant. Mechanical damage and loss of cell viability associated with HIFU-induced acoustic cavitation may pose a risk during these treatments or may hinder their success. Here we present a method that achieves mild heating and reduces cavitation by using a multi-focused HIFU beam. We quantify cavitation level and temperature rise in multi-focal sonications and compare it to single-focus sonications at the transducer geometric focus.

Methods

Continuous wave sonications were performed with the Sonalleve V2 transducer in gel phantoms and pork at 5, 10, 20, 40, 60, 80 acoustic watts for 30 s. Cavitation activity was measured with two ultrasound (US) imaging probes, both by computing the raw channel variance and using passive acoustic mapping (PAM). Temperature rise was measured with MR thermometry at 3 T. Cavitation and heating were compared for single- and multi-focal sonication geometries. Multi-focal sonications used four points equally spaced on a ring of either 4 mm or 8 mm diameter. Single-focus sonications were not steered.

Results

Multi-focal sonication generated distinct foci that were visible in MRI thermal maps in both phantoms and pork, and visible in PAM images in phantoms only. Cavitation activity (measured by channel variance) and mean PAM image value were highly correlated (r > 0.9). In phantoms, cavitation exponentially decreased over the 30-second sonication, consistent with depletion of cavitation nuclei. In pork, sporadic spikes signaling cavitation were observed with single focusing only. In both materials, the widest beam reduced average and peak cavitation level by a factor of two or more at each power tested when compared to a single focus. The widest beam reduced peak temperature by at least 10 °C at powers above 5 W, and created heating that was more spatially diffuse than single focus, resulting in more voxels in the mild heating (3–8 °C) range.

Conclusions

Multi-focal HIFU can be used to achieve mild temperature elevation and reduce cavitation activity.

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.

Quantitative Frequency-Domain Passive Cavitation Imaging

Passive cavitation detection has been an instrumental technique for measuring cavitation dynamics, elucidating concomitant bioeffects, and guiding ultrasound therapies. Recently, techniques have been developed to create images of cavitation activity to provide investigators with a more complete set of information. These techniques use arrays to record and subsequently beamform received cavitation emissions, rather than processing emissions received on a single-element transducer. In this paper, the methods for performing frequency-domain delay, sum, and integrate passive imaging are outlined. The method can be applied to any passively acquired acoustic scattering or emissions, including cavitation emissions. To compare data across different systems, techniques for normalizing Fourier transformed data and converting the data to the acoustic energy received by the array are described. A discussion of hardware requirements and alternative imaging approaches is additionally outlined. Examples are provided in MATLAB.