Investigation of standing-wave formation in a human skull for a clinical prototype of a large-aperture, transcranial MR-guided focused ultrasound (MRgFUS) phased array: an experimental and simulation study

Focal volume reduction in transcranial focused ultrasound using spherical wave expansions

Transcranial focused ultrasound (tFUS) has been gaining increased attention as a non-invasive modality for treating brain diseases. However, accurately focusing on brain structures remains a challenge as the ultrasound is severely distorted by the presence of the skull. In this article, we propose a promising distortion correction method based on spherical wave expansions. It is demonstrated that the focal gain is directly related to the zero-order spherical harmonic coefficient, and suppressing higher-order coefficients significantly reduces the focal volume. Simulation results show that this method can correct distortion and effectively balance focal gain and volume, achieving a smaller focal spot with lower grating lobes compared to the commonly used time reversal technique. We also verified the capability of shifting the focal position in real time without additional simulations. This work provides an effective approach for tFUS treatments, offering enhanced precision and reduced focal volume.

Treatment envelope of transcranial histotripsy: challenges and strategies to maximize the treatment location profile

A 750 kHz, 360-element ultrasound array has been built for transcranial histotripsy applications. This study aims to evaluate its performance to determine whether this array is adequate for treating a wide range of brain locations through a human skull. Treatment location profiles in 2 excised human skulls were experimentally characterized based on passive cavitation mapping. Full-wave acoustic simulations were performed in 8 human skulls to analyze the ultrasound propagation at shallow targets in skulls with different properties. Results showed that histotripsy successfully generated cavitation from deep to shallow targets within 5 mm from the skull surface in the skull with high SDR and small thickness, whereas in the skull with low SDR and large thickness, the treatment envelope was limited up to 16 mm from the skull surface. Simulation results demonstrated that the treatment envelope was highly dependent on the skull acoustic properties. Pre-focal pressure hotspots were observed in both simulation and experiments when targeting near the skull. For each skull, the acoustic pressure loss increases significantly for shallow targets compared to central targets due to high attenuation, large incident angles, and pre-focal pressure hotspots. Strategies including array design optimization, pose optimization, and amplitude correction, are proposed to broaden the treatment envelope. This study identifies the capabilities and limitations of the 360-element transcranial histotripsy array and suggests strategies for designing the next-generation transcranial histotripsy array to expand the treatment location profile for a future clinical trial.

Revealing physical interactions of ultrasound waves with the body through photoelasticity imaging

Ultrasound is a ubiquitous technology in medicine for screening, diagnosis, and treatment of disease. The functionality and efficacy of different ultrasound modes relies strongly on our understanding of the physical interactions between ultrasound waves and biological tissue structures. This article reviews the use of photoelasticity imaging for investigating ultrasound fields and interactions. Physical interactions are described for different ultrasound technologies, including those using linear and nonlinear ultrasound waves, as well as shock waves. The use of optical modulation of light by ultrasound is presented for shadowgraphic and photoelastic techniques. Investigations into shock wave and burst wave lithotripsy using photoelastic methods are summarized, along with other endoscopic forms of lithotripsy. Photoelasticity in soft tissue surrogate materials is reviewed, and its deployment in investigating tissue-bubble interactions, generated ultrasound waves, and traumatic brain injury, are discussed. With the continued growth of medical ultrasound, photoelasticity imaging can play a role in elucidating the physical mechanisms leading to useful bioeffects of ultrasound for imaging and therapy.

Cavitation monitoring, treatment strategy, and acoustic simulations of focused ultrasound blood-brain barrier disruption in patients with glioblastoma

PURPOSE

We report our experience disrupting the blood-brain barrier (BBB) to improve drug delivery in glioblastoma patients receiving temozolomide chemotherapy. The goals of this retrospective analysis were to compare MRI-based measures of BBB disruption and vascular damage to the exposure levels, acoustic emissions data, and acoustic simulations. We also simulated the cavitation detectors.

METHODS

Monthly BBB disruption (BBBD) was performed using a 220 kHz hemispherical phased array focused ultrasound system (Exablate Neuro, InSightec) and Definity microbubbles (Lantheus) over 38 sessions in nine patients. Exposure levels were actively controlled via the cavitation dose obtained by monitoring subharmonic acoustic emissions. The acoustic field and sensitivity profile of the cavitation detection system were simulated. Exposure levels and cavitation metrics were compared to the level of BBBD evident in contrast-enhanced MRI and to hypointense regions in T2*-weighted MRI.

RESULTS

Our treatment strategy evolved from using a relatively high cavitation dose goal to a lower goal and longer sonication duration and ultimately resulted in BBBD across the treatment volume with minimal petechiae. Subsonication-level feedback control of the exposure using acoustic emissions also improved consistency. Simulations of the acoustic field suggest that reflections and standing waves appear when the focus is placed near the skull, but their effects can be mitigated with aberration correction. Simulating the cavitation detectors suggest variations in the sensitivity profile across the treatment volume and between patients. A correlation was observed with the cavitation dose, BBBD and petechial hemorrhage in 8/9 patients, but substantial variability was evident. Analysis of the cavitation spectra found that most bursts did not contain wideband emissions, a signature of inertial cavitation, but biggest contribution to the cavitation dose - the metric used to control the procedure - came from bursts with wideband emissions.

CONCLUSION

Using a low subharmonic cavitation dose with a longer duration resulted in BBBD with minimal petechiae. The correlation between cavitation dose and outcomes demonstrates the benefits of feedback control based on acoustic emissions, although more work is needed to reduce variability. Acoustic simulations could improve focusing near the skull and inform our analysis of acoustic emissions. Monitoring additional frequency bands and improving the sensitivity of the cavitation detection could provide signatures of microbubble activity associated with BBB disruption that were undetected here and could improve our ability to achieve BBB disruption without vascular damage.

Global sonication of the human intracranial space via a jumbo planar transducer

Contrary to conditioning a Focused Ultrasound (FUS) beam to sonicate a localized region of the human brain, the goal of this investigation was to explore the prospect of distributing homogeneous ultrasound energy over the entire brain space with a large cranium-wide ultrasound beam. Recent ultrasound preclincal studies utilizing large or whole brain stimulation regions create a demand for expanding the treatment envelope of transcranial pulsed-low intensity ultrasound towards Global Brain Sonication (GBS) for potential human investigation. Here, we conduct ultrasound field characterizations when transmitting pulsed ultrasound through human skull specimens using a 1-3 piezocomposite planar transducer operating at 464 kHz with an active single-element surface of 30 × 30 cm. Through computational simulation and hydrophone scanning methodology, ultrasound wave behavior and dose homogeneity in the brain space were evaluated under various trajectories of sonication using the planar transducer. Clinically relevant pulse parameters used for transcranial therapeutic ultrasound applications were used in the experiments. Simulations and empirical testing revealed that dose homogeneity and acoustic intensity over the brain space are influenced by sonication trajectory, skull lens effects, and acoustic wave reflections. The transducer can emit a spatial peak pulse average intensity of 4.03 W/cm² (0.24 MPa) measured in the free-field at 464 kHz with electrical power of 1 kW. The simulation showed that approximately 99 % of the cranial volume was exposed with <30 % of the maximum external acoustic intensity being transmitted into the skull. The transmission loss across all sonication trajectories is similar to previously reported FUS studies. A marker for GBS dose homogeneity is introduced to score the ultrasound pressure field uniformity in the intracranial space. Results of this study identify the initial challenges of exposing the entire human brain space with ultrasound using a large cranium-wide sonication beam intended for global brain therapeutic modulation.

An Affordable and Easy-to-Use Focused Ultrasound Device for Noninvasive and High Precision Drug Delivery to the Mouse Brain

OBJECTIVE

Focused ultrasound (FUS) combined with microbubble-mediated blood-brain barrier (BBB) opening (FUS-BBBO) is not only a promising technique for clinical applications but also a powerful tool for preclinical research. However, existing FUS devices for preclinical research are expensive, bulky, and lack the precision needed for small animal research, which limits the broad adoption of this promising technique by the research community. Our objective was to design and fabricate an affordable, easy-to-use, high-precision FUS device for small animal research.

METHODS

We designed and fabricated in-house mini-FUS transducers (∼$80 each in material cost) with three frequencies (1.5, 3.0, and 6.0 MHz) and integrated them with a stereotactic frame for precise mouse brain targeting using established stereotactic procedures. The BBB opening volume by FUS at different acoustic pressures (0.20-0.57 MPa) was quantified using T1-weighted contrast-enhanced magnetic resonance imaging of gadolinium leakage and fluorescence imaging of Evans blue extravasation.

RESULTS

The targeting accuracy of the device as measured by the offset between the desired target location and the centroid of BBBO was 0.63 ± 0.19 mm. The spatial precision of the device in targeting individual brain structures was improved by the use of higher frequency FUS transducers. The BBB opening volume had high linear correlations with the cavitation index (defined by the ratio between acoustic pressure and frequency) and mechanical index (defined by the ratio between acoustic pressure and the square root of frequency). The correlation coefficient of the cavitation index was slightly higher than that of the mechanical index.

CONCLUSION

This study demonstrated that spatially accurate and precise BBB opening was achievable using an affordable and easy-to-use FUS device. The BBB opening volume was tunable by modulating the cavitation index. This device is expected to decrease the barriers to the adoption of the FUS-BBBO technique by the broad research community.

Feasibility of Upper Cranial Nerve Sonication in Human Application via Neuronavigated Single-Element Pulsed Focused Ultrasound

Sonicating deep brain regions with pulsed focused ultrasound using magnetic resonance imaging-guided neuronavigation single-element piezoelectric transducers is a new area of exploration for neuromodulation. Upper cranial nerves such as the trigeminal nerve and other nerves responsible for sensory/motor functions in the head may be potential targets for ultrasound pain therapy. The location of upper cranial nerves close to the skull base poses additional challenges when compared with conventional cortical or middle brain targets. In the work described here, a series of computational and empirical testing methods using human skull specimens were conducted to assess the feasibility of sonicating the trigeminal pathway near the sphenoid bone region. The results indicate a transducer with a focal length of 120 mm and diameter of 85 mm (350 kHz) can deliver sonication to upper cranial nerve regions with spatial accuracy comparable to that of focused ultrasound brain targets used in previous human studies. Temperature measurements in cortical bone and in the skull base with embedded thermocouples yield evidence of minimal bone heating. Conventional pulse parameters were found to cause reverberation interference patterns near the cranial floor; therefore, changes in pulse cycles and pulse repetition frequency were examined for reducing standing waves. Limitations and considerations for conducting ultradeep focal targeting in human applications are discussed.

Microscale concert hall acoustics to produce uniform ultrasound stimulation for targeted sonogenetics in hsTRPA1-transfected cells

The field of ultrasound neuromodulation has rapidly developed over the past decade, a consequence of the discovery of strain-sensitive structures in the membrane and organelles of cells extending into the brain, heart, and other organs. Notably, clinical trials are underway for treating epilepsy using focused ultrasound to elicit an organized local electrical response. A key limitation to this approach is the formation of standing waves within the skull. In standing acoustic waves, the maximum ultrasound intensity spatially varies from near zero to double the mean in one half a wavelength, and has lead to localized tissue damage and disruption of normal brain function while attempting to evoke a broader response. This phenomenon also produces a large spatial variation in the actual ultrasound exposure in tissue, leading to heterogeneous results and challenges with interpreting these effects. One approach to overcome this limitation is presented herein: transducer-mounted diffusers that result in spatiotemporally incoherent ultrasound. Herein, we numerically and experimentally quantified the effect of a diffuser in an enclosed domain, and show that adding the diffuser leads to a two-fold increase in ultrasound responsiveness of hsTRPA1 transfected HEK cells. Furthermore, we demonstrate the diffuser allow us to produce an uniform spatial distribution of pressure in the rodent skull. Collectively, we propose that our approach leads to a means to deliver uniform ultrasound into irregular cavities for sonogenetics.

Feasibility of ultrasound-induced blood-brain barrier disruption with a single-element transducer under three different frequencies in two non-human primates in vivo: Case report

BACKGROUND

Single-element focused transducers applied in blood-brain barrier (BBB) disruption experiments to optimize intravascular therapies in CNS diseases have the advantage of low cost and portability. Most of the in vivo studies on non-human primates report the use of single-element transducers with an annular spherical shape and a central frequency of 500 kHz. High-frequency ultrasound has smaller focal area and less standing-wave effect but lower transcranial penetration efficiency. Our study reports the feasibility and safety concerns of BBB opening by single-element spherical transducers with central frequencies of 300, 650 and 800 kHz on two rhesus macaques.

METHODS

Pulsed ultrasound exposure (3-minute duration, 0.5-1% duty cycle) combined with microbubble injection (SonoVue, 0.2uL/g) was used to disrupt the BBB of the monkeys under the magnetic resonance imaging (MRI) guidance. Gadolinium contrast-enhanced MRI was used to confirm and evaluate the BBB opening after sonication. T2-weighted fast spin echo and T2 * -weighted gradient echo sequences were used to check the post-sonication complications, such as edema and micro-bleeding.

RESULTS

Contrast enhancement was found on the post-sonication T1 weighted images for all experiments, showing that the BBB was successfully opened under all the three frequencies on both monkeys. The enhanced area was largest at the lowest frequency. No obvious hypo-intensity or hyper-intensity was observed on either the T2 * weighted gradient echo images or T2-weighted fast-spin echo images, implying the safety of the opening procedure. However, signal enhancement was also observed in the subarachnoid space of the sulci for all frequencies, indicating that the BBB was also disrupted in the propagation path outside the focal area.

CONCLUSIONS

The feasibility of BBB opening with single-element transducer under frequencies ranging from 300 kHz to 800 kHz was confirmed by experiments in two non-human primates in vivo. Further investigation into the off-target effects and transducer configurations is needed for safety optimization.

Implementation of a Skull-Conformal Phased Array for Transcranial Focused Ultrasound Therapy

OBJECTIVE

To implement a skull-conformal phased array for ultrasound-guided transcranial focused ultrasound therapy with improved patient comfort.

METHODS

Using patient-specific computed tomography and MRI neuroimaging data, tightly-conforming helmet scaffolds were designed computationally. The helmet scaffolds were designed to hold reusable transducer modules at near-normal incidence in an optimal configuration for the treatment location(s) of interest. Numerical simulations of trans-skull ultrasound propagation were performed to evaluate different conformal array designs and to compare with hemispherical arrays similar to those employed clinically. A 4096-element phased array was constructed by 3D printing a helmet scaffold optimised for an ex vivo human skullcap, and its performance was evaluated via benchtop and in vivo experiments.

RESULTS

Acoustic field measurements confirmed the system's ability to focus through human skull bone using simulation-based transcranial aberration corrections. Preliminary in vivo testing demonstrated safe trans-human skull blood-brain barrier (BBB) opening in rodents.

CONCLUSION

Patient-specific conformal ultrasound phased arrays appear to be a feasible and safe approach for conducting transcranial BBB opening procedures.

SIGNIFICANCE

Skull-conformal phased arrays stand to improve patient comfort and have the potential to accelerate the adoption of transcranial FUS therapy by improving access to the technology.

A Generalized Split-Step Angular Spectrum Method for Efficient Simulation of Wave Propagation in Heterogeneous Media

Angular spectrum (AS) methods enable efficient calculation of wave propagation from one plane to another inside homogeneous media. For wave propagation in heterogeneous media such as biological tissues, AS methods cannot be applied directly. Split-stepping techniques decompose the heterogeneous domain into homogeneous and perturbation parts, and provide a solution for forward wave propagation by propagating the incident wave in both frequency-space and frequency-wavenumber domains. Recently, a split-step hybrid angular spectrum (HAS) method was proposed for plane wave propagation of focused ultrasound beams. In this study, we extend these methods to enable simulation of acoustic pressure field for an arbitrary source distribution, by decomposing the source and reflection spectra into orthogonal propagation direction components, propagating each component separately, and summing all components to get the total field. We show that our method can efficiently simulate the pressure field of arbitrary sources in heterogeneous media. The accuracy of the method was analyzed comparing the resultant pressure field with pseudospectral time domain (PSTD) solution for breast tomography and hemispherical transcranial-focused ultrasound simulation models. Eighty times acceleration was achieved for a 3-D breast simulation model compared to PSTD solution with 0.005 normalized root mean-squared difference (NRMSD) between two solutions. For the hemispherical phased array, aberrations due to skull were accurately calculated in a single simulation run as evidenced by the resultant-focused ultrasound beam simulations, which had 0.001 NRMSD with 40 times acceleration factor compared to the PSTD method.

Comparison Between Ray-Tracing and Full-Wave Simulation for Transcranial Ultrasound Focusing on a Clinical System Using the Transfer Matrix Formalism

Only one high-intensity focused ultrasound device has been clinically approved for transcranial brain surgery at the time of writing. The device operates within 650 and 720 kHz and corrects the phase distortions induced by the skull of each patient using a multielement phased array. Phase correction is estimated adaptively using a proprietary algorithm based on computed-tomography (CT) images of the patient's skull. In this article, we assess the performance of the phase correction computed by the clinical device and compare it to: 1) the correction obtained with a previously validated full-wave simulation algorithm using an open-source pseudo-spectral toolbox and 2) a hydrophone-based correction performed invasively to measure the aberrations induced by the skull at 650 kHz. For the full-wave simulation, three different mappings between CT Hounsfield units and the longitudinal speed of sound inside the skull were tested. All methods are compared with the exact same setup due to transfer matrices acquired with the clinical system for N = 5 skulls and T = 2 different targets for each skull. We show that the clinical ray-tracing software and the full-wave simulation restore, respectively, 84% ± 5% and 86% ± 5% of the pressure obtained with hydrophone-based correction for targets located in central brain regions. On the second target (off-center), we also report that the performance of both algorithms degrades when the average incident angles of the acoustic beam at the skull surface increase. When incident angles are higher than 20°, the restored pressure drops below 75% of the pressure restored with hydrophone-based correction.

Transcranial focused ultrasound stimulation with high spatial resolution

BACKGROUND

Low-intensity transcranial focused ultrasound stimulation is a promising candidate for noninvasive brain stimulation and accurate targeting of brain circuits because of its focusing capability and long penetration depth. However, achieving a sufficiently high spatial resolution to target small animal sub-regions is still challenging, especially in the axial direction.

OBJECTIVE

To achieve high axial resolution, we designed a dual-crossed transducer system that achieved high spatial resolution in the axial direction without complex microfabrication, beamforming circuitry, and signal processing.

METHODS

High axial resolution was achieved by crossing two ultrasound beams of commercially available piezoelectric curved transducers at the focal length of each transducer. After implementation of the fixture for the dual-crossed transducer system, three sets of in vivo animal experiments were conducted to demonstrate high target specificity of ultrasound neuromodulation using the dual-crossed transducer system (n = 38).

RESULTS

The full-width at half maximum (FWHM) focal volume of our dual-crossed transducer system was under 0.52 μm³. We report a focal diameter in both lateral and axial directions of 1 mm. To demonstrate successful in vivo brain stimulation of wild-type mice, we observed the movement of the forepaws. In addition, we targeted the habenula and verified the high spatial specificity of our dual-crossed transducer system.

CONCLUSIONS

Our results demonstrate the ability of the dual-crossed transducer system to target highly specific regions of mice brains using ultrasound stimulation. The proposed system is a valuable tool to study the complex neurological circuitry of the brain noninvasively.

Measurement and simulation of steered acoustic fields generated by a multielement array for therapeutic ultrasound

Modelling of fields generated by therapeutic ultrasound arrays can be prone to errors arising from differences from nominal transducer parameters, and variations in relative outputs of array elements when driven under different conditions, especially when simulating steered fields. Here, the effect of element size, element positions, relative source pressure variations, and electrical crosstalk on the accuracy of modelling pressure fields generated by a 555 kHz 32-element ultrasonic array were investigated. For this transducer, errors in pressure amplitude and focal position were respectively reduced from 20% to 4% and 3.3 mm to 1.5 mm using crosstalk prediction, and experimentally determined positions.

Computationally Efficient Transcranial Ultrasonic Focusing: Taking Advantage of the High Correlation Length of the Human Skull

The phase correction necessary for transcranial ultrasound therapy requires numerical simulation to noninvasively assess the phase shift induced by the skull bone. Ideally, the numerical simulations need to be fast enough for clinical implementation in a brain therapy protocol and to provide accurate estimation of the phase shift to optimize the refocusing through the skull. In this article, we experimentally performed transcranial ultrasound focusing at 900 kHz on N = 5 human skulls. To reduce the computation time, we propose here to perform the numerical simulation at 450 kHz and use the corresponding phase shifts experimentally at 900 kHz. We demonstrate that a 450-kHz simulation restores 94.2% of the pressure when compared with a simulation performed at 900 kHz and 85.0% of the gold standard pressure obtained by an invasive time reversal procedure based on the signal recorded by a hydrophone placed at the target. From a 900- to 450-kHz simulation, the grid size is divided by 8, and the computation time is divided by 10.

Magnetic resonance-guided, high-intensity focused ultrasound sonolysis: potential applications for stroke

Stroke is one of the leading causes of death worldwide and a significant source of long-term morbidity. Unfortunately, a substantial number of stroke patients either are ineligible or do not significantly benefit from contemporary medical and interventional therapies. To address this void, investigators recently made technological advances to render transcranial MR-guided, high-intensity focused ultrasound (MRg-HIFU) sonolysis a potential therapeutic option for both acute ischemic stroke (AIS)-as an alternative for patients with emergent large-vessel occlusion (ELVO) who are ineligible for endovascular mechanical thrombectomy (EMT) or as salvage therapy for patients in whom EMT fails-and intracerebral hemorrhage (ICH)-as a neoadjuvant means of clot lysis prior to surgical evacuation. Herein, the authors review the technological principles behind MRg-HIFU sonolysis, its results in in vitro and in vivo stroke models, and its potential clinical applications. As a noninvasive transcranial technique that affords rapid clot lysis, MRg-HIFU thrombolysis may develop into a therapeutic option for patients with AIS or ICH. However, additional studies of transcranial MRg-HIFU are necessary to ascertain the merit of this treatment approach for thrombolysis in both AIS and ICH, as well as its technical limitations and risks.

Intracranial Non-thermal Ablation Mediated by Transcranial Focused Ultrasound and Phase-Shift Nanoemulsions

High intensity focused ultrasound (HIFU) mechanical ablation is an emerging technique for non-invasive transcranial surgery. Lesions are created by driving inertial cavitation in tissue, which requires significantly less peak pressure and time-averaged power compared with traditional thermal ablation. The utility of mechanical ablation could be extended to the brain provided the pressure threshold for inertial cavitation can be reduced. In this study, the utility of perfluorobutane (PFB)-based phase-shift nanoemulsions (PSNEs) for lowering the inertial cavitation threshold and enabling focal mechanical ablation in the brain was investigated. We successfully achieved vaporization of PFB-based PSNEs at 1.8 MPa with a 740 kHz focused transducer with a pulsed sonication protocol (duty cycle = 1.5%, 10 min sonication) within intact CD-1 mice brains. Evidence is provided showing that a single bolus injection of PSNEs could be used to initiate and sustain inertial cavitation in cerebrovasculature for at least 10 min. Histologic analysis of brain slices after HIFU exposure revealed ischemic and hemorrhagic lesions with dimensions that were comparable to the focal zone of the transducer. These results suggest that PFB-based PSNEs may be used to significantly reduce the inertial cavitation threshold in the cerebrovasculature and, when combined with transcranial focused ultrasound, enable focal intracranial mechanical ablation.

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.

Improved scheduling algorithm for signal processing in asynchronous distributed ultrasonic total-focusing-method system

Compared to the conventional ultrasonic phased-array system, a large-element phased-array system employing the total focusing method (TFM) can yield improved image resolution and accuracy, providing more flexible scanning methods and image merging functionality. In order to meet various forms of ultrasonic multi-group scanning, an architecture for multi-group scan integration called the "asynchronous distributed ultrasonic TFM system" is proposed, and a novel scheduling algorithm called "the sum of start time and processing time adjacent (SSPA) algorithm" is presented. The architecture adds a focus and group scheduler (FGS) and signal processing scheduler (SPS) to the traditional ultrasonic phased array system and constructs a signal processing arbitration (SPA) with several signal processing modules (SPMs). The FGS provides the focus parameters, pixel memory range, and number of pixels in each group. The SPS controls the SPA for the ultrasonic scanning data obtained from the elements, with SPM-sharing output data; hence, the optimal priority order and SPM assignment are realized, enabling switching of reading operations among the first-in-first-out memories for signal processing and minimal time-slot waiting. The SSPA algorithm is used to solve the job-shop scheduling problem with start time, which considers the processing time and start time, in order to reduce the time slot after each scheduling using adjacent operations. Therefore, the architecture enhances the flexibility of the multi-group scan, and this algorithm decreases the makespan, achieving higher efficiency compared to conventional scheduling algorithms. The reliability and validity of the algorithm are substantiated after its implementation using FPGA technology. The SPM utilization rate and the real-time performance of the ultrasonic TFM are improved. Thus, the proposed algorithm and architecture have considerable potential application in multi-sensor systems.

Analysis of Multifrequency and Phase Keying Strategies for Focusing Ultrasound to the Human Vertebral Canal

Focused ultrasound has been shown to increase the permeability of the blood-brain barrier and its feasibility for opening the blood-spinal cord barrier has also been demonstrated in small animal models, with great potential to impact the treatment of spinal cord (SC) disorders. For clinical translation, challenges to transvertebral focusing of ultrasound energy on the human spinal canal, such as a focal depth of field and standing-wave formation, must be addressed. A dual-aperture approach using multifrequency and phase-shift keying (PSK) strategies for achieving a controlled focus in human thoracic vertebrae was investigated through numerical simulations and benchtop experiments in ex vivo human vertebrae. An ~85% reduction in the focal depth of field was achieved compared to a single-aperture approach at 564 kHz. Short-burst (two-cycle) excitations in combination with PSK were found to suppress the formation of standing waves in ex vivo human thoracic vertebrae when focusing through the vertebral laminae. The results make an important contribution toward the development of a clinical-scale approach for targeting ultrasound therapy to the SC.

MRI-Guided High-Intensity Focused Ultrasound as an Emerging Therapy for Stroke: A Review

Stroke, either ischemic or hemorrhagic, accounts for significantly high morbidity and mortality rates around the globe effecting millions of lives annually. For the past few decades, ultrasound has been extensively investigated to promote clot lysis for the treatment of stroke, myocardial infarction, and acute peripheral arterial occlusions, with or without the use of tPA or contrast agents. In the age of modern minimal invasive techniques, magnetic resonance imaging-guided high-intensity focused ultrasound is a new emerging modality that seems to promise therapeutic utilities for both ischemic and hemorrhagic stroke. High-intensity focused ultrasound causes thermal heating as the tissue absorbs the mechanical energy transmitted by the ultrasonic waves leading to tissue denaturation and coagulation. Several in-vitro and in-vivo studies have demonstrated the viability of this technology for sonothrombolysis in both types of stroke and have warranted clinical trials. Apart from safety and efficacy, initiation of trials would further enable answers regarding its practical application in a clinical setup. Though this technology has been under study for treatment of various brain diseases for some decades now, relatively very few neurologists and even neurosurgeons seem to be acquainted with it. The aim of this review is to provide basic understanding of this powerful technology and discuss its clinical application and potential role as an emerging viable therapeutic option for the future management of stroke.

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.

Numerical evaluation of the skull for human neuromodulation with transcranial focused ultrasound

OBJECTIVE

Transcranial focused ultrasound is an emerging field for human non-invasive neuromodulation, but its dosing in humans is difficult to know due to the skull. The objective of the present study was to establish modeling methods based on medical images to assess skull differences between individuals on the wave propagation of ultrasound.

APPROACH

Computational models of transcranial focused ultrasound were constructed using CT and MR scans to solve for intracranial pressure. We explored the effect of including the skull base in models, different transducer placements on the head, and differences between 250 kHz or 500 kHz acoustic frequency for both female and male models. We further tested these features using linear, nonlinear, and elastic simulations. To better understand inter-subject skull thickness and composition effects we evaluated the intracranial pressure maps between twelve individuals at two different skull sites.

MAIN RESULTS

Nonlinear acoustic simulations resulted in virtually identical intracranial pressure maps with linear acoustic simulations. Elastic simulations showed a difference in max pressures and full width half maximum volumes of 15% at most. Ultrasound at an acoustic frequency of 250 kHz resulted in the creation of more prominent intracranial standing waves compared to 500 kHz. Finally, across twelve model human skulls, a significant linear relationship to characterize intracranial pressure maps was not found.

SIGNIFICANCE

Despite its appeal, an inherent problem with the use of a noninvasive transcranial ultrasound method is the difficulty of knowing intracranial effects because of the skull. Here we develop detailed computational models derived from medical images of individuals to simulate the propagation of neuromodulatory ultrasound across the skull and solve for intracranial pressure maps. These methods allow for a much better understanding of the intracranial effects of ultrasound for an individual in order to ensure proper targeting and more tightly control dosing.

A convenient, reliable, and fast acoustic pressure field measurement method for magnetic resonance-guided high-intensity focused ultrasound systems with phased array transducers

BACKGROUND

With the expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), there is an urgent need for a convenient, reliable, and fast acoustic pressure field measurement method to aid treatment protocol design, ensure consistent and safe operation of the transducer, and facilitate regulatory approval of new techniques. Herein, we report a method for acoustic pressure field characterization of MR-HIFU systems with multi-element phased array transducers. This method integrates fiber-optic hydrophone measurements and electronic steering of the ultrasound beam with MRI-assisted HIFU focus alignment to the fiber tip.

METHODS

A clinical MR-HIFU system (Sonalleve V2, Profound Medical Inc., Mississauga, Canada) was used to assess the proposed method. A fiber-optic hydrophone was submerged in a degassed water bath, and the fiber tip location was traced using MRI. Subsequently, the nominal transducer focal point indicated on the MR-HIFU therapy planning software was positioned at the fiber tip, and the HIFU focus was electronically steered around the fiber tip within a 3D volume for 3D pressure field mapping, eliminating the need for an additional, expensive, and MRI-compatible 3D positioning stage. The peak positive and negative pressures were measured at the focus and validated using a standard hydrophone measurement setup outside the MRI magnet room.

RESULTS

We found that the initial MRI-assisted HIFU focus alignment had an average offset of 2.23 ± 1.33 mm from the fiber tip as identified by the 3D pressure field mapping. MRI guidance and electronic beam steering allowed 3D focus localization within ~ 1 h, i.e., faster than the typical time required using the standard laboratory setup (~ 3-4 h). Acoustic pressures measured using the proposed method were not significantly different from those obtained with the standard laboratory hydrophone measurements.

CONCLUSIONS

In conclusion, our method offers a convenient, reliable, and fast acoustic pressure field characterization tool for MR-HIFU systems with phased array transducers.

The reduction in treatment efficiency at high acoustic powers during MR-guided transcranial focused ultrasound thalamotomy for Essential Tremor

PURPOSE

To analyze clinical data indicating a reduction in the induced energy-temperature efficiency relationship during transcranial focused ultrasound (FUS) Essential Tremor (ET) thalamotomy treatments at higher acoustic powers, establish its relationship with the spatial distribution of the focal temperature elevation, and explore its cause.

METHODS

A retrospective observational study of patients (n = 19) treated between July 2015 and August 2016 for (ET) by FUS thalamotomy was performed. These data were analyzed to compare the relationships between the applied power, the applied energy, the resultant peak temperature achieved in the brain, and the dispersion of the focal volume. Full ethics approval was received and all patients provided signed informed consent forms before the initiation of the study. Computer simulations, animal experiments, and clinical system tests were performed to determine the effects of skull heating, changes in brain properties and transducer acoustic output, respectively. All animal procedures were approved by the Animal Care and Use Committee and conformed to the guidelines set out by the Canadian Council on Animal Care. MATLAB was used to perform statistical analysis.

RESULTS

The reduction in the energy efficiency relationship during treatment correlates with the increase in size of the focal volume at higher sonication powers. A linear relationship exists showing that a decrease in treatment efficiency correlates positively with an increase in the focal size over the course of treatment (P < 0.01), supporting the hypothesis of transient skull and tissue heating causing acoustic aberrations leading to a decrease in efficiency. Changes in thermal conductivity, perfusion, absorption rates in the brain, as well as ultrasound transducer acoustic output levels were found to have minimal effects on the observed reduction in efficiency.

CONCLUSIONS

The reduction in energy-temperature efficiency during high-power FUS treatments correlated with observed increases in the size of the focal volume and is likely caused by transient changes in the tissue and skull during heating.

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.

An Improved Scheduling Algorithm for Data Transmission in Ultrasonic Phased Arrays with Multi-Group Ultrasonic Sensors

High data transmission efficiency is a key requirement for an ultrasonic phased array with multi-group ultrasonic sensors. Here, a novel FIFOs scheduling algorithm was proposed and the data transmission efficiency with hardware technology was improved. This algorithm includes FIFOs as caches for the ultrasonic scanning data obtained from the sensors with the output data in a bandwidth-sharing way, on the basis of which an optimal length ratio of all the FIFOs is achieved, allowing the reading operations to be switched among all the FIFOs without time slot waiting. Therefore, this algorithm enhances the utilization ratio of the reading bandwidth resources so as to obtain higher efficiency than the traditional scheduling algorithms. The reliability and validity of the algorithm are substantiated after its implementation in the field programmable gate array (FPGA) technology, and the bandwidth utilization ratio and the real-time performance of the ultrasonic phased array are enhanced.

Design of patient-specific focused ultrasound arrays for non-invasive brain therapy with increased trans-skull transmission and steering range

The use of a phased array of ultrasound transducer elements to sonicate through the skull has opened the way for new treatments and the delivery of therapeutics beyond the blood-brain barrier. The limited steering range of current clinical devices, particularly at higher frequencies, limits the regions of the brain that are considered treatable by ultrasound. A new array design is introduced that allows for high levels of beam steering and increased transmission throughout the brain. These improvements are achieved using concave transducers normal to the outer-skull surface in a patient-specific configuration to target within the skull, so that the far-field of each beam is within the brain. It is shown that by using pulsed ultrasound waves timed to arrive in-phase at the desired target, sufficient levels of acoustic energy are delivered for blood-brain barrier opening throughout the brain.

An uncovered risk factor of sonothrombolysis: Substantial fluctuation of ultrasound transmittance through the human skull

Sonothrombolysis is one of the most feasible methods for enhancing clot lysis with a recombinant tissue plasminogen activator (rt-PA) in cases of acute ischemic strokes. For safe and efficient clinical practices of sonothrombolysis, accurate estimation of ultrasound transmittance through the human skull is critical. Previously, we reported substantial and periodic fluctuation of ultrasound transmittance through a bone-phantom plate following changes to ultrasound frequency, the thickness of the bone-phantom plate, and the distance between a transducer and the bone-phantom plate. In the present study, we clarify the transmittance behavior of medium-frequency ultrasound (from 400kHz to 600kHz) through the human skull, and examine reduction of the transmittance fluctuation. For the study, we measured transmittance of sinusoidal ultrasound waves at 400kHz, 500kHz, and 600kHz at 13 temple spots on 3 human skulls by changing the distance between a transducer and the skull bone, and found substantial and periodic fluctuation in the transmittance behaviors for these sinusoidal voltage excitations. Degrees of the fluctuation varied depending on the measurement spots. A fluctuation ratio between the maximum transmittance and the minimum transmittance reached 3 in some spots. This large transmittance fluctuation is considered to be a risk factor for sonothrombolysis therapies. We examined a modulated ultrasound wave to reduce the fluctuation, and succeeded in obtaining considerable reduction. The average fluctuation ratios for 400-kHz, 500-kHz, and 600-kHz waves were 2.38, 2.38, and 2.07, respectively. We successfully reduced the ratio to 1.72 by using a periodic selection of random frequency (PSRF)-type of modulation wave. The thus obtained results indicate that attention to the fluctuation in ultrasound transmittance through the skull is necessary for safe and effective sonothrombolysis therapies, and that modulated ultrasound waves constitute a powerful method for reducing the risk of fluctuation.

Ex vivo optimisation of a heterogeneous speed of sound model of the human skull for non-invasive transcranial focused ultrasound at 1 MHz

Transcranial brain therapy has recently emerged as a non-invasive strategy for the treatment of various neurological diseases, such as essential tremor or neurogenic pain. However, treatments require millimetre-scale accuracy. The use of high frequencies (typically ≥1 MHz) decreases the ultrasonic wavelength to the millimetre scale, thereby increasing the clinical accuracy and lowering the probability of cavitation, which improves the safety of the technique compared with the use of low-frequency devices that operate at 220 kHz. Nevertheless, the skull produces greater distortions of high-frequency waves relative to low-frequency waves. High-frequency waves require high-performance adaptive focusing techniques, based on modelling the wave propagation through the skull. This study sought to optimise the acoustical modelling of the skull based on computed tomography (CT) for a 1 MHz clinical brain therapy system. The best model tested in this article corresponded to a maximum speed of sound of 4000 m.s^-1 in the skull bone, and it restored 86% of the optimal pressure amplitude on average in a collection of six human skulls. Compared with uncorrected focusing, the optimised non-invasive correction led to an average increase of 99% in the maximum pressure amplitude around the target and an average decrease of 48% in the distance between the peak pressure and the selected target. The attenuation through the skulls was also assessed within the bandwidth of the transducers, and it was found to vary in the range of 10 ± 3 dB at 800 kHz and 16 ± 3 dB at 1.3 MHz.

A Tikhonov Regularization Scheme for Focus Rotations With Focused Ultrasound-Phased Arrays

Phased arrays have a wide range of applications in focused ultrasound therapy. By using an array of individually driven transducer elements, it is possible to steer a focus through space electronically and compensate for acoustically heterogeneous media with phase delays. In this paper, the concept of focusing an ultrasound-phased array is expanded to include a method to control the orientation of the focus using a Tikhonov regularization scheme. It is then shown that the Tikhonov regularization parameter used to solve the ill-posed focus rotation problem plays an important role in the balance between quality focusing and array efficiency. Finally, the technique is applied to the synthesis of multiple foci, showing that this method allows for multiple independent spatial rotations.

Nonthermal ablation of deep brain targets: A simulation study on a large animal model

PURPOSE

Thermal ablation with transcranial MRI-guided focused ultrasound (FUS) is currently limited to central brain targets because of heating and other beam effects caused by the presence of the skull. Recently, it was shown that it is possible to ablate tissues without depositing thermal energy by driving intravenously administered microbubbles to inertial cavitation using low-duty-cycle burst sonications. A recent study demonstrated that this ablation method could ablate tissue volumes near the skull base in nonhuman primates without thermally damaging the nearby bone. However, blood-brain disruption was observed in the prefocal region, and in some cases, this region contained small areas of tissue damage. The objective of this study was to analyze the experimental model with simulations and to interpret the cause of these effects.

METHODS

The authors simulated prior experiments where nonthermal ablation was performed in the brain in anesthetized rhesus macaques using a 220 kHz clinical prototype transcranial MRI-guided FUS system. Low-duty-cycle sonications were applied at deep brain targets with the ultrasound contrast agent Definity. For simulations, a 3D pseudospectral finite difference time domain tool was used. The effects of shear mode conversion, focal steering, skull aberrations, nonlinear propagation, and the presence of skull base on the pressure field were investigated using acoustic and elastic wave propagation models.

RESULTS

The simulation results were in agreement with the experimental findings in the prefocal region. In the postfocal region, however, side lobes were predicted by the simulations, but no effects were evident in the experiments. The main beam was not affected by the different simulated scenarios except for a shift of about 1 mm in peak position due to skull aberrations. However, the authors observed differences in the volume, amplitude, and distribution of the side lobes. In the experiments, a single element passive cavitation detector was used to measure the inertial cavitation threshold and to determine the pressure amplitude to use for ablation. Simulations of the detector's acoustic field suggest that its maximum sensitivity was in the lower part of the main beam, which may have led to excessive exposure levels in the experiments that may have contributed to damage in the prefocal area.

CONCLUSIONS

Overall, these results suggest that case-specific full wave simulations before the procedure can be useful to predict the focal and the prefocal side lobes and the extent of the resulting bioeffects produced by nonthermal ablation. Such simulations can also be used to optimally position passive cavitation detectors. The disagreement between the simulations and the experiments in the postfocal region may have been due to shielding of the ultrasound field due to microbubble activity in the focal region. Future efforts should include the effects of microbubble activity and vascularization on the pressure field.

Preclinical evaluation of a low-frequency transcranial MRI-guided focused ultrasound system in a primate model

This study investigated thermal ablation and skull-induced heating with a 230 kHz transcranial MRI-guided focused ultrasound (TcMRgFUS) system in nonhuman primates. We evaluated real-time acoustic feedback and aimed to understand whether cavitation contributed to the heating and the lesion formation. In four macaques, we sonicated thalamic targets at acoustic powers of 34-560 W (896-7590 J). Tissue effects evaluated with MRI and histology were compared to MRI-based temperature and thermal dose measurements, acoustic emissions recorded during the experiments, and acoustic and thermal simulations. Peak temperatures ranged from 46 to 57 °C, and lesions were produced in 5/8 sonicated targets. A linear relationship was observed between the applied acoustic energy and both the focal and brain surface heating. Thermal dose thresholds were 15-50 cumulative equivalent minutes at 43 °C, similar to prior studies at higher frequencies. Histology was also consistent with earlier studies of thermal effects in the brain. The system successfully controlled the power level and maintained a low level of cavitation activity. Increased acoustic emissions observed in 3/4 animals occurred when the focal temperature rise exceeded approximately 16 °C. Thresholds for thermally-significant subharmonic and wideband emissions were 129 and 140 W, respectively, corresponding to estimated pressure amplitudes of 2.1 and 2.2 MPa. Simulated focal heating was consistent with the measurements for sonications without thermally-significant acoustic emissions; otherwise it was consistently lower than the measurements. Overall, these results suggest that the lesions were produced by thermal mechanisms. The detected acoustic emissions, however, and their association with heating suggest that cavitation might have contributed to the focal heating. Compared to earlier work with a 670 kHz TcMRgFUS system, the brain surface heating was substantially reduced and the focal heating was higher with this 230 kHz system, suggesting that a reduced frequency can increase the treatment envelope for TcMRgFUS and potentially reduce the risk of skull heating.

Cavitation-enhanced nonthermal ablation in deep brain targets: feasibility in a large animal model

OBJECT Transcranial MRI-guided focused ultrasound (TcMRgFUS) is an emerging noninvasive alternative to surgery and radiosurgery that is undergoing testing for tumor ablation and functional neurosurgery. The method is currently limited to central brain targets due to skull heating and other factors. An alternative ablative approach combines very low intensity ultrasound bursts and an intravenously administered microbubble agent to locally destroy the vasculature. The objective of this work was to investigate whether it is feasible to use this approach at deep brain targets near the skull base in nonhuman primates. METHODS In 4 rhesus macaques, targets near the skull base were ablated using a clinical TcMRgFUS system operating at 220 kHz. Low-duty-cycle ultrasound exposures (sonications) were applied for 5 minutes in conjunction with the ultrasound contrast agent Definity, which was administered as a bolus injection or continuous infusion. The acoustic power level was set to be near the inertial cavitation threshold, which was measured using passive monitoring of the acoustic emissions. The resulting tissue effects were investigated with MRI and with histological analysis performed 3 hours to 1 week after sonication. RESULTS Thirteen targets were sonicated in regions next to the optic tract in the 4 animals. Inertial cavitation, indicated by broadband acoustic emissions, occurred at acoustic pressure amplitudes ranging from 340 to 540 kPa. MRI analysis suggested that the lesions had a central region containing red blood cell extravasations that was surrounded by edema. Blood-brain barrier disruption was observed on contrast-enhanced MRI in the lesions and in a surrounding region corresponding to the prefocal area of the FUS system. In histology, lesions consisting of tissue undergoing ischemic necrosis were found in all regions that were sonicated above the inertial cavitation threshold. Tissue damage in prefocal areas was found in several cases, suggesting that in those cases the sonication exceeded the inertial cavitation threshold in the beam path. CONCLUSIONS It is feasible to use a clinical TcMRgFUS system to ablate skull base targets in nonhuman primates at time-averaged acoustic power levels at least 2 orders of magnitude below what is needed for thermal ablation with this device. The results point to the risks associated with the method if the exposure levels are not carefully controlled to avoid inertial cavitation in the acoustic beam path. If methods can be developed to provide this control, this nonthermal approach could greatly expand the use of TcMRgFUS for precisely targeted ablation to locations across the entire brain.

Full-wave acoustic and thermal modeling of transcranial ultrasound propagation and investigation of skull-induced aberration correction techniques: a feasibility study

Background

Transcranial focused ultrasound (tcFUS) is an attractive noninvasive modality for neurosurgical interventions. The presence of the skull, however, compromises the efficiency of tcFUS therapy, as its heterogeneous nature and acoustic characteristics induce significant distortion of the acoustic energy deposition, focal shifts, and thermal gain decrease. Phased-array transducers allow for partial compensation of skull-induced aberrations by application of precalculated phase and amplitude corrections.

Methods

An integrated numerical framework allowing for 3D full-wave, nonlinear acoustic and thermal simulations has been developed and applied to tcFUS. Simulations were performed to investigate the impact of skull aberrations, the possibility of extending the treatment envelope, and adverse secondary effects. The simulated setup comprised an idealized model of the ExAblate Neuro and a detailed MR-based anatomical head model. Four different approaches were employed to calculate aberration corrections (analytical calculation of the aberration corrections disregarding tissue heterogeneities; a semi-analytical ray-tracing approach compensating for the presence of the skull; two simulation-based time-reversal approaches with and without pressure amplitude corrections which account for the entire anatomy). These impact of these approaches on the pressure and temperature distributions were evaluated for 22 brain-targets

Results

While (semi-)analytical approaches failed to induced high pressure or ablative temperatures in any but the targets in the close vicinity of the geometric focus, simulation-based approaches indicate the possibility of considerably extending the treatment envelope (including targets below the transducer level and locations several centimeters off the geometric focus), generation of sharper foci, and increased targeting accuracy. While the prediction of achievable aberration correction appears to be unaffected by the detailed bone-structure, proper consideration of inhomogeneity is required to predict the pressure distribution for given steering parameters

Conclusions

Simulation-based approaches to calculate aberration corrections may aid in the extension of the tcFUS treatment envelope as well as predict and avoid secondary effects (standing waves, skull heating). Due to their superior performance, simulationbased techniques may prove invaluable in the amelioration of skull-induced aberration effects in tcFUS therapy. The next steps are to investigate shear-wave-induced effects in order to reliably exclude secondary hot-spots, and to develop comprehensive uncertainty assessment and validation procedures.

Substantial fluctuation of acoustic intensity transmittance through a bone-phantom plate and its equalization by modulation of ultrasound frequency

For safe and efficient sonothrombolysis therapies, accurate estimation of ultrasound transmittance through the human skull is essential. The present study clarifies uncertainty surrounding this transmittance and experimentally verifies the equalization of transmittance through the modulation of ultrasound frequency. By changing three factors (ultrasound frequency, the thickness of a bone-phantom plate, and the distance between a transducer and a bone-phantom plate), we measured the intensity of ultrasound passing through the plate. Two activating methods, sinusoidal waves at 500 kHz and modulated waves, were compared. When we changed (1) the distance between a transducer and a bone-phantom plate and (2) the thickness of the bone-phantom plate, ultrasound transmittance through the plates substantially fluctuated. The substantial fluctuation in transmittance was observed also for a cut piece of human temporal skull bone. This fluctuation significantly declined for the modulated wave. In conclusion, modulation of ultrasound frequency can equalize the transmittance with an approximately 30-65% fluctuation drop and an approximately 40% fluctuation drop for a bone-phantom plate and for a cut piece of skull bone, respectively. By using modulated waves, we can develop safer and more effective sonothrombolysis therapies.

High-definition transcranial direct current stimulation induces both acute and persistent changes in broadband cortical synchronization: a simultaneous tDCS-EEG study

The goal of this study was to develop methods for simultaneously acquiring electrophysiological data during high-definition transcranial direct current stimulation (tDCS) using high-resolution electroencephalography (EEG). Previous studies have pointed to the after-effects of tDCS on both motor and cognitive performance, and there appears to be potential for using tDCS in a variety of clinical applications. However, little is known about the real-time effects of tDCS on rhythmic cortical activity in humans due to the technical challenges of simultaneously obtaining electrophysiological data during ongoing stimulation. Furthermore, the mechanisms of action of tDCS in humans are not well understood. We have conducted a simultaneous tDCS-EEG study in a group of healthy human subjects. Significant acute and persistent changes in spontaneous neural activity and event-related synchronization (ERS) were observed during and after the application of high-definition tDCS over the left sensorimotor cortex. Both anodal and cathodal stimulation resulted in acute global changes in broadband cortical activity which were significantly different than the changes observed in response to sham stimulation. For the group of eight subjects studied, broadband individual changes in spontaneous activity during stimulation were apparent both locally and globally. In addition, we found that high-definition tDCS of the left sensorimotor cortex can induce significant ipsilateral and contralateral changes in event-related desynchronization and ERS during motor imagination following the end of the stimulation period. Overall, our results demonstrate the feasibility of acquiring high-resolution EEG during high-definition tDCS and provide evidence that tDCS in humans directly modulates rhythmic cortical synchronization during and after its administration.

Design and experimental evaluation of a 256-channel dual-frequency ultrasound phased-array system for transcranial blood-brain barrier opening and brain drug delivery

Focused ultrasound (FUS) in the presence of microbubbles can bring about transcranial and local opening of the blood-brain barrier (BBB) for potential noninvasive delivery of drugs to the brain. A phased-array ultrasound system is essential for FUS-BBB opening to enable electronic steering and correction of the focal beam which is distorted by cranial bone. Here, we demonstrate our prototype design of a 256-channel ultrasound phased-array system for large-region transcranial BBB opening in the brains of large animals. One of the unique features of this system is the capability of generating concurrent dual-frequency ultrasound signals from the driving system for potential enhancement of BBB opening. A wide range of signal frequencies can be generated (frequency = 0.2-1.2 MHz) with controllable driving burst patterns. Precise output power can be controlled for individual channels via 8-bit duty-cycle control of transistor-transistor logic signals and the 8-bit microcontroller-controlled buck converter power supply output voltage. The prototype system was found to be in compliance with the electromagnetic compatibility standard. Moreover, large animal experiments confirmed the phase switching effectiveness of this system, and induction of either a precise spot or large region of BBB opening through fast focal-beam switching. We also demonstrated the capability of dual-frequency exposure to potentially enhance the BBB-opening effect. This study contributes to the design of ultrasound phased arrays for future clinical applications, and provides a new direction toward optimizing FUS brain drug delivery.

The application of sparse arrays in high frequency transcranial focused ultrasound therapy: a simulation study

PURPOSE

Transcranial focused ultrasound is an emerging therapeutic modality that can be used to perform noninvasive neurosurgical procedures. The current clinical transcranial phased array operates at 650 kHz, however the development of a higher frequency array would enable more precision, while reducing the risk of standing waves. However, the smaller wavelength and the skull's increased distortion at this frequency are problematic. It would require an order of magnitude more elements to create such an array. Random sparse arrays enable steering of a therapeutic array with fewer elements. However, the tradeoffs inherent in the use of sparsity in a transcranial phased array have not been systematically investigated and so the objective of this simulation study is to investigate the effect of sparsity on transcranial arrays at a frequency of 1.5 MHz that provides small focal spots for precise exposure control.

METHODS

Transcranial sonication simulations were conducted using a multilayer Rayleigh-Sommerfeld propagation model. Element size and element population were varied and the phased array's ability to steer was assessed.

RESULTS

The focal pressures decreased proportionally as elements were removed. However, off-focus hotspots were generated if a high degree of steering was attempted with very sparse arrays. A phased array consisting of 1588 elements 3 mm in size, a 10% population, was appropriate for steering up to 4 cm in all directions. However, a higher element population would be required if near-skull sonication is desired.

CONCLUSIONS

This study demonstrated that the development of a sparse, hemispherical array at 1.5 MHz could enable more precision in therapies that utilize lower intensity sonications.

Numerical simulations of clinical focused ultrasound functional neurosurgery

A computational model utilizing grid and finite difference methods were developed to simulate focused ultrasound functional neurosurgery interventions. The model couples the propagation of ultrasound in fluids (soft tissues) and solids (skull) with acoustic and visco-elastic wave equations. The computational model was applied to simulate clinical focused ultrasound functional neurosurgery treatments performed in patients suffering from therapy resistant chronic neuropathic pain. Datasets of five patients were used to derive the treatment geometry. Eight sonications performed in the treatments were then simulated with the developed model. Computations were performed by driving the simulated phased array ultrasound transducer with the acoustic parameters used in the treatments. Resulting focal temperatures and size of the thermal foci were compared quantitatively, in addition to qualitative inspection of the simulated pressure and temperature fields. This study found that the computational model and the simulation parameters predicted an average of 24 ± 13% lower focal temperature elevations than observed in the treatments. The size of the simulated thermal focus was found to be 40 ± 13% smaller in the anterior-posterior direction and 22 ± 14% smaller in the inferior-superior direction than in the treatments. The location of the simulated thermal focus was off from the prescribed target by 0.3 ± 0.1 mm, while the peak focal temperature elevation observed in the measurements was off by 1.6 ± 0.6 mm. Although the results of the simulations suggest that there could be some inaccuracies in either the tissue parameters used, or in the simulation methods, the simulations were able to predict the focal spot locations and temperature elevations adequately for initial treatment planning performed to assess, for example, the feasibility of sonication. The accuracy of the simulations could be improved if more precise ultrasound tissue properties (especially of the skull bone) could be obtained.

A review of numerical and experimental compensation techniques for skull-induced phase aberrations in transcranial focused ultrasound

The development of phased array transducers and their integration with magnetic resonance (MR) guidance and thermal monitoring has established transcranial MR-guided focused ultrasound (tcMRgFUS) as an attractive non-invasive modality for neurosurgical interventions. The presence of the skull, however, compromises the efficiency of transcranial FUS (tcFUS) therapy, as its heterogeneous nature and acoustic characteristics induce significant phase aberrations and energy attenuation, especially at the higher acoustic frequencies employed in tcFUS thermal therapy. These aberrations may distort and shift the acoustic focus as well as induce heating at the patient's scalp and skull bone. Phased array transducers feature hundreds of elements that can be driven individually, each with its own phase and amplitude. This feature allows for compensation of skull-induced aberrations by calculation and application of appropriate phase and amplitude corrections. In this paper, we illustrate the importance of precise refocusing and provide a comprehensive review of the wide variety of numerical and experimental techniques that have been used to estimate these corrections.

Comparative study of standing wave reduction methods using random modulation for transcranial ultrasonication

Various transcranial sonotherapeutic technologies have risks related to standing waves in the skull. In this study, we present a comparative study on standing waves using four different activation methods: sinusoidal (SIN), frequency modulation by noise (FMN), periodic selection of random frequency (PSRF), and random switching of both inverse carriers (RSBIC). The standing wave was produced and monitored by the schlieren method using a flat plane and a human skull. The minimum ratio RSW, which is defined by the ratio of the mean of the difference between local maximal value and local minimal value of amplitude to the average value of the amplitude, was 36% for SIN, 24% for FMN, 13% for PSRF, and 4%for RSBIC for the flat reflective plate, and it was 25% for SIN, 11% for FMN, 13% for PSRF, and 5% for RSBIC for the inner surface of the human skull. This study is expected to have a role in the development of safer therapeutic equipment.

Creating brain lesions with low-intensity focused ultrasound with microbubbles: a rat study at half a megahertz

Low-intensity focused ultrasound was applied with microbubbles (Definity, Lantheus Medical Imaging, North Billerica, MA, USA; 0.02 mL/kg) to produce brain lesions in 50 rats at 558 kHz. Burst sonications (burst length: 10 ms; pulse repetition frequency: 1 Hz; total exposure: 5 min; acoustic power: 0.47-1.3 W) generated ischemic or hemorrhagic lesions at the focal volume revealed by both magnetic resonance imaging and histology. Shorter burst time (2 ms) or shorter sonication time (1 min) reduced the probability of lesion production. Longer pulses (200 ms, 500 ms and continuous wave) caused significant near-field damage. Using microbubbles with focused ultrasound significantly reduced acoustic power levels and, therefore, avoided skull heating issues and potentially can extend the treatable volume of transcranial focused ultrasound to brain tissues close to the skull.

Transcranial passive acoustic mapping with hemispherical sparse arrays using CT-based skull-specific aberration corrections: a simulation study

The feasibility of transcranial passive acoustic mapping with hemispherical sparse arrays (30 cm diameter, 16 to 1372 elements, 2.48 mm receiver diameter) using CT-based aberration corrections was investigated via numerical simulations. A multi-layered ray acoustic transcranial ultrasound propagation model based on CT-derived skull morphology was developed. By incorporating skull-specific aberration corrections into a conventional passive beamforming algorithm (Norton and Won 2000 IEEE Trans. Geosci. Remote Sens. 38 1337-43), simulated acoustic source fields representing the emissions from acoustically-stimulated microbubbles were spatially mapped through three digitized human skulls, with the transskull reconstructions closely matching the water-path control images. Image quality was quantified based on main lobe beamwidths, peak sidelobe ratio, and image signal-to-noise ratio. The effects on the resulting image quality of the source's emission frequency and location within the skull cavity, the array sparsity and element configuration, the receiver element sensitivity, and the specific skull morphology were all investigated. The system's resolution capabilities were also estimated for various degrees of array sparsity. Passive imaging of acoustic sources through an intact skull was shown possible with sparse hemispherical imaging arrays. This technique may be useful for the monitoring and control of transcranial focused ultrasound (FUS) treatments, particularly non-thermal, cavitation-mediated applications such as FUS-induced blood-brain barrier disruption or sonothrombolysis, for which no real-time monitoring techniques currently exist.

Ultrasound enhanced drug delivery to the brain and central nervous system

There is an increasing interest in the use of ultrasound to enhance drug delivery to the brain and central nervous system. Disorders of the brain and CNS historically have had poor response to drug therapy due to the presence of the blood-brain barrier (BBB). Techniques for circumventing the BBB are typically highly invasive or involve disrupting large portions of the BBB, exposing the brain to pathogens. Ultrasound can be non-invasively delivered to the brain through the intact skull. When combined with preformed microbubbles, ultrasound can safely induce transient, localised and reversible disruption of the BBB, allowing therapeutics to be delivered. Investigations to date have shown positive response to ultrasound BBB disruption combined with therapeutic agent delivery in rodent models of primary and metastatic brain cancer and Alzheimer's disease. Recent work in non-human primates has demonstrated that the technique is feasible for use in humans. This review examines the current status of drug delivery to the brain and CNS both by disruption of the BBB, and by ultrasound enhancement of drug delivery through the already compromised BBB. Cellular and physical mechanisms of disruption are discussed, as well as treatment technique, safety and monitoring.

Controlled ultrasound-induced blood-brain barrier disruption using passive acoustic emissions monitoring

The ability of ultrasonically-induced oscillations of circulating microbubbles to permeabilize vascular barriers such as the blood-brain barrier (BBB) holds great promise for noninvasive targeted drug delivery. A major issue has been a lack of control over the procedure to ensure both safe and effective treatment. Here, we evaluated the use of passively-recorded acoustic emissions as a means to achieve this control. An acoustic emissions monitoring system was constructed and integrated into a clinical transcranial MRI-guided focused ultrasound system. Recordings were analyzed using a spectroscopic method that isolates the acoustic emissions caused by the microbubbles during sonication. This analysis characterized and quantified harmonic oscillations that occur when the BBB is disrupted, and broadband emissions that occur when tissue damage occurs. After validating the system's performance in pilot studies that explored a wide range of exposure levels, the measurements were used to control the ultrasound exposure level during transcranial sonications at 104 volumes over 22 weekly sessions in four macaques. We found that increasing the exposure level until a large harmonic emissions signal was observed was an effective means to ensure BBB disruption without broadband emissions. We had a success rate of 96% in inducing BBB disruption as measured by in contrast-enhanced MRI, and we detected broadband emissions in less than 0.2% of the applied bursts. The magnitude of the harmonic emissions signals was significantly (P<0.001) larger for sonications where BBB disruption was detected, and it correlated with BBB permeabilization as indicated by the magnitude of the MRI signal enhancement after MRI contrast administration (R(2) = 0.78). Overall, the results indicate that harmonic emissions can be a used to control focused ultrasound-induced BBB disruption. These results are promising for clinical translation of this technology.

Controlled ultrasound-induced blood-brain barrier disruption using passive acoustic emissions monitoring

The ability of ultrasonically-induced oscillations of circulating microbubbles to permeabilize vascular barriers such as the blood-brain barrier (BBB) holds great promise for noninvasive targeted drug delivery. A major issue has been a lack of control over the procedure to ensure both safe and effective treatment. Here, we evaluated the use of passively-recorded acoustic emissions as a means to achieve this control. An acoustic emissions monitoring system was constructed and integrated into a clinical transcranial MRI-guided focused ultrasound system. Recordings were analyzed using a spectroscopic method that isolates the acoustic emissions caused by the microbubbles during sonication. This analysis characterized and quantified harmonic oscillations that occur when the BBB is disrupted, and broadband emissions that occur when tissue damage occurs. After validating the system's performance in pilot studies that explored a wide range of exposure levels, the measurements were used to control the ultrasound exposure level during transcranial sonications at 104 volumes over 22 weekly sessions in four macaques. We found that increasing the exposure level until a large harmonic emissions signal was observed was an effective means to ensure BBB disruption without broadband emissions. We had a success rate of 96% in inducing BBB disruption as measured by in contrast-enhanced MRI, and we detected broadband emissions in less than 0.2% of the applied bursts. The magnitude of the harmonic emissions signals was significantly (P<0.001) larger for sonications where BBB disruption was detected, and it correlated with BBB permeabilization as indicated by the magnitude of the MRI signal enhancement after MRI contrast administration (R(2) = 0.78). Overall, the results indicate that harmonic emissions can be a used to control focused ultrasound-induced BBB disruption. These results are promising for clinical translation of this technology.

High-intensity focused ultrasound (HIFU) for dissolution of clots in a rabbit model of embolic stroke

It is estimated that only 2-6% of patients receive thrombolytic therapy for acute ischemic stroke suggesting that alternative therapies are necessary. In this study, we investigate the potential for high intensity focused ultrasound (HIFU) to initiate thrombolysis in an embolic model of stroke. Iron-loaded blood clots were injected into the middle cerebral artery (MCA) of New Zealand White rabbits, through the internal carotid artery and blockages were confirmed by angiography. MRI was used to localize the iron-loaded clot and target the HIFU beam for treatment. HIFU pulses (1.5 MHz, 1 ms bursts, 1 Hz pulse repetition frequency, 20 s duration) were applied to initiate thrombolysis. Repeat angiograms and histology were used to assess reperfusion and vessel damage. Using 275 W of acoustic power, there was no evidence of reperfusion in post-treatment angiograms of 3 rabbits tested. In a separate group of animals, 415 W of acoustic power was applied and reperfusion was observed in 2 of the 4 (50%) animals treated. In the last group of animals, acoustic power was further increased to 550 W, which led to the reperfusion in 5 of 7 (∼70%) animals tested. Histological analysis confirmed that the sonicated vessels remained intact after HIFU treatment. Hemorrhage was detected outside of the sonication site, likely due to the proximity of the target vessel with the base of the rabbit skull. These results demonstrate the feasibility of using HIFU, as a stand-alone method, to cause effective thrombolysis without immediate damage to the targeted vessels. HIFU, combined with imaging modalities used to identify and assess stroke patients, could dramatically reduce the time to achieve flow restoration in patients thereby significantly increasing the number of patients which benefit from thrombolysis treatments.

The design of a focused ultrasound transducer array for the treatment of stroke: a simulation study

High intensity focused ultrasound (HIFU) is capable of mechanically disintegrating blood clots at high pressures. Safe thrombolysis may require frequencies higher than those currently utilized by transcranial HIFU. Since the attenuation and focal distortion of ultrasound in bone increases at higher frequencies, resulting focal pressures are diminished. This study investigated the feasibility of using transcranial HIFU for the non-invasive treatment of ischemic stroke. The use of large aperture, 1.1-1.5 MHz phased arrays in targeting four clinically relevant vessel locations was simulated. Resulting focal sizes decreased with frequency, producing a maximum -3 dB depth of field and lateral width of 2.0 and 1.2 mm, respectively. Mean focal gains above an order of magnitude were observed in three of four targets and transducer intensities required to achieve thrombolysis were determined. Required transducer element counts are about an order of magnitude higher than what currently exists and so, although technically feasible, new arrays would need to be developed to realize this as a treatment modality for stroke.