Transcranial Functional Ultrasound Imaging Detects Focused Ultrasound Neuromodulation Induced Hemodynamic Changes in Mouse and Nonhuman Primate Brains In Vivo.

Focused ultrasound (FUS) is an emerging noinvasive technique for neuromodulation in the central nervous system (CNS). To evaluate the effects of FUS-induced neuromodulation, many studies used behavioral changes, functional magnetic resonance imaging (fMRI) or electroencephalography (EEG). However, behavioral readouts are often not easily mapped to specific brain activity, EEG has low spatial resolution limited to the surface of the brain and fMRI requires a large importable scanner that limits additional readouts and manipulations. In this context, functional ultrasound imaging (fUSI) holds promise to directly monitor the effects of FUS neuromodulation with high spatiotemporal resolution in a large field of view, with a comparatively simple and flexible setup. fUSI uses ultrafast Power Doppler Imaging (PDI) to measure changes in cerebral blood volume, which correlates well with neuronal activity and local field potentials. We designed a setup that aligns a FUS transducer with a linear array to allow immediate subsequent monitoring of the hemodynamic response with fUSI during and after FUS neuromodulation. We established a positive correlation between FUS pressure and the size of the activated area, as well as changes in cerebral blood volume (CBV) and found that unilateral sonications produce bilateral hemodynamic changes with ipsilateral accentuation in mice. We further demonstrated the ability to perform fully noninvasive, transcranial FUS-fUSI in nonhuman primates for the first time by using a lower-frequency transducer configuration.

Transcranial Blood-Brain Barrier Opening in Alzheimer's Disease Patients Using a Portable Focused Ultrasound System with Real-Time 2-D Cavitation Mapping.

Background: Focused ultrasound (FUS) in combination with microbubbles has recently shown great promise in facilitating blood-brain barrier (BBB) opening for drug delivery and immunotherapy in Alzheimer's disease (AD). However, it is currently limited to systems integrated within the MRI suites or requiring post-surgical implants, thus restricting its widespread clinical adoption. In this pilot study, we investigate the clinical safety and feasibility of a portable, non-invasive neuronavigation-guided FUS (NgFUS) system with integrated real-time 2-D microbubble cavitation mapping. Methods: A phase 1 clinical study with mild to moderate AD patients (N=6) underwent a single session of microbubble-mediated NgFUS to induce transient BBB opening (BBBO). Microbubble activity under FUS was monitored with real-time 2-D cavitation maps and dosing to ensure the efficacy and safety of the NgFUS treatment. Post-operative MRI was used for BBB opening and closure confirmation as well as safety assessment. Changes in AD biomarker levels in both blood serum and extracellular vesicles (EVs) were evaluated, while changes in amyloid-beta (Aß) load in the brain were assessed through 18F-Florbetapir PET. Results: BBBO was achieved in 5 out of 6 subjects with an average volume of 983±626 mm3 following FUS at the right frontal lobe both in white and gray matter regions. The outpatient treatment was completed within 34.8±10.7 min. Cavitation dose significantly correlated with the BBBO volume (R2>0.9, N=4), demonstrating the portable NgFUS system's capability of predicting opening volumes. The cavitation maps co-localized closely with the BBBO location, representing the first report of real-time transcranial 2-D cavitation mapping in the human brain. Larger opening volumes correlated with increased levels of AD biomarkers, including Aß42 (R2=0.74), Tau (R2=0.95), and P-Tau181 (R2=0.86), assayed in serum-derived EVs sampled 3 days after FUS (N=5). From PET scans, subjects showed a lower Aß load increase in the treated frontal lobe region compared to the contralateral region. Reduction in asymmetry standardized uptake value ratios (SUVR) correlated with the cavitation dose (R2>0.9, N=3). Clinical changes in the mini-mental state examination over 6 months were within the expected range of cognitive decline with no additional changes observed as a result of FUS. Conclusion: We showed the safety and feasibility of this cost-effective and time-efficient portable NgFUS treatment for BBBO in AD patients with the first demonstration of real-time 2-D cavitation mapping. The cavitation dose correlated with BBBO volume, a slowed increase in pathology, and serum detection of AD proteins. Our study highlights the potential for accessible FUS treatment in AD, with or without drug delivery.

Real-Time Passive Acoustic Mapping With Enhanced Spatial Resolution in Neuronavigation-Guided Focused Ultrasound for Blood-Brain Barrier Opening.

OBJECTIVE: Passive acoustic mapping (PAM) provides the spatial information of acoustic energy emitted from microbubbles during focused ultrasound (FUS), which can be used for safety and efficacy monitoring of blood-brain barrier (BBB) opening. In our previous work with a neuronavigation-guided FUS system, only part of the cavitation signal could be monitored in real time due to the computational burden although full-burst analysis is required to detect transient and stochastic cavitation activity. In addition, the spatial resolution of PAM can be limited for a small-aperture receiving array transducer. For full-burst real-time PAM with enhanced resolution, we developed a parallel processing scheme for coherence-factor-based PAM (CF-PAM) and implemented it onto the neuronavigation-guided FUS system using a co-axial phased-array imaging transducer. METHODS: Simulation and in-vitro human skull studies were conducted for the performance evaluation of the proposed method in terms of spatial resolution and processing speed. We also carried out real-time cavitation mapping during BBB opening in non-human primates (NHPs). RESULTS: CF-PAM with the proposed processing scheme provided better resolution than that of traditional time-exposure-acoustics PAM with a higher processing speed than that of eigenspace-based robust Capon beamformer, which facilitated the full-burst PAM with the integration time of 10 ms at a rate of 2 Hz. In vivo feasibility of PAM with the co-axial imaging transducer was also demonstrated in two NHPs, showing the advantages of using real-time B-mode and full-burst PAM for accurate targeting and safe treatment monitoring. SIGNIFICANCE: This full-burst PAM with enhanced resolution will facilitate the clinical translation of online cavitation monitoring for safe and efficient BBB opening.

Using a novel rapid alternating steering angles pulse sequence to evaluate the impact of theranostic ultrasound-mediated ultra-short pulse length on blood-brain barrier opening volume and closure, cavitation mapping, drug delivery feasibility, and safety.

Background: Focused ultrasound (FUS)-mediated blood-brain barrier (BBB) opening is a noninvasive, safe and reversible technique for targeted drug delivery to the brain. Most preclinical systems developed to perform and monitor BBB opening are comprised of a separate geometrically focused transducer and passive cavitation detector (PCD) or imaging array. This study builds upon previous work from our group developing a single imaging phased array configuration for simultaneous BBB opening and monitoring called theranostic ultrasound (ThUS), leveraging ultra-short pulse lengths (USPLs) and a novel rapid alternating steering angles (RASTA) pulse sequence design for simultaneous bilateral sonications with target-specific USPL. The RASTA sequence was further employed to evaluate the impact of USPL on BBB opening volume, power cavitation imaging (PCI) pixel intensity, BBB closing timeline, drug delivery efficiency, and safety. Methods: A P4-1 phased array transducer driven by a Verasonics Vantage ultrasound system was operated using a custom script to run the RASTA sequence which consisted of interleaved steered, focused transmits and passive imaging. Contrast-enhanced magnetic resonance imaging (MRI) confirmed initial opening volume and closure of the BBB by longitudinal imaging through 72 hours post-BBB opening. For drug delivery experiments, mice were systemically administered a 70 kDa fluorescent dextran or adeno-associated virus serotype 9 (AAV9) for fluorescence microscopy or enzyme-linked immunosorbent assay (ELISA) to evaluate ThUS-mediated molecular therapeutic delivery. Additional brain sections were also H&E-stained to evaluate histological damage, and IBA1- and GFAP-stained to elucidate the effects of ThUS-mediated BBB opening on stimulation of key cell types involved in the neuro-immune response, microglia and astrocytes. Results: The ThUS RASTA sequence induced distinct BBB openings simultaneously in the same mouse where volume, PCI pixel intensity, level of dextran delivery, and AAV reporter transgene expression were correlated with brain hemisphere-specific USPL, consistent with statistically significant differences between 1.5, 5, and 10-cycle USPL groups. BBB closure after ThUS required 2-48 hours depending on USPL. The potential for acute damage and neuro-immune activation increased with USPL, but such observable damage was nearly reversed 96 hours post-ThUS. Conclusion: ThUS is a versatile single-array technique which exhibits the potential for investigating a variety of non-invasive therapeutic delivery applications in the brain.

Experimental Study of Aperiodic Plane Wave Imaging for Ultrafast 3-D Ultrasound Imaging.

OBJECTIVE: Although plane wave imaging (PWI) with multiple plane waves (PWs) steered at different angles enables ultrafast three-dimensional (3-D) ultrasonic imaging, there is still a challenging tradeoff between image quality and frame rate. To address this challenge, we recently proposed the aperiodic PWI (APWI) with mathematical analysis and simulation study. In this paper, we demonstrate the feasibility of APWI and evaluate the performance with phantom and in vivo experiments. METHODS: APWI with a concentric ring angle pattern (APWI-C) and APWI with a sunflower pattern (APWI-S) are evaluated. For experimental verification of the methods, the experimental results are compared with simulation results in terms of the spatial resolution and the mainlobe-to-sidelobe ratio. In addition, the performance of APWI is compared with that of conventional PWI by using a commercial phantom. To examine the potential for clinical use of APWI, a gallstone phantom study and an in vivo carotid artery experiment are also conducted. RESULTS: In the phantom study, the APWI methods provide a contrast ratio approximately 2-3 dB higher than that of PWI. In a gallstone experiment, the proposed methods yield 3-D rendered stone images more similar to the real stones than PWI. In the in vivo carotid artery images, APWI reduces the clutter artifacts inside the artery. CONCLUSION: Phantom and in vivo studies show that the APWI enhances the contrast without compromising the spatial resolution and frame rate. SIGNIFICANCE: This study experimentally demonstrates the feasibility and advantage of APWI for ultrafast 3-D ultrasonic imaging.

Analysis of the Time and Phase Delay Resolutions in Ultrasound Baseband I/Q Beamformers.

OBJECTIVE: The ultrasound baseband in-phase/quadrature beamformer (IQBF) has been widely employed in medical ultrasound imaging to reduce the amount of channel data or to decrease the data rate of the beamforming process. The aim of this study is to assess the effect of the time and phase delay compensation accuracies on the IQBF and thereby to suggest the criteria for selecting the delay resolutions of the IQBF. METHODS: Mathematical models of the gain loss (GL) and sidelobe level (SL) in closed form are suggested, and the relationships between the parameters (time and phase delay resolutions of the IQBF and the signal bandwidth) and the errors (GL and SL) are investigated. The performance of the IQBF is compared with that of the traditional radio-frequency beamformer (RFBF). Simulation and phantom and in vivo experimental results are shown to corroborate the theoretical analysis. RESULTS AND CONCLUSION: Theoretical analysis and simulation and experimental results show that a phase delay resolution with a quantization step of 2π/64 is sufficient for phase compensation and that a time delay resolution with a sampling rate of 4f0 and 2f0 in the IQBF is sufficient for data with a -6 dB bandwidth of 50% and 25%, respectively, for similar performance as the RFBF with a sampling rate of 16f0, where f0 is the center frequency of the ultrasound signal. SIGNIFICANCE: The suggested criteria have the potential to be used for designing an efficient IQBF satisfying the desired specifications and beamforming accuracy.

In Vivo Evaluation of Plane Wave Imaging for Abdominal Ultrasonography.

Although plane wave imaging (PWI) has been extensively employed for ultrafast ultrasound imaging, its potential for sectorial B-mode imaging with a convex array transducer has not yet been widely recognized. Recently, we reported an optimized PWI approach for sector scanning that exploits the dynamic transmit focusing capability. In this paper, we first report the clinical applicability of the optimized PWI for abdominal ultrasonography by in vivo image and video evaluations and compare it with conventional focusing (CF) and diverging wave imaging (DWI), which is another dynamic transmit focusing technique generally used for sectorial imaging. In vivo images and videos of the liver, kidney, and gallbladder were obtained from 30 healthy volunteers using PWI, DWI, and CF. Three radiologists assessed the phantom images, 156 in vivo images, and 66 in vivo videos. PWI showed significantly enhanced (p < 0.05) spatial resolution, contrast, and noise and artifact reduction, and a 4-fold higher acquisition rate compared to CF and provided similar performances compared to DWI. Because the computations required for PWI are considerably lower than that for DWI, PWI may represent a promising technique for sectorial imaging in abdominal ultrasonography that provides better image quality and eliminates the need for focal depth adjustment.

Contrast and Volume Rate Enhancement of 3-D Ultrasound Imaging Using Aperiodic Plane Wave Angles: A Simulation Study.

Three-dimensional plane wave imaging (PWI) with a 2-D array has been studied for ultrafast volumetric imaging in medical ultrasound. Compared to 2-D PWI, 3-D PWI requires the transmission of an increased number of plane waves (PWs) to scan a volume of interest and achieve transmit dynamic focusing in both the lateral and elevational directions. To reduce the number of PW angles for a given 2-D angular range by mitigating the grating lobe level, we propose two aperiodic patterns of PW angles: concentric rings with a uniform radial interval and the well-known sunflower pattern. Both patterns are validated to provide uniform angle distributions without regular periodicity, and thereby reduce the grating lobe level compared to a periodic angle distribution with the same number of PW angles. Simulation studies show that the aperiodic patterns enhance the contrast of B-mode images by approximately 3-6 dB over all depths. This enhancement implies that the aperiodic angle sets can increase the volume rate by approximately 2-6 times compared to the periodic angle set at the same contrast and spatial resolution.

Comparison study of passive acoustic mapping and high-speed photography for monitoring in situ cavitation bubbles.

The spatiotemporal accuracy of passive acoustic mapping (PAM) for monitoring in situ cavitation bubbles has not been assessed directly via optical means. Here, the cavitation bubbles are monitored from two image sequences obtained simultaneously with PAM and high-speed photography (HSP). The temporal accuracy of PAM for detecting cavitation nucleation and the spatial resolution for cavitation localization are compared with those measured from HSP. The results show that PAM has a temporal accuracy of 20 µs. Mean differences in the spatial locations of PAM and HSP are as small as 10.0 and 30.5 µm along the lateral and axial directions, respectively.

Ultrasonic sector imaging using plane wave synthetic focusing with a convex array transducer.

Synthetic transmit focusing (STF) methods using unfocused waves or plane waves (PWs) have recently been investigated extensively. However, STF using PWs with a convex array (PWSTF-CA) has not been rigorously studied for high-resolution sector imaging. In this paper, the authors suggest an analytical model for accurate characterization of the spatial beam patterns of PWSTF-CA using a large range of either uniformly or non-uniformly distributed PW angles. On the basis of the model, a frame-based PWSTF-CA approach with non-uniform PW angles is suggested to achieve superior image quality at a higher frame rate than conventional transmit focusing (CTF). The analytical model can also be used for optimal selection of a set of PW angles to scan the entire sectorial field of view and its subsets employed for STF at each imaging point. The authors also investigate how to select transmit subarrays for each of the PWs to obtain the best spatial resolution. A theoretical analysis and simulations are conducted for the verification of the analytical model and the optimal utilization strategy of PWSTF-CA. The results indicate that the PWSTF-CA improves not only the frame rate but also the contrast, signal-to-noise ratio, and resolution compared with the CTF, as in the case of PWSTF with linear arrays.