A High-Frequency Phased Array System for Transcranial Ultrasound Delivery in Small Animals

A Transmit-Receive Phased Array for Microbubble-Mediated Focused Ultrasound Brain Therapy in Small Animals

Focused ultrasound (FUS) combined with circulating microbubbles (MBs) can be employed for non-invasive, localized agent delivery across the blood-brain barrier (BBB). Previous work has demonstrated the feasibility of clinical-scale transmit-receive phased arrays for performing transcranial therapies under MB imaging feedback.

OBJECTIVE

This study aimed to design, construct, and evaluate a dual-mode phased array for MB-mediated FUS brain therapy in small animals.

METHODS

A 256-element sparse hemispherical array (100 mm diameter) was fabricated by installing 128 PZT cylinder transmitters (f0 = 1.16 MHz) and 128 broadband PVDF receivers within a 3D-printed scaffold.

RESULTS

The transmit array's focal size at the geometric focus was 0.8 mm × 0.8 mm × 1.7 mm, with a 31 mm/27 mm (lateral/axial) steering range. The receive array's point spread function was 0.6 mm × 0.6 mm × 1.5 mm (1.16 MHz source) at the geometric focus, and sources were localized up to 30 mm/16 mm (lateral/axial) from geometric focus. The array was able to spatially map MB cloud activity in 3D throughout a vessel-mimicking phantom at sub-, ultra-, and second-harmonic frequencies. Preliminary in-vivo work demonstrated its ability to induce localized BBB permeability changes under 3D sub-harmonic MB imaging feedback in a mouse model.

CONCLUSION

Small form factor transmit-receive phased arrays enable acoustic imaging-controlled FUS and MB-mediated brain therapies with high targeting precision required for rodent studies.

SIGNIFICANCE

Dual-mode phased arrays dedicated for small animal use will facilitate high-throughput studies of FUS-mediated BBB permeability enhancement to explore novel therapeutic strategies for future clinical application.

Machine Learning-Enhanced Skull-Universal Acoustic Hologram for Efficient Transcranial Ultrasound Neuromodulation Across Varied Rodent Skulls

Ultrasound neuromodulation (UNM) has gained significant interest in brain science due to its non-invasive nature, precision, and deep brain stimulation capabilities. However, the skull poses challenges along the acoustic path, leading to beam distortion and necessitating effective acoustic aberration correction. Acoustic holograms used with single-element ultrasound transducers offer a promising solution by enabling both aberration correction and multi-focal stimulation. A major limitation, however, is that hologram lenses designed for specific skulls may not perform well on other skulls, requiring multiple custom lenses for scaled studies. To address this, we introduce the Skull-Universal Acoustic Hologram (SUAH), which enables efficient transcranial UNM across various skull types. Our hologram generation framework integrates a physics-based acoustic hologram, differentiable acoustic simulation in heterogeneous media, and a gradient accumulation technique. SUAH, trained on a range of rodent skull shapes, demonstrated remarkable generalizability and robustness, even outperforming the Skull-Specific Acoustic Hologram (SSAH). Through comprehensive analyses, we showed that SUAH performs exceptionally well-even when trained on smaller datasets-significantly outperforming training based on individual skulls. In conclusion, SUAH shows promise as a scalable, versatile, and accurate tool for ultrasound neuromodulation, representing a significant advancement over conventional single-skull hologram lenses. Its ability to adapt to different skull types without the need for multiple custom lenses has the potential to greatly facilitate research in ultrasound neuromodulation.

High-spatial-resolution transcranial focused ultrasound neuromodulation using frequency-modulated pattern interference radiation force

Stimulating the brain in a precise location is crucial in ultrasound neuromodulation. However, improving the resolution proves a challenge owing to the characteristics of transcranial focused ultrasound. In this paper, we present a new neuromodulation system that overcomes the existing limitations based on an acoustic radiation force with a frequency-modulated waveform and standing waves. By using the frequency-modulated pattern interference radiation force (FM-PIRF), the axial spatial resolution can be reduced to a single wavelength level and the target location can be controlled in axial direction electronically. A linear frequency-modulated chirp waveform used in the experiment was designed based on the simulation results. The displacement of the polydimethylsiloxane (PDMS) cantilever was measured at intervals of 0.1 mm to visualize the distribution of radiation force. These results and methods experimentally show that FM-PIRF has improved spatial resolution and capability of electrical movement.

Correction of a transcranial acoustic field using a transient ultrasound field visualization technique

Ultrasound, due to its noninvasive nature, has the potential to enhance or suppress neural activity, making it highly promising for regulating intractable brain disorders. Precise ultrasound stimulation is crucial for improving the efficiency of neural modulation and studying its mechanisms. However, the presence of the skull can cause distortion in the ultrasound field, thereby affecting the accuracy of stimulation. Existing correction methods primarily rely on magnetic resonance guidance and numerical simulation. Due to the large size and high cost, the MR-guided transcranial ultrasound is difficult to be widely applied in small animals. The numerical simulation usually requires further validation and optimization before application, and the most effective method is to visualize the excited ultrasound field. However, the ultrasound field correction methods based on acoustic field visualization are still lacking. Therefore, a shadowgraph-based transient ultrasonic field visualization system is developed, and an ex vivo transcranial ultrasound field correction is performed. By visualizing the ultrasound field with or without a rat skull and then calculating the time difference of each element's ultrasound wavefront, the parameters for ultrasound field correction can be achieved. The experimental results show that this method can improve both the shape and the size of the focal spot, as well as enhance the acoustic pressure at the focus. Overall, the results demonstrate that the ultrasonic field visualization technology can effectively improve the transcranial ultrasound focusing effect and provide a new tool for achieving precise ultrasonic neural modulation.

Higher harmonics dynamic focalization in single-element ring transducers using biaxial driving

Biaxial driving is a new driving technique that allows the steering of the ultrasound field generated by a single-element piezoceramic transducer. Because of their natural axisymmetric geometry, ultrasound generation with ring transducers can take advantage of the biaxial driving to change the focus of the beam generated by this type of transducer using only two driving signals. In this study, we applied the biaxial driving technique into a single-element PZT ring transducer operating at 500 kHz to produce a change in size and position of the focal spot while using the 1st (482 kHz), 3rd (1.362 MHz) and 5th (2.62 MHz) harmonic excitation. The transducer had a thickness of 2.85 mm, an inner diameter of 9.75 mm and a ring width of 2.0 mm, and two pairs of electrodes as required for biaxial driving. Simulation and experimental results showed that both the focal area and the distance at which the focal area centre was located changed as a function of the phase and power difference between the two driving signals. Experimental results showed that the focal area could be reduced from 31.6 mm² (conventional driving) to 3.4 mm² (89 % reduction) when using the first harmonic excitation. For the third harmonic, the focal area could be reduced from 4.0 mm² (conventional driving) to 3.3 mm² (17.5 % reduction). For the fifth harmonic, the focal area could be reduced from 1.7 mm² (conventional driving) to 1 mm² (41.7 % reduction). Results also demonstrated the centre of the focus could be displaced between 3.0 mm and 9.3 mm from the surface of the transducer when using the first harmonic, between 7.3 mm and 8.4 mm at the third harmonic, and between 4.9 mm and 8.2 mm at the fifth harmonic. The reduction in the focus area, as well as the possibility to displace the focus dynamically will be advantageous for preclinical applications of focused ultrasound, especially on drug delivery and neuromodulation studies in small rodents.

Multifocal skull-compensated transcranial focused ultrasound system for neuromodulation applications based on acoustic holography

Transcranial focused ultrasound stimulation is a promising therapeutic modality for human brain disorders because of its noninvasiveness, long penetration depth, and versatile spatial control capability through beamforming and beam steering. However, the skull presents a major hurdle for successful applications of ultrasound stimulation. Specifically, skull-induced focal aberration limits the capability for accurate and versatile targeting of brain subregions. In addition, there lacks a fully functional preclinical neuromodulation system suitable to conduct behavioral studies. Here, we report a miniature ultrasound system for neuromodulation applications that is capable of highly accurate multiregion targeting based on acoustic holography. Our work includes the design and implementation of an acoustic lens for targeting brain regions with compensation for skull aberration through time-reversal recording and a phase conjugation mirror. Moreover, we utilize MEMS and 3D-printing technology to implement a 0.75-g lightweight neuromodulation system and present in vivo characterization of the packaged system in freely moving mice. This preclinical system is capable of accurately targeting the desired individual or multitude of brain regions, which will enable versatile and explorative behavior studies using ultrasound neuromodulation to facilitate widespread clinical adoption.

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.

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

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

Simulation, Implementation and Measurement of Defined Sound Fields for Blood-Brain Barrier Opening in Rats

The blood-brain barrier (BBB) is the most important obstacle to delivery of therapeutics to the central nervous system. Low-intensity pulsed focused ultrasound (FUS) in combination with microbubbles applied under magnetic resonance imaging (MRI) control provides a non-invasive and safe technique for BBB opening (BBBo). In rodent models, however, settings and application protocols differ significantly. Depending on the strain and size, important variables include ultrasound attenuation and sound field distortion caused by the skull. We examined the ultrasound attenuation of the skull of Wistar rats using a targeted FUS system. By modifying the transducer elements and by varying and simulating the acoustic field of the FUS system, we measured a skull attenuation of about 60%. To evaluate potential application of the targeted FUS system in genetically modified animals with increased sensitivity to brain hemorrhage caused by vascular dysfunction, we assessed safety in healthy animals. Histological and MRI analyses of the central nervous system revealed an increase in the number and severity of hyperacute bleeds with focal pressure. At a pressure of 0.4 MPa, no bleeds were induced, albeit at the cost of a weaker hyperintense MRI signal post BBBo. These results indicate a relationship between pressure and the dimension of permeabilization.

An Acoustic Measurement Library for Non-Invasive Trans-Rodent Skull Ultrasonic Focusing at High Frequency

OBJECTIVE

To investigate the feasibility of developing an acoustic measurement library for non-invasive trans-rodent skull ultrasonic focusing at high frequency.

METHODS

A fiber-optic hydrophone (FOH) was positioned at the geometric focus of a spherically-curved phased array (64 elements, 25 mm diameter, 20 mm radius of curvature). Elements were driven sequentially (3.3 MHz driving frequency) and FOH waveforms were recorded with and without intervening ex-vivo rodent skullcaps. Measurements were carried out on 15 skullcaps (Sprague-Dawley rats, 182-209 g) across 3 fixed transmission regions per specimen. An element-wise measurement library of skull-induced phase differences was constructed using mean values across all specimens for each transmission region. Library-based transcranial phase differences were compared with direct FOH-based measurements across 5 additional skullcaps not included in the library.

RESULTS

Library-based phase corrections deviated less from FOH-based trans-skull phase difference values than those calculated for the water-path case, and restored partial transcranial focal quality relative to that recovered using invasive hydrophone-based corrections. Retrospective analysis suggests comparable performance can be obtained using smaller library sizes.

CONCLUSION

An acoustic measurement library can facilitate non-invasive transcranial aberration correction in rodents at high frequency.

SIGNIFICANCE

Library-based focusing represents a practical approach for delivering high-frequency ultrasound brain treatments in small animals.

Acoustic holograms for bilateral blood-brain barrier opening in a mouse model

Transcranial focused ultrasound (FUS) in conjunction with circulating microbubbles injection is the sole non-invasive technique that temporally and locally opens the blood-brain barrier (BBB), allowing targeted drug delivery into the central nervous system (CNS). However, single-element FUS technologies do not allow the simultaneous targeting of several brain structures with high-resolution, and multi-element devices are required to compensate the aberrations introduced by the skull. In this work, we present the first preclinical application of acoustic holograms to perform a bilateral BBB opening in two mirrored regions in mice. The system consisted of a single-element focused transducer working at 1.68~MHz, coupled to a 3D-printed acoustic hologram designed to produce two symmetric foci in anesthetized mice \textit{in vivo} and, simultaneously, compensate the aberrations of the wavefront caused by the skull bones. T1-weighed MR images showed gadolinium extravasation at two symmetric quasi-spherical focal spots. By encoding time-reversed fields, holograms are capable of focusing acoustic energy with a resolution near the diffraction limit at multiple spots inside the skull of small preclinical animals. This work demonstrates the feasibility of hologram-assisted BBB opening for low-cost and highly-localized targeted drug delivery in the CNS in symmetric regions of separate hemispheres.