Enhanced Numerical Method for the Design of 3-D-Printed Holographic Acoustic Lenses for Aberration Correction of Single-Element Transcranial Focused Ultrasound

Focused ultrasound heating in brain tissue/skull phantoms with 1 MHz single-element transducer

PURPOSE

The study aims to provide insights on the practicality of using single-element transducers for transcranial Focused Ultrasound (tFUS) thermal applications.

METHODS

FUS sonications were performed through skull phantoms embedding agar-based tissue mimicking gels using a 1 MHz single-element spherically focused transducer. The skull phantoms were 3D printed with Acrylonitrile Butadiene Styrene (ABS) and Resin thermoplastics having the exact skull bone geometry of a healthy volunteer. The temperature field distribution during and after heating was monitored in a 3 T Magnetic Resonance Imaging (MRI) scanner using MR thermometry. The effect of the skull's thickness on intracranial heating was investigated.

RESULTS

A single FUS sonication at focal acoustic intensities close to 1580 W/cm2 for 60 s in free field heated up the agar phantom to ablative temperatures reaching about 90 °C (baseline of 37 °C). The ABS skull strongly blocked the ultrasonic waves, resulting in zero temperature increase within the phantom. Considerable heating was achieved through the Resin skull, but it remained at hyperthermia levels. Conversely, tFUS through a 1 mm Resin skull showed enhanced ultrasonic penetration and heating, with the focal temperature reaching 70 °C.

CONCLUSIONS

The ABS skull demonstrated poorer performance in terms of tFUS compared to the Resin skull owing to its higher ultrasonic attenuation and porosity. The thin Resin phantom of 1 mm thickness provided an efficient acoustic window for delivering tFUS and heating up deep phantom areas. The results of such studies could be particularly useful for accelerating the establishment of a wider range of tFUS applications.

Feasibility of Ultrasonic Heating through Skull Phantom Using Single-element Transducer

Background

Noninvasive neurosurgery has become possible through the use of transcranial focused ultrasound (FUS). This study assessed the heating ability of single element spherically focused transducers operating at 0.4 and 1.1 MHz through three-dimensional (3D) printed thermoplastic skull phantoms.

Methods

Phantoms with precise skull bone geometry of a male patient were 3D printed using common thermoplastic materials following segmentation on a computed tomography head scan image. The brain tissue was mimicked by an agar-based gel phantom developed in-house. The selection of phantom materials was mainly based on transmission-through attenuation measurements. Phantom sonications were performed through water, and then, with the skull phantoms intervening the beam path. In each case, thermometry was performed at the focal spot using thermocouples.

Results

The focal temperature change in the presence of the skull phantoms was reduced to less than 20 % of that recorded in free field when using the 0.4 MHz transducer, whereas the 1.1 MHz trans-skull sonication produced minimal or no change in focal temperature. The 0.4 MHz transducer showed better performance in trans-skull transmission but still not efficient.

Conclusion

The inability of both tested single element transducers to steer the beam through the high attenuating skull phantoms and raise the temperature at the focus was confirmed, underlying the necessity to use a correction technique to compensate for energy losses, such those provided by phased arrays. The proposed phantom could be used as a cost-effective and ergonomic tool for trans-skull FUS preclinical studies.

Foam gratings as an alternative to customized acoustic lenses

This article describes a method of manipulating acoustic fields using transmission through foam gratings. The approach is investigated with an analytical model, a numerical model simulating full wave ultrasound propagation through the gratings, and experimental measurements. A grating is demonstrated that mimics a conventional ultrasound lens, modulating the phase of transmitted ultrasound while maximizing the transmitted amplitude. The performance of a foam grating is compared to a lens made of polydimethylsiloxane or three-dimensional printed resin. Using two gratings, independent control of amplitude and phase is demonstrated, with increased insertion loss. The primary advantages of this technique over conventional lenses are very rapid manufacture (<30 min), high repeatability due to the simplicity of manufacture, and the ability to control the amplitude of the transmitted ultrasound. Potential applications include generation of complex ultrasound fields for patient specific treatments in ultrasound therapy.

Differential evolution method to find optimal location of a single-element transducer for transcranial focused ultrasound therapy

BACKGROUND AND OBJECTIVE

Focused ultrasound (FUS) has been receiving growing attention as a noninvasive brain stimulation tool because of its superior spatial specificity and depth penetrability. However, the large mismatch of acoustic properties between the skull and water can disrupt and shift the acoustic focus in the brain. In this paper, we present a numerical method to find the optimal location of a single-element FUS transducer, which creates focus on the target region.

METHODS

The score function, representing the superposition of acoustic waves according to the relative phase difference and transmissibility, was defined based on time-reversal invariance of acoustic waves and depending on the spatial location of the transducer. The optimal location of the transducer was then determined using a differential evolution algorithm. To assess the proposed method, we conducted a forward simulation and compared the resulting focal location to the desired target point. We also performed experimental validation by measuring the acoustic pressure field through an ex vivo human skull in a water tank.

RESULTS

The numerical results indicated that the score function had a positive proportional relationship with the acoustic pressure at the target. Moreover, for the optimized transducer location, both the numerical and experimental results showed that the normalized acoustic pressure at the target was higher than 0.9.

CONCLUSIONS

In this study, we developed an optimization method to place a single-element transducer that effectively transmits acoustic energy to the targeted region in the brain. Our numerical and experimental results demonstrate that the proposed method can provide an optimal transducer location for safe and efficient FUS treatment.

Adjustable Acoustic Field Controlled by "Ultrasonic Projector" on Ultrasound Application

The control of acoustic field has great potential in many applications such as medical treatment, neuro-modulation, and bio-imaging. Recently, acoustic lenses and phased arrays have become common ways of controlling acoustic fields. However, the shortcomings of the two ways are obvious. Acoustic lenses lack flexibility after design, and phased arrays have complicated structures and need to adjust the parameters of each array element. In this work, we propose an alternative for sound field control using a flexible and adjustable "acoustic projector," and two symmetric mirrors are used to change the direction of propagation of an acoustic wave produced by a piezoelectric element and realize acoustic focusing in the target region. The 2-D "acoustic projector" model was built in finite element simulation, and the feasibility was verified with an actual prototype. The sound intensity produced by the piezoelectric element at different horizontal and vertical positions along the target area can be accurately controlled by two adjustable mirrors. When the angle of the mirror ranges from 30° to 40°, the focal depth can change from 39 to 140 mm. Furthermore, the focus can be controlled in a sector with an angle of 60°. The "acoustic projector" demonstrates simple but precise control of acoustic fields and may broaden their applicability. To show its imaging ability, the three groups of target balls at different positions were imaged and their position information by scanning the mirrors in simulation was given.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.

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.

Ultrasound Technologies for Imaging and Modulating Neural Activity

Visualizing and perturbing neural activity on a brain-wide scale in model animals and humans is a major goal of neuroscience technology development. Established electrical and optical techniques typically break down at this scale due to inherent physical limitations. In contrast, ultrasound readily permeates the brain, and in some cases the skull, and interacts with tissue with a fundamental resolution on the order of 100 μm and 1 ms. This basic ability has motivated major efforts to harness ultrasound as a modality for large-scale brain imaging and modulation. These efforts have resulted in already-useful neuroscience tools, including high-resolution hemodynamic functional imaging, focused ultrasound neuromodulation, and local drug delivery. Furthermore, recent breakthroughs promise to connect ultrasound to neurons at the genetic level for biomolecular imaging and sonogenetic control. In this article, we review the state of the art and ongoing developments in ultrasonic neurotechnology, building from fundamental principles to current utility, open questions, and future potential.

A Clinical System for Non-invasive Blood-Brain Barrier Opening Using a Neuronavigation-Guided Single-Element Focused Ultrasound Transducer

Focused ultrasound (FUS)-mediated blood-brain barrier (BBB) opening is currently being investigated in clinical trials. Here, we describe a portable clinical system with a therapeutic transducer suitable for humans, which eliminates the need for in-line magnetic resonance imaging (MRI) guidance. A neuronavigation-guided 0.25-MHz single-element FUS transducer was developed for non-invasive clinical BBB opening. Numerical simulations and experiments were performed to determine the characteristics of the FUS beam within a human skull. We also validated the feasibility of BBB opening obtained with this system in two non-human primates using U.S. Food and Drug Administration (FDA)-approved treatment parameters. Ultrasound propagation through a human skull fragment caused 44.4 ± 1% pressure attenuation at a normal incidence angle, while the focal size decreased by 3.3 ± 1.4% and 3.9 ± 1.8% along the lateral and axial dimension, respectively. Measured lateral and axial shifts were 0.5 ± 0.4 mm and 2.1 ± 1.1 mm, while simulated shifts were 0.1 ± 0.2 mm and 6.1 ± 2.4 mm, respectively. A 1.5-MHz passive cavitation detector transcranially detected cavitation signals of Definity microbubbles flowing through a vessel-mimicking phantom. T₁-weighted MRI confirmed a 153 ± 5.5 mm³ BBB opening in two non-human primates at a mechanical index of 0.4, using Definity microbubbles at the FDA-approved dose for imaging applications, without edema or hemorrhage. In conclusion, we developed a portable system for non-invasive BBB opening in humans, which can be achieved at clinically relevant ultrasound exposures without the need for in-line MRI guidance. The proposed FUS system may accelerate the adoption of non-invasive FUS-mediated therapies due to its fast application, low cost and portability.

On the Evaluation of the Suitability of the Materials Used to 3D Print Holographic Acoustic Lenses to Correct Transcranial Focused Ultrasound Aberrations

The correction of transcranial focused ultrasound aberrations is a relevant topic for enhancing various non-invasive medical treatments. Presently, the most widely accepted method to improve focusing is the emission through multi-element phased arrays; however, a new disruptive technology, based on 3D printed holographic acoustic lenses, has recently been proposed, overcoming the spatial limitations of phased arrays due to the submillimetric precision of the latest generation of 3D printers. This work aims to optimize this recent solution. Particularly, the preferred acoustic properties of the polymers used for printing the lenses are systematically analyzed, paying special attention to the effect of p-wave speed and its relationship to the achievable voxel size of 3D printers. Results from simulations and experiments clearly show that, given a particular voxel size, there are optimal ranges for lens thickness and p-wave speed, fairly independent of the emitted frequency, the transducer aperture, or the transducer-target distance.