Thermal Therapy With a Fully Electronically Steerable HIFU Phased Array Using Ultrasound Guidance and Local Harmonic Motion Monitoring

Technical note: Evaluation of the acoustic radiation force imaging for predicting HIFU focus with in vitro and ex vivo experiments

BACKGROUND

High-intensity focused ultrasound (HIFU) is currently used for the treatment of various diseases, but it still lacks a reliable technique in the preoperative stage to accurately place its "energy blade" onto diseased targets. Acoustic radiation force imaging (ARFI) was recently introduced to tackle this issue, but its applicability and limitations were not clear.

PURPOSE

The aim of this study was to evaluate the performance of ARFI method in prediction of HIFU focal location at the preoperative stage.

METHODS

A point spread function (PSF) localization method, which was borrowed from the ultrasound super resolution field, was used to validate the core autocorrelation-based motion estimation algorithm in the ARFI procedure. Accuracy of the ARFI method for estimating the HIFU focus were tested with in vitro and ex vivo experiments with a clinically equivalent HIFU system. Comparisons were made between the estimated focal locations and those of the damaged area after the testing objects were cut open.

RESULTS

Results showed that the PSF localization was able to serve as a validating method for motion detection only when the tissue displacement was large. With the ARFI method, location of the HIFU focus could be accurately predicted by a 2D motion map preoperatively, and the axial spatial errors were less than 0.5 mm. However, the derived 2D motion maps can only be valuable when the acoustic stimulation in ARFI were strong enough, which was probably due to the fact that the HIFU focal locations were at large depths and the ultrasound imaging signal had low signal to noise ratio.

CONCLUSION

The ARFI method was indeed an accurate technique for preoperatively predicting HIFU focus in vitro and ex vivo. If clinical applications were to be considered, particularly in deep tissues, efforts might need to be made to improve ability of the ultrasound motion estimation technique.

A Modular, Kerf-Minimizing Approach for Therapeutic Ultrasound Phased Array Construction

A novel method for fabricating a modular, kerf-minimizing histotripsy phased array was developed and tested. The method utilizes arbitrarily shaped elements, 3-D printing, water jet cutting, and a thin, 125- [Formula: see text] electrically insulating epoxy coating to maximize aperture utilization while allowing for replacement of individual transducer modules. The method was used to fabricate a 750-kHz truncated circular aperture array (165 mm ×234 mm) transducer with a focal length of 142 mm. The aperture was segmented into 260 arc-shaped modular elements, each approximately 11.5 mm ×11.5 mm, arranged in concentric rings. The resulting aperture utilization was 92%. The full-width-half-maximum (FWHM) focal zone of the array was measured to be 1.6 mm ×1.1 mm ×4.5 mm, and the FWHM electrical steering range was measured to be 38.5 mm ×33 mm 40 mm. The array was estimated to be capable of generating approximately 120-MPa peak negative pressure at the geometric focus. In addition, the array was used to ablate a 5-cm3 volume of tissue with electric focal steering.

FUS-Net: U-Net-Based FUS Interference Filtering

Imaging applications tailored towards ultrasound-based treatment, such as high intensity focused ultrasound (FUS), where higher power ultrasound generates a radiation force for ultrasound elasticity imaging or therapeutics/theranostics, are affected by interference from FUS. The artifact becomes more pronounced with intensity and power. To overcome this limitation, we propose FUS-net, a method that incorporates a CNN-based U-net autoencoder trained end-to-end on 'clean' and 'corrupted' RF data in Tensorflow 2.3 for FUS artifact removal. The network learns the representation of RF data and FUS artifacts in latent space so that the output of corrupted RF input is clean RF data. We find that FUS-net perform 15% better than stacked autoencoders (SAE) on evaluated test datasets. B-mode images beamformed from FUS-net RF shows superior speckle quality and better contrast-to-noise (CNR) than both notch-filtered and adaptive least means squares filtered RF data. Furthermore, FUS-net filtered images had lower errors and higher similarity to clean images collected from unseen scans at all pressure levels. Lastly, FUS-net RF can be used with existing cross-correlation speckle-tracking algorithms to generate displacement maps. FUS-net currently outperforms conventional filtering and SAEs for removing high pressure FUS interference from RF data, and hence may be applicable to all FUS-based imaging and therapeutic methods.

Multiple-Focus Patterns of Sparse Random Array Using Particle Swarm Optimization for Ultrasound Surgery

This study aims to investigate the feasibility and potential of sparse random arrays driven by the particle swarm optimization (PSO) algorithm to generate multiple-focus patterns and a large scanning range without grating lobes, which extends the scanning range of focused ultrasound in the treatment of brain tumors, opening the blood-brain barrier, and neuromodulation. Operating at 1.1 MHz, a random spherical array with 200 square elements (sparseness 58%) and a sparse random array with 660 square elements (sparseness 41%) driven by PSO are employed to simulate different focus patterns. With the same radius of curvature and diameter of transducer and element size, the scanning range of the off-axis single focus of a random 200-element array is two times that of an ordinary array using symmetric arrangement. The focal volume of multiple-focus patterns of the random array is 18 times that of the single focus. The single focus of the sparse random array with 660 elements could steer up to ±23 mm in the radial direction, without grating lobes. The maximum distance between two foci in a multiple-focus "S"-shaped deflection is approximately 25 mm. Simulation results illustrate the capability of a focused beam steered in 3-D space. Multiple-focus patterns could significantly increase the focal volume and shorten the treatment time for large target volumes. Simulation results show the feasibility and potential of the method combining PSO with a sparse random array to generate flexible focus patterns that can adapt to different needs in different tissue treatments.

Acoustic beam mapping for guiding HIFU therapy in vivo using sub-therapeutic sound pulse and passive beamforming

OBJECTIVE

Although HIFU has been successfully applied in various clinical applications in the past two decades for the ablation of many types of tumors, one bottleneck in its wider applications is the lack of a reliable and affordable strategy to guide the therapy. This study aims at estimating the therapeutic beam path at the pre-treatment stage to guide the therapeutic procedure.

METHODS

An incident beam mapping technique using passive beamforming was proposed based on a clinical HIFU system and an ultrasound imaging research system. An optimization model was created to map the cross-like beam pattern by maximizing the total energy within the mapped area. This beam mapping technique was validated by comparing the estimated focal region with the HIFU-induced actual focal region (damaged region) through simulation, in-vitro, ex-vivo and in-vivo experiments.

RESULTS

The results of this study showed that the proposed technique was, to a large extent, tolerant of sound speed inhomogeneities, being able to estimate the focal location with errors of 0.15 mm and 0.93 mm under in-vitro and ex-vivo situations respectively, and slightly over 1 mm under the in-vivo situation. It should be noted that the corresponding errors were 6.8 mm, 3.2 mm, and 9.9 mm respectively when the conventional geometrical method was used.

CONCLUSION

This beam mapping technique can be very helpful in guiding the HIFU therapy and can be easily applied in clinical environments with an ultrasound-guided HIFU system.

SIGNIFICANCE

The technique is non-invasive and can potentially be adapted to other ultrasound-related beam manipulating applications.

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.

High-Power Phased-Array Transducer Module for the Construction of a System for the Treatment of Deep Vein Thrombosis

Blood clot can be disintegrated by high-intensity focused ultrasound alone through inertial cavitation. There are limitations in using single-element ultrasound transducers for this purpose such as lack of steerability and control of the focus in terms of shape and location. Phased-array transducers being able to rapidly scan over the clots can alleviate this problem. A full 3-D control of the ultrasound beam can be achieved by 2-D electronically steerable arrays. However, the required high-pressure amplitude has not been possible with such arrays. In this work, a 2-D 64-element fully populated phased-array transducer module was designed and fabricated for the high-pressure amplitude required for deep vein thrombosis (DVT). Lateral coupling was considered for the transducer design to decrease the electrical impedance and eliminate the need for electrical matching circuit. PZT-4 with a thickness of 0.35 mm, an element surface area of [Formula: see text] mm, and a length of 6 mm showed a mean electrical impedance of 60.4 ± 11.5 measured for each transducer element facilitating effective electric power transfer from the driving electronics. No breakdown was observed when the voltage was increased gradually to 180 ± 3 V_pp. Operation at 180 V_pp was found to be safe over 10,000 repetitions without reduction in the power, resulting in the average pressure amplitude of 1.01 ± 0.09 MPa at 2 mm from the element surface. These pressure amplitude values indicate that an array of eight modules (80 [Formula: see text] mm) is required to reach to the pressure amplitude needed for DVT. Such arrays are practical with the current technology.

Image-Guided Treatment of Primary Liver Cancer in Mice Leads to Vascular Disruption and Increased Drug Penetration

Despite advances in interventional procedures and chemotherapeutic drug development, hepatocellular carcinoma (HCC) is still the fourth leading cause of cancer-related deaths worldwide with a <30% 5-year survival rate. This poor prognosis can be attributed to the fact that HCC most commonly occurs in patients with pre-existing liver conditions, rendering many treatment options too aggressive. Patient survival rates could be improved by a more targeted approach. Ultrasound-induced cavitation can provide a means for overcoming traditional barriers defining drug uptake. The goal of this work was to evaluate preclinical efficacy of image-guided, cavitation-enabled drug delivery with a clinical ultrasound scanner. To this end, ultrasound conditions (unique from those used in imaging) were designed and implemented on a Philips EPIQ and S5-1 phased array probe to produced focused ultrasound for cavitation treatment. Sonovue^® microbubbles which are clinically approved as an ultrasound contrast agent were used for both imaging and cavitation treatment. A genetically engineered mouse model was bred and used as a physiologically relevant preclinical analog to human HCC. It was observed that image-guided and targeted microbubble cavitation resulted in selective disruption of the tumor blood flow and enhanced doxorubicin uptake and penetration. Histology results indicate that no gross morphological damage occurred as a result of this process. The combination of these effects may be exploited to treat HCC and other challenging malignancies and could be implemented with currently available ultrasound scanners and reagents.