Soft-Tissue Aberration Correction for Histotripsy

Transducer module apodization to reduce bone heating during focused ultrasound uterine fibroid ablation with phased arrays: A numerical study

BACKGROUND

During magnetic resonance-guided focused ultrasound (MRgFUS) surgery for uterine fibroids, ablation of fibrous tissues in proximity to the hips and spine is challenging due to heating within the bone that can cause patients to experience pain and potentially damage nerves. This far-field bone heating limits the volume of fibroid tissue that is treatable via MRgFUS.

PURPOSE

To investigate transducer module apodization for improving the ratio of focal-to-bone heating (Δ T ratio $\Delta T_{\mathrm{ratio}}$ ) when targeting fibroid tissue close to the hips and spine, to enable MRgFUS treatments closer to the bone.

METHODS

Acoustic and thermal simulations were performed using 3D magnetic resonance imaging (MRI)-derived anatomies of ten patients who underwent MRgFUS ablation for uterine fibroids using a low-frequency (0.5 MHz $0.5 \ \text{MHz}$ ) 6144-element flat fully-populated modular phased array system (Arrayus Technologies Inc., Burlington, Canada) at our institution as part of a larger clinical trial (NCT03323905). Transducer modules (64 elements $64 \ \text{elements}$ per module) whose beams intersected with no-pass zones delineated within the field were identified, their output power levels were reduced by varying blocking percentage levels, and the resulting temperature field distributions were evaluated across multiple sonications near the hip and spine bones in each patient. Acoustic and thermal simulations took approximately20 min $20 \ \text{min}$ (7 min $7 \ \text{min}$ ) and1 min $1 \ \text{min}$ (30 s $30 \ \text{s}$ ) to run for a single near-spine (near-hip) target, respectively.

RESULTS

For all simulated sonications, transducer module blocking improvedΔ T ratio $\Delta T_{\mathrm{ratio}}$ compared to the no blocking case. In just over half of sonications, full module blocking maximizedΔ T ratio $\Delta T_{\mathrm{ratio}}$ (increase of 82% ± $\pm$ 38% in 50% of hip targets and 49% ± $\pm$ 30% in 62% of spine targets vs. no blocking; mean ± SD), at the cost of more diffuse focusing (focal heating volumes increased by 13% ± 13% for hip targets and 39% ± 27% for spine targets) and thus requiring elevated total (hip: 6% ± 17%, spine: 37% ± 17%) and peak module-wise (hip: 65% ± 36%, spine: 101% ± 56%) acoustic power levels to achieve equivalent focal heating as the no blocking control case. In the remaining sonications, partial module blocking provided further improvements in bothΔ T ratio $\Delta T_{\mathrm{ratio}}$ (increased by 29% ± 25% in the hip and 15% ± 12% in the spine) and focal heating volume (decrease of 20% ± 10% in the hip and 34% ± 17% in the spine) relative to the full blocking case. The optimal blocking percentage value was dependent on the specific patient geometry and target location of interest. Although not all individual target locations saw the benefit, element-wise phase aberration corrections improved the averageΔ T ratio $\Delta T_{\mathrm{ratio}}$ compared to the no correction case (increase of 52% ± 47% in the hip, 35% ± 24% in the spine) and impacted the optimal blocking percentage value. Transducer module blocking enabled ablative treatments to be carried out closer to both hip and spine without overheating or damaging the bone (no blocking:42 ± 1 mm $42\pm 1 \ \text{mm}$ /17 ± 2 mm $17 \pm 2 \ \text{mm}$ , full blocking:38 ± 1 mm $38\pm 1 \ \text{mm}$ /8 ± 1 mm $8\pm 1 \ \text{mm}$ , optimal partial blocking:36 ± 1 mm $36\pm 1 \ \text{mm}$ /7 ± 1 mm $7\pm 1 \ \text{mm}$ for hip/spine).

CONCLUSION

The proposed transducer apodization scheme shows promise for improving MRgFUS treatments of uterine fibroids, and may ultimately increase the effective treatment envelope of MRgFUS surgery in the body by enabling tissue ablation closer to bony structures.

Histotripsy: A Method for Mechanical Tissue Ablation with Ultrasound

Histotripsy is a relatively new therapeutic ultrasound technology to mechanically liquefy tissue into subcellular debris using high-amplitude focused ultrasound pulses. In contrast to conventional high-intensity focused ultrasound thermal therapy, histotripsy has specific clinical advantages: the capacity for real-time monitoring using ultrasound imaging, diminished heat sink effects resulting in lesions with sharp margins, effective removal of the treated tissue, a tissue-selective feature to preserve crucial structures, and immunostimulation. The technology is being evaluated in small and large animal models for treating cancer, thrombosis, hematomas, abscesses, and biofilms; enhancing tumor-specific immune response; and neurological applications. Histotripsy has been recently approved by the US Food and Drug Administration to treat liver tumors, with clinical trials undertaken for benign prostatic hyperplasia and renal tumors. This review outlines the physical principles of various types of histotripsy; presents major parameters of the technology and corresponding hardware and software, imaging methods, and bioeffects; and discusses the most promising preclinical and clinical applications.

Safety and efficacy of histotripsy delivery through overlying gas-filled small bowel in an ex vivo swine model

PURPOSE

To evaluate the safety and efficacy of performing histotripsy through overlying gas-filled bowel in an ex vivo swine model.

METHODS

An ex vivo model was created to simulate histotripsy treatment of solid organs through gas-filled bowel. Spherical 2.5 cm histotripsy treatments were performed in agar phantoms for each of five treatment groups: 1) control with no overlying bowel (n = 6), 2) bowel 0 cm above phantom (n = 6), 3) bowel 1 cm above phantom (n = 6), 4) bowel 2 cm above phantom (n = 6), and 5) bowel 0 cm above the phantom with increased treatment amplitude (n = 6). Bowel was inspected for gross and microscopic damage, and treatment zones were measured. A ray-tracing simulation estimated the percentage of therapeutic beam path blockage by bowel in each scenario.

RESULTS

All histotripsy treatments through partial blockage were successful (24/24). No visible or microscopic damage was observed to intervening bowel. Partial blockage resulted in a small increase in treatment volume compared to controls (p = 0.002 and p = 0.036 for groups with bowel 0 cm above the phantom, p > 0.3 for bowel 1 cm and 2 cm above the phantom). Gas-filled bowel was estimated to have blocked 49.6%, 35.0%, and 27.3% of the therapeutic beam at 0, 1, and 2 cm, respectively.

CONCLUSION

Histotripsy has the potential to be applied through partial gas blockage of the therapeutic beam path, as shown by this ex vivo small bowel model. Further work in an in vivo survival model appears indicated.

Treatment Planning and Aberration Correction Algorithm for HIFU Ablation of Renal Tumors

High-intensity focused ultrasound (HIFU) applications for thermal or mechanical ablation of renal tumors often encounter challenges due to significant beam aberration and refraction caused by oblique beam incidence, inhomogeneous tissue layers, and presence of gas and bones within the beam. These losses can be significantly mitigated through sonication geometry planning, patient positioning, and aberration correction using multielement phased arrays. Here, a sonication planning algorithm is introduced, which uses the simulations to select the optimal transducer position and evaluate the effect of aberrations and acoustic field quality at the target region after aberration correction. Optimization of transducer positioning is implemented using a graphical user interface (GUI) to visualize a segmented 3-D computed tomography (CT)-based acoustic model of the body and to select sonication geometry through a combination of manual and automated approaches. An HIFU array (1.5 MHz, 256 elements) and three renal cell carcinoma (RCC) cases with different tumor locations and patient body habitus were considered. After array positioning, the correction of aberrations was performed using a combination of backpropagation from the focus with an ordinary least squares (OLS) optimization of phases at the array elements. The forward propagation was simulated using a combination of the Rayleigh integral and k-space pseudospectral method (k-Wave toolbox). After correction, simulated HIFU fields showed tight focusing and up to threefold higher maximum pressure within the target region. The addition of OLS optimization to the aberration correction method yielded up to 30% higher maximum pressure compared to the conventional backpropagation and up to 250% higher maximum pressure compared to the ray-tracing method, particularly in strongly distorted cases.

Aberration correction in abdominal histotripsy

In transabdominal histotripsy, ultrasound pulses are focused on the body to noninvasively destroy soft tissues via cavitation. However, the ability to focus is limited by phase aberration, or decorrelation of the ultrasound pulses due to spatial variation in the speed of sound throughout heterogeneous tissue. Phase aberration shifts, broadens, and weakens the focus, thereby reducing the safety and efficacy of histotripsy therapy. This paper reviews and discusses aberration effects in histotripsy and in related therapeutic ultrasound techniques (e.g., high intensity focused ultrasound), with an emphasis on aberration by soft tissues. Methods for aberration correction are reviewed and can be classified into two groups: model-based methods, which use segmented images of the tissue as input to an acoustic propagation model to predict and compensate phase differences, and signal-based methods, which use a receive-capable therapy array to detect phase differences by sensing acoustic signals backpropagating from the focus. The relative advantages and disadvantages of both groups of methods are discussed. Importantly, model-based methods can correct focal shift, while signal-based methods can restore substantial focal pressure, suggesting that both methods should be combined in a 2-step approach. Aberration correction will be critical to improving histotripsy treatments and expanding the histotripsy treatment envelope to enable non-invasive, non-thermal histotripsy therapy for more patients.

Histotripsy Treatment of Murine Brain and Glioma: Temporal Profile of Magnetic Resonance Imaging and Histological Characteristics Post-treatment

OBJECTIVE

Currently, there is a knowledge gap in our understanding of the magnetic resonance imaging (MRI) characteristics of brain tumors treated with histotripsy to evaluate treatment response as well as treatment-related injuries. Our aim was to bridge this gap by investigating and correlating MRI with histological analysis after histotripsy treatment of mouse brain with and without brain tumors and evaluating the evolution of the histotripsy ablation zone on MRI over time.

METHODS

An eight-element, 1 MHz histotripsy transducer with a focal distance of 32.5 mm was used to treat orthotopic glioma-bearing mice and normal mice. The tumor burden at the time of treatment was ∼5 mm3. T2, T2*, T1 and T1-gadolinium (Gd) MR images and histology of the brain were acquired on days 0, 2 and 7 for tumor-bearing mice and days 0, 2, 7, 14, 21 and 28 post-histotripsy for normal mice.

RESULTS

T2 and T2* sequences most accurately correlated with histotripsy treatment zone. The treatment-induced blood products, T1 along with T2, revealed blood product evolution from oxygenated, de-oxygenated blood and methemoglobin to hemosiderin. And T1-Gd revealed the state of the blood-brain barrier arising from the tumor or histotripsy ablation. Histotripsy leads to minor localized bleeding, which resolves within the first 7 d as evident on hematoxylin and eosin staining. By day 14, the ablation zone could be distinguished only by the macrophage-laden hemosiderin, which resides around the ablation zone, rendering the treated zone hypo-intense on all MR sequences.

CONCLUSION

These results provide a library of radiological features on MRI sequences correlated to histology, thus allowing for non-invasive evaluation of histotripsy treatment effects in in vivo experiments.

Soft Tissue Aberration Correction for Histotripsy Using Acoustic Emissions From Cavitation Cloud Nucleation and Collapse

OBJECTIVE

Phase aberration from soft tissue limits the efficacy of histotripsy, a therapeutic ultrasound technique based on acoustic cavitation. Previous work has shown that the acoustic emissions from cavitation can serve as "point sources" for aberration correction (AC). This study compared the efficacy of soft tissue AC for histotripsy using acoustic cavitation emissions (ACE) from bubble cloud nucleation and collapse.

METHODS

A 750-kHz, receive-capable histotripsy array was pulsed to generate cavitation in ex vivo porcine liver through an intervening abdominal wall. Received ACE signals were used to determine the arrival time differences to the focus and compute corrective delays. Corrections from single pulses and from the median of multiple pulses were tested.

DISCUSSION

On average, ACE AC obtained 96% ± 3% of the pressure amplitude obtained by hydrophone-based correction (compared with 71% ± 5% without AC). Both nucleation- and collapse-based corrections obtained >96% of the hydrophone-corrected pressure when using medians of ≥10 pulses. When using single-pulse corrections, nucleation obtained a range of 49%-99% of the hydrophone-corrected pressure, while collapse obtained 95%-99%.

CONCLUSION

The results suggest that (i) ACE AC can recover nearly all pressure amplitude lost owing to soft tissue aberration and that (ii) the collapse signal permits robust AC using a small number of pulses.

Influence of Plastination on Ultrasound Transmission Through the Human Skull

Development, optimization and validation of transcranial ultrasound methods require the use of fresh human or animal skulls. However, to avoid fresh skull degradation over time, fixation methods are required for conservation, such as formaldehyde buffer solution. This method allows for conservation of the skull properties over a relatively long period, but requires specific conditioning (de-gassing) and storage conditions, such that its practical use is limited. Plastination appears to be a unique solution for the preservation and transportation of body parts without constraints. However, the influence of this conservation process has yet to be characterized with respect to ultrasound transmission to verify that the acoustic and mechanical properties of the skulls are not altered by the plastination process. The objective of the study described here was to quantify the effect of plastination on ultrasound transmission through the temporal and parietal areas of the human skull between 200 kHz and 2 MHz. To achieve this, transmission measurements were performed on three different skulls and four areas before and after plastination. It was found that the plastination process results in a transmission loss of 5 dB. Moreover, results indicate that the plastination process does not induce any phase shift in the transmitted signal, validating the proper use of plastinated skulls for in vitro measurements and development of new transcranial ultrasound methods.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

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.

Effects of phase aberration on transabdominal focusing for a large aperture, lowf-number histotripsy transducer

Objective. Soft tissue phase aberration may be particularly severe for histotripsy due to large aperture and lowf-number transducer geometries. This study investigated how phase aberration from human abdominal tissue affects focusing of a large, strongly curved histotripsy transducer.Approach.A computational model (k-Wave) was experimentally validated withex vivoporcine abdominal tissue and used to simulate focusing a histotripsy transducer (radius: 14.2 cm,f-number: 0.62, central frequencyfc: 750 kHz) through the human abdomen. Abdominal computed tomography images from 10 human subjects were segmented to create three-dimensional acoustic property maps. Simulations were performed focusing at 3 target locations in the liver of each subject with ideal phase correction, without phase correction, and after separately matching the sound speed of water and fat to non-fat soft tissue.Main results.Experimental validation in porcine abdominal tissue showed that simulated and measured arrival time differences agreed well (average error, ∼0.10 acoustic cycles atfc). In simulations with human tissue, aberration created arrival time differences of 0.65μs (∼0.5 cycles) at the target and shifted the focus from the target by 6.8 mm (6.4 mm pre-focally along depth direction), on average. Ideal phase correction increased maximum pressure amplitude by 95%, on average. Matching the sound speed of water and fat to non-fat soft tissue decreased the average pre-focal shift by 3.6 and 0.5 mm and increased pressure amplitude by 2% and 69%, respectively.Significance.Soft tissue phase aberration of large aperture, lowf-number histotripsy transducers is substantial despite low therapeutic frequencies. Phase correction could potentially recover substantial pressure amplitude for transabdominal histotripsy. Additionally, different heterogeneity sources distinctly affect focusing quality. The water path strongly affects the focal shift, while irregular tissue boundaries (e.g. fat) dominate pressure loss.

Two-step aberration correction: application to transcranial histotripsy

Objective: Phase aberration correction is essential in transcranial histotripsy to compensate for focal distortion caused by the heterogeneity of the intact skull bone. This paper improves the 2-step aberration correction (AC) method that has been previously presented and develops an AC workflow that fits in the clinical environment, in which the computed tomography (CT)-based analytical approach was first implemented, followed by a cavitation-based approach using the shockwaves from the acoustic cavitation emission (ACE).Approach:A 700 kHz, 360-element hemispherical transducer array capable of transmit-and-receive on all channels was used to transcranially generate histotripsy-induced cavitation and acquire ACE shockwaves. For CT-AC, two ray-tracing models were investigated: a forward ray-tracing model (transducer-to-focus) in the open-source software Kranion, and an in-house backward ray-tracing model (focus-to-transducer) accounting for refraction and the sound speed variation in skulls. Co-registration was achieved by aligning the skull CT data to the skull surface map reconstructed using the acoustic pulse-echo method. For ACE-AC, the ACE signals from the collapses of generated bubbles were aligned by cross-correlation to estimate the corresponding time delays.Main results:The performance of the 2-step method was tested with 3 excised human calvariums placed at 2 different locations in the transducer array. Results showed that the 2-step AC achieved 90 ± 7% peak focal pressure compared to the gold standard hydrophone correction. It also reduced the focal shift from 0.84 to 0.30 mm and the focal volume from 10.6 to 2.0 mm3on average compared to the no AC cases.Significance:The 2-step AC yielded better refocusing compared to either CT-AC or ACE-AC alone and can be implemented in real-time for transcranial histotripsy brain therapy.

Robust and durable aberrative and absorptive phantom for therapeutic ultrasound applications

Phase aberration induced by soft tissue inhomogeneities often complicates high-intensity focused ultrasound (HIFU) therapies by distorting the field and, previously, we designed and fabricated a bilayer gel phantom to reproducibly mimic that effect. A surface pattern containing size scales relevant to inhomogeneities of a porcine body wall was introduced between gel materials with fat- and muscle-like acoustic properties-ballistic and polyvinyl alcohol gels. Here, the phantom design was refined to achieve relevant values of ultrasound absorption and scattering and make it more robust, facilitating frequent handling and use in various experimental arrangements. The fidelity of the interfacial surface of the fabricated phantom to the design was confirmed by three-dimensional ultrasound imaging. The HIFU field distortions-displacement of the focus, enlargement of the focal region, and reduction of focal pressure-produced by the phantom were characterized using hydrophone measurements with a 1.5 MHz 256-element HIFU array and found to be similar to those induced by an ex vivo porcine body wall. A phase correction approach was used to mitigate the aberration effect on nonlinear focal waveforms and enable boiling histotripsy treatments through the phantom or body wall. The refined phantom represents a practical tool to explore HIFU therapy systems capabilities.

Transcranial MR-Guided Histotripsy System

Histotripsy has been previously shown to treat a wide range of locations through excised human skulls in vitro. In this article, a transcranial magnetic resonance (MR)-guided histotripsy (tcMRgHt) system was developed, characterized, and tested in the in vivo pig brain through an excised human skull. A 700-kHz, 128-element MR-compatible phased-array ultrasound transducer with a focal depth of 15 cm was designed and fabricated in-house. Support structures were also constructed to facilitate transcranial treatment. The tcMRgHt array was acoustically characterized with a peak negative pressure up to 137 MPa in free field, 72 MPa through an excised human skull with aberration correction, and 48.4 MPa without aberration correction. The electronic focal steering range through the skull was 33.5 mm laterally and 50 mm axially, where a peak negative pressure above the 26-MPa cavitation intrinsic threshold can be achieved. The MR compatibility of the tcMRgHt system was assessed quantitatively using SNR, B0 field map, and B1 field map in a clinical 3T magnetic resonance imaging (MRI) scanner. Transcranial treatment using electronic focal steering was validated in red blood cell phantoms and in vivo pig brain through an excised human skull. In two pigs, targeted cerebral tissue was successfully treated through the human skull as confirmed by MRI. Excessive bleeding or edema was not observed in the peri-target zones by the time of pig euthanasia. These results demonstrated the feasibility of using this preclinical tcMRgHt system for in vivo transcranial treatment in a swine model.

Transcostal Histotripsy Ablation in an In Vivo Acute Hepatic Porcine Model

PURPOSE

To determine whether histotripsy can create human-scale transcostal ablations in porcine liver without causing severe thermal wall injuries along the beam path.

MATERIALS AND METHODS

Histotripsy was applied to the liver using a preclinical prototype robotic system through a transcostal window in six female swine. A 3.0 cm spherical ablation zone was prescribed. Duration of treatment (75 min) was longer than a prior subcostal treatment study (24 min, 15 s) to minimize beam path heating. Animals then underwent contrast-enhanced MRI, necropsy, and histopathology. Images and tissue were analyzed for ablation zone size, shape, completeness of necrosis, and off-target effects.

RESULTS

Ablation zones demonstrated complete necrosis with no viable tissue remaining in 6/6 animals by histopathology. Ablation zone volume was close to prescribed (13.8 ± 1.8 cm³ vs. prescribed 14.1 cm³). Edema was noted in the body wall overlying the ablation on T2 MRI in 5/5 (one animal did not receive MRI), though there was no gross or histologic evidence of injury to the chest wall at necropsy. At gross inspection, lung discoloration in the right lower lobe was present in 5/6 animals (mean size: 1 × 2 × 4 cm) with alveolar hemorrhage, preservation of blood vessels and bronchioles, and minor injuries to pneumocytes noted at histology.

CONCLUSION

Transcostal hepatic histotripsy ablation appears feasible, effective, and no severe injuries were identified in an acute porcine model when prolonged cooling time is added to minimize body wall heating.

Noninvasive thyroid histotripsy treatment: proof of concept study in a porcine model

INTRODUCTION

This study was performed to determine the feasibility and safety of creating superficial histotripsy treatment in a live porcine thyroid model.

METHODS

The porcine thymus comparable in size, shape and location to the human thyroid was used for this study. This model has been used for thyroid surgery studies due to the diminutive size of the porcine thyroid. Four female swine underwent a total of eight histotripsy treatments performed with a prototype therapy system (HistoSonics, Inc., Ann Arbor, MI). Two treatments were performed in each animal: a spherical 1.0 × 1.0 × 1.0 cm and ovoid 1.0 × 1.0 × 2.0 cm treatment zones. MRI immediately post-procedure was evaluated for histotripsy treatment zone size and imaging appearance, followed immediately by sacrifice. Tissue was then reviewed for percent cellular destruction and precision.

RESULTS

Treatment zones measured on post treatment MRI were similar to prescribed volumes (spherical = 0.60 (+/- 0.11) cm³, ovoid = 1.23 (+/- 0.40) cm³, p > 0.05 vs. prescribed). MRI demonstrated well demarcated treatment zones and imaging findings consistent with cellular destruction. Histology demonstrated sharp transitions to normal tissue (mean 0.33 (+/- 0.13) cm), and high degrees of cellular destruction (mean 76% (+/- 12.5), range of 50-100%) in the treated tissue. Edema within the overlying muscle was seen in 2/8 treatments.

CONCLUSION

Histotripsy is capable of safely creating precise histotripsy treatments within the superficial neck of a porcine thyroid model without evidence of considerable complications.

A Prototype Therapy System for Boiling Histotripsy in Abdominal Targets Based on a 256-Element Spiral Array

Boiling histotripsy (BH) uses millisecond-long ultrasound (US) pulses with high-amplitude shocks to mechanically fractionate tissue with potential for real-time lesion monitoring by US imaging. For BH treatments of abdominal organs, a high-power multielement phased array system capable of electronic focus steering and aberration correction for body wall inhomogeneities is needed. In this work, a preclinical BH system was built comprising a custom 256-element 1.5-MHz phased array (Imasonic, Besançon, France) with a central opening for mounting an imaging probe. The array was electronically matched to a Verasonics research US system with a 1.2-kW external power source. Driving electronics and software of the system were modified to provide a pulse average acoustic power of 2.2 kW sustained for 10 ms with a 1-2-Hz repetition rate for delivering BH exposures. System performance was characterized by hydrophone measurements in water combined with nonlinear wave simulations based on the Westervelt equation. Fully developed shocks of 100-MPa amplitude are formed at the focus at 275-W acoustic power. Electronic steering capabilities of the array were evaluated for shock-producing conditions to determine power compensation strategies that equalize BH exposures at multiple focal locations across the planned treatment volume. The system was used to produce continuous volumetric BH lesions in ex vivo bovine liver with 1-mm focus spacing, 10-ms pulselength, five pulses/focus, and 1% duty cycle.

Phase-Aberration Correction for HIFU Therapy Using a Multielement Array and Backscattering of Nonlinear Pulses

Phase aberrations induced by heterogeneities in body wall tissues introduce a shift and broadening of the high-intensity focused ultrasound (HIFU) focus, associated with decreased focal intensity. This effect is particularly detrimental for HIFU therapies that rely on shock front formation at the focus, such as boiling histotripsy (BH). In this article, an aberration correction method based on the backscattering of nonlinear ultrasound pulses from the focus is proposed and evaluated in tissue-mimicking phantoms. A custom BH system comprising a 1.5-MHz 256-element array connected to a Verasonics V1 engine was used as a pulse/echo probe. Pulse inversion imaging was implemented to visualize the second harmonic of the backscattered signal from the focus inside a phantom when propagating through an aberrating layer. Phase correction for each array element was derived from an aberration-correction method for ultrasound imaging that combines both the beamsum and the nearest neighbor correlation method and adapted it to the unique configuration of the array. The results were confirmed by replacing the target tissue with a fiber-optic hydrophone. Comparing the shock amplitude before and after phase-aberration correction showed that the majority of losses due to tissue heterogeneity were compensated, enabling fully developed shocks to be generated while focusing through aberrating layers. The feasibility of using a HIFU phased-array transducer as a pulse-echo probe in harmonic imaging mode to correct for phase aberrations was demonstrated.

Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications

Aberrations induced by soft tissue inhomogeneities often complicate high-intensity focused ultrasound (HIFU) therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel and ballistic gel was built to mimic alternating layers of water-based and lipid tissues characteristic of an abdominal body wall and to reproducibly distort HIFU fields. The density, sound speed, and attenuation coefficient of each material were measured using a homogeneous gel layer. A surface with random topographical features was designed as an interface between gel layers using a 2D Fourier spectrum approach and replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for linear and nonlinear beam focusing and compared to the corresponding distortion induced by an ex vivo porcine body wall of the same thickness. Both spatial shift and widening of the focal lobe were observed, as well as dramatic reduction in focal pressures caused by aberrations. The results suggest that the phantom produced levels of aberration that are similar to a real body wall and can serve as a research tool for studying HIFU effects as well as for developing algorithms for aberration correction.

Real-Time Transcranial Histotripsy Treatment Localization and Mapping Using Acoustic Cavitation Emission Feedback

Cavitation events generated during histotripsy therapy generate large acoustic cavitation emission (ACE) signals that can be detected through the skull. This article investigates the feasibility of using these ACE signals, acquired using the elements of a 500-kHz, 256-element hemispherical histotripsy transducer as receivers, to localize and map the cavitation activity in real time through the human skullcap during transcranial histotripsy therapy. The locations of the generated cavitation events predicted using the ACE feedback signals in this study were found to be accurate to within <1.5 mm of the centers of masses detected by optical imaging and found to lie to within the measured volumes of the generated cavitation events in >~80 % of cases. Localization results were observed to be biased in the prefocal direction of the histotripsy array and toward its transverse origin but were only weakly affected by focal steering location. The choice of skullcap and treatment pulse repetition frequency (PRF) were both observed to affect the accuracy of the localization results in the low PRF regime (1-10 Hz), but the localization accuracy was seen to stabilize at higher PRFs (≥10 Hz). Tests of the localization algorithm in vitro, for treatment delivered to a bovine brain sample mounted within the skullcap, revealed good agreement between the ACE feedback-generated treatment map and the morphological characteristics of the treated volume of the brain sample. Localization during experiments was achieved in real time for pulses delivered at rates up to 70 Hz, but benchmark tests indicate that the localization algorithm is scalable, indicating that higher rates are possible with more powerful hardware. The results of this article demonstrate the feasibility of using ACE feedback signals to localize and map transcranially generated cavitation events during histotripsy. Such capability has the potential to greatly simplify transcranial histotripsy treatments, as it may provide a non-MRI-based method for monitoring and localizing transcranial histotripsy treatments in real time.

Enhanced Shock Scattering Histotripsy With Pseudomonopolar Ultrasound Pulses

Shock scattering histotripsy involves a complex interaction between positive and negative phases of an acoustic burst to initiate a robust cavitation bubble cloud. To more precisely study these effects and optimize shock scattering histotripsy therapy, we constructed a frequency compounding transducer to generate pseudomonopolar ultrasound pulses. The transducer consisted of 113 individual piezoelectric elements with various resonant frequencies (250 kHz, 500 kHz, 750 kHz, 1 MHz, 1.5 MHz, 2 MHz, and 3 MHz). For each resonant frequency, an extremely short pulse could be generated. Pseudomonopolar peak positive pulses were generated by aligning the principal peak positive pressures of individual frequency components temporally, so that they added constructively, and destructive interference occurred outside the peak-positive-overlapped temporal window. After inverting the polarity of the excitation signals, pseudomonopolar peak negative pulses were generated similarly by aligning principal peak negative pressures. Decoupling the positive and negative acoustic phases could have significant advantages for therapeutic applications enhancing precision and avoiding cavitation at tissue interfaces by using mostly positive pressure pulses. For example, we show that 16 shock scattering bubble clouds can be generated using only peak positive pulses following a single peak negative pulse that initiates a pressure release "seed cloud" from which the first shock front is "scattered." Subsequent positive only pulses result in a precise elongated lesion within red blood cell phantoms.