Transcranial MR-Guided Histotripsy System

A novel transcranial MR Guided focused ultrasound method based on the ultrashort echo time skull acoustic model and phase retrieval techniques

Transcranial ultrasound stimulation (TUS) has been clinically applied as a neuromodulation tool. Particularly, the phase array ultrasound can be applied in TUS to non-invasively focus on the cortex or deep brain. However, the vital phase distortion of the ultrasound induced by the skull limits its clinical application. In the current study, we aimed to develop a hybrid method that combines the ultrashort echo time (UTE) magnetic resonance imaging (MRI) sequences with the prDeep technique to achieve focusing ventral intermediate thalamic nucleus (VIM). The time-reversal (TR) approach of the UTE numerical acoustic model of the skull combined with the prDeep algorithm was used to reduce the number of iterations. The skull acoustic model simulation therapy process was establish to valid this method's prediction and focus performance, and the classical TR method were considered as the gold standard (GS). Our approach could restore 75% of the GS intensity in 25 iteration steps, with a superior the noise immunity. Our findings demonstrate that the phase aberration caused by the skull can be estimated using phase retrieval techniques to achieve a fast and accurate transcranial focus. The method has excellent adaptability and anti-noise capacity for satisfying complex and changeable scenarios.

Neuronavigation-Guided Transcranial Histotripsy (NaviTH) System

OBJECTIVE

The goal of the work described here was to develop the first neuronavigation-guided transcranial histotripsy (NaviTH) system and associated workflow for transcranial ablation.

METHODS

The NaviTH system consists of a 360-element, 700 kHz transmitter-receiver-capable transcranial histotripsy array, a clinical neuronavigation system and associated equipment for patient-to-array co-registration and therapy planning and targeting software systems. A workflow for NaviTH treatments, including pre-treatment aberration correction, was developed. Targeting errors stemming from target registration errors (TREs) during the patient-to-array co-registration process, as well as focal shifts caused by skull-induced aberrations, were investigated and characterized. The NaviTH system was used in treatments of two <96 h post-mortem human cadavers and in experiments in two excised human skullcaps.

RESULTS

The NaviTH was successfully used to create ablations in the cadaver brains as confirmed in post-treatment magnetic resonance imaging A total of three ablations were created in the cadaver brains, and targeting errors of 9, 3.4 and 4.4 mm were observed in corpus callosum, septum and thalamus targets, respectively. Errors were found to be caused primarily by TREs resulting from transducer tracking instrument design flaws and imperfections in the treatment workflow. Transducer tracking instrument design and workflow improvements reduced TREs to <2 mm, and skull-induced focal shifts, following pre-treatment aberration correction, were 0.3 mm. Total targeting errors of the NaviTH system following the noted improvements were 2.5 mm.

CONCLUSIONS

The feasibility of using the first NaviTH system in a human cadaver model has been determined. Although accuracy still needs to be improved, the proposed system has the potential to allow for transcranial histotripsy therapies without requiring active magnetic resonance treatment guidance.

Characterization of Blood-Brain Barrier Opening Induced by Transcranial Histotripsy in Murine Brains

OBJECTIVE

Transcranial histotripsy has shown promise as a non-invasive neurosurgical tool, as it has the ability to treat a wide range of locations in the brain without overheating the skull. One important effect of histotripsy in the brain is the blood-brain barrier (BBB) opening (BBBO) at the ablation site, but there is a knowledge gap concerning the extent of histotripsy-induced BBBO. Here we describe induction of BBBO by transcranial histotripsy and use of magnetic resonance imaging (MRI) and histology to quantify changes in BBBO at the periphery of the histotripsy ablation zone over time in the healthy mouse brain.

METHODS

An eight-element, 1 MHz histotripsy transducer with a focal distance of 32.5 mm was used to treat the brains of 23 healthy female BL6 mice. T1-gadolinium (T1-Gd) MR images were acquired immediately following histotripsy treatment and during each of the subsequent 4 wk to quantify the size and intensity of BBB leakage.

RESULTS

The T1-Gd MRI results revealed that the hyperintense BBBO volume increased over the first week and subsided gradually over the following 3 wk. Histology revealed complete loss of tight junction proteins and blood vessels in the center of the ablation region immediately after histotripsy, partial recovery in the periphery of the ablation zone 1 wk following histotripsy and near-complete recovery of tight junction complex after 4 wk.

CONCLUSION

These results provide the first evidence of transcranial histotripsy-induced BBBO and repair at the periphery of the ablation zone.

Ultrasound imaging guided precision histotripsy: Effects of pulse settings on ablation properties in rat brain

A high-frequency 6 MHz miniature handheld histotripsy device with an endoscopic form factor and co-registered high-resolution ultrasound imaging was developed. This device could allow precision histotripsy ablation during minimally invasive brain tumor surgeries with real-time image guidance. This study characterized the outcome of acute histotripsy in the normal in vivo rat brain using the device with a range of histotripsy pulse settings, including number of cycles, pulse repetition frequency, and pressure, as well as other experimental factors. The stability and shape of the bubble cloud were measured during ablations, as well as the post-histotripsy ablation shape in ultrasound B-mode and histology. The results were compared between histological images and the ultrasound imaging data to determine how well ultrasound data reflected observable damage in histology. The results indicated that while pulse settings can have some influence on ablation shape, sample-to-sample variation had a larger influence on ablation shape. This suggests that real-time ablation monitoring is essential for accurate knowledge of outcomes. Ultrasound imaging provided an accurate real-time indication of ablation shape both during ablation and post-ablation.

Revolutionizing brain interventions: the multifaceted potential of histotripsy

Histotripsy, a non-thermal ultrasound technique, holds significant promise in various applications within the realm of brain interventions. While its use for treating brain tumors is somewhat limited, focused ultrasound technology has been extensively investigated for a wide range of purposes within the brain, including disrupting the blood-brain barrier, supporting immunotherapy, addressing conditions like essential tremor, Parkinson's disease, Alzheimer's disease, epilepsy, and neuropathic pain. Research findings indicate that histotripsy can reduce tumor cells with fewer pulses, minimizing the risk of bleeding and cellular injury. The use of MRI sequences such as T2 and T2* enhances the evaluation of the effects of histotripsy treatment, facilitating non-invasive assessment of treated areas. Furthermore, histotripsy displays promise in creating precise brain lesions with minimal edema and inflammation, particularly in porcine models, suggesting considerable progress in the treatment of brain lesions. Moreover, studies confirm its feasibility, safety, and effectiveness in treating intracerebral hemorrhage by safely liquefying clots without causing significant harm to surrounding brain tissue., opening exciting possibilities for clinical applications. The development of transcranial MR-guided focused ultrasound systems based on histotripsy represents a significant breakthrough in overcoming the limitations associated with thermal ablation techniques. Histotripsy's ability to efficiently liquefy clots, minimize skull heating, and target shallow lesions near the skull establishes it as a promising alternative for various brain treatments. In conclusion, histotripsy offers diverse potential in the field of brain interventions, encompassing applications ranging from tumor treatment to the management of intracerebral hemorrhage. While challenges such as accurate monitoring and differentiation of treatment effects persist, ongoing research efforts and technological advancements continue to expand the role of histotripsy in both neurology and neurosurgery.

Influence of the skull bone and brain tissue on the sound field in transcranial extracorporeal shock wave therapy: an ex vivo study

OBJECTIVES

Focused ultrasound is mainly known for focal ablation and localized hyperthermia of tissue. During the last decade new treatment options were developed for neurological indications based on blood-brain-barrier opening or neuromodulation. Recently, the transcranial application of shock waves has been a subject of research. However, the mechanisms of action are not yet understood. Hence, it is necessary to know the energy that reaches the brain during the treatment and the focusing characteristics within the tissue.

METHODS

The sound field of a therapeutic extracorporeal shock wave transducer was investigated after passing human skull bone (n=5) or skull bone with brain tissue (n=2) in this ex vivo study. The maximum and minimum pressure distribution and the focal pressure curves were measured at different intensity levels and penetration depths, and compared to measurements in water.

RESULTS

Mean peak negative pressures of up to -4.97 MPa were reached behind the brain tissue. The positive peak pressure was attenuated by between 20.85 and 25.38 dB/cm by the skull bone. Additional damping by the brain tissue corresponded to between 0.29 and 0.83 dB/cm. Compared to the measurements in water, the pulse intensity integral in the focal spot was reduced by 84 % by the skull bone and by additional 2 % due to the brain tissue, resulting in a total damping of up to 86 %. The focal position was shifted up to 8 mm, whereas the basic shape of the pressure curves was preserved.

CONCLUSIONS

Positive effects may be stimulated by transcranial shock wave therapy but damage cannot be excluded.

An Ultrasound Array of Emitter-Receiver Stacks for Microbubble-Based Therapy

Most therapeutic ultrasound devices place emitters and receivers in separate locations, so that the long therapeutic pulses (>1 ms) can be emitted while receivers monitor the procedure. However, with such placement, emitters and receivers are competing for the same space, producing a trade-off between emission efficiency and reception sensitivity. Taking advantage of recent studies demonstrating that short-pulse ultrasound can be used therapeutically, we aimed to develop a device that overcomes such trade-offs. The array was composed of emitter-receiver stacks, which enabled both emission and reception from the same location. Each element was made of a lead zirconate titanate (PZT)-polyvinylidene fluoride (PVDF) stack. The PZT (frequency: 500 kHz, diameter: 16 mm) was used for emission and the PVDF (thickness: 28 μm, diameter: 16 mm) for broadband reception. 32 elements were assembled in a 3D-printed dome-shaped frame (focal length: 150 mm; [Formula: see text]-number: 1) and was tested in free-field and through an ex-vivo human skull. In free-field, the array had a 4.5 × 4.5 × 32 mm focus and produced a peak-negative pressure (PNP) of 2.12 MPa at its geometric center. The electronic steering range was ±15 mm laterally and larger than ±15 mm axially. Through the skull, the array produced a PNP of 0.63 MPa. The PVDF elements were able to localize broadband microbubble emissions across the skull. We built the first multi-element array for short-pulse and microbubble-based therapeutic applications. Stacked arrays overcome traditional trade-offs between the transmission and reception quality and have the potential to create a step change in treatment safety and efficacy.

Probability of Cavitation in a Custom Iron-Based Coupling Medium for Transcranial Magnetic Resonance-Guided Focused Ultrasound Procedures

OBJECTIVE

A coupling bath of circulating, chilled, degassed water is essential to safe and precise acoustic transmittance during transcranial magnetic resonance-guided focused ultrasound (tMRgFUS) procedures, but the circulating water impairs the critical real-time magnetic resonance imaging (MRI). An iron-based coupling medium (IBCM) using iron oxide nanoparticles previously developed by our group increased the relaxivity of the coupling bath such that it appears to be invisible on MRI compared with degassed water. However, the nanoparticles also reduced the pressure threshold for cavitation. To address this concern for prefocal cavitation, our group recently developed an IBCM of electrosterically stabilized and aggregation-resistant poly(methacrylic acid)-coated iron oxide nanoparticles (PMAA-FeOX) with a similar capability to reduce the MR signal of degassed water. This study examines the effect of the PMAA-FeOX IBCM on the cavitation threshold.

METHODS

Increasing concentrations of PMAA-FeOX nanoparticles in degassed, deionized water were placed at the focus of two different transducers to assess low and high duty-cycle pulsing parameters which are representative of two modes of focused ultrasound being investigated for tMRgFUS. Passive cavitation detection and high-speed optical imaging were used to measure cavitation threshold pressures.

RESULTS

The mean cavitation threshold was determined in both cases to be indistinguishable from the degassed water control, between 6-8 MPa for high duty-cycle pulsing (CW) and between 25.5-26.5 MPa for very low duty-cycle pulsing.

CONCLUSION

The findings of this study indicate that an IBCM of PMAA-FeOX nanoparticles is a possible solution to reducing MRI interference from the coupling bath without increasing the risk of prefocal cavitation.

A review of the development of histotripsy for extremity tumor ablation with a canine comparative oncology model to inform human treatments

Cancer is a devasting disease resulting in millions of deaths worldwide in both humans and companion animals, including dogs. Treatment of cancer is complex and challenging and therefore often multifaceted, as in the case of osteosarcoma (OS) and soft tissue sarcoma (STS). OS predominantly involves the appendicular skeleton and STS commonly develops in the extremities, resulting in treatment challenges due to the need to balance wide-margin resections to achieve local oncological control against the functional outcomes for the patient. To achieve wide tumor resection, invasive limb salvage surgery is often required, and the patient is at risk for numerous complications which can ultimately lead to impaired limb function and mobility. The advent of tumor ablation techniques offers the exciting potential of developing noninvasive or minimally invasive treatment options for extremity tumors. One promising innovative tumor ablation technique with strong potential to serve as a noninvasive limb salvage treatment for extremity tumor patients is histotripsy. Histotripsy is a novel, noninvasive, non-thermal, and non-ionizing focused ultrasound technique which uses controlled acoustic cavitation to mechanically disintegrate tissue with high precision. In this review, we present the ongoing development of histotripsy as a non-surgical alternative for extremity tumors and highlight the value of spontaneously occurring OS and STS in the pet dog as a comparative oncology research model to advance this field of histotripsy research.

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.

Magnetic Resonance Thermometry Targeting for Magnetic Resonance-Guided Histotripsy Treatments

OBJECTIVE

The potential of transcranial magnetic resonance (MR)-guided histotripsy for brain applications has been described in prior in vivo studies in the swine brain through an excised human skull. The safety and accuracy of transcranial MR-guided histotripsy (tcMRgHt) rely on pre-treatment targeting guidance. In the work described here, we investigated the feasibility and accuracy of using ultrasound-induced low-temperature heating and MR thermometry for histotripsy pre-treatment targeting in ex vivo bovine brain.

METHODS

A 15-element, 750-kHz MRI-compatible ultrasound transducer with modified drivers that can deliver both low-temperature heating and histotripsy acoustic pulses was used to treat seven bovine brain samples. The samples were first heated to an approximately 1.6°C temperature increase at the focus, and MR thermometry was used to localize the target. Once the targeting was confirmed, a histotripsy lesion was generated at the focus and visualized on post-histotripsy MR images.

DISCUSSION

The accuracy of MR thermometry targeting was evaluated with the mean/standard deviation of the difference between the locus of peak heating identified by MR thermometry and the center of mass of the post-treatment histotripsy lesion, which was 0.59/0.31 mm and 1.31/0.93 mm in the transverse and longitudinal directions, respectively.

CONCLUSION

This study determined that MR thermometry could provide reliable pre-treatment targeting for transcranial MR-guided histotripsy treatment.

Focused ultrasound for treatment of peripheral brain tumors

Malignant brain tumors are the leading cause of cancer-related death in children and remain a significant cause of morbidity and mortality throughout all demographics. Central nervous system (CNS) tumors are classically treated with surgical resection and radiotherapy in addition to adjuvant chemotherapy. However, the therapeutic efficacy of chemotherapeutic agents is limited due to the blood-brain barrier (BBB). Magnetic resonance guided focused ultrasound (MRgFUS) is a new and promising intervention for CNS tumors, which has shown success in preclinical trials. High-intensity focused ultrasound (HIFU) has the capacity to serve as a direct therapeutic agent in the form of thermoablation and mechanical destruction of the tumor. Low-intensity focused ultrasound (LIFU) has been shown to disrupt the BBB and enhance the uptake of therapeutic agents in the brain and CNS. The authors present a review of MRgFUS in the treatment of CNS tumors. This treatment method has shown promising results in preclinical trials including minimal adverse effects, increased infiltration of the therapeutic agents into the CNS, decreased tumor progression, and improved survival rates.

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.

Transcranial histotripsy parameter study in primary and metastatic murine brain tumor models

OBJECTIVE

This study investigated the effect of various histotripsy dosages on tumor cell kill and associated bleeding in two murine brain tumor models (glioma [Gl261] and lung metastasis [LL/2-Luc2]).

METHODS AND MATERIALS

GL261 or LL/2-Luc2 cells were cultured and implanted into the brains of C57BL/6 mice. Histotripsy (1-cycle pulses, 5 Hz PRF, 30 MPa-P) was performed using a 1 MHz transducer for five different dosages for each cell line: 5, 20 or 200 pulses per location (PPL) at a single treatment point, or 5 or 10-20 PPL at multiple treatment points. MRI, bioluminescence imaging and histology were used to assess tumor ablation and treatment effects within 4-6 h post-treatment.

RESULTS

All treatment groups resulted in a reduction of BLI intensity for the LL/2-Luc2 tumors, with significant signal reductions for the multi-point groups. The average pre-/post-treatment BLI flux (photons/s, ×108) for the different treatment groups were: 4.39/2.19 (5 PPL single-point), 5.49/1.80 (20 PPL single-point), 3.86/1.73 (200 PPL single-point), 2.44/1.11 (5 PPL multi-point) and 5.85/0.80 (10 PPL multi-point). MRI and H&E staining showed increased tumor damage and hemorrhagic effects with increasing histotripsy dose for both GL261 and LL/2-Luc2 tumors, but the increase in tumor damage was diminished beyond 10-20 PPL for single-point treatments and outweighed by increased hemorrhage. In general, hemorrhage was confined to be within 1 mm of the treatment boundary for all groups.

CONCLUSIONS

Our results suggest that a lower number of histotripsy pulses at fewer focal locations can achieve substantial tumor kill while minimizing hemorrhage.

Monitoring cavitation dynamics evolution in tissue mimicking hydrogels for repeated exposures via acoustic cavitation emissions

A 700 kHz histotripsy array is used to generate repeated cavitation events in agarose, gelatin, and polyacrylamide hydrogels. High-speed optical imaging, a broadband hydrophone, and the narrow-band receive elements of the histotripsy array are used to capture bubble dynamics and acoustic cavitation emissions. Bubble radii, lifespan, shockwave amplitudes are noted to be measured in close agreement between the different observation methods. These features also decrease with increasing hydrogel stiffness for all of the tested materials. However, the evolutions of these properties during the repeated irradiations vary significantly across the different material subjects. Bubble maximum radius initially increases, then plateaus, and finally decreases in agarose, but remains constant across exposures in gelatin and polyacrylamide. The bubble lifespan increases monotonically in agarose and gelatin but decreases in polyacrylamide. Collapse shockwave amplitudes were measured to have different-shaped evolutions between all three of the tested materials. Bubble maximum radii, lifespans, and collapse shockwave amplitudes were observed to express evolutions that are dependent on the structure and stiffness of the nucleation medium.

A Rapid Prototyping Method for Sub-MHz Single-Element Piezoelectric Transducers by Using 3D-Printed Components

We present a rapid prototyping method for sub-megahertz single-element piezoelectric transducers by using 3D-printed components. In most of the early research phases of applying new sonication ideas, the prototyping quickness is prioritized over the final packaging quality, since the quickness of preliminary demonstration is crucial for promptly determining specific aims and feasible research approaches. We aim to develop a rapid prototyping method for functional ultrasonic transducers to overcome the current long lead time (>a few weeks). Here, we used 3D-printed external housing parts considering a single matching layer and either air backing or epoxy-composite backing (acoustic impedance > 5 MRayl). By molding a single matching layer on the top surface of a piezoceramic in a 3D-printed housing, an entire packaging time was significantly reduced (<26 h) compared to the conventional methods with grinding, stacking, and bonding. We demonstrated this prototyping method for 590-kHz single-element, rectangular-aperture transducers for moderate pressure amplitudes (mechanical index > 1) at focus with temporal pulse controllability (maximum amplitude by <5-cycle burst). We adopted an air-backing design (Type A) for efficient pressure outputs, and bandwidth improvement was tested by a tungsten-composite-backing (Type B) design. The acoustic characterization results showed that the type A prototype provided 3.3 kPa/Vpp far-field transmitting sensitivity with 25.3% fractional bandwidth whereas the type B transducer showed 2.1 kPa/Vpp transmitting sensitivity with 43.3% fractional bandwidth. As this method provided discernable quickness and cost efficiency, this detailed rapid prototyping guideline can be useful for early-phase sonication projects, such as multi-element therapeutic ultrasound array and micro/nanomedicine testing benchtop device prototyping.

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.

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.

Transcranial Magnetic Resonance-Guided Histotripsy for Brain Surgery: Pre-clinical Investigation

Histotripsy has been previously applied to target various cranial locations in vitro through an excised human skull. Recently, a transcranial magnetic resonance (MR)-guided histotripsy (tcMRgHt) system was developed, enabling pre-clinical investigations of tcMRgHt for brain surgery. To determine the feasibility of in vivo transcranial histotripsy, tcMRgHt treatment was delivered to eight pigs using a 700-kHz, 128-element, MR-compatible phased-array transducer inside a 3-T magnetic resonance imaging (MRI) scanner. After craniotomy to open an acoustic window to the brain, histotripsy was applied through an excised human calvarium to target the inside of the pig brain based on pre-treatment MRI and fiducial markers. MR images were acquired pre-treatment, immediately post-treatment and 2-4 h post-treatment to evaluate the acute treatment outcome. Successful histotripsy ablation was observed in all pigs. The MR-evident lesions were well confined within the targeted volume, without evidence of excessive brain edema or hemorrhage outside of the target zone. Histology revealed tissue homogenization in the ablation zones with a sharp demarcation between destroyed and unaffected tissue, which correlated well with the radiographic treatment zones on MRI. These results are the first to support the in vivo feasibility of tcMRgHt in the pig brain, enabling further investigation of the use of tcMRgHt for brain surgery.

Histotripsy Ablation of Bone Tumors: Feasibility Study in Excised Canine Osteosarcoma Tumors

Osteosarcoma (OS) is a primary bone tumor affecting both dogs and humans. Histotripsy is a non-thermal, non-invasive focused ultrasound method using controlled acoustic cavitation to mechanically disintegrate tissue. In this study, we investigated the feasibility of treating primary OS tumors with histotripsy using a 500-kHz transducer on excised canine OS samples harvested after surgery at the Veterinary Teaching Hospital at Virginia Tech. Samples were embedded in gelatin tissue phantoms and treated with the 500-kHz histotripsy system using one- or two-cycle pulses at a pulse repetition frequency of 250 Hz and a dosage of 4000 pulses/point. Separate experiments also assessed histotripsy effects on normal canine bone and nerve using the same pulsing parameters. After treatment, histopathological evaluation of the samples was completed. To determine the feasibility of treating OS through intact skin/soft tissue, additional histotripsy experiments assessed OS with overlying tissues. Generation of bubble clouds was achieved at the focus in all tumor samples at peak negative pressures of 26.2 ± 4.5 MPa. Histopathology revealed effective cell ablation in treated areas for OS tumors, with no evidence of cell death or tissue damage in normal tissues. Treatment through tissue/skin resulted in generation of well-confined bubble clouds and ablation zones inside OS tumors. Results illustrate the feasibility of treating OS tumors with histotripsy.

Current state of therapeutic focused ultrasound applications in neuro-oncology

INTRODUCTION

Despite manifold advances in oncology, cancers of the central nervous system remain among the most lethal. Unique features of the brain, including distinct cellular composition, immunological privilege, and physical barriers to therapeutic delivery, likely contribute to the poor prognosis of patients with neuro-oncological disease. Focused ultrasound is an emerging technology that allows transcranial delivery of ultrasound energy to focal brain targets with great precision.

METHODS

A review of the clinical and preclinical focused ultrasound literature was performed to obtain data regarding the current state of the focused ultrasound in context of neuro-oncology. A narrative review was then constructed to provide an overview of current and future applications of this technology.

RESULTS

Focused ultrasound can facilitate direct control of tumors by thermal or mechanical ablation, as well as enhance delivery of diverse therapeutics by disruption of the blood-brain barrier without local tissue damage. Indeed, ultrasound-sensitive drug formulations or sonosensitizers may be combined with ultrasound blood-brain barrier disruption to achieve high local drug concentration while limiting systemic exposure to therapeutics. Furthermore, focused ultrasound can induce radiosensitization, immunomodulation, and neuromodulation. Here we review applications of focused ultrasound with a focus on approaches currently under clinical investigation for the treatment of neuro-oncological disease, such as blood-brain barrier disruption for drug delivery and thermal ablation. We also discuss design of clinical trials, selection of patient cohorts, and emerging approaches to improve the efficacy of transcranial ultrasound, such as histotripsy, as well as combinatorial strategies to exploit synergistic biological effects of existing cancer therapies and ultrasound.

CONCLUSIONS

Focused ultrasound is a promising and actively expanding therapeutic modality for diverse neuro-oncological diseases.