Catheter Hydrophone Aberration Correction for Transcranial Histotripsy Treatment of Intracerebral Hemorrhage: Proof-of-Concept

Histotripsy - hype or hope? Review of innovation and future implications

BACKGROUND

Histotripsy is a novel, ultrasound-based ablative technique that was recently approved by the Food and Drug Administration for hepatic targets. It has several promising additional theoretical applications that need to be further investigated. Its basis as a nonthermal cavitational technology presents a unique advantage over existing thermal ablation techniques in maximizing local effects while minimizing adjacent tissue destruction. This review discusses the technical basis and current preclinical and clinical data surrounding histotripsy.

METHODS

This was a comprehensive review of the literature surrounding histotripsy and the clinical landscape of existing ablative techniques using the PubMed database. A technical summary of histotripsy's physics and cellular effect was described. Moreover, data from recent clinical trials, including Hope4Liver, and future implications regarding its application in various benign and malignant conditions were discussed.

RESULTS

Preclinical data demonstrated the efficacy of histotripsy ablation in various organ systems with minimal tissue destruction when examined at the histologic level. The first prospective clinical trial involving histotripsy in hepatocellular carcinoma and liver metastases, Hope4Liver, demonstrated a primary efficacy of 95.5% with minimal complications (6.8%). This efficacy was replicated in similar trials involving the treatment of benign prostatic hypertrophy.

DISCUSSION

In addition to the noninvasive ability to ablate lesions in the liver, histotripsy offers additional therapeutic potential. Early data suggest a potential complementary therapeutic effect when combining histotripsy with existing immunologic therapies because of the technology's theoretical ability to sensitize tumors to adaptive immunity. As with most novel therapies, the effect of histotripsy on the oncologic therapeutic landscape remains uncertain.

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.

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.

Nominal Versus Actual Spatial Resolution: Comparison of Directivity and Frequency-Dependent Effective Sensitive Element Size for Membrane, Needle, Capsule, and Fiber-Optic Hydrophones

Frequency-dependent effective sensitive element radius [Formula: see text] is a key parameter for elucidating physical mechanisms of hydrophone operation. In addition, it is essential to know [Formula: see text] to correct for hydrophone output voltage reduction due to spatial averaging across the hydrophone sensitive element surface. At low frequencies, [Formula: see text] is greater than geometrical sensitive element radius a_g . Consequently, at low frequencies, investigators can overrate their hydrophone spatial resolution. Empirical models for [Formula: see text] for membrane, needle, and fiber-optic hydrophones have been obtained previously. In this article, an empirical model for [Formula: see text] for capsule hydrophones is presented, so that models are now available for the four most common hydrophone types used in biomedical ultrasound. The [Formula: see text] value was estimated from directivity measurements (over the range from 1 to 20 MHz) for five capsule hydrophones (three with [Formula: see text] and two with [Formula: see text]). The results suggest that capsule hydrophones behave according to a "rigid piston" model for k a _g ≥ 0.7 ( k = 2π /wavelength). Comparing the four hydrophone types, the low-frequency discrepancy between [Formula: see text] and a_g was found to be greatest for membrane hydrophones, followed by capsule hydrophones, and smallest for needle and fiber-optic hydrophones. Empirical models for [Formula: see text] are helpful for choosing an appropriate hydrophone for an experiment and for correcting for spatial averaging (over the sensitive element surface) in pressure and beamwidth measurements. When reporting hydrophone-based pressure measurements, investigators should specify [Formula: see text] at the center frequency (which may be estimated from the models presented here) in addition to a_g .

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.

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.

Bubble Cloud Behavior and Ablation Capacity for Histotripsy Generated from Intrinsic or Artificial Cavitation Nuclei

The study described here examined the effects of cavitation nuclei characteristics on histotripsy. High-speed optical imaging was used to compare bubble cloud behavior and ablation capacity for histotripsy generated from intrinsic and artificial cavitation nuclei (gas-filled microbubbles, fluid-filled nanocones). Results showed a significant decrease in the cavitation threshold for microbubbles and nanocones compared with intrinsic-nuclei controls, with predictable and well-defined bubble clouds generated in all cases. Red blood cell experiments showed complete ablations for intrinsic and nanocone phantoms, but only partial ablation in microbubble phantoms. Results also revealed a lower rate of ablation in artificial-nuclei phantoms because of reduced bubble expansion (and corresponding decreases in stress and strain). Overall, this study demonstrates the potential of using artificial nuclei to reduce the histotripsy cavitation threshold while highlighting differences in the bubble cloud behavior and ablation capacity that need to be considered in the future development of these approaches.

Histotripsy: the first noninvasive, non-ionizing, non-thermal ablation technique based on ultrasound

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technology guided by real-time imaging. Using focused ultrasound delivered from outside the body, histotripsy mechanically destroys tissue through cavitation, rendering the target into acellular debris. The material in the histotripsy ablation zone is absorbed by the body within 1-2 months, leaving a minimal remnant scar. Histotripsy has also been shown to stimulate an immune response and induce abscopal effects in animal models, which may have positive implications for future cancer treatment. Histotripsy has been investigated for a wide range of applications in preclinical studies, including the treatment of cancer, neurological diseases, and cardiovascular diseases. Three human clinical trials have been undertaken using histotripsy for the treatment of benign prostatic hyperplasia, liver cancer, and calcified valve stenosis. This review provides a comprehensive overview of histotripsy covering the origin, mechanism, bioeffects, parameters, instruments, and the latest results on preclinical and human studies.

Stereotactic Transcranial Focused Ultrasound Targeting System for Murine Brain Models

An inexpensive, accurate focused ultrasound stereotactic targeting method guided by pretreatment magnetic resonance imaging (MRI) images for murine brain models is presented. An uncertainty of each sub-component of the stereotactic system was analyzed. The entire system was calibrated using clot phantoms. The targeting accuracy of the system was demonstrated with an in vivo mouse glioblastoma (GBM) model. The accuracy was quantified by the absolute distance difference between the prescribed and ablated points visible on the pre treatment and posttreatment MR images, respectively. A precalibration phantom study ( N = 6 ) resulted in an error of 0.32 ± 0.31, 0.72 ± 0.16, and 1.06 ± 0.38 mm in axial, lateral, and elevational axes, respectively. A postcalibration phantom study ( N = 8 ) demonstrated a residual error of 0.09 ± 0.01, 0.15 ± 0.09, and 0.47 ± 0.18 mm in axial, lateral, and elevational axes, respectively. The calibrated system showed significantly reduced ( ) error of 0.20 ± 0.21, 0.34 ± 0.24, and 0.28 ± 0.21 mm in axial, lateral, and elevational axes, respectively, in the in vivo GBM tumor-bearing mice ( N = 10 ).

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.

Histotripsy Liquefaction of Large Hematoma for Intracerebral Hemorrhage Using Millisecond-Length Ultrasound Pulse Groups Combined With Fundamental and Second Harmonic Superposition: A Preliminary Study

Intracerebral hemorrhage is a life-threatening acute cerebrovascular disease characterized by a 30-d mortality rate of 40% and substantial disability for those who survive. The objective of this study is to investigate the feasibility of histotripsy-mediated efficient and fine liquefaction of large-volume hematoma by utilizing a protocol of millisecond-length ultrasound pulse groups combined with fundamental and second harmonic superposition. Experiments were initially performed in an in vitro hematoma phantom, using a two-element confocal-annular array. Results showed that a single ellipsoid shape, histotripsy lesion with major dimensions of 10.8 ± 1.2 mm axially and 4.8 ± 0.2 mm laterally was successfully generated. Controllability of the lesion shape and size could be realized by modulating treatment parameters in single-spot experiments. Large-volume hematomas were efficiently and finely liquefied through multisonications via a treatment strategy under the relatively optimized treatment parameters. Liquefied contents were evacuated and analyzed using a particle sizing system. The size of the lysates for the most part ranged from 4-8 μm, with more than 99% of them being smaller than 10 μm. Experiments were then conducted in an optically transparent tissue phantom to explore the liquefaction mechanisms. The phantom was composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA), and a thin phantom layer consisted of red blood cells in the BSA polyacrylamide gel was inlayed in the BSA gel phantom. The related mechanisms, such as the frequent boiling that occurred at multiple positions and the enhanced cavitation, revealed the quick development of the lesion in the phantom and the efficient liquefaction of the clot. These results indicated that the proposed histotripsy approach is feasible for the efficient, precise and fine liquefaction of large-volume hematoma and may be developed as a useful tool for intracerebral hemorrhage treatment.

Histotripsy Clot Liquefaction in a Porcine Intracerebral Hemorrhage Model

BACKGROUND

Intracerebral hemorrhage (ICH) is characterized by a 30-d mortality rate of 40% and significant disability for those who survive.

OBJECTIVE

To investigate the initial safety concerns of histotripsy mediated clot liquefaction and aspiration in a porcine ICH model. Histotripsy is a noninvasive, focused ultrasound technique that generates cavitation to mechanically fractionate tissue. Histotripsy has the potential to liquefy clot in the brain and facilitate minimally invasive aspiration.

METHODS

About 1.75-mL clots were formed in the frontal lobe of the brain (n = 18; n = 6/group). The centers of the clots were liquefied with histotripsy 48 h after formation, and the content was either evacuated or left within the brain. A control group was left untreated. Pigs underwent magnetic resonance imaging (MRI) 7 to 8 d after clot formation and were subsequently euthanized. Neurological behavior was assessed throughout. Histological analysis was performed on harvested brains. A subset of pigs underwent acute analysis (≤6 h).

RESULTS

Histotripsy was able to liquefy the center of clots without direct damage to the perihematomal brain tissue. An average volume of 0.9 ± 0.5 mL was drained after histotripsy treatment. All groups showed mild ischemia and gliosis in the perihematomal region; however, there were no deaths or signs of neurological dysfunction in any groups.

CONCLUSION

This study presents the first analysis of histotripsy-based liquefaction of ICH in vivo. Histotripsy safely liquefies clots without significant additional damage to the perihematomal region. The liquefied content of the clot can be easily evacuated, and the undrained clot has no effect on pig survival or neurological behavior.

Soft-Tissue Aberration Correction for Histotripsy

Acoustic aberrations caused by natural heterogeneities of biological soft tissue are a substantial problem for histotripsy, a therapeutic ultrasound technique that uses acoustic cavitation to mechanically fractionate and destroy unwanted target tissue without damaging surrounding tissue. These aberrations, primarily caused by sound speed variations, result in severe defocusing of histotripsy pulses, thereby decreasing treatment efficacy. The gold standard for aberration correction (AC) is to place a hydrophone at the desired focal location to directly measure phase aberrations, which is a method that is infeasible in vivo. We hypothesized that the acoustic cavitation emission (ACE) shockwaves from the initial expansion of inertially cavitating microbubbles generated by histotripsy can be used as a point source for AC. In this study, a 500-kHz, 112-element histotripsy phased array capable of transmitting and receiving ultrasound on all channels was used to acquire ACE shockwaves. These shockwaves were first characterized optically and acoustically. It was found that the shockwave pressure increases significantly as the source changes from a single bubble to a dense cavitation cloud. The first arrival of the shockwave received by the histotripsy array was from the outer-most cavitation bubbles located closest to the histotripsy array. Hydrophone and ACE AC methods were then tested on ex vivo porcine abdominal tissue samples. Without AC, the focal pressure is reduced by 49.7% through the abdominal tissue. The hydrophone AC approach recovered 55.5% of the lost pressure. Using the ACE AC method, over 20% of the lost pressure was recovered, and the array power required to induce cavitation was reduced by approximately 31.5% compared to without AC. These results supported our hypothesis that the ACE shockwaves coupled with a histotripsy array with transmit and receive capability can be used for AC for histotripsy through soft tissue.

The influence of medium elasticity on the prediction of histotripsy-induced bubble expansion and erythrocyte viability

Histotripsy is a form of therapeutic ultrasound that liquefies tissue mechanically via acoustic cavitation. Bubble expansion is paramount in the efficacy of histotripsy therapy, and the cavitation dynamics are strongly influenced by the medium elasticity. In this study, an analytic model to predict histotripsy-induced bubble expansion in a fluid was extended to include the effects of medium elasticity. Good agreement was observed between the predictions of the analytic model and numerical computations utilizing highly nonlinear excitations (shock-scattering histotripsy) and purely tensile pulses (microtripsy). No bubble expansion was computed for either form of histotripsy when the elastic modulus was greater than 20 MPa and the peak negative pressure was less than 50 MPa. Strain in the medium due to the expansion of a single bubble was also tabulated. The viability of red blood cells was calculated as a function of distance from the bubble wall based on empirical data of impulsive stretching of erythrocytes. Red blood cells remained viable at distances further than 44 µm from the bubble wall. As the medium elasticity increased, the distance over which bubble expansion-induced strain influenced red blood cells was found to decrease sigmoidally. These results highlight the relationship between tissue elasticity and the efficacy of histotripsy. In addition, an upper medium elasticity limit was identified, above which histotripsy may not be effective for tissue liquefaction.