Refining histotripsy: defining the parameter space for the creation of nonthermal lesions with high intensity, pulsed focused ultrasound of the in vitro kidney

Therapeutic ultrasound to noninvasively create intracardiac communications in an intact animal model

OBJECTIVE

To determine if pulsed cavitational ultrasound therapy (histotripsy) can accurately and safely generate ventricular septal defects (VSDs) through the intact chest of a neonatal animal, with the eventual goal of developing a noninvasive technique of creating intra-cardiac communications in patients with congenital heart disease.

BACKGROUND

Histotripsy is an innovative ultrasonic technique that generates demarcated, mechanical tissue fractionation utilizing high intensity ultrasound pulses. Previous work has shown that histotripsy can create atrial septal defects in a beating heart in an open-chest canine model.

METHODS

Nine neonatal pigs were treated with transcutaneous histotripsy targeting the ventricular septum. Ultrasound pulses of 5-μsec duration at a peak negative pressure of 13 MPa and a pulse repetition frequency of 1 kHz were generated by a 1 MHz focused transducer. The procedure was guided by real-time ultrasound imaging.

RESULTS

VSDs were created in all pigs with diameters ranging from 2 to 6.5 mm. Six pigs were euthanized within 2 hrs of treatment, while three were recovered and maintained for 2-3 days to evaluate lesion maturation and clinical side effects. There were only transient clinical effects and pathology revealed mild collateral damage around the VSD with no significant damage to other cardiac or extra-cardiac structures.

CONCLUSIONS

Histotripsy can accurately and safely generate VSDs through the intact chest in a neonatal animal model. These results suggest that with further advances, histotripsy can be a useful, noninvasive technique to create intracardiac communications, which currently require invasive catheter-based or surgical procedures, to clinically stabilize newborn infants with complex congenital heart disease.

Acoustic access to the prostate for extracorporeal ultrasound ablation

This study aimed to measure acoustic access to the prostate for extracorporeal ultrasound ablation. Both transabdominal and transperineal approaches were considered. The objective was to measure the size and shape of the aperture available for unobstructed targeting of the prostate. CT images of 17 randomly selected men > 56 years of age were used to create 3D reconstructions of the lower abdomen and pelvis. Rays were traced from target locations in the prostate toward the perineum and the abdomen. The maximum CT density encountered along each path was recorded; those paths that traversed structures with CT density exceeding a soft tissue threshold were considered to be blocked by bone. Unblocked rays comprised the accessible aperture. The aperture through the perineum was found to be a triangular-shaped region bounded by the lower bones of the pelvis varying significantly in size between subjects. The free aperture through the abdomen was wedge shaped limited by the pubis and also with great subject-to-subject variability. Average unblocked fractions of an f/1 transducer to target base, middle, and apex of the prostate along the urethra from the perineum were 77.0%, 94.4%, and 99.6%, respectively. Averages targeting from the abdomen were 86.1%, 52.3%, and 11.0%. Acoustic access to the prostate for therapy through the perineum was judged to be feasible. Access from the abdomen was judged to be sufficient for the base of the prostate, but likely inadequate for the middle and apex.

Noninvasive thrombolysis using pulsed ultrasound cavitation therapy - histotripsy

Clinically available thrombolysis techniques are limited by either slow reperfusion (drugs) or invasiveness (catheters) and carry significant risks of bleeding. In this study, the feasibility of using histotripsy as an efficient and noninvasive thrombolysis technique was investigated. Histotripsy fractionates soft tissue through controlled cavitation using focused, short, high-intensity ultrasound pulses. In vitro blood clots formed from fresh canine blood were treated by histotripsy. The treatment was applied using a focused 1-MHz transducer, with five-cycle pulses at a pulse repetition rate of 1kHz. Acoustic pressures varying from 2 to 12MPa peak negative pressure were tested. Our results show that histotripsy can perform effective thrombolysis with ultrasound energy alone. Histotripsy thrombolysis only occurred at peak negative pressure >or=6MPa when initiation of a cavitating bubble cloud was detected using acoustic backscatter monitoring. Blood clots weighing 330mg were completely broken down by histotripsy in 1.5 to 5min. The clot was fractionated to debris with >96% weight smaller than 5mum diameter. Histotripsy thrombolysis treatment remained effective under a fast, pulsating flow (a circulatory model) as well as in static saline. Additionally, we observed that fluid flow generated by a cavitation cloud can attract, trap and further break down clot fragments. This phenomenon may provide a noninvasive method to filter and eliminate hazardous emboli during thrombolysis.

Prostate histotripsy in an anticoagulated model

OBJECTIVES

To further explore the phenomenon of minimal bleeding after histotripsy by performing extensive prostate histotripsy treatments in anticoagulated canines. Histotripsy is a noninvasive ultrasound technology which induces microbubble formation (cavitation) within tissues producing mechanical tissue fractionation. During initial in vivo feasibility canine studies of prostate ablation, minimal hematuria was observed.

METHODS

Histotripsy was performed on 9 canine subjects pretreated with 6 mg of oral warfarin for 3-5 days using an extracorporeal 750 kHz therapeutic ultrasound transducer delivering acoustic pulses to the prostatic urethra and periurethral parenchyma. After 7-28 days, the subjects were euthanized, transrectal prostate ultrasound was performed, and the prostate was harvested. Serum hemoglobin and International Normalization Ratio were measured immediately before histotripsy treatment and at euthanasia.

RESULTS

Mean treatment International Normalization Ratio was 4.6 (median, 2.4; range, 1.2-11.3). There was no clinically significant change in hemoglobin concentration at euthanasia compared with baseline. At harvest, histologic sections of the prostate revealed a large cavity corresponding to the planned treatment volume incorporating the prostatic urethra and parenchyma in all subjects. Urine was clear within 2 days of treatment, and no blood clots were seen.

CONCLUSIONS

Despite therapeutic and supratherapeutic anticoagulation, histotripsy resulted in minimal bleeding despite significant fractionation and tissue debulking of the prostate. These results have prompted further studies to understand the mechanism of nonthermal hemostasis underlying histotripsy.

Histotripsy of the prostate: dose effects in a chronic canine model

OBJECTIVES

To develop the technique of histotripsy ultrasound therapy as a noninvasive treatment for benign prostatic hyperplasia and to examine the histotripsy dose-tissue response effect over time to provide an insight for treatment optimization. We have previously demonstrated the feasibility of prostate histotripsy fractionation in a canine model.

METHODS

Various doses of histotripsy were applied transabdominally to the prostates of 20 canine subjects. Treated prostates were then harvested at interval time points from 0 to 28 days and assessed for histologic treatment response.

RESULTS

The lowest dose applied was found to produce only scattered cellular disruption and necrosis, whereas higher doses produced more significant regions of tissue effect that resulted in sufficient fractionation of tissue so the material could be voided with urination. Urethral tissue was more resistant to the lower histotripsy doses than was parenchymal tissue. Treatment of the urethra at the lowest doses appeared to heal, with minimal long-term sequelae.

CONCLUSIONS

Histotripsy was effective at fractionating parenchymal and urethral tissue in the prostate, in the presence of a sufficient dose. Further development of this technique could lead to a noninvasive method for debulking the prostate to relieve symptoms associated with benign prostatic hyperplasia.

High-intensity focused ultrasound for the treatment of renal masses: current status and future potential

PURPOSE OF REVIEW

To outline the current status of high-intensity focused ultrasound (HIFU) for the treatment of renal tumors.

RECENT FINDINGS

Application of extracorporeal HIFU for renal tumors is well tolerated with no serious perioperative complications. However, the techniques available do not permit sufficient tumor destruction that can be considered as an alternative to surgical extirpation. Laparoscopic HIFU avoids problems with respiratory movement and interphases and may achieve a greater rate of tumor destruction.

SUMMARY

At the current time, HIFU of renal tumors has to be considered an experimental treatment approach. Prospective evaluation of laparoscopic HIFU is necessary to evaluate its oncologic efficacy.

Size measurement of tissue debris particles generated from pulsed ultrasound cavitational therapy-histotripsy

Extensive mechanical tissue fractionation can be achieved using successive high intensity ultrasound pulses ("histotripsy"). Histotripsy has many potential medical applications where noninvasive tissue removal is desired (e.g., tumor ablation). There is a concern that debris generated by histotripsy-induced tissue fractionation might be an embolization hazard. The aim of this study is to measure the size distribution of these tissue debris particles. Histotripsy pulses were produced by a 513-element 1 MHz array transducer, an 18-element 750 kHz array transducer and a 788 kHz single element transducer. Peak negative pressures of 11 to 25 MPa, pulse durations of 3 to 50 cycles, pulse repetition frequencies of 100 Hz to 2 kHz were used. Tissue debris particles created by histotripsy were collected and measured with a particle sizing system. In the resulting samples, debris <6 microm in diameter constituted >99% of the total number of tissue particles. The largest particle generated by one of the parameter sets tested was 54 microm in diameter, which is smaller than the clinic filter size (100 microm) used to prevent embolization. The largest particles generated using other parameter sets were larger than 60 microm but the value could not be specified using our current setup. Exposures with shorter pulses produced lower percentages of large tissue debris (>30 microm) in comparison to longer pulses. These results suggest that the tissue debris particle size distribution is adjustable by altering acoustic parameters if necessary.

Renal thermal ablative therapy

Energy targeting is greatly enhanced through imaging modalities, which greatly assist needle placement or energy delivery to the optimal location for maximal effectiveness. When vital structures obscure access to the renal lesion, laparoscopic mobilization of these structures with direct visualization of the tumor can increase the likelihood of ablation success and minimize complication risk. Ablative therapies are attractive because of their minimal impact on patient quality of life in addition to their morbidity and cost. Although they show promise of efficacy, they must be evaluated with long-term follow-up before they are considered the standard of oncologic care. Renal masses can be treated with a laparoscopic or percutaneous approach depending on tumor location, size, and the available technology and experience of the center.

Non-thermal ablation of rabbit liver VX2 tumor by pulsed high intensity focused ultrasound with ultrasound contrast agent: Pathological characteristics

AIM: To investigate the pathological characteristics of non-thermal damage induced by pulsed high intensity focused ultrasound (PHIFU) combined with ultrasound contrast agent (UCA), SonoVue (Bracco SpA, Milan, Italy) in rabbit liver VX2 tumor.

METHODS: Liver VX2 tumor models were established in 20 rabbits, which were divided randomly into PHIFU combined with ultrasound contrast agent group (PHIFU + UCA group) and sham group. In the PHIFU + UCA group, 0.2 mL of SonoVue was injected intravenously into the tumor, followed by ultrasound exposure of I_SP 5900 W/cm². The rabbits were sacrificed one day after ultrasound exposure. Specimens of the exposed tumor tissues were obtained and observed pathologically under light microscope and transmission electron microscope. The remaining tumor tissues were sent for 2,3,5-Triphenyltetrazolium chloride (TTC) staining.

RESULTS: Before TTC staining, tumor tissues in both the sham and the PHIFU + UCA groups resembled gray fish meat. After TTC staining, the tumor tissues were uniformly stained red, with a clear boundary between tumor tissue and normal tissue. Histological examination showed signs of tumor cell injury in PHIFU + UCA group, with cytoplasmic vacuoles of various sizes, chromatin margination and karyopyknosis. Electron microscopic examination revealed tumor cell volume reduction, karyopyknosis, chromatin margination, intercellular space widening, the presence of high electron-density apoptotic bodies and vacuoles in cytoplasm.

CONCLUSION: The non-thermal effects of PHIFU combined with UCA can be used to ablate rabbit liver VX2 tumors.

Renal ablation by histotripsy--does it spare the collecting system?

PURPOSE

The feasibility of histotripsy (transcutaneous nonthermal mechanical tissue fractionation) was previously demonstrated in an in vivo rabbit renal cortex model. We explored the spectrum of histotripsy bio-effects on different tissue types in an in vitro porcine kidney model.

MATERIALS AND METHODS

Using an 18 element focused annular array ultrasound system we performed histotripsy treatments in 5 in vitro porcine kidneys, targeting 7 cortical volumes and 17 tissue volumes bridging the cortex, medulla and/or collecting system. Treated areas were observed using ultrasound. In 5 lesions methylene blue was infused into the collecting system to evaluate the preservation of collecting system integrity. Kidneys were sectioned and examined grossly for evidence of tissue fractionation, ie the presence of histotripsy paste, or fixed in formalin and prepared for histological analysis.

RESULTS

Histotripsy of renal cortical tissue created tissue defects in the cortical area treated. Histotripsy targeting the renal collecting system, medulla and renal cortex resulted in tissue fractionation in the area of the cortex, intermediate damage in the medulla and minimal damage to the collecting system.

CONCLUSIONS

There is a differential histotripsy treatment effect when comparing renal cortical tissue to renal collecting system. There is no significant architectural disruption of the renal collecting system after histotripsy. This differential effect is a notable finding that may prove useful in future planning of ablative treatments for renal tissue.

Histotripsy: minimally invasive technology for prostatic tissue ablation in an in vivo canine model

OBJECTIVES

Symptoms of benign prostatic hyperplasia affect men increasingly as they age. Minimally invasive therapies for the treatment of benign prostatic hyperplasia continue to evolve. We describe histotripsy, a noninvasive, nonthermal, focused ultrasound technology for precise tissue ablation, and report the initial results of using histotripsy for prostatic tissue ablation in an in vivo canine model.

METHODS

An annular 18-element, 750-kHz, phased-array ultrasound system delivered high-intensity (22 kW/cm(2)), ultrasound pulses (15 cycles in 20 ms) at pulse repetition frequencies of 100 to 500 Hz to canine prostates. Eight lateral lobe and nine periurethral treatments were performed in 11 anesthetized dogs. Diagnostic ultrasound transducers provided in-line and transrectal imaging. Retrograde urethrography was performed before and after the periurethral treatments. After treatment, the prostates were grossly examined, sectioned, and submitted for histologic examination.

RESULTS

In the lateral lobe treatments, a well-demarcated cavity containing liquefied material was present at the ablation site. Microscopically, the targeted volume was characterized by the presence of histotripsy paste (debris, absent cellular structures). A narrow margin of cellular injury was noted, beyond which no tissue damage was apparent. The periurethral treatments resulted in total urethral ablation or significant urethral wall damage, with visible prostatic urethral defects on retrograde urethrography. Real-time ultrasound imaging demonstrated a dynamic hyperechoic zone at the focus, indicative of cavitation and suggesting effective tissue ablation.

CONCLUSIONS

The results of our study have shown that histotripsy is capable of precise prostatic tissue destruction and results in subcellular fractionation of prostate parenchyma. Histotripsy can also produce prostatic urethral damage and thereby facilitate drainage of finely fractionated material per urethra, producing immediate debulking.

High speed imaging of bubble clouds generated in pulsed ultrasound cavitational therapy--histotripsy

Our recent studies have demonstrated that mechanical fractionation of tissue structure with sharply demarcated boundaries can be achieved using short (< 20 micros), high intensity ultrasound pulses delivered at low duty cycles. We have called this technique histotripsy. Histotripsy has potential clinical applications where noninvasive tissue fractionation and/or tissue removal are desired. The primary mechanism of histotripsy is thought to be acoustic cavitation, which is supported by a temporally changing acoustic backscatter observed during the histotripsy process. In this paper, a fast-gated digital camera was used to image the hypothesized cavitating bubble cloud generated by histotripsy pulses. The bubble cloud was produced at a tissue-water interface and inside an optically transparent gelatin phantom which mimics bulk tissue. The imaging shows the following: (1) Initiation of a temporally changing acoustic backscatter was due to the formation of a bubble cloud; (2) The pressure threshold to generate a bubble cloud was lower at a tissue-fluid interface than inside bulk tissue; and (3) at higher pulse pressure, the bubble cloud lasted longer and grew larger. The results add further support to the hypothesis that the histotripsy process is due to a cavitating bubble cloud and may provide insight into the sharp boundaries of histotripsy lesions.