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.

A fast full-wave solver for calculating ultrasound propagation in the body

Therapeutic ultrasound is a promising non-invasive method for inducing various beneficial biological effects in the human body. In cancer treatment applications, high-power ultrasound is focused at a target tissue volume to ablate the malignant tumour. The success of the procedure depends on the ability to accurately focus ultrasound and destroy the target tissue volume through coagulative necrosis whilst preserving the surrounding healthy tissue. Patient-specific treatment planning strategies are therefore being developed to increase the efficacy of such therapies, while reducing any damage to healthy tissue. These strategies require to use high-performance computing methods to solve ultrasound wave propagation in the body quickly and accurately. For realistic clinical scenarios, all numerical methods which employ volumetric meshes require several hours or days to solve the full-wave propagation on a computer cluster. The boundary element method (BEM) is an efficient approach for modelling the wave field because only the boundaries of the hard and soft tissue regions require discretisation. This paper presents a multiple-domain BEM formulation with a novel preconditioner for solving the Helmholtz transmission problem (HTP). This new formulation is efficient at high-frequencies and where high-contrast materials are present. Numerical experiments are performed to solve the HTP in multiple domains comprising: (i) human ribs, an idealised abdominal fat layer and liver tissue, (ii) a human kidney with a perinephric fat layer, exposed to the acoustic field generated by a high-intensity focused ultrasound (HIFU) array transducer. The time required to solve the equations associated with these problems on a single workstation is of the order of minutes. These results demonstrate the great potential of this new BEM formulation for accurately and quickly solving ultrasound wave propagation problems in large anatomical domains which is essential for developing treatment planning strategies.

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

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

Fast and Selective Ablation of Liver Tumors by High-Intensity Focused Ultrasound Using a Toroidal Transducer Guided by Ultrasound Imaging: The Results of Animal Experiments

This study demonstrated that high-intensity focused ultrasound (HIFU) produced with an intra-operative toroidal-shaped transducer causes fast, selective liver tumor ablations in an animal model. The HIFU device is composed of 256 emitters working at 3 MHz. A 7.5 MHz ultrasound imaging probe centered on the HIFU transducer guided treatment. VX2 tumor segments (25 mg) were implanted into the right lateral liver lobes of 45 New Zealand rabbits. The animals were evenly divided into groups 1 (toroidal HIFU ablation), 2 (surgical resection) and 3 (untreated control). Therapeutic responses were evaluated with gross pathology and histology 11 d post-treatment. Toroidal transducer-produced HIFU ablation (average ablation rate 10.5 cc/min) allowed fast and homogeneous tumor treatment. Sonograms showed all ablations. VX2 tumors were completely coagulated and surrounded by safety margins without surrounding-organ secondary HIFU lesions. HIFU group tumor volumes at autopsy (39 mm³) were significantly lower than control group volumes (2610 mm³, p < 0.0001). HIFU group tumor metastasis (27%) was lower than resected (33%) and control (67%) group metastasis. Ultrasound imaging, gross pathology and histology results supported these outcomes. HIFU procedures had no complications. Rabbit liver tumor ablation using a toroidal HIFU transducer under ultrasound imaging guidance might therefore be an effective intra-operative treatment for localized liver metastases.

Minimally invasive therapeutic ultrasound: Ultrasound-guided ultrasound ablation in neuro-oncology

INTRODUCTION

To improve patient outcomes (eg, reducing blood loss and infection), practitioners have gravitated toward noninvasive and minimally invasive surgeries (MIS), which demand specialized toolkits. Focused ultrasound, for example, facilitates thermal ablation from a distance, thereby reducing injury to surrounding tissue. Focused ultrasound can often be performed noninvasively; however, it is more difficult to carry out in neuro-oncological tumors, as ultrasound is dramatically attenuated while propagating through the skull. This shortcoming has prompted exploration of MIS options for intracranial placement of focused ultrasound probes, such as within the BrainPath™ (NICO Corporation, Indianapolis, IN). Herein, we present the design, development, and in vitro testing of an image-guided, focused ultrasound prototype designed for use in MIS procedures. This probe can ablate neuro-oncological lesions despite its small size.

MATERIALS & METHODS

Preliminary prototypes were iteratively designed, built, and tested. The final prototype consisted of three 8-mm-diameter therapeutic elements guided by an imaging probe. Probe functionality was validated on a series of tissue-mimicking phantoms.

RESULTS

Lesions were created in tissue-mimicking phantoms with average dimensions of 2.5 × 1.2 × 6.5 mm and 3.4 × 3.25 × 9.36 mm after 10- and 30-second sonification, respectively. 30 s sonification with 118 W power at 50% duty cycle generated a peak temperature of 68 °C. Each ablation was visualized in real time by the built-in imaging probe.

CONCLUSION

We developed and validated an ultrasound-guided focused ultrasound probe for use in MIS procedures. The dimensional constraints of the prototype were designed to reflect those of BrainPath trocars, which are MIS tools used to create atraumatic access to deep-seated brain pathologies.

Ultrafast three-dimensional microbubble imaging in vivo predicts tissue damage volume distributions during nonthermal brain ablation

Transcranial magnetic resonance imaging (MRI)-guided focused ultrasound (FUS) thermal ablation is under clinical investigation for non-invasive neurosurgery, though its use is restricted to central brain targets due primarily to skull heating effects. The combination of FUS and contrast agent microbubbles greatly reduces the ultrasound exposure levels needed to ablate brain tissue and may help facilitate the use of transcranial FUS ablation throughout the brain. However, sources of variability exist during microbubble-mediated FUS procedures that necessitate the continued development of systems and methods for online treatment monitoring and control, to ensure that excessive and/or off-target bioeffects are not induced from the exposures. Methods: Megahertz-rate three-dimensional (3D) microbubble imaging in vivo was performed during nonthermal ablation in rabbit brain using a clinical-scale prototype transmit/receive hemispherical phased array system. Results:In-vivo volumetric acoustic imaging over microsecond timescales uncovered spatiotemporal microbubble dynamics hidden by conventional whole-burst temporal averaging. Sonication-aggregate ultrafast 3D source field intensity data were predictive of microbubble-mediated tissue damage volume distributions measured post-treatment using MRI and confirmed via histopathology. Temporal under-sampling of acoustic emissions, which is common practice in the field, was found to impede performance and highlighted the importance of capturing adequate data for treatment monitoring and control purposes. Conclusion: The predictive capability of ultrafast 3D microbubble imaging, reported here for the first time, will enable future microbubble-mediated FUS treatments with unparalleled precision and accuracy, and will accelerate the clinical translation of nonthermal tissue ablation procedures both in the brain and throughout the body.

Deployable tessellated transducer array for ultrasound focusing and bio-heat generation in a multilayer environment

High intensity focused ultrasound (HIFU) has great potential to thermally ablate diseased tissues with minimal invasion. Yet, HIFU practice has limited cancer treatment potential since the absorption, diffusion, and reflection of ultrasound prevent HIFU from penetrating the body to deep and concealed diseased tissue. To explore a vision of deployable HIFU transducers, this research introduces an origami-inspired concept wherein a deployable tessellated acoustic array is employed to reduce the distance between the HIFU transducer and diseased tissues. A flat-foldable HIFU transducer array is considered, such that the compact shape is used to pass through the human body and then deployed into the operational form for treatment. Here a theoretical framework is developed to study the focusing and thermal heating capabilities of the tessellated array in a multilayer environment. It is observed that the wavefield and thermal elevation realized by the foldable array are functionally similar to those of an ideal arc-shaped transducer. Folding patterns that permit adequate curvature and high quality factor, and that balance slenderness and conformability are found to be beneficial for an ultrasound focusing practice. The efficacy of the analytical predictions are verified through direct numerical simulations. All together, the results encourage attention to foldable array concepts as potential means to advance in-vivo HIFU-based procedures.

HIFU Power Monitoring Using Combined Instantaneous Current and Voltage Measurement

During high-intensity focused ultrasound (HIFU) therapy, it is important that the electrical power delivered to the transducer is monitored to avoid underexposure or overexposure, ensure patient safety, and to protect the transducer itself. Due to ease of measurement, the transducer's potential difference may be as an indicator of power delivery. However, even when a transducer's complex impedance is well characterized at small amplitudes and matching networks are used, voltage-only (VO) monitoring cannot account for the presence of drive waveform distortion, changes to the acoustic path, or damage to the transducer. In this study, combined current and voltage (CCV) is proposed as a magnetic resonance imaging (MRI)-compatible, miniature alternative to bidirectional power couplers, which is compatible with switched amplifiers. For CCV power measurement, current probe data were multiplied by the voltage waveform and integrated in the frequency domain. Transducer efficiency was taken into account to predict acoustic power. The technique was validated with a radiation force balance (RFB). When using a typical HIFU transducer and amplifier, VO predictions and acoustic power had a maximum difference of 20%. However, under the same conditions, CCV only had a maximum difference of 5%. The technique was applied to several lesioning experiments and it was shown that when VO was used as a control between two amplifiers, there was up to a 38% difference in lesion area. This greatly reduced to a maximum of 5% once CCV was used instead. These results demonstrate that CCV can accurately predict real-time electrical power delivery, leading to safer HIFU treatments.

Real-Time Photoacoustic Thermometry Combined With Clinical Ultrasound Imaging and High-Intensity Focused Ultrasound

High-intensity focused ultrasound (HIFU) treatment is a promising non-invasive method for killing or destroying the diseased tissues by locally delivering thermal and mechanical energy without damaging surrounding normal tissues. In HIFU, measuring the temperature at the site of delivery is important for improving therapeutic efficacy, controlling safety, and appropriately planning a treatment. Several researchers have proposed photoacoustic thermometry for monitoring HIFU treatment, but they had many limitations, including the inability to image while the HIFU is on, inability to provide two-dimensional monitoring, and the inability to be used clinically. In this paper, we propose a novel integrated real-time photoacoustic thermometry system for HIFU treatment monitoring. The system provides ultrasound B-mode imaging, photoacoustic structural imaging, and photoacoustic thermometry during HIFU treatment in real-time for both in vitro and in vivo environments, without any interference from the strong therapeutic HIFU waves. We have successfully tested the real-time photoacoustic thermometry by investigating the relationship between the photoacoustic amplitude and the measured temperature with in vitro phantoms and in vivo tumor-bearing mice. The results show the feasibility of a real-time photoacoustic thermometry system for safe and effective monitoring of HIFU treatment.

Enhanced Numerical Method for the Design of 3-D-Printed Holographic Acoustic Lenses for Aberration Correction of Single-Element Transcranial Focused Ultrasound

The correction of transcranial focused ultrasound aberrations is a relevant issue for enhancing various non-invasive medical treatments. The emission through multi-element phased arrays has been the most widely accepted method to improve focusing in recent years; however, the number and size of transducers represent a bottleneck that limits the focusing accuracy of the technique. To overcome this limitation, a new disruptive technology, based on 3-D-printed acoustic lenses, has recently been proposed. As the submillimeter precision of the latest generation of 3-D printers has been proven to overcome the spatial limitations of phased arrays, a new challenge is to improve the accuracy of the numerical simulations required to design this type of ultrasound lens. In the study described here, we evaluated two improvements in the numerical model applied in previous works for the design of 3-D-printed lenses: (i) allowing the propagation of shear waves in the skull by means of its simulation as an isotropic solid and (ii) introduction of absorption into the set of equations that describes the dynamics of the wave in both fluid and solid media. The results obtained in the numerical simulations are evidence that the inclusion of both s-waves and absorption significantly improves focusing.

Quasi Class-DE Driving of HIFU Transducer Arrays

Recently, a method was proposed to determine the parameters for each Class DE driver in high-intensity focused ultrasound arrays for efficient operation and to compensate for variations in the impedance of each array element. This work extends that method to consider the effect of switch resistance and to provide limited control on the power delivered to the transducers with a constant supply voltage while keeping a good efficiency. The method is experimentally validated using an integrated driver developed by the authors. This paper also shows that the frequency range for efficient electrical operation is close to the frequency where the transducer array presents a peak in the conversion efficiency.

Experimental study in vivo ablation of swine pancreas using high-intensity focused ultrasound

OBJECTIVE

The objective of this study is to evaluate the feasibility and safety of high-intensity focused ultrasound (HIFU) for ablation of swine pancreas and to detect the pathological variations in pancreas damage.

MATERIALS AND METHODS

(a) Eight swine were involved and randomly divided into two groups (Group A and B). HIFU was applied on swine in both groups for in vivo ablation of pancreas. The animals were anesthetized, and the artificial acoustic window was built. Then, the irradiation of FEP-BY02 type HIFU on pancreas was applied. (b) Swine in Group A were euthanized immediately after treating with HIFU to examine variations in pancreas. The biochemical evidence of pancreatitis was evaluated by blood samples collected from swine in Group B before and after HIFU. Then, the pancreas of swine in Group B was euthanized on day 5 after treatment to examine the pancreas. All specimens were visually inspected for both ultrasonic focal damage region (UFDR) and pathological routine by a skilled pathologist.

RESULTS

(a) The vital signs of all animals were stable during HIFU treatment and recovered well after treatment. (b) UFDR were observed in all HIFU irradiation region of the specimens, without significant size difference between the two groups. The coagulation nucleus pyknosis, cytochylema vacuolation, and nucleus membrane disruption were observed after HIFU in both groups. Membranous structure dissolution and inflammatory cell infiltration were also found after HIFU in swine of Group B. (c) There was no significant difference in the levels of blood amylase in swine of Group B before and after HIFU treatment.

CONCLUSIONS

It was feasible and safe to use HIFU for ablation of the pancreas in swine.

HIFU Drive System Miniaturization Using Harmonic Reduced Pulsewidth Modulation

Switched excitation has the potential to improve on the cost, efficiency, and size of the linear amplifier circuitry currently used in high-intensity focused ultrasound (HIFU) systems. Existing switching schemes are impaired by high harmonic distortion or lack array apodisation capability, so require adjustable supplies and/or large power filters to be useful. A multilevel pulsewidth modulation (PWM) topology could address both of these issues but the switching-speed limitations of transistors mean that there are a limited number of pulses available in each waveform cycle. In this study, harmonic reduction PWM (HRPWM) is proposed as an algorithmic solution to the design of switched waveforms. Its appropriateness for HIFU was assessed by design of a high power five-level unfiltered amplifier and subsequent thermal-only lesioning of ex vivo chicken breast. Three switched waveforms of different electrical powers (16, 26, 35 W) were generated using the HRPWM algorithm. Lesion sizes were measured and compared with those made at the same electrical power using a linear amplifier and bi-level excitation. HRPWM produced symmetric, thermal-only lesions that were the same size as their linear amplifier equivalents ( ). At 16 W, bi-level excitation produced smaller lesions but at higher power levels large transients in the acoustic waveform nucleated undesired cavitation. These results demonstrate that HRPWM can minimize HIFU drive circuity size without the need for filters to remove harmonics or adjustable power supplies to achieve array apodisation.

Weighting the Passive Acoustic Mapping Technique With the Phase Coherence Factor for Passive Ultrasound Imaging of Ultrasound-Induced Cavitation

Ultrasound (US) cavitation is currently being explored for low-invasive therapy techniques applied to a wide panel of pathologies. Because of the random behavior of cavitation, a real-time spatial monitoring system may be required. For this purpose, the US passive imaging techniques have been recently investigated. In particular, the passive acoustic mapping (PAM) beamforming method enables the reconstruction of cavitation activity maps by beamforming acoustic signals passively recorded by an array transducer. In this paper, an optimized version of PAM, PAM weighted with a phase coherence factor (PAM-PCF), is considered. A general validation process is developed including simulations on a point source and experiments on a wire. Furthermore, using a focused regulated US-induced cavitation generator, reproducible cavitation experiments are conducted in water and in agar gel. The spatial behavior of a bubble cavitation cloud is determined using the PAM-PCF beamforming method to localize the focal cavitation point in two perpendicular imaging planes.

A Low-Cost Miniature Histotripsy Transducer for Precision Tissue Ablation

A miniature, 10 mm aperture histotripsy transducer with an f-number of 0.7 was fabricated using an elliptically shaped aluminum lens, which was epoxy-bonded to an air-backed 5.0 MHz, PTZ-5A, 1-3 dice-and-fill piezoelectric composite, and the lens coupled to water using a quarter-wavelength matching layer of Parylene-C. A Krimholtz-Leedom-Matthaei model of the device and curved lens was developed. The epoxy layer resulted in an increased power output at 6.8 MHz compared to the 5 MHz composite design. Cavitation was observed in water by driving the composite with a 173 V single-cycle, unipolar 6.8 MHz pulse at a pulse repetition frequency of 50 Hz, and a bubble cloud 264 long by 124 wide was measured. A coregistered imaging and ablation device was also fabricated and characterized. The coregistered device was modified to include a mm square hole through the center, allowing access for a high-frequency imaging array, and both imaging and ablation are demonstrated in cerebral tissue with this device. Radial -3 dB beam widths were measured as 0.145 and 0.116 mm, and axial -3 dB depths of field were 0.698 and 0.752 mm for the noncoregistered and coregistered transducers, respectively. Total material cost for the transducer and pulser board is below $200 USD.

Integrated Histotripsy and Bubble Coalescence Transducer for Rapid Tissue Ablation

Residual bubbles produced after collapse of a cavitation cloud provide cavitation nuclei for subsequent cavitation events, causing cavitation to occur repeatedly at the same discrete set of sites. This effect, referred to as cavitation memory, limits the efficiency of histotripsy soft tissue fractionation. Besides passively mitigating cavitation memory by using a low pulse repetition frequency (~1 Hz), an active strategy was developed by our group. In this strategy, low-amplitude ultrasound sequences were used to stimulate coalescence of residual bubbles. The goal of this work is to remove cavitation memory and achieve rapid, homogeneous lesion formation using a single phased array transducer. A 1-MHz integrated histotripsy and bubble coalescing (BC) transducer system with a specialized electronic driving system was built in house. High-amplitude ( MPa) histotripsy pulses and subsequent low-amplitude (~1-2 MPa) BC sequences were applied to a red blood cell tissue-mimicking phantom at a single focal site. Significant reduction of the cavitation memory effect and increase in the fractionation rate were observed by introducing BC sequence. Effects of BC pulsing parameters were further studied. The optimal BC parameters were then utilized to homogenize a mm² region at high rate.

Phase-Inverted Multifrequency HIFU Transducer for Lesion Expansion: A Simulation Study

It has been well known that the treatment time of high-intensity focused ultrasound (HIFU) surgery can be reduced by expanding the focal area per sonication. Previously, a dual-concentric transducer using phase-inverted signals was proposed to axially extend the focal area, but it has suffered from the deep notch point between two focal lobes. In this paper, we propose the improved HIFU transducer with dual-concentric aperture driven by phase-inverted multifrequency signals based on an inversion layer technique. The proposed transducer can generate the expanded focal zone with a significantly reduced level of the notch point between two focal lobes in the axial direction. The performance of the proposed transducer was investigated using finite element analysis simulation. The electrical impedance, one-way impulse response, and acoustic field of the transducer were simulated. Subsequently, the lesion volume was investigated by heat transfer simulation. In the proposed method, the level of the notch point was increased above -6 dB due to various phase interactions between the fundamental and harmonic frequency combinations and the inverted and noninverted frequency combinations. The -6-dB depth of field related to the necrotic lesion size was increased by 141% compared with the conventional single element transducer. Thus, the proposed transducer can be a potential way to enlarge coagulated lesion size resulting in a reduced overall treatment time of HIFU surgery.

Evaluation of a Three Hydrophones Method for 2-Dimensional Cavitation Localization

Cavitation is a critical parameter in various therapeutic applications involving ultrasound (US) such as histotrispy, lithothripsy, drug delivery, and cavitation-enhanced hyperthermia. A cavitation exposure outside the region of interest may lead to suboptimal treatment efficacy or in a worse case, to safety issues. Current methods of localizing cavitation are based on imaging approaches, such as beamforming the cavitation signals received passively by a US imager. These methods, although efficient, require expensive equipment, which may discourage potential future developments. We propose a threehydrophone method to localize the cavitation cloud source. Firstly, the delays between the three receptors are measured by detecting the maximum of their inter-correlations. Then, the position of the source is calculated by either minimizing a cost function or solving hyperbolic equations. After a numerical validation, the method was assessed experimentally. This method was able to track a source displacement with accuracy similar to the size of the cavitation cloud (2-4 millimeters). This light and versatile method provides interesting perspectives since localization can be executed in real time and the extension to three-dimensional localization seems straightforward.

Method for Designing Multielement Fully Populated Random Phased Arrays for Ultrasound Surgery Applications

Maximizing the power of multielement phased arrays is a critical factor for high-intensity focused ultrasound (HIFU) applications such as histotripsy and transcostal sonications. This can be achieved by a tight packing of the array elements. Good electronic focusing capabilities are also required. Currently used quasi-random arrays with a relatively low filling factor of about 60% have this focusing ability. Here, a novel method of designing random HIFU arrays with the maximum possible filling factor (100% if no gaps between elements needed in practice are introduced) and polygonal elements of equal area and slightly different shapes based on the capacity-constrained tessellation is described. The method is validated by comparing designs of two arrays with the same geometric and physical parameters: an existing 256-element array with a compact 16-spirals layout of circular elements and the proposed array with the maximum possible filling factor. Introduction of a 0.5-mm gap between the elements of the new array resulted in a reduction of its filling factor to 86%, as compared with 61% for the spiral array. It is shown that for the same intensity at the elements, the proposed array provides two times higher total power while maintaining the same electronic focusing capabilities as compared to the spiral one. Furthermore, the surface of the capacity-constrained tessellation array, its boundary, and a central opening can have arbitrary shapes.

High-intensity focused ultrasound ablation around the tubing

High-intensity focused ultrasound (HIFU) has been emerging as an effective and noninvasive modality in cancer treatment with very promising clinical results. However, a small vessel in the focal region could be ruptured, which is an important concern for the safety of HIFU ablation. In this study, lesion formation in the polyacrylamide gel phantom embedded with different tubing (inner diameters of 0.76 mm and 3 mm) at varied flow speeds (17–339 cm/s) by HIFU ablation was photographically recorded. Produced lesions have decreased length (~30%) but slightly increased width (~6%) in comparison to that without the embedded tubing. Meanwhile, bubble activities during the exposures were measured by passive cavitation detection (PCD) at the varied pulse repetition frequency (PRF, 10–30 Hz) and duty cycle (DC, 10%-20%) of the HIFU bursts. High DC and low flow speed were found to produce stronger bubble cavitation whereas no significant influence of the PRF. In addition, high-speed photography illustrated that the rupture of tubing was produced consistently after the first HIFU burst within 20 ms and then multiple bubbles would penetrate into the intraluminal space of tubing through the rupture site by the acoustic radiation force. Alignment of HIFU focus to the anterior surface, middle, and posterior surface of tubing led to different characteristics of vessel rupture and bubble introduction. In summary, HIFU-induced vessel rupture is possible as shown in this phantom study; produced lesion sizes and shapes are dependent on the focus alignment to the tubing, flow speed, and tubing properties; and bubble cavitation and the formation liquid jet may be one of the major mechanisms of tubing rupture as shown in the high-speed photography.

Medical Devices; Gastroenterology-Urology Devices; Classification of the High Intensity Ultrasound System for Prostate Tissue Ablation. Final order

The Food and Drug Administration (FDA or we) is classifying the high intensity ultrasound system for prostate tissue ablation into class II (special controls). The special controls that apply to the device type are identified in this order and will be part of the codified language for the high intensity ultrasound system for prostate tissue ablation’s classification. We are taking this action because we have determined that classifying the device into class II (special controls) will provide a reasonable assurance of safety and effectiveness of the device. We believe this action will also enhance patients’ access to beneficial innovative devices, in part by reducing regulatory burdens.

A Prototype Therapy System for Transcutaneous Application of Boiling Histotripsy

Boiling histotripsy (BH) is a method of focused ultrasound surgery that noninvasively applies millisecond-length pulses with high-amplitude shock fronts to generate liquefied lesions in tissue. Such a technique requires unique outputs compared to a focused ultrasound thermal therapy apparatus, particularly to achieve high in situ pressure levels through intervening tissue. This paper describes the design and characterization of a system capable of producing the necessary pressure to transcutaneously administer BH therapy through clinically relevant overlying tissue paths using pulses with duration up to 10 ms. A high-voltage electronic pulser was constructed to drive a 1-MHz focused ultrasound transducer to produce shock waves with amplitude capable of generating boiling within the pulse duration in tissue. The system output was characterized by numerical modeling with the 3-D Westervelt equation using boundary conditions established by acoustic holography measurements of the source field. Such simulations were found to be in agreement with directly measured focal waveforms. An existing derating method for nonlinear therapeutic fields was used to estimate in situ pressure levels at different tissue depths. The system was tested in ex vivo bovine liver samples to create BH lesions at depths up to 7 cm. Lesions were also created through excised porcine body wall (skin, adipose, and muscle) with 3-5 cm thickness. These results indicate that the system is capable of producing the necessary output for transcutaneous ablation with BH.

Variation of High-Intensity Therapeutic Ultrasound (HITU) Pressure Field Characterization: Effects of Hydrophone Choice, Nonlinearity, Spatial Averaging and Complex Deconvolution

Reliable acoustic characterization is fundamental for patient safety and clinical efficacy during high-intensity therapeutic ultrasound (HITU) treatment. Technical challenges, such as measurement variation and signal analysis, still exist for HITU exposimetry using ultrasound hydrophones. In this work, four hydrophones were compared for pressure measurement: a robust needle hydrophone, a small polyvinylidene fluoride capsule hydrophone and two fiberoptic hydrophones. The focal waveform and beam distribution of a single-element HITU transducer (1.05 MHz and 3.3 MHz) were evaluated. Complex deconvolution between the hydrophone voltage signal and frequency-dependent complex sensitivity was performed to obtain pressure waveforms. Compressional pressure (p₊), rarefactional pressure (p_-) and focal beam distribution were compared up to 10.6/-6.0 MPa (p₊/p_-) (1.05 MHz) and 20.65/-7.20 MPa (3.3 MHz). The effects of spatial averaging, local non-linear distortion, complex deconvolution and hydrophone damage thresholds were investigated. This study showed a variation of no better than 10%-15% among hydrophones during HITU pressure characterization.

Assessment of Gold Nanoparticle-Mediated-Enhanced Hyperthermia Using MR-Guided High-Intensity Focused Ultrasound Ablation Procedure

High-intensity focused ultrasound (HIFU) has gained increasing popularity as a noninvasive therapeutic procedure to treat solid tumors. However, collateral damage due to the use of high acoustic powers during HIFU procedures remains a challenge. The objective of this study is to assess the utility of using gold nanoparticles (gNPs) during HIFU procedures to locally enhance heating at low powers, thereby reducing the likelihood of collateral damage. Phantoms containing tissue-mimicking material (TMM) and physiologically relevant concentrations (0%, 0.0625%, and 0.125%) of gNPs were fabricated. Sonications at acoustic powers of 10, 15, and 20 W were performed for a duration of 16 s using an MR-HIFU system. Temperature rises and lesion volumes were calculated and compared for phantoms with and without gNPs. For an acoustic power of 10 W, the maximum temperature rise increased by 32% and 43% for gNPs concentrations of 0.0625% and 0.125%, respectively, when compared to the 0% gNPs concentration. For the power of 15 W, a lesion volume of 0, 44.5 ± 7, and 63.4 ± 32 mm³ was calculated for the gNPs concentration of 0%, 0.0625%, and 0.125%, respectively. For a power of 20 W, it was found that the lesion volume doubled and tripled for concentrations of 0.0625% and 0.125% gNPs, respectively, when compared to the concentration of 0% gNPs. We conclude that gNPs have the potential to locally enhance the heating and reduce damage to healthy tissue during tumor ablation using HIFU.

Annular phased array transducer for preclinical testing of anti-cancer drug efficacy on small animals

A technique using pulsed High Intensity Focused Ultrasound (HIFU) to destroy deep-seated solid tumors is a promising noninvasive therapeutic approach. A main purpose of this study was to design and test a HIFU transducer suitable for preclinical studies of efficacy of tested, anti-cancer drugs, activated by HIFU beams, in the treatment of a variety of solid tumors implanted to various organs of small animals at the depth of the order of 1-2cm under the skin. To allow focusing of the beam, generated by such transducer, within treated tissue at different depths, a spherical, 2-MHz, 29-mm diameter annular phased array transducer was designed and built. To prove its potential for preclinical studies on small animals, multiple thermal lesions were induced in a pork loin ex vivo by heating beams of the same: 6W, or 12W, or 18W acoustic power and 25mm, 30mm, and 35mm focal lengths. Time delay for each annulus was controlled electronically to provide beam focusing within tissue at the depths of 10mm, 15mm, and 20mm. The exposure time required to induce local necrosis was determined at different depths using thermocouples. Location and extent of thermal lesions determined from numerical simulations were compared with those measured using ultrasound and magnetic resonance imaging techniques and verified by a digital caliper after cutting the tested tissue samples. Quantitative analysis of the results showed that the location and extent of necrotic lesions on the magnetic resonance images are consistent with those predicted numerically and measured by caliper. The edges of lesions were clearly outlined although on ultrasound images they were fuzzy. This allows to conclude that the use of the transducer designed offers an effective noninvasive tool not only to induce local necrotic lesions within treated tissue without damaging the surrounding tissue structures but also to test various chemotherapeutics activated by the HIFU beams in preclinical studies on small animals.

[Prevention of surgery-related complications of D2+ lymphadenectomy for gastric cancer]

D2 lymphadenectomy is currently the worldwide standard operation for locally advanced gastric cancer and D2+ is an option for some selected patients. The D2 plus lymphadenectomy includes No.8p, No.10, No.11d, No.12b, No.12p, No.13, No.14v, No.16a2 and No.16b1. Dissection of these groups of lymph nodes may cause related complications. Postoperative complications that can cause prolonged inflammation have significant impact not only on mortality but also on overall survival of patients with gastric cancer even if the tumor is resected curatively. D2 plus lymphadenectomy is recommended only in high volume medical center by experienced surgeon. The adequate exposure of the operative field, right anatomical space, use of ultrasound scalpel and operator with enough patience are proved to be pivotal to prevent the complications.

[The key points of prevention for special surgical complications after radical operation of gastric cancer]

Incidence of gastric cancer is high in China and standard radical operation is currently the main treatment for gastric cancer. Postoperative complications, especially some special complications, can directly affect the prognosis of patients, even result in the increase of mortality. But the incidences of these special complications are low, so these complications are often misdiagnosed and delayed in treatment owing to insufficient recognition of medical staff. These special complications include (1) Peterson hernia: It is an abdominal hernia developed in the space between Roux loop and transverse colon mesentery after Roux-Y reconstruction of digestive tract. Peterson hernia is rare and can quickly result in gangrenous ileus. Because of low incidence and without specific clinical symptoms, this hernia does not attract enough attention in clinical practice, so the outcome will be very serious. Once the diagnosis is made, an emergent operation must be performed immediately. Peterson space should be closed routinely in order to avoid the development of hernia. (2) Lymphatic leakage: It is also called chyle leakage. Cisterna chylus is formed by gradual concentration of extensive lymphatic net to diaphragm angle within abdominal cavity. Lymphadenectomy during operation may easily damage lymphatic net and result in leakage. The use of ultrasonic scalpel can decrease the risk of lymphatic leakage in certain degree. If lymphatic leakage is found during operation, transfixion should be performed in time. Treatment includes total parenteral nutrition, maintenance of internal environment, supplement of protein, and observation by clamp as an attempt. (3)Duodenal stump leakage: It is one of serious complications affecting the recovery and leading to death after subtotal gastrectomy. Correct management of duodenal stump during operation is one of key points of the prevention of duodenal stump leakage. Routine purse embedding of duodenal stump is recommend during operation. The key treatment of this complication is to promt diagnosis and effective hemostasis.(4) Blood supply disorder of Roux-Y intestinal loop: Main preventive principle of this complication is to pay attention to the blood supply of vascular arch in intestinal edge. (5) Anastomotic obstruction by big purse of jejunal stump: When Roux-en-Y anastomosis is performed after distal radical operation for gastric cancer, anvil is placed in the remnant stomach and anastomat from distal jejunal stump is placed to make gastrojejunal anastomosis, and the stump is closed with big purse embedding. The embedding jejunal stump may enter gastric cavity leading to internal hernia and anastomotic obstruction. We suggest that application of interruptable and interlocking suture and fixation of stump on the gastric wall can avoid the development of this complication.

Modeling-based design and assessment of an acousto-optic guided high-intensity focused ultrasound system

Real-time acousto-optic (AO) sensing has been shown to noninvasively detect changes in ex vivo tissue optical properties during high-intensity focused ultrasound (HIFU) exposures. The technique is particularly appropriate for monitoring noncavitating lesions that offer minimal acoustic contrast. A numerical model is presented for an AO-guided HIFU system with an illumination wavelength of 1064 nm and an acoustic frequency of 1.1 MHz. To confirm the model’s accuracy, it is compared to previously published experimental data gathered during AO-guided HIFU in chicken breast. The model is used to determine an optimal design for an AO-guided HIFU system, to assess its robustness, and to predict its efficacy for the ablation of large volumes. It was found that a through transmission geometry results in the best performance, and an optical wavelength around 800 nm was optimal as it provided sufficient contrast with low absorption. Finally, it was shown that the strategy employed while treating large volumes with AO guidance has a major impact on the resulting necrotic volume and symmetry.

High intensity focused ultrasound with Focal-One® device: Prostate-specific antigen impact and morbidity evaluation during the initial experience

OBJECTIVE

We report our initial experience in the treatment of prostate cancer (PCa) with high-intensity focused ultrasound (HIFU) using the Focal-One^® device.

MATERIAL AND METHODS

Retrospective review of the prospectively populated database. Between June 2014 to October 2015, 85 patients underwent HIFU (focal/whole-gland) treatment for localized PCa. Preoperative cancer localization was done with multiparametric magnetic resonance imaging (mpMRI) and transperineal mapping biopsies. Treatment was carried out using the Focal-One^® device under general anesthesia. Oncological follow-up: PSA measurement and control biopsy with mpMRI according to protocol. Questionnaire-based functional outcome assessment was done. Complications were reported using Clavien classification.

RESULTS

The median PSA was 7.79ng/ml (IQR 6.32-9.16), with a median prostate volume of 38cc (IQR: 33-49.75). Focal and whole-gland therapy was performed in 64 and 21 patients respectively. Ten patients received salvage HIFU. Complications were encountered in 15% of cases, all Clavien 2 graded. Mean hospital stay was 1.8 days (0-7) and bladder catheter was removed on day 2 (1-6). Mean percentage reduction of PSA was 54%. Median follow-up was 3 months (IQR: 2-8). Functional outcomes: All patients were continents at 3 months and potency was maintained in 83% of the preoperatively potent.

CONCLUSIONS

Focal-One^® HIFU treatment appears to be a safe procedure with few complications. Functional outcomes proved no urinary incontinence and sexual function were maintained in 83%.

Ex Vivo HIFU Experiments Using a $32 \times 32$ -Element CMUT Array

High-intensity focused ultrasound (HIFU) has been used as noninvasive treatment for various diseases. For these therapeutic applications, capacitive micromachined ultrasonic transducers (CMUTs) have advantages that make them potentially preferred transducers over traditional piezoelectric transducers. In this paper, we present the design and the fabrication process of an 8 ×8 -mm 2 32 ×32 -element 2-D CMUT array for HIFU applications. To reduce the system complexity for addressing the 1024 transducer elements, we propose to group the CMUT array elements into eight HIFU channels based on the phase delay from the CMUT element to the targeted focal point. Designed to focus at an 8-mm depth with a 5-MHz exciting frequency, this grouping scheme was realized using a custom application-specific integrated circuit. With a 40-V dc bias and a 60-V peak-to-peak ac excitation, the surface pressure was measured 1.2 MPa peak-to-peak and stayed stable for a long enough time to create a lesion. With this dc and ac voltage combination, the measured peak-to-peak output pressure at the focus was 8.5 MPa, which is expected to generate a lesion in a minute according to the temperature simulation. The following ex vivo tissue experiments successfully demonstrated its capability to make lesions in both bovine muscle and liver tissue.

Class-DE Ultrasound Transducer Driver for HIFU Therapy

This paper presents a practical implementation of an integrated MRI-compatible CMOS amplifier capable of directly driving a piezoelectric ultrasound transducer suitable for high-intensity focused ultrasound (HIFU) therapy. The amplifier operates in Class DE mode without the need for an output matching network. The integrated amplifier has been implemented with the AMS AG H35 CMOS process. A class DE amplifier design methodology for driving unmatched piezoelectric loads is presented along with simulation and experimental results. The proposed design achieves approximately 90% efficiency with over 800 mW of output power at 1010 kHz. The total die area including pads is 2 mm(2). Compatibility with MRI was validated with B1 imaging of a phantom and the amplifier circuit.

HIFU for Palliative Treatment of Pancreatic Cancer

Pancreatic cancer is one of the deadliest malignancies, with only a 6 % 5-year survival rate and over 50 % of patients being diagnosed at the advanced stage. Current therapies are ineffective, and the treatment of patients with advanced disease is palliative. In the past decade, HIFU ablation has emerged as a modality for palliative treatment of pancreatic tumors. Multiple preclinical and non-randomized clinical trials have been performed to evaluate the safety and efficacy of this procedure. Substantial tumor-related pain reduction was achieved in most cases after HIFU treatment and few significant side effects were observed. In addition, some studies indicate that combination of HIFU ablation with chemotherapy may provide a survival benefit. This chapter summarizes the pre-clinical and clinical experience obtained to date in HIFU treatment of pancreatic tumors and discusses the challenges, limitations and new approaches in this modality.

HIFU Tissue Ablation: Concept and Devices

High intensity focused ultrasound (HIFU) is rapidly gaining clinical acceptance as a technique capable of providing non-invasive heating and ablation for a wide range of applications. Usually requiring only a single session, treatments are often conducted as day case procedures, with the patient either fully conscious, lightly sedated or under light general anesthesia. HIFU scores over other thermal ablation techniques because of the lack of necessity for the transcutaneous insertion of probes into the target tissue. Sources placed either outside the body (for treatment of tumors or abnormalities of the liver, kidney, breast, uterus, pancreas brain and bone), or in the rectum (for treatment of the prostate), provide rapid heating of a target tissue volume, the highly focused nature of the field leaving tissue in the ultrasound propagation path relatively unaffected. Numerous extra-corporeal, transrectal and interstitial devices have been designed to optimize application-specific treatment delivery for the wide-ranging areas of application that are now being explored with HIFU. Their principle of operation is described here, and an overview of their design principles is given.

In Vivo application and localization of transcranial focused ultrasound using dual-mode ultrasound arrays

Focused ultrasound (FUS) has been proposed for a variety of transcranial applications, including neuromodulation, tumor ablation, and blood-brain barrier opening. A flurry of activity in recent years has generated encouraging results demonstrating its feasibility in these and other applications. To date, monitoring of FUS beams has been primarily accomplished using MR guidance, where both MR thermography and elastography have been used. The recent introduction of real-time dual-mode ultrasound array (DMUA) systems offers a new paradigm in transcranial focusing. In this paper, we present first experimental results of ultrasound-guided transcranial FUS (tFUS) application in a rodent brain, both ex vivo and in vivo. DMUA imaging is used for visualization of the treatment region for placement of the focal spot within the brain. This includes the detection and localization of pulsating blood vessels at or near the target point(s). In addition, DMUA imaging is used to monitor and localize the FUS-tissue interactions in real time. In particular, a concave (40 mm radius of curvature), 32-element, 3.5-MHz DMUA prototype was used for imaging and tFUS application in ex vivo and in vivo rat models. The ex vivo experiments were used to evaluate the point spread function of the transcranial DMUA imaging at various points within the brain. In addition, DMUA-based transcranial ultrasound thermography measurements were compared with thermocouple measurements of subtherapeutic tFUS heating in rat brain ex vivo. The ex vivo setting was also used to demonstrate the capability of DMUA to produce localized thermal lesions. The in vivo experiments were designed to demonstrate the ability of the DMUA to apply, monitor, and localize subtherapeutic tFUS patterns that could be beneficial in transient blood-brain barrier opening. The results show that although the DMUA focus is degraded due to the propagation through the skull, it still produces localized heating effects within a sub-millimeter volume. In addition, DMUA transcranial echo data from brain tissue allow for reliable estimation of temperature change.

Efficient Driving of Piezoelectric Transducers Using a Biaxial Driving Technique

Efficient driving of piezoelectric materials is desirable when operating transducers for biomedical applications such as high intensity focused ultrasound (HIFU) or ultrasound imaging. More efficient operation reduces the electric power required to produce the desired bioeffect or contrast. Our preliminary work [Cole et al. Journal of Physics: Condensed Matter. 2014;26(13):135901.] suggested that driving transducers by applying orthogonal electric fields can significantly reduce the coercivity that opposes ferroelectric switching. We present here the experimental validation of this biaxial driving technique using piezoelectric ceramics typically used in HIFU. A set of narrow-band transducers was fabricated with two sets of electrodes placed in an orthogonal configuration (following the propagation and the lateral mode). The geometry of the ceramic was chosen to have a resonance frequency similar for the propagation and the lateral mode. The average (± s.d.) resonance frequency of the samples was 465.1 (± 1.5) kHz. Experiments were conducted in which each pair of electrodes was driven independently and measurements of effective acoustic power were obtained using the radiation force method. The efficiency (acoustic/electric power) of the biaxial driving method was compared to the results obtained when driving the ceramic using electrodes placed only in the pole direction. Our results indicate that the biaxial method increases efficiency from 50% to 125% relative to the using a single electric field.

Histotripsy Lesion Formation Using an Ultrasound Imaging Probe Enabled by a Low-Frequency Pump Transducer

When histotripsy pulses shorter than 2 cycles are applied, the formation of a dense bubble cloud relies only on the applied peak negative pressure (p-) exceeding the "intrinsic threshold" of the medium (absolute value of 26-30 MPa in most soft tissues). It has been found that a sub-threshold high-frequency probe pulse (3 MHz) can be enabled by a sub-threshold low-frequency pump pulse (500 kHz) where the sum exceeds the intrinsic threshold, thus generating lesion-producing dense bubble clouds ("dual-beam histotripsy"). Here, the feasibility of using an imaging transducer to provide the high-frequency probe pulse in the dual-beam histotripsy approach is investigated. More specifically, an ATL L7-4 imaging transducer (Philips Healthcare, Andover, MA, USA), pulsed by a V-1 Data Acquisition System (Verasonics, Redmond, WA, USA), was used to generate the high-frequency probe pulses. The low-frequency pump pulses were generated by a 20-element 345-kHz array transducer, driven by a custom high-voltage pulser. These dual-beam histotripsy pulses were applied to red blood cell tissue-mimicking phantoms at a pulse repetition frequency of 1 Hz, and optical imaging was used to visualize bubble clouds and lesions generated in the red blood cell phantoms. The results indicated that dense bubble clouds (and resulting lesions) were generated when the p- of the sub-threshold pump and probe pulses combined constructively to exceed the intrinsic threshold. The average size of the smallest reproducible lesions using the imaging probe pulse enabled by the sub-threshold pump pulse was 0.7 × 1.7 mm, whereas that using the supra-threshold pump pulse alone was 1.4 × 3.7 mm. When the imaging transducer was steered laterally, bubble clouds and lesions were steered correspondingly until the combined p- no longer exceeded the intrinsic threshold. These results were also validated with ex vivo porcine liver experiments. Using an imaging transducer for dual-beam histotripsy can have two advantages: (i) lesion steering can be achieved using the steering of the imaging transducer (implemented with the beamformer of the accompanying programmable ultrasound system), and (ii) treatment can be simultaneously monitored when the imaging transducer is used in conjunction with an ultrasound imaging system.

Spatial-temporal ultrasound imaging of residual cavitation bubbles around a fluid-tissue interface in histotripsy

Cavitation is considered as the primary mechanism of soft tissue fragmentation (histotripsy) by pulsed high-intensity focused ultrasound. The residual cavitation bubbles have a dual influence on the histotripsy pulses: these serve as nuclei for easy generation of new cavitation, and act as strong scatterers causing energy "shadowing." To monitor the residual cavitation bubbles in histotripsy, an ultrafast active cavitation imaging method with relatively high signal-to-noise ratio and good spatial-temporal resolution was proposed in this paper, which combined plane wave transmission, minimum variance beamforming, and coherence factor weighting. The spatial-temporal evolutions of residual cavitation bubbles around a fluid-tissue interface in histotripsy under pulse duration (PD) of 10-40 μs and pulse repetition frequency (PRF) of 0.67-2 kHz were monitored by this method. The integrated bubble area curves inside the tissue interface were acquired from the bubble image sequence, and the formation process of histotripsy damage was estimated. It was observed that the histotripsy efficiency decreased with both longer PDs and higher PRFs. A direct relationship with a coefficient of 1.0365 between histotripsy lesion area and inner residual bubble area was found. These results can assist in monitoring and optimization of the histotripsy treatment further.

Magnetic resonance imaging assessment of effective ablated volume following high intensity focused ultrasound

Under magnetic resonance (MR) guidance, high intensity focused ultrasound (HIFU) is capable of precise and accurate delivery of thermal dose to tissues. Given the excellent soft tissue imaging capabilities of MRI, but the lack of data on the correlation of MRI findings to histology following HIFU, we sought to examine tumor response to HIFU ablation to determine whether there was a correlation between histological findings and common MR imaging protocols in the assessment of the extent of thermal damage. Female FVB mice (n = 34), bearing bilateral neu deletion tumors, were unilaterally insonated under MR guidance, with the contralateral tumor as a control. Between one and five spots (focal size 0.5 × 0.5 × 2.5 mm³) were insonated per tumor with each spot receiving approximately 74.2 J of acoustic energy over a period of 7 seconds. Animals were then imaged on a 7T MR scanner with several protocols. T1 weighted images (with and without gadolinium contrast) were collected in addition to a series of T2 weighted and diffusion weighted images (for later reconstruction into T2 and apparent diffusion coefficient maps), immediately following ablation and at 6, 24, and 48 hours post treatment. Animals were sacrificed at each time point and both insonated/treated and contralateral tumors removed and stained for NADH-diaphorase, caspase 3, or with hematoxylin and eosin (H&E). We found the area of non-enhancement on contrast enhanced T1 weighted imaging immediately post ablation correlated with the region of tissue receiving a thermal dose CEM43 ≥ 240 min. Moreover, while both tumor T2 and apparent diffusion coefficient values changed from pre-ablation values, contrast enhanced T1 weighted images appeared to be more senstive to changes in tissue viability following HIFU ablation.

[OPPORTUNITIES OF HIFU TECHNOLOGY FOR TREATMENT FIBROIDS DISEASES SUCH AS NON-INVASIVE AND ALTERNATIVE METHOD TO SURGERY]

Concepts of medical treatment are constantly evolving and improving. This opportunity provides treatment with High Intensity Focused Ultrasound (HIFU). In Europe and Asia more than 10,000 patients with uterine fibroids have been successfully treated with HIFU technology until now. This is completely innovative technology for non-invasive extracorporeal treatment of benign and malignant tumors. Neighboring healthy tissue is not damaged. The main indication in HIFU-Center in Pleven is uterine fibroid. It is the most common solid tumor in the female pelvis and is the leading cause of hysterectomy. The methods of treatment are hysterectomy, myomectomy or embolization of uterine arteries. HIFU-methodology allows non-invasive treatment of fibroids disease.

Robotized High Intensity Focused Ultrasound (HIFU) system for treatment of mobile organs using motion tracking by ultrasound imaging: An in vitro study

High Intensity Focused Ultrasound (HIFU) therapy is a very promising method for ablation of solid tumors. However, intra-abdominal organ motion, principally due to breathing, is a substantial limitation that results in incorrect tumor targeting. The objective of this work is to develop an all-in-one robotized HIFU system that can compensate motion in real-time during HIFU treatment. To this end, an ultrasound visual servoing scheme working at 20 Hz was designed. It relies on the motion estimation by using a fast ultrasonic speckle tracking algorithm and on the use of an interleaved imaging/HIFU sonication sequence for avoiding ultrasonic wave interferences. The robotized HIFU system was tested on a sample of chicken breast undergoing a vertical sinusoidal motion at 0.25 Hz. Sonications with and without motion compensation were performed in order to assess the effect of motion compensation on thermal lesions induced by HIFU. Motion was reduced by more than 80% thanks to this ultrasonic visual servoing system.

Ultrafast 2-dimensional image monitoring and array-based passive cavitation detection for ultrasound contrast agent destruction in a variably sized region

OBJECTIVES

A combined approach was proposed, based on programmable ultrasound equipment, to simultaneously monitor surviving microbubbles and detect cavitation activity during microbubble destruction in a variably sized region for use in ultrasound contrast agent (UCA)-enhanced therapeutic ultrasound applications.

METHODS

A variably sized focal region wherein the acoustic pressure was above the UCA fragmentation threshold was synthesized at frequencies of 3, 4, 5, and 6 MHz with a linear broadband imaging probe. The UCAs' temporal and spatial distribution during the microbubbles' destruction was monitored in a 2-dimensional imaging plane at 5 MHz and a frame rate of 400 Hz, and simultaneously, broadband noise emissions during the microbubbles' fragmentation were extracted by using the backscattered signals produced by the focused release bursts (ie, destruction pulses) themselves. Afterward, the temporal evolution of broadband noise emission, the surviving microbubbles in a region of interest (ROI), and the destruction area in a static UCA suspension were computed. Then the inertial cavitation dose, destruction rate of microbubbles in the ROI, and area of the destruction region were determined.

RESULTS

It was found that an increasing pulse length and a decreasing transmit aperture and excitation frequency were correlated with an increased inertial cavitation dose, microbubble destruction rate, and destruction area. Furthermore, it was obvious that the microbubble destruction rate was significantly correlated with the inertial cavitation dose (P < .05). In addition, the intensity decrease in the ROI was significantly correlated with the destruction area (P < .05).

CONCLUSIONS

By the proposed strategy, microbubbles could be destroyed in a variably sized region, and destruction efficiency as well as the corresponding inertial cavitation dose could be regulated by manipulating the transmission parameters.

Temperature-dependent MR signals in cortical bone: potential for monitoring temperature changes during high-intensity focused ultrasound treatment in bone

PURPOSE

Because existing magnetic resonance thermometry techniques do not provide temperature information within bone, high-intensity focused ultrasound (HIFU) exposures in bone are monitored using temperature changes in adjacent soft tissues. In this study, the potential to monitor temperature changes in cortical bone using a short TE gradient echo sequence is evaluated.

METHODS

The feasibility of this proposed method was initially evaluated by measuring the temperature dependence of the gradient echo signal during cooling of cortical bone samples implanted with fiber-optic temperature sensors. A subsequent experiment involved heating a cortical bone sample using a clinical MR-HIFU system.

RESULTS

A consistent relationship between temperature change and the change in magnitude signal was observed within and between cortical bone samples. For the two-dimensional gradient echo sequence implemented in this study, a least-squares linear fit determined the percentage change in signal to be (0.90 ± 0.01)%/°C. This relationship was used to estimate temperature changes observed in the HIFU experiment and these temperatures agreed well with those measured from an implanted fiber-optic sensor.

CONCLUSION

This method appears capable of displaying changes related to temperature in cortical bone and could improve the safety of MR-HIFU treatments. Further investigations into the sensitivity of the technique in vivo are warranted.

Quantitative ultrasound imaging for monitoring in situ high-intensity focused ultrasound exposure

Quantitative ultrasound (QUS) imaging is hypothesized to map temperature elevations induced in tissue with high spatial and temporal resolution. To test this hypothesis, QUS techniques were examined to monitor high-intensity focused ultrasound (HIFU) exposure of tissue. In situ experiments were conducted on mammary adenocarcinoma tumors grown in rats and lesions were formed using a HIFU system. A thermocouple was inserted into the tumor to provide estimates of temperature at one location. Backscattered time-domain waveforms from the tissue during exposure were recorded using a clinical ultrasonic imaging system. Backscatter coefficients were estimated using a reference phantom technique. Two parameters were estimated from the backscatter coefficient (effective scatterer diameter (ESD) and effective acoustic concentration (EAC). The changes in the average parameters in the regions corresponding to the HIFU focus over time were correlated to the temperature readings from the thermocouple. The changes in the EAC parameter were consistently correlated to temperature during both heating and cooling of the tumors. The changes in the ESD did not have a consistent trend with temperature. The mean ESD and EAC before exposure were 120 ± 16 μm and 32 ± 3 dB/cm3, respectively, and changed to 144 ± 9 μm and 51 ± 7 dB/cm3, respectively, just before the last HIFU pulse was delivered to the tissue. After the tissue cooled down to 37 °C, the mean ESD and EAC were 126 ± 8 μm and 35 ± 4 dB/cm3, respectively. Peak temperature in the range of 50-60 °C was recorded by a thermocouple placed just behind the tumor. These results suggest that QUS techniques have the potential to be used for non-invasive monitoring of HIFU exposure.

A new active cavitation mapping technique for pulsed HIFU applications--bubble Doppler

In this work, a new active cavitation mapping technique for pulsed high-intensity focused ultrasound (pHIFU) applications termed bubble Doppler is proposed and its feasibility is tested in tissue-mimicking gel phantoms. pHIFU therapy uses short pulses, delivered at low pulse repetition frequency, to cause transient bubble activity that has been shown to enhance drug and gene delivery to tissues. The current gold standard for detecting and monitoring cavitation activity during pHIFU treatments is passive cavitation detection (PCD), which provides minimal information on the spatial distribution of the bubbles. B-mode imaging can detect hyperecho formation, but has very limited sensitivity, especially to small, transient microbubbles. The bubble Doppler method proposed here is based on a fusion of the adaptations of three Doppler techniques that had been previously developed for imaging of ultrasound contrast agents-color Doppler, pulse-inversion Doppler, and decorrelation Doppler. Doppler ensemble pulses were interleaved with therapeutic pHIFU pulses using three different pulse sequences and standard Doppler processing was applied to the received echoes. The information yielded by each of the techniques on the distribution and characteristics of pHIFU-induced cavitation bubbles was evaluated separately, and found to be complementary. The unified approach-bubble Doppler-was then proposed to both spatially map the presence of transient bubbles and to estimate their sizes and the degree of nonlinearity.

Rapid prototyping fabrication of focused ultrasound transducers

Rapid prototyping (RP) fabrication techniques are currently widely used in diverse industrial and medical fields, providing substantial advantages in development time and costs in comparison to more traditional manufacturing processes. This paper presents a new method for the fabrication of high-intensity focused ultrasound transducers using RP technology. The construction of a large-aperture hemispherical transducer designed by computer software is described to demonstrate the process. The transducer was conceived as a modular design consisting of 32 individually focused 50.8-mm (2-in) PZT-8 element modules distributed in a 300-mm hemispherical scaffold with a geometric focus of 150 mm. The entire structure of the array, including the module housings and the hemispherical scaffold was fabricated through a stereolithography (SLA) system using a proprietary photopolymer. The PZT elements were bonded to the lenses through a quarter-wave tungsten-epoxy matching layer developed in-house specifically for this purpose. Modules constructed in this manner displayed a high degree of electroacoustic consistency, with an electrical impedance mean and standard deviation of 109 ± 10.2 Ω for the 32 elements. Time-of-flight measurements for individually pulsed modules mounted on the hemispherical scaffold showed that all pulses arrived at the focus within a 350 ns range, indicating a good degree of element alignment. Pressure profile measurements of the fully assembled transducer also showed close agreement with simulated results. The measured focal beam FWHM dimensions were 1.9 × 4.0 mm (1.9 × 3.9 mm simulated) in the transversal and axial directions respectively. Total material expenses associated with the construction of the transducer were approximately 5000 USD (as of 2011). The versatility and lower fabrication costs afforded by RP methods may be beneficial in the development of complex transducer geometries suitable for a variety of research and clinical applications.

Hollow iron-silica nanoshells for enhanced high intensity focused ultrasound

BACKGROUND

High intensity-focused ultrasound (HIFU) is an alterative ablative technique currently being investigated for local treatment of breast cancer and fibroadenomas. Current HIFU therapies require concurrent magnetic resonance imaging monitoring. Biodegradable 500 nm perfluoropentane-filled iron-silica nanoshells have been synthesized as a sensitizing agent for HIFU therapies, which aid both mechanical and thermal ablation of tissues. In low duty cycle high-intensity applications, rapid tissue damage occurs from mechanical rather than thermal effects, which can be monitored closely by ultrasound obviating the need for concurrent magnetic resonance imaging.

MATERIALS AND METHODS

Iron-silica nanoshells were synthesized by a sol-gel method on polystyrene templates and calcined to yield hollow nanoshells. The nanoshells were filled with perfluoropentane and injected directly into excised human breast tumor, and intravenously (IV) into healthy rabbits and Py8119 tumor-bearing athymic nude mice. HIFU was applied at 1.1 MHz and 3.5 MPa at a 2% duty cycle to achieve mechanical ablation.

RESULTS

Ex vivo in excised rabbit livers, the time to visually observable damage with HIFU was 20 s without nanoshells and only 2 s with nanoshells administered IV before sacrifice. Nanoshells administered IV into nude mice with xenograft tumors were activated in vivo by HIFU 24 h after administration. In this xenograft model, applied HIFU resulted in a 13.6 ± 6.1 mm(3) bubble cloud with the IV injected particles and no bubble cloud without particles.

CONCLUSIONS

Iron-silica nanoshells can reduce the power and time to perform HIFU ablative therapy and can be monitored by ultrasound during low duty cycle operation.

Treatment of refractory open-angle glaucoma using ultrasonic circular cyclocoagulation: a prospective case series

OBJECTIVE

To evaluate the efficacy and safety of the ultrasonic circular cyclocoagulation (UC(3)) procedure in patients with refractory primary open-angle glaucoma.

RESEARCH DESIGN AND METHODS

Prospective non-comparative interventional case series performed in two French glaucoma centers. Twenty-eight eyes of 28 patients with primary open-angle glaucoma, intraocular pressure (IOP) >21 mmHg, an average of 1.4 failed previous surgeries and an average of 3.8 hypotensive medications were insonified with a therapy probe comprising six piezoelectric transducers. The six transducers were activated and all patients were treated with a 6 second exposure time. Complete ophthalmic examinations were performed before the procedure, and at 1 day, 1 week, 1, 2, 3, 6 and 12 months after.

MAIN OUTCOME MEASURES

Primary outcomes were surgical success (defined as IOP reduction from baseline ≥20% and IOP >5 mmHg) at the last follow-up visit, and vision-threatening complications. Secondary outcomes were mean IOP at each follow-up visit compared to baseline, medication use, complications, and re-interventions.

RESULTS

IOP was significantly reduced (P < 0.05), from a mean preoperative value of 29.0 ± 7.2 mmHg (n = 3.8 hypotensive medications) to a mean value of 21.6 ± 9.4 mmHg at last follow-up (n = 3.8 hypotensive medications and n = 1.29 procedures) (mean IOP reduction of 26%). Complete success (IOP reduction >20% without re-intervention and without hypotensive medications adjunction) was achieved in 50% of eyes at last follow-up (mean IOP reduction of 45% in these same eyes) and qualified success (IOP reduction >20% with possible re-interventions) was achieved in 68% of eyes at last follow-up. No major intra- or post-operative complications occurred.

CONCLUSIONS

UC(3) procedure seems to be an effective and well tolerated method to reduce intraocular pressure in patients with primary open-angle glaucoma. Studies directly comparing the efficacy and safety of the UC(3) procedure with that of trabeculectomy or diode laser are needed.

[Overview of ablation devices for atrial fibrillation surgery]

Since Cox-maze procedure was first reported in 1987, ablation technology has made great progress and revolutionized atrial fibrillation surgery. Various energy sources have been introduced to facilitate creating conduction block within the atria. These sources include radiofrequency, cryothermia, microwave, laser, and high-frequency focused ultrasound( HIFU). They have been utilized both endocardially in the arrested heart and epicardially in the beating heart. The goal of all these sources is to create transmural lesions which enable irreversible conduction block. They have been reported to have clinical efficacy, but each of them have their own advantages and disadvantages, and their ability of creating transmural lesions are also different. It is important for surgeons that they fully understand these differences to make appropriate use of these technologies for achieving safe atrial fibrillation surgery and developing less invasive new procedures.

Improved sonothrombolysis from a modified diagnostic transducer delivering impulses containing a longer pulse duration

Although guided high-mechanical-index (MI) impulses from a diagnostic ultrasound transducer have been used in preclinical studies to dissolve coronary arterial and microvascular thrombi in the presence of intravenously infused microbubbles, it is possible that pulse durations (PDs) longer than that used for diagnostic imaging may further improve the effectiveness of this approach. By use of an established in vitro model flow system, a total of 90 occlusive porcine arterial thrombi (thrombus age: 3-4 h) within a vascular mimicking system were randomized to 10-min treatments with two different PDs (5 and 20 μs) using a Philips S5-1 transducer (1.6-MHz center frequency) at a range of MIs (from 0.2 to 1.4). All impulses were delivered in an intermittent fashion to permit microbubble replenishment within the thrombosed vessel. Diluted lipid-encapsulated microbubbles (0.5% Definity) were infused during the entire treatment period. A tissue-mimicking phantom 5 cm thick was placed between the transducer and thrombosed vessel to mimic transthoracic attenuation. Two 20-MHz passive cavitation detection systems were placed confocal to the insonified vessel to assess for inertial cavitational activity. Percentage thrombus dissolution was calculated by weighing the thrombi before and after each treatment. Percentage thrombus dissolution was significantly higher with a 20-μs PD already at the 0.2 and 0.4 MI therapeutic impulses (54 ± 12% vs. 33 ± 17% and 54 ± 22% vs. 34 ± 17%, p < 0.05 compared with the 5-μs PD group, respectively), and where passive cavitation detection systems detected only low intensities of inertial cavitation. At higher MI settings and 20-μs PDs, percentage thrombus dissolution decreased most likely from high-intensity cavitation shielding of the thrombus. Slightly prolonging the PD on a diagnostic transducer improves the degree of sonothrombolysis that can be achieved without fibrinolytic agents at a lower mechanical index.