Image-guided acoustic therapy

A novel gas embolotherapy using microbubbles electrocoalescence for cancer treatment

BACKGROUND AND OBJECTIVE

Embolotherapy has been increasingly used to disrupt tumor growth. Despite its success in the occlusion of microvessels, it has drawbacks such as limited access to the target location, limited control of the blocker size, and inattention to the tumor characteristics, especially high interstitial fluid pressure. The present work introduces a novel numerical method of gas embolotherapy for cancer treatment through tumor vessel occlusion.

METHODS

The gas microbubbles are generated from Levovist bolus injection into the tumor microvessel. The microbubble movement in the blood flow is innovatively controlled by an electric field applied to the tumor-feeding vessel. The interaction between the Levovist microbubbles and the electric field is resolved by developing a fully coupled model using the phase-field model, Carreau model for non-Newtonian blood, Navier-Stokes equations and Maxwell stress tensor. Additionally, the critical effect of high interstitial fluid pressure as a characteristic of solid tumors is included.

RESULTS

The findings of this study indicate that the rates of microbubble deformation and displacement increase with the applied potential intensity to the microvessel wall. Accordingly, the required time for a microbubble to join the upper microvessel wall reduces from 1.97ms to 22 μs with an increase of the electric potential from 3.5V to 12.5V. Additionally, an electric potential of 12.5V causes the microbubbles coalescence and formation of a gas column against the bloodstream.

CONCLUSIONS

Clinically, our novel embolization procedure can be considered a non-invasive targeted therapy, and under a controlled electric field, the blocker size can be precisely controlled. Also, the proposed method has the potential to be used as a gradual treatment in advanced cancers as tumors develop resistance and relapse.

Dual-modality fibre optic probe for simultaneous ablation and ultrasound imaging

All-optical ultrasound (OpUS) is an emerging high resolution imaging paradigm utilising optical fibres. This allows both therapeutic and imaging modalities to be integrated into devices with dimensions small enough for minimally invasive surgical applications. Here we report a dual-modality fibre optic probe that synchronously performs laser ablation and real-time all-optical ultrasound imaging for ablation monitoring. The device comprises three optical fibres: one each for transmission and reception of ultrasound, and one for the delivery of laser light for ablation. The total device diameter is < 1 mm. Ablation monitoring was carried out on porcine liver and heart tissue ex vivo with ablation depth tracked using all-optical M-mode ultrasound imaging and lesion boundary identification using a segmentation algorithm. Ablation depths up to 2.1 mm were visualised with a good correspondence between the ultrasound depth measurements and visual inspection of the lesions using stereomicroscopy. This work demonstrates the potential for OpUS probes to guide minimally invasive ablation procedures in real time.

Lysozyme microspheres incorporated with anisotropic gold nanorods for ultrasound activated drug delivery

We report on the fabrication of lysozyme microspheres (LyMs) incorporated with gold nanorods (NRs) as a distinctive approach for the encapsulation and release of an anticancer drug, 5-Fluorouracil (5-FU). LyMs with an average size of 4.0 ± 1.0 µm were prepared by a sonochemical method and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier-transform infrared spectroscopy (FTIR). The LyMs were examined using hydrophobic (nile red) as well as hydrophilic (trypan blue) dyes under confocal laser scanning microscopy (CLSM) to obtain information about the preferential distribution of fluorescent molecules. Notably, the fluorescent molecules were accumulated in the inner lining of LyMs as the core was occupied with air. The encapsulation efficiency of 5-FU for LyMs-NR was found to be ∼64%. The drug release from control LyMs as well as LyMs incorporated with NRs was investigated under the influence of ultrasound (US) at 200 kHz. The total release for control LyMs and LyMs incorporated with gold NRs was found to be ∼70 and 95% after 1 h, respectively. The density difference caused by NR incorporation on the shell played a key role in rupturing the LyMs-NR under US irradiation. Furthermore, 5-FU loaded LyMs-NR exhibited excellent anti-cancer activity against the THP-1 cell line (∼90% cell death) when irradiated with US of 200 kHz. The enhanced anti-cancer activity of LyMs-NR was caused by the transfer of released 5-FU molecules from bulk to the interior of the cell via temporary pores formed on the surface of cancer cells, i.e., sonoporation. Thus, LyMs-NR demonstrated here has a high potential for use as carriers in the field of drug delivery, bio-imaging and therapy.

Oncologic outcome of salvage high-intensity focused ultrasound (HIFU) in radiorecurrent prostate cancer. A systematic review

INTRODUCTION

External Beam Radiation Therapy (EBRT) is one of the option available for the treatment of clinically localized prostate cancer. In patients with radiorecurrent localized prostate cancer, Androgen Deprivation Therapy (ADT) is one of the most common therapeutic strategies. However, in the last decades, other salvage treatment options have been investigated, such as brachytherapy, cryoablation and High Intensity Focused Ultrasound (Hifu).

MATERIAL AND METHODS

The oncologic outcome of Hifu in a salvage setting after EBRT failure was investigated. We reviewed the literature from 2005 to 2020 in order to report the oncologic outcome of the technique.

RESULTS

A total of 1241 patients were analyzed, with a mean age of 68.6 years and a PSA value of 5.87 ng/mL before treatment. Mean follow-up was 24.3 months after treatment, ranging from 3 to 168 months.

CONCLUSION

Our review of the literature revealed that salvage Hifu is effective in the treatment of radiorecurrent clinically localized prostate cancer, with an overall survival of 85.2% at 5 years.

Potential Alternatives to Conventional Cancer Therapeutic Approaches: The Way Forward

onventional cancer therapeutic approaches broadly include chemotherapy, radiation therapy and surgery. These established approaches have evolved over several decades of clinical experience. For a complex disease like cancer, satisfactory treatment remains an enigma for the simple fact that the causal factors for cancer are extremely diverse. In order to overcome existing therapeutic limitations, consistent scientific endeavors have evolved several potential therapeutic approaches, majority of which focuses essentially on targeted drug delivery, minimal concomitant ramification, and selective high cytotoxicity. The current review focuses on highlighting some of these potential alternatives that are currently in various stages of in vitro, in vivo, and clinical trials. These include physical, chemical and biological entities that are avidly being explored for therapeutic alternatives. Some of these entities include suicide gene, micro RNA, modulatory peptides, ultrasonic waves, free radicals, nanoparticles, phytochemicals, and gene knockout, and stem cells. Each of these techniques may be exploited exclusively and in combination with conventional therapeutic approaches thereby enhancing the therapeutic efficacy of the treatment. The review intends to briefly discuss the mechanism of action, pros, and cons of potential alternatives to conventional therapeutic approaches.

Internal-Illumination Photoacoustic Tomography Enhanced by a Graded-Scattering Fiber Diffuser

The penetration depth of photoacoustic imaging in biological tissues has been fundamentally limited by the strong optical attenuation when light is delivered externally through the tissue surface. To address this issue, we previously reported internal-illumination photoacoustic imaging using a customized radial-emission optical fiber diffuser, which, however, has complex fabrication, high cost, and non-uniform light emission. To overcome these shortcomings, we have developed a new type of low-cost fiber diffusers based on a graded-scattering method in which the optical scattering of the fiber diffuser is gradually increased as the light travels. The graded scattering can compensate for the optical attenuation and provide relatively uniform light emission along the diffuser. We performed Monte Carlo numerical simulations to optimize several key design parameters, including the number of scattering segments, scattering anisotropy factor, divergence angle of the optical fiber, and reflective index of the surrounding medium. These optimized parameters collectively result in uniform light emission along the fiber diffuser and can be flexibly adjusted to accommodate different applications. We fabricated and characterized the prototype fiber diffuser made of agarose gel and intralipid. Equipped with the new fiber diffuser, we performed thorough proof-of-concept studies on ex vivo tissue phantoms and an in vivo swine model to demonstrate the deep-imaging capability (~10 cm achieved ex vivo) of photoacoustic tomography. We believe that the internal light delivery via the optimized fiber diffuser is an effective strategy to image deep targets (e.g., kidney) in large animals or humans.

Oncologic outcome, side effects and comorbidity of high-intensity focused ultrasound (HIFU) for localized prostate cancer. A review

Introduction

Prostate cancer is considered one of the most important health problems. Due to the increased number of diagnosed patients and the inability to distinguish aggressive tumors, minimally-invasive procedures have become increasingly interesting. High-intensity focused ultrasound (HIFU) is an alternative option to radical surgery to treat prostate cancer. To date, however, data on side effects and comorbidities of this technique are still not conclusive.

Methods and results

We reviewed the literature to concentrate on side effects and comorbidities of HIFU treatment of prostate cancer with the following key words: hifu, high intensity focused ultrasound, ultrasonic therapy, transrectal hifu, prostate ablation, side effects, comorbidities. MedLine and Embase via Ovid database were searched. Selection criteria were: English language, articles published between 2001 and 2015, case series including at least 100 participants and reported data on side effects and comorbidities. Sixteen uncontrolled studies were identified. No randomized controlled trials (RCT) were found in the literature comparing side effects and comorbidities of HIFU to other routine approaches to prostate cancer treatment.

Conclusion

HIFU seems to be a promising minimally-invasive treatment for low- and intermediate-risk prostate cancer, especially for patients who are unfit for radical surgery. Prospective studies with longer follow-up periods and RCT are required to properly assess the impact of side effects and comobidities related to the HIFU technique in comparison with other therapies to treat prostate cancer.

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HIFU is a promising minimally-invasive treatment for prostate cancer, especially in patients with low- and intermediate-risk disease.

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To date, the most proper indication to HIFU is for patients who are not fit for, or are unwilling to undergo, radical surgery.

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The most common complications are impotence, urinary incontinence, acute urinary retention and urethral fistula.

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High-intensity focused ultrasound is a safe and effective procedure.

Simultaneous Photoacoustic Imaging and Cavitation Mapping in Shockwave Lithotripsy

Kidney stone disease is a major health problem worldwide. Shockwave lithotripsy (SWL), which uses high-energy shockwave pulses to break up kidney stones, is extensively used in clinic. However, despite its noninvasiveness, SWL can produce cavitation in vivo. The rapid expansion and violent collapse of cavitation bubbles in small blood vessels may result in renal vascular injury. To better understand the mechanism of tissue injury and improve treatment safety and efficiency, it is highly desirable to concurrently detect cavitation and vascular injury during SWL. Current imaging modalities used in SWL ( e.g. , C-arm fluoroscopy and B-mode ultrasound) are not sensitive to vascular injuries. By contrast, photoacoustic imaging is a non-invasive and non-radiative imaging modality that is sensitive to blood, by using hemoglobin as the endogenous contrast. Moreover, photoacoustic imaging is also compatible with passive cavitation detection by sharing the ultrasound detection system. Here, we have integrated shockwave treatment, photoacoustic imaging, and passive cavitation detection into a single system. Our experimental results on phantoms and in vivo small animals have collectively demonstrated that the integrated system is capable of capturing shockwave-induced cavitation and the resultant vascular injury simultaneously. We expect that the integrated system, when combined with our recently developed internal-light-illumination photoacoustic imaging, will find important applications for monitoring shockwave-induced vascular injury in deep tissues during SWL.

Systemically Administered Hemostatic Nanoparticles for Identification and Treatment of Internal Bleeding

Internal bleeding is an injury that can be difficult to localize and effectively treat without invasive surgeries. Injectable polymeric nanoparticles have been developed that can reduce clotting times and blood loss, but they have yet to incorporate sufficient diagnostic capabilities to assist in identifying bleeding sources. Herein, polymeric nanoparticles were developed to simultaneously treat internal bleeding while incorporating tracers for visualization of the nanoparticles by standard clinical imaging modalities. Addition of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate (DiD; a fluorescent dye), biotin functionality, and gold nanoparticles to hemostatic polymeric nanoparticles resulted in nanoparticles amenable to imaging with near-infrared (NIR) imaging, immunohistochemistry, and X-ray computed tomography (CT), respectively. Following a lethal liver resection injury, visualization of accumulated nanoparticles by multiple imaging methods was achieved in rodents, with the highest accumulation observed at the liver injury site, resulting in improved survival rates. Tracer addition to therapeutic nanoparticles allows for an expansion of their applicability, during stabilization by first responders to diagnosis and identification of unknown internal bleeding sites by clinicians using standard clinical imaging modalities.

Preclinical MRI-Guided Focused Ultrasound: A Review of Systems and Current Practices

Effective preclinical research is a vital component in the development of MRI-guided focused ultrasound (MRgFUS) and its translation to clinic. In this review, we seek to outline the challenges at hand for effective preclinical research, survey different solutions, and underline best practices. Furthermore, we summarize efforts to build and characterize dedicated preclinical MRgFUS equipment, including lab prototypes and available commercial products. Finally, we discuss constraints and considerations specific to using clinical MRgFUS equipment in preclinical research. Specifically, we examine additional hardware that has been used to adapt clinical MRgFUS equipment to better position, constrain, and image preclinical subjects, as well as software solutions that have been used to extend the potential and capabilities of clinical devices.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

Development of a spherically focused phased array transducer for ultrasonic image-guided hyperthermia

A 1.5 MHz prolate spheroidal therapeutic array with 128 circular elements was designed to accommodate standard imaging arrays for ultrasonic image-guided hyperthermia. The implementation of this dual-array system integrates real-time therapeutic and imaging functions with a single ultrasound system (Vantage 256, Verasonics). To facilitate applications involving small animal imaging and therapy the array was designed to have a beam depth of field smaller than 3.5 mm and to electronically steer over distances greater than 1 cm in both the axial and lateral directions. In order to achieve the required f number of 0.69, 1-3 piezocomposite modules were mated within the transducer housing. The performance of the prototype array was experimentally evaluated with excellent agreement with numerical simulation. A focal volume (2.70 mm (axial)  ×  0.65 mm (transverse)  ×  0.35 mm (transverse)) defined by the  -6 dB focal intensity was obtained to address the dimensions needed for small animal therapy. An electronic beam steering range defined by the  -3 dB focal peak intensity (17 mm (axial)  ×  14 mm (transverse)  ×  12 mm (transverse)) and  -8 dB lateral grating lobes (24 mm (axial)  ×  18 mm (transverse)  ×  16 mm (transverse)) was achieved. The combined testing of imaging and therapeutic functions confirmed well-controlled local heating generation and imaging in a tissue mimicking phantom. This dual-array implementation offers a practical means to achieve hyperthermia and ablation in small animal models and can be incorporated within protocols for ultrasound-mediated drug delivery.

A fast boundary element method for the scattering analysis of high-intensity focused ultrasound

High-intensity focused ultrasound (HIFU) techniques are promising modalities for the non-invasive treatment of cancer. For HIFU therapies of, e.g., liver cancer, one of the main challenges is the accurate focusing of the acoustic field inside a ribcage. Computational methods can play an important role in the patient-specific planning of these transcostal HIFU treatments. This requires the accurate modeling of acoustic scattering at ribcages. The use of a boundary element method (BEM) is an effective approach for this purpose because only the boundaries of the ribs have to be discretized instead of the standard approach to model the entire volume around the ribcage. This paper combines fast algorithms that improve the efficiency of BEM specifically for the high-frequency range necessary for transcostal HIFU applications. That is, a Galerkin discretized Burton-Miller formulation is used in combination with preconditioning and matrix compression techniques. In particular, quick convergence is achieved with the operator preconditioner that has been designed with on-surface radiation conditions for the high-frequency approximation of the Neumann-to-Dirichlet map. Realistic computations of acoustic scattering at 1 MHz on a human ribcage model demonstrate the effectiveness of this dedicated BEM algorithm for HIFU scattering analysis.

Traumatic intra-abdominal hemorrhage control: has current technology tipped the balance toward a role for prehospital intervention?

BACKGROUND

The identification and control of traumatic hemorrhage from the torso remains a major challenge and carries a significant mortality despite the reduction of transfer times. This review examines the current technologies that are available for abdominal hemorrhage control within the prehospital setting and evaluates their effectiveness.

METHODS

A systematic search of online databases was undertaken. Where appropriate, evidence was highlighted using the Oxford levels of clinical evidence. The primary outcome assessed was mortality, and secondary outcomes included blood loss and complications associated with each technique.

RESULTS

Of 89 studies, 34 met the inclusion criteria, of which 29 were preclinical in vivo trials and 5 were clinical. Techniques were subdivided into mechanical compression, endovascular control, and energy-based hemostatic devices. Gas insufflation and manual pressure techniques had no associated mortalities. There was one mortality with high intensity focused ultrasound. The intra-abdominal infiltration of foam treatment had 64% and the resuscitative endovascular balloon occlusion of the aorta had 74% mortality risk reduction. In the majority of cases, morbidity and blood loss associated with each interventional procedure were less than their respective controls.

CONCLUSION

Mortality from traumatic intra-abdominal hemorrhage could be reduced through early intervention at the scene by emerging technology. Manual pressure or the resuscitative endovascular balloon occlusion of the aorta techniques have demonstrated clinical effectiveness for the control of major vessel bleeding, although complications need to be carefully considered before advocating clinical use. At present, fast transfer to the trauma center remains paramount.

LEVEL OF EVIDENCE

Systematic review, level IV.

Advantages and Challenges of Relaxor-PbTiO3 Ferroelectric Crystals for Electroacoustic Transducers- A Review

Relaxor-PbTiO3 (PT) based ferroelectric crystals with the perovskite structure have been investigated over the last few decades due to their ultrahigh piezoelectric coefficients (d33 > 1500 pC/N) and electromechanical coupling factors (k33 > 90%), far outperforming state-of-the-art ferroelectric polycrystalline Pb(Zr,Ti)O3 ceramics, and are at the forefront of advanced electroacoustic applications. In this review, the performance merits of relaxor-PT crystals in various electroacoustic devices are presented from a piezoelectric material viewpoint. Opportunities come from not only the ultrahigh properties, specifically coupling and piezoelectric coefficients, but through novel vibration modes and crystallographic/domain engineering. Figure of merits (FOMs) of crystals with various compositions and phases were established for various applications, including medical ultrasonic transducers, underwater transducers, acoustic sensors and tweezers. For each device application, recent developments in relaxor-PT ferroelectric crystals were surveyed and compared with state-of-the-art polycrystalline piezoelectrics, with an emphasis on their strong anisotropic features and crystallographic uniqueness, including engineered domain - property relationships. This review starts with an introduction on electroacoustic transducers and the history of piezoelectric materials. The development of the high performance relaxor-PT single crystals, with a focus on their uniqueness in transducer applications, is then discussed. In the third part, various FOMs of piezoelectric materials for a wide range of ultrasound applications, including diagnostic ultrasound, therapeutic ultrasound, underwater acoustic and passive sensors, tactile sensors and acoustic tweezers, are evaluated to provide a thorough understanding of the materials' behavior under operational conditions. Structure-property-performance relationships are then established. Finally, the impacts and challenges of relaxor-PT crystals are summarized to guide on-going and future research in the development of relaxor-PT crystals for the next generation electroacoustic transducers.

Photoacoustic detection and optical spectroscopy of high-intensity focused ultrasound-induced thermal lesions in biologic tissue

PURPOSE

The aims of this study are: (a) to investigate the capability of photoacoustic (PA) method in detecting high-intensity focused ultrasound (HIFU) treatments in muscle tissues in vitro; and (b) to determine the optical properties of HIFU-treated and native tissues in order to assist in the interpretation of the observed contrast in PA detection of HIFU treatments.

METHODS

A single-element, spherically concaved HIFU transducer with a centre frequency of 1 MHz was utilized to create thermal lesions in chicken breast tissues in vitro. To investigate the detectability of HIFU treatments photoacoustically, PA detection was performed at 720 and 845 nm on seven HIFU-treated tissue samples. Within each tissue sample, PA signals were acquired from 22 locations equally divided between two regions of interest within two volumes in tissue - a HIFU-treated volume and an untreated volume. Optical spectroscopy was then carried out on 10 HIFU-treated chicken breast specimens in the wavelength range of 500-900 nm, in 1-nm increments, using a spectrophotometer with an integrating sphere attachment. The authors' optical spectroscopy raw data (total transmittance and diffuse reflectance) were used to obtain the optical absorption and reduced scattering coefficients of HIFU-induced thermal lesions and native tissues by employing the inverse adding-doubling method. The aforementioned interaction coefficients were subsequently used to calculate the effective attenuation coefficient and light penetration depth of HIFU-treated and native tissues in the wavelength range of 500-900 nm.

RESULTS

HIFU-treated tissues produced greater PA signals than native tissues at 720 and 845 nm. At 720 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.68 ± 0.25 (mean ± standard error of the mean). At 845 nm, the averaged ratio of the peak-to-peak PA signal amplitude of HIFU-treated tissue to that of native tissue was 3.75 ± 0.26 (mean ± standard error of the mean). The authors' spectroscopic investigation has shown that HIFU-treated tissues have a greater optical absorption and reduced scattering coefficients than native tissues in the wavelength range of 500-900 nm. In fact, at 720 and 845 nm, the ratio of the optical absorption coefficient of HIFU-treated tissues to that of native tissues was 1.13 and 1.17, respectively; on the other hand, the ratio of the reduced scattering coefficient of HIFU-treated tissues to that of native tissues was 13.22 and 14.67 at 720 and 845 nm, respectively. Consequently, HIFU-treated tissues have a higher effective attenuation coefficient and a lower light penetration depth than native tissues in the wavelength range 500-900 nm.

CONCLUSIONS

Using a PA approach, HIFU-treated tissues interrogated at 720 and 845 nm optical wavelengths can be differentiated from untreated tissues. Based on the authors' spectroscopic investigation, the authors conclude that the observed PA contrast between HIFU-induced thermal lesions and untreated tissue is due, in part, to the increase in the optical absorption coefficient, the reduced scattering coefficient and, therefore, the deposited laser energy fluence in HIFU-treated tissues.

Real-time monitoring of high-intensity focused ultrasound thermal therapy using the manifold learning method

High-intensity focused ultrasound (HIFU) induces thermal lesions by increasing the tissue temperature in a tight focal region. The main ultrasound imaging techniques currently used to monitor HIFU treatment are standard pulse-echo B-mode ultrasound imaging, ultrasound temperature estimation and elastography-based methods. The present study was carried out on ex vivo animal tissue samples, in which backscattered radiofrequency (RF) signals were acquired in real time at time instances before, during and after HIFU treatment. The manifold learning algorithm, a non-linear dimensionality reduction method, was applied to RF signals whichconstruct B-mode images to detect the HIFU-induced changes among the image frames obtained during HIFU treatment. In this approach, the embedded non-linear information in the region of interest of sequential images is represented in a 2-D manifold with the Isomap algorithm, and each image is depicted as a point on the reconstructed manifold. Four distinct regions are chosen in the manifold corresponding to the four phases of HIFU treatment (before HIFU treatment, during HIFU treatment, immediately after HIFU treatment and 10-min after HIFU treatment). It was found that disorganization of the points is achieved by increasing the acoustic power, and if the thermal lesion has been formed, the regions of points related to pre- and post-HIFU significantly differ. Moreover, the manifold embedding was repeated on 2-D moving windows in RF data envelopes related to pre- and post-HIFU exposure data frames. It was concluded that if mean values of the points related to pre- and post-exposure frames in the reconstructed manifold are estimated, and if the Euclidean distance between these two mean values is calculated and the sliding window is moved and this procedure is repeated for the whole image, a new image based on the Euclidean distance can be formed in which the HIFU thermal lesion is detectable.

Evaluation of superparamagnetic iron oxide-polymer composite microcapsules for magnetic resonance-guided high-intensity focused ultrasound cancer surgery

Background

Superparamagnetic poly (lactic-co-glycolic acid) (PLGA)-coated Fe₃O₄ microcapsules are receiving increased attention as potential diagnostic and therapeutic modalities in the field of oncology. In this study, PLGA-coated Fe₃O₄ microcapsules were combined with a magnetic resonance imaging-guided high-intensity focused ultrasound (MR-guided HIFU) platform, with the objective of investigating the effects of these composite microcapsules regarding MR-guided HIFU liver cancer surgery in vivo.

Methods

PLGA-coated Fe₃O₄ microcapsules consisting of a liquid core and a PLGA-Fe₃O₄ shell were fabricated using a modified double emulsion evaporation method. Their acute biosafety was confirmed in vitro using MDA cells and in vivo using rabbits. To perform MR-guided HIFU surgery, the microcapsules were intravenously injected into a rabbit liver tumor model before MR-guided HIFU. T₂-weighted images and MR signal intensity in normal liver parenchyma and tumor tissue were acquired before and after injection, to assess the MR imaging ability of the microcapsules. After MR-guided HIFU ablation tissue temperature mapping, the coagulative volume and histopathology of the tumor tissue were analyzed to investigate the ablation effects of MR-guided HIFUs.

Results

Scanning and transmission electron microscopy showed that the microcapsules displayed a spherical morphology and a shell-core structure (mean diameter, 587 nm). The hysteresis curve displayed the typical superparamagnetic properties of the microcapsules, which are critical to their application in MR-guided HIFU surgery. In MR-guided HIFU surgery, these microcapsules functioned as an MRI contrast agent, induced significant hyperthermal enhancement (P < 0.05) and significantly enhanced the volume of coagulative necrosis (P < 0.05).

Conclusions

The administration of PLGA-coated Fe₃O₄ microcapsules is a potentially synergistic technique regarding the enhancement of MR-guided HIFU cancer surgery.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-800) contains supplementary material, which is available to authorized users.

The potential utility of high-intensity ultrasound to treat osteoarthritis

Osteoarthritis (OA) is a widespread musculoskeletal disease that reduces quality of life and for which there is no cure. The treatment of OA is challenging since cartilage impedes the local and systemic delivery of therapeutic compounds (TCs). This review identifies high-intensity ultrasound (HIU) as a non-contact technique to modify articular cartilage and subchondral bone. HIU enables new approaches to overcome challenges associated with drug delivery to cartilage and new non-invasive approaches for the treatment of joint disease. Specifically, HIU has the potential to facilitate targeted drug delivery and release deep within cartilage, to repair soft tissue damage, and to physically alter tissue structures including cartilage and bone. The localized, non-invasive ultrasonic delivery of TCs to articular cartilage and subchondral bone appears to be a promising technique in the immediate future.

Characterization of macrolesions induced by myocardial cavitation-enabled therapy

Intermittent high intensity ultrasound pulses with circulating contrast agent microbubbles can induce scattered cavitation caused myocardial microlesions of potential value for tissue reduction therapy. Here, computer-aided histological evaluation of the effective treated volume was implemented to optimize ultrasound pulse parameters, exposure duration, and contrast agent dose. Rats were treated with 1.5 MHz focused ultrasound bursts and Evans blue staining indicates lethal cardiomyocytic injury. Each heart was sectioned to provide samples covering the entire exposed myocardial volume. Both brightfield and fluorescence images were taken for up to 40 tissue sections. Tissue identification and microlesion detection were first done based on 2-D images to form microlesion masks containing the outline of the heart and the stained cell regions. Image registration was then performed on the microlesion masks to reconstruct a volume-based model according to the morphology of the heart. The therapeutic beam path was estimated from the 3-D stacked microlesions, and finally the total microlesion volume, here termed macrolesion, was characterized along the therapeutic beam axis. Radially symmetric fractional macrolesions were characterized via stepping disks of variable radius determined by the local distribution of microlesions. Treated groups showed significant macrolesions of a median volume of 87.3 μL, 2.7 mm radius, 4.8 mm length, and 14.0% lesion density compared to zero radius, length, and lesion density for sham. The proposed radially symmetric lesion model is a robust evaluation for myocardial cavitation-enabled therapy. Future work will include validating the proposed method with varying acoustic exposures and optimizing involved parameters to provide macrolesion characterization.

Variations in temperature distribution and tissue lesion formation induced by tissue inhomogeneity for therapeutic ultrasound

Tissue inhomogeneity might have an important effect on the treatment accuracy of therapeutic ultrasound. Both computer simulation and measurement were performed to study the influence of tissue inhomogeneity on the temperature distribution and tissue lesion formation induced by focused ultrasound. The inhomogeneous tissue is considered a combination of a homogeneous medium and a phase aberration screen in this article. Temperature distributions and lesion dimensions were predicted using the combination of acoustic non-linear and bio-heat transfer equations. To verify the theoretical predictions, polyethylene plates with phase distributions of different correlation lengths and standard deviations were made to mimic inhomogeneous tissues such as human abdominal tissue, and a series of experiments were performed, including acoustic and thermal measurements. The results indicate that the tissue inhomogeneity caused phase aberration of the ultrasound beam. With increasing standard deviation and correlation length of phase aberration, the scattering level of the acoustic field increased, while ultrasound-induced peak temperature and lesion size decreased. This study provides a theoretical and experimental basis for future development of accurate treatment plans for high-intensity focused ultrasound.

A comparison of methods for focusing the field of a HIFU array transducer through human ribs

A forward model, which predicts the scattering by human ribs of a multi-element high-intensity focused ultrasound transducer, was used to investigate the efficacy of a range of focusing approaches described in the literature. This forward model is based on the boundary element method and was described by Gélat et al (2011 Phys. Med. Biol. 56 5553-81; 2012 Phys. Med. Biol. 57 8471-97). The model has since been improved and features a complex surface impedance condition at the surface of the ribs. The inverse problem of focusing through the ribs was implemented on six transducer array-rib topologies and five methods of focusing were investigated, including spherical focusing, binarized apodization based on geometric ray tracing, phase conjugation and the decomposition of the time-reversal operator method. The excitation frequency was 1 MHz and the array was of spherical-section type. Both human and idealized rib topologies were considered. The merit of each method of focusing was examined. It was concluded that the constrained optimization approach offers greater potential than the other focusing methods in terms of maximizing the ratio of acoustic pressure magnitudes at the focus to those on the surface of the ribs whilst taking full advantage of the dynamic range of the phased array.

Nakagami imaging for detecting thermal lesions induced by high-intensity focused ultrasound in tissue

High-intensity focused ultrasound induces focalized tissue coagulation by increasing the tissue temperature in a tight focal region. Several methods have been proposed to monitor high-intensity focused ultrasound-induced thermal lesions. Currently, ultrasound imaging techniques that are clinically used for monitoring high-intensity focused ultrasound treatment are standard pulse-echo B-mode ultrasound imaging, ultrasound temperature estimation, and elastography-based methods. On the contrary, the efficacy of two-dimensional Nakagami parametric imaging based on the distribution of the ultrasound backscattered signals to quantify properties of soft tissue has recently been evaluated. In this study, ultrasound radio frequency echo signals from ex vivo tissue samples were acquired before and after high-intensity focused ultrasound exposures and then their Nakagami parameter and scaling parameter of Nakagami distribution were estimated. These parameters were used to detect high-intensity focused ultrasound-induced thermal lesions. Also, the effects of changing the acoustic power of the high-intensity focused ultrasound transducer on the Nakagami parameters were studied. The results obtained suggest that the Nakagami distribution's scaling and Nakagami parameters can effectively be used to detect high-intensity focused ultrasound-induced thermal lesions in tissue ex vivo. These parameters can also be used to understand the degree of change in tissue caused by high-intensity focused ultrasound exposures, which could be interpreted as a measure of degree of variability in scatterer concentration in various parts of the high-intensity focused ultrasound lesion.

Estimating dynamic changes of tissue attenuation coefficient during high-intensity focused ultrasound treatment

BACKGROUND

This study investigated the dynamic changes of tissue attenuation coefficients before, during, and after high-intensity focused ultrasound (HIFU) treatment at different total acoustic powers (TAP) in ex vivo porcine muscle tissue. It further assessed the reliability of employing changes in tissue attenuation coefficient parameters as potential indicators of tissue thermal damage.

METHODS

Two-dimensional pulse-echo radio frequency (RF) data were acquired before, during, and after HIFU exposure to estimate changes in least squares attenuation coefficient slope (Δβ) and attenuation coefficient intercept (Δα 0). Using the acquired RF data, Δβ and Δα 0 images, along with conventional B-mode ultrasound images, were constructed. The dynamic changes of Δβ and Δα 0, averaged in the region of interest, were correlated with B-mode images obtained during the HIFU treatment process.

RESULTS

At a HIFU exposure duration of 40 s and various HIFU intensities (737-1,068 W/cm(2)), Δβ and Δα 0 increased rapidly to values in the ranges 1.5-2.5 dB/(MHz.cm) and 4-5 dB/cm, respectively. This rapid increase was accompanied with the appearance of bubble clouds in the B-mode images. Bubble activities appeared as strong hyperechoic regions in the B-mode images and caused fluctuations in the estimated Δβ and Δα 0 values. After the treatment, Δβ and Δα 0 values gradually decreased, accompanied by fade-out of hyperechoic spots in the B-mode images. At 10 min after the treatment, they reached values in ranges 0.75-1 dB/(MHz.cm) and 1-1.5 dB/cm, respectively, and remained stable within those ranges. At a long HIFU exposure duration of around 10 min and low HIFU intensity (117 W/cm(2)), Δβ and Δα 0 gradually increased to values of 2.2 dB/(MHz.cm) and 2.2 dB/cm, respectively. This increase was not accompanied with the appearance of bubble clouds in the B-mode images. After HIFU treatment, Δβ and Δα 0 gradually decreased to values of 1.8 dB/(MHz.cm) and 1.5 dB/cm, respectively, and remained stable at those values.

CONCLUSIONS

Δβ and Δα 0 estimations were both potentially reliable indicators of tissue thermal damage. In addition, Δβ and Δα 0 images both had significantly higher contrast-to-speckle ratios compared to the conventional B-mode images and outperformed the B-mode images in detecting HIFU thermal lesions at all investigated TAPs and exposure durations.

Ultrasound imparted air-recoil resonance (UIAR) method for acoustic power estimation: theory and experiment

Ultrasound imparted air-recoil resonance (UIAR), a new method for acoustic power estimation, is introduced with emphasis on therapeutic high-intensity focused ultrasound (HIFU) monitoring applications. Advantages of this approach over existing practices include fast response; electrical and magnetic inertness, and hence MRI compatibility; portability; high damage threshold and immunity to vibration and interference; low cost; etc. The angle of incidence should be fixed for accurate measurement. However, the transducer-detector pair can be aligned in any direction with respect to the force of gravity. In this sense, the operation of the device is orientation independent. The acoustic response of a pneumatically coupled pair of Helmholtz resonators, with one of them acting as the sensor head, is used for the estimation of acoustic power. The principle is valid in the case of pulsed/ burst as well as continuous ultrasound exposure, the former being more sensitive and accurate. An electro-acoustic theory has been developed for describing the dynamics of pressure flow and resonance in the system considering various thermo- viscous loss mechanisms. Experimental observations are found to be in agreement with theoretical results. Assuming the window damage threshold (~10 J·mm(-2)) and accuracy of RF power estimation are the upper and lower scale-limiting factors, the performance of the device was examined for an RF power range of 5 mW to 100 W with a HIFU transducer operating at 1.70 MHz, and an average nonlinearity of ~1.5% was observed. The device is also sensitive to sub-milliwatt powers. The frequency response was analyzed at 0.85, 1.70, 2.55, and 3.40 MHz and the results are presented with respective theoretical estimates. Typical response time is in the millisecond regime. Output drift is about 3% for resonant and 5% for nonresonant modes. The principle has been optimized to demonstrate a general-purpose acoustic power meter.

Estimating dynamic changes of tissue attenuation coefficient during high-intensity focused ultrasound treatment

BACKGROUND

This study investigated the dynamic changes of tissue attenuation coefficients before, during, and after high-intensity focused ultrasound (HIFU) treatment at different total acoustic powers (TAP) in ex vivo porcine muscle tissue. It further assessed the reliability of employing changes in tissue attenuation coefficient parameters as potential indicators of tissue thermal damage.

METHODS

Two-dimensional pulse-echo radio frequency (RF) data were acquired before, during, and after HIFU exposure to estimate changes in least squares attenuation coefficient slope (Δβ) and attenuation coefficient intercept (Δα 0). Using the acquired RF data, Δβ and Δα 0 images, along with conventional B-mode ultrasound images, were constructed. The dynamic changes of Δβ and Δα 0, averaged in the region of interest, were correlated with B-mode images obtained during the HIFU treatment process.

RESULTS

At a HIFU exposure duration of 40 s and various HIFU intensities (737-1,068 W/cm(2)), Δβ and Δα 0 increased rapidly to values in the ranges 1.5-2.5 dB/(MHz.cm) and 4-5 dB/cm, respectively. This rapid increase was accompanied with the appearance of bubble clouds in the B-mode images. Bubble activities appeared as strong hyperechoic regions in the B-mode images and caused fluctuations in the estimated Δβ and Δα 0 values. After the treatment, Δβ and Δα 0 values gradually decreased, accompanied by fade-out of hyperechoic spots in the B-mode images. At 10 min after the treatment, they reached values in ranges 0.75-1 dB/(MHz.cm) and 1-1.5 dB/cm, respectively, and remained stable within those ranges. At a long HIFU exposure duration of around 10 min and low HIFU intensity (117 W/cm(2)), Δβ and Δα 0 gradually increased to values of 2.2 dB/(MHz.cm) and 2.2 dB/cm, respectively. This increase was not accompanied with the appearance of bubble clouds in the B-mode images. After HIFU treatment, Δβ and Δα 0 gradually decreased to values of 1.8 dB/(MHz.cm) and 1.5 dB/cm, respectively, and remained stable at those values.

CONCLUSIONS

Δβ and Δα 0 estimations were both potentially reliable indicators of tissue thermal damage. In addition, Δβ and Δα 0 images both had significantly higher contrast-to-speckle ratios compared to the conventional B-mode images and outperformed the B-mode images in detecting HIFU thermal lesions at all investigated TAPs and exposure durations.

Controllable in vivo hyperthermia effect induced by pulsed high intensity focused ultrasound with low duty cycles

High intensity focused ultrasound (HIFU)-induced hyperthermia is a promising tool for cancer therapy. Three-dimensional nonlinear acoustic-bioheat transfer-blood flow-coupling model simulations and in vivo thermocouple measurements were performed to study hyperthermia effects in rabbit auricular vein exposed to pulsed HIFU (pHIFU) at varied duty cycles (DCs). pHIFU-induced temperature elevations are shown to increase with increasing DC. A critical DC of 6.9% is estimated for temperature at distal vessel wall exceeding 44 °C, although different tissue depths and inclusions could affect the DC threshold. The results demonstrate clinic potentials of achieving controllable hyperthermia by adjusting pHIFU DCs, while minimizing perivascular thermal injury.

High intensity focused ultrasound treatment for patients with advanced pancreatic cancer: a preliminary dosimetric analysis

OBJECTIVE

To analyse the clinical dosimetry of high intensity focused ultrasound (HIFU) for the treatment of inoperable pancreatic cancer in humans.

METHODS

136 patients with advanced pancreatic cancer were treated with HIFU, including 89 male and 47 female patients. The median targeted volume (V(t)) was 31.1 cm(3) (range: 9.8-102.1). The median of the average ultrasound power (P(avg)) was 225 W (range: 117-399), and the median energy of the ultrasound (E(total)) was 278.3 kJ (range: 70.5-1195.2). Spearman rank correlation analysis for HIFU dosimetric analysis was conducted.

RESULTS

There was a significant correlation between greyscale changes after HIFU ablation and HIFU dose intensity (DI), P(avg), and unit time (T(u)). However, no correlation was found between greyscale changes after HIFU ablation and gender, age, pancreatic cancer position, or depth of tumour.

CONCLUSIONS

We preliminarily deem that dose intensity and sound power can act as good reference points for HIFU dosimetry in the treatment of pancreatic tumours using the Chongqing system. If there was no obvious change in the ultrasound-monitored image following HIFU treatment for pancreatic cancer, the P(avg) and DI should be no less than 260 W and 11 kJ/cm(3), respectively.

High-frequency ultrasound m-mode imaging for identifying lesion and bubble activity during high-intensity focused ultrasound ablation

Effective real-time monitoring of high-intensity focused ultrasound (HIFU) ablation is important for application of HIFU technology in interventional electrophysiology. This study investigated rapid, high-frequency M-mode ultrasound imaging for monitoring spatiotemporal changes during HIFU application. HIFU (4.33 MHz, 1 kHz PRF, 50% duty cycle, 1 s, 2600‒6100 W/cm²) was applied to ex vivo porcine cardiac tissue specimens with a confocally and perpendicularly aligned high-frequency imaging system (Visualsonics Vevo 770, 55 MHz center frequency). Radio-frequency (RF) data from M-mode imaging (1 kHz PRF, 2 s × 7 mm) was acquired before, during and after HIFU treatment (n = 12). Among several strategies, the temporal maximum integrated backscatter with a threshold of +12 dB change showed the best results for identifying final lesion width (receiver-operating characteristic curve area 0.91 ± 0.04, accuracy 85 ± 8%, compared with macroscopic images of lesions). A criterion based on a line-to-line decorrelation coefficient is proposed for identification of transient gas bodies.

MR-guided focused ultrasound for the treatment of uterine fibroids

Magnetic resonance imaging-guided focused ultrasound (MRgFUS) ablation of uterine fibroids provides a minimally invasive outpatient technique for targeting and treating symptomatic uterine fibroids. Magnetic resonance imaging provides a guidance platform that has high temporal and spatial resolution for guiding, as well as thermal monitoring of the procedure. The high-intensity focused ultrasound provides a mechanism for delivering large amounts of energy directly into the fibroid without causing detrimental effects to the nontarget tissues. Early and intermediate follow-up of patients treated with MRgFUS provided promising results on the efficacy of the technique for providing symptom relief to patients. As more long-term follow-up data are published, the efficacy of this technique can be compared to more invasive surgical and minimally invasive catheter treatments.

A fast tissue stiffness-dependent elastography for HIFU-induced lesions inspection

To monitor HIFU-induced lesion with elastography in quasi-real time, a fast correlation based elastographic algorithm using tissue stiffness-dependent displacement estimation (SdDE) is developed in this paper. The high time efficiency of the proposed method contributes to the reduction on both the number of the displacement points and the computational time of most of the points by utilizing local uniformity of the tissue under HIFU treatment. To obtain admirable comprehensive performance, the key algorithm parameter, a threshold to densify the displacement points, is optimized with simulation over a wedge-inclusion tissue model by compromising the axial resolution (AR) and the computational cost. With the optimum parameter, results from both simulations and phantom experiments show that the SdDE is faster in about one order of magnitude than the traditional correlation based algorithm. At the same time, other performance parameters, such as the signal-to-noise ratio (SNRe), the contrast-to-noise ratio (CNRe) and the axial resolution (AR), are superior to or comparable with that obtained from the traditional algorithm. In vitro experiments on bovine livers validate the improvement on the time efficiency under the circumstances of real tissue and real radio frequency (RF) signal. This preliminary work implies potential of the SdDE in dynamic or close real time guidance and monitoring of HIFU treatment.

Controlled tissue emulsification produced by high intensity focused ultrasound shock waves and millisecond boiling

In high intensity focused ultrasound (HIFU) applications, tissue may be thermally necrosed by heating, emulsified by cavitation, or, as was recently discovered, emulsified using repetitive millisecond boiling caused by shock wave heating. Here, this last approach was further investigated. Experiments were performed in transparent gels and ex vivo bovine heart tissue using 1, 2, and 3 MHz focused transducers and different pulsing schemes in which the pressure, duty factor, and pulse duration were varied. A previously developed derating procedure to determine in situ shock amplitudes and the time-to-boil was refined. Treatments were monitored using B-mode ultrasound. Both inertial cavitation and boiling were observed during exposures, but emulsification occurred only when shocks and boiling were present. Emulsified lesions without thermal denaturation were produced with shock amplitudes sufficient to induce boiling in less than 20 ms, duty factors of less than 0.02, and pulse lengths shorter than 30 ms. Higher duty factors or longer pulses produced varying degrees of thermal denaturation combined with mechanical emulsification. Larger lesions were obtained using lower ultrasound frequencies. The results show that shock wave heating and millisecond boiling is an effective and reliable way to emulsify tissue while monitoring the treatment with ultrasound.

Comparative study of lesions created by high-intensity focused ultrasound using sequential discrete and continuous scanning strategies

Lesion formation and temperature distribution induced by high-intensity focused ultrasound (HIFU) were investigated both numerically and experimentally via two energy-delivering strategies, i.e., sequential discrete and continuous scanning modes. Simulations were presented based on the combination of Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation and bioheat equation. Measurements were performed on tissue-mimicking phantoms sonicated by a 1.12-MHz single-element focused transducer working at an acoustic power of 75 W. Both the simulated and experimental results show that, in the sequential discrete mode, obvious saw-tooth-like contours could be observed for the peak temperature distribution and the lesion boundaries, with the increasing interval space between two adjacent exposure points. In the continuous scanning mode, more uniform peak temperature distributions and lesion boundaries would be produced, and the peak temperature values would decrease significantly with the increasing scanning speed. In addition, compared to the sequential discrete mode, the continuous scanning mode could achieve higher treatment efficiency (lesion area generated per second) with a lower peak temperature. The present studies suggest that the peak temperature and tissue lesion resulting from the HIFU exposure could be controlled by adjusting the transducer scanning speed, which is important for improving the HIFU treatment efficiency.

The effect of the scanning pathway in high-intensity focused ultrasound therapy on lesion production

Because tumors are much larger in size compared with the beam width of high-intensity focused ultrasound (HIFU), raster scanning throughout the entire target is conventionally performed for HIFU thermal ablation. Thermal diffusion affects the temperature elevation and the consequent lesion formation. As a result, the lesion will grow continuously over the course of HIFU therapy. The purpose of this study was to investigate the influence of scanning pathways on the overall thermal lesion. Two new scanning pathways, spiral scanning from the center to the outside and spiral scanning from the outside to the center, were proposed with the same HIFU parameters (power and exposure time) for each treatment spot. The lesions produced in the gel phantom and bovine liver were compared with those using raster scanning. Although more uniform lesions can be achieved using the new scanning pathways, the produced lesion areas (27.5 ± 12.3 mm(2) and 65.2 ± 9.6 mm(2), respectively) in the gel phantom are significantly smaller (p < 0.05) than those using raster scanning (92.9 ± 11.8 mm(2)). Furthermore, the lesion patterns in the gel phantom and bovine liver were similar to the simulations using temperature and thermal dose-threshold models, respectively. Thermal diffusion, the scanning pathway and the biophysical aspects of the target all play important roles in HIFU lesion production. By selecting the appropriate scanning pathway and varying the parameters as ablation progresses, HIFU therapy can achieve uniform lesions while minimizing the total delivered energy and treatment time.

Dual-focus therapeutic ultrasound transducer for production of broad tissue lesions

In noninvasive high-intensity focused ultrasound (HIFU) treatment, formation of a large tissue lesion per sonication is desirable for reducing the overall treatment time. The goal of this study is to show the feasibility of enlarging tissue lesion size with a dual-focus therapeutic ultrasound transducer (DFTUT) by increasing the depth-of-focus (DOF). The proposed transducer consists of a disc- and an annular-type element of different radii of curvatures to produce two focal zones. To increase focal depth and to maintain uniform beamwidth of the elongated DOF, each element transmits ultrasound of a different center frequency: the inner element at a higher frequency for near field focusing and the outer element at a lower frequency for far field focusing. By activating two elements at the same time with a single transmitter capable of generating a dual-frequency mixed signal, the overall DOF of the proposed transducer may be extended considerably. A prototype transducer composed of a 4.1 MHz inner element and a 2.7 MHz outer element was fabricated to obtain preliminary experimental results. The feasibility the proposed technique was demonstrated through sound field, temperature and thermal dose simulations. The performance of the prototype transducer was verified by hydrophone measurements and tissue ablation experiments on a beef liver specimen. When several factors affecting the length and the uniformity of elongated DOF of the DFTUT are optimized, the proposed therapeutic ultrasound transducer design may increase the size of ablated tissues in the axial direction and, thus, decreasing the treatment time for a large volume of malignant tissues especially deep-seated targets.

Adaptive HIFU noise cancellation for simultaneous therapy and imaging using an integrated HIFU/imaging transducer

It was previously demonstrated that it is feasible to simultaneously perform ultrasound therapy and imaging of a coagulated lesion during treatment with an integrated transducer that is capable of high intensity focused ultrasound (HIFU) and B-mode ultrasound imaging. It was found that coded excitation and fixed notch filtering upon reception could significantly reduce interference caused by the therapeutic transducer. During HIFU sonication, the imaging signal generated with coded excitation and fixed notch filtering had a range side-lobe level of less than -40 dB, while traditional short-pulse excitation and fixed notch filtering produced a range side-lobe level of -20 dB. The shortcoming is, however, that relatively complicated electronics may be needed to utilize coded excitation in an array imaging system. It is for this reason that in this paper an adaptive noise canceling technique is proposed to improve image quality by minimizing not only the therapeutic interference, but also the remnant side-lobe 'ripples' when using the traditional short-pulse excitation. The performance of this technique was verified through simulation and experiments using a prototype integrated HIFU/imaging transducer. Although it is known that the remnant ripples are related to the notch attenuation value of the fixed notch filter, in reality, it is difficult to find the optimal notch attenuation value due to the change in targets or the media resulted from motion or different acoustic properties even during one sonication pulse. In contrast, the proposed adaptive noise canceling technique is capable of optimally minimizing both the therapeutic interference and residual ripples without such constraints. The prototype integrated HIFU/imaging transducer is composed of three rectangular elements. The 6 MHz center element is used for imaging and the outer two identical 4 MHz elements work together to transmit the HIFU beam. Two HIFU elements of 14.4 mm x 20.0 mm dimensions could increase the temperature of the soft biological tissue from 55 degrees C to 71 degrees C within 60 s. Two types of experiments for simultaneous therapy and imaging were conducted to acquire a single scan-line and B-mode image with an aluminum plate and a slice of porcine muscle, respectively. The B-mode image was obtained using the single element imaging system during HIFU beam transmission. The experimental results proved that the combination of the traditional short-pulse excitation and the adaptive noise canceling method could significantly reduce therapeutic interference and remnant ripples and thus may be a better way to implement real-time simultaneous therapy and imaging.

Shock-induced heating and millisecond boiling in gels and tissue due to high intensity focused ultrasound

Nonlinear propagation causes high-intensity ultrasound waves to distort and generate higher harmonics, which are more readily absorbed and converted to heat than the fundamental frequency. Although such nonlinear effects have been investigated previously and found to not significantly alter high-intensity focused ultrasound (HIFU) treatments, two results reported here change this paradigm. One is that at clinically relevant intensity levels, HIFU waves not only become distorted but form shock waves in tissue. The other is that the generated shock waves heat the tissue to boiling in much less time than predicted for undistorted or weakly distorted waves. In this study, a 2-MHz HIFU source operating at peak intensities up to 25,000 W/cm(2) was used to heat transparent tissue-mimicking phantoms and ex vivo bovine liver samples. Initiation of boiling was detected using high-speed photography, a 20-MHz passive cavitation detector and fluctuation of the drive voltage at the HIFU source. The time to boil obtained experimentally was used to quantify heating rates and was compared with calculations using weak shock theory and the shock amplitudes obtained from nonlinear modeling and measurements with a fiber optic hydrophone. As observed experimentally and predicted by calculations, shocked focal waveforms produced boiling in as little as 3 ms and the time to initiate boiling was sensitive to small changes in HIFU output. Nonlinear heating as a result of shock waves is therefore important to HIFU, and clinicians should be aware of the potential for very rapid boiling because it alters treatments.

Sonothrombolysis: an emerging modality for the management of stroke

OBJECTIVE

Ischemic stroke and intracranial hemorrhage remain a persistent scourge in Western civilization. Therefore, novel therapeutic modalities are desperately needed to expand the current limitations of treatment. Sonothrombolysis possesses the potential to fill this void because it has experienced a dramatic evolution from the time of early conceptualization in the 1960s. This process began in the realm of peripheral and cardiovascular disease and has since progressed to encompass intracranial pathologies. Our purpose is to provide a comprehensive review of the historical progression and existing state of knowledge, including underlying mechanisms as well as evidence for clinical application of ultrasound thrombolysis.

METHODS

Using MEDLINE, in addition to cross-referencing existing publications, a meticulous appraisal of the literature was conducted. Additionally, personal communications were used as appropriate.

RESULTS

This appraisal revealed several different technologies close to broad clinical use. However, fundamental questions remain, especially in regard to transcranial high-intensity focused ultrasound. Currently, the evidence supporting low intensity ultrasound's potential in isolation, without tissue plasminogen, remains uncertain; however, possibilities exist in the form of microbubbles to allow for focal augmentation with minimal systemic consequences. Alternatively, the literature clearly demonstrates, the efficacy of high-intensity focused ultrasound for independent thrombolysis.

CONCLUSION

Sonothrombolysis exists as a promising modality for the noninvasive or minimally invasive management of stroke, both ischemic and hemorrhagic. Further research facilitating clinical application is warranted.

MR-guided transcranial brain HIFU in small animal models

Recent studies have demonstrated the feasibility of transcranial high-intensity focused ultrasound (HIFU) therapy in the brain using adaptive focusing techniques. However, the complexity of the procedures imposes provision of accurate targeting, monitoring and control of this emerging therapeutic modality in order to ensure the safety of the treatment and avoid potential damaging effects of ultrasound on healthy tissues. For these purposes, a complete workflow and setup for HIFU treatment under magnetic resonance (MR) guidance is proposed and implemented in rats. For the first time, tissue displacements induced by the acoustic radiation force are detected in vivo in brain tissues and measured quantitatively using motion-sensitive MR sequences. Such a valuable target control prior to treatment assesses the quality of the focusing pattern in situ and enables us to estimate the acoustic intensity at focus. This MR-acoustic radiation force imaging is then correlated with conventional MR-thermometry sequences which are used to follow the temperature changes during the HIFU therapeutic session. Last, pre- and post-treatment magnetic resonance elastography (MRE) datasets are acquired and evaluated as a new potential way to non-invasively control the stiffness changes due to the presence of thermal necrosis. As a proof of concept, MR-guided HIFU is performed in vitro in turkey breast samples and in vivo in transcranial rat brain experiments. The experiments are conducted using a dedicated MR-compatible HIFU setup in a high-field MRI scanner (7 T). Results obtained on rats confirmed that both the MR localization of the US focal point and the pre- and post-HIFU measurement of the tissue stiffness, together with temperature control during HIFU are feasible and valuable techniques for efficient monitoring of HIFU in the brain. Brain elasticity appears to be more sensitive to the presence of oedema than to tissue necrosis.

Noninvasive determination of in situ heating rate using kHz acoustic emissions and focused ultrasound

For high-intensity focused ultrasound (HIFU) to be widely applicable in the clinic, robust methods of treatment planning, guidance and delivery need to be developed. These technologies would greatly benefit if patient specific tissue parameters could be provided as inputs so that the treatment planning and monitoring schemes are customized and tailored on a case by case basis. A noninvasive method of estimating the local in situ acoustic heating rate using the heat transfer equation (HTE) and applying novel signal processing techniques is presented in this article. The heating rate is obtained by experimentally measuring the time required to raise the temperature of the therapeutic focus from a baseline temperature to boiling (here assumed to be 100 degrees C for aqueous media) and then solving the heat transfer equation iteratively to find the heating rate that results in the onset of boiling. The onset of boiling is noninvasively detected by measuring the time instant of onset of acoustic emissions in the audible frequency range due to violent collapse of bubbles. In vitro experiments performed in a tissue mimicking alginate phantom and excised turkey breast muscle tissue demonstrate that the noninvasive estimates of heating rate are in good agreement with those obtained independently using established methods. The results show potential for the applicability of these techniques in therapy planning and monitoring for therapeutic dose optimization using real-time acoustic feedback.

Ultrasound transducer and system for real-time simultaneous therapy and diagnosis for noninvasive surgery of prostate tissue

For noninvasive treatment of prostate tissue using high-intensity focused ultrasound this paper proposes a design of an integrated multifunctional confocal phased array (IMCPA) and a strategy to perform both imaging and therapy simultaneously with this array. IMCPA is composed of triple-row phased arrays: a 6-MHz array in the center row for imaging and two 4-MHz arrays in the outer rows for therapy. Different types of piezoelectric materials and stack configurations may be employed to maximize their respective functionalities, i.e., therapy and imaging. Fabrication complexity of IMCPA may be reduced by assembling already constructed arrays. In IMCPA, reflected therapeutic signals may corrupt the quality of imaging signals received by the center-row array. This problem can be overcome by implementing a coded excitation approach and/or a notch filter when B-mode images are formed during therapy. The 13-bit Barker code, which is a binary code with unique autocorrelation properties, is preferred for implementing coded excitation, although other codes may also be used. From both Field II simulation and experimental results, we verified whether these remedial approaches would make it feasible to simultaneously carry out imaging and therapy by IMCPA. The results showed that the 13-bit Barker code with 3 cycles per bit provided acceptable performances. The measured -6 dB and -20 dB range mainlobe widths were 0.52 mm and 0.91 mm, respectively, and a range sidelobe level was measured to be -48 dB regardless of whether a notch filter was used. The 13-bit Barker code with 2 cycles per bit yielded -6 dB and -20 dB range mainlobe widths of 0.39 mm and 0.67 mm. Its range sidelobe level was found to be -40 dB after notch filtering. These results indicate the feasibility of the proposed transducer design and system for real-time imaging during therapy.

High-intensity focused ultrasound therapy: an overview for radiologists

High-intensity focused ultrasound therapy is a novel, emerging, therapeutic modality that uses ultrasound waves, propagated through tissue media, as carriers of energy. This completely non-invasive technology has great potential for tumor ablation as well as hemostasis, thrombolysis and targeted drug/gene delivery. However, the application of this technology still has many drawbacks. It is expected that current obstacles to implementation will be resolved in the near future. In this review, we provide an overview of high-intensity focused ultrasound therapy from the basic physics to recent clinical studies with an interventional radiologist's perspective for the purpose of improving the general understanding of this cutting-edge technology as well as speculating on future developments.

Prevention of post-focal thermal damage by formation of bubbles at the focus during high intensity focused ultrasound therapy

Safety concerns exist for potential thermal damage at tissue-air or tissue-bone interfaces located in the post-focal region during high intensity focused ultrasound (HIFU) treatments. We tested the feasibility of reducing thermal energy deposited at the post-focal tissue-air interfaces by producing bubbles (due to acoustic cavitation and/or boiling) at the HIFU focus. HIFU (in-situ intensities of 460-3500 W/cm2, frequencies of 3.2-5.5 MHz) was applied for 30 s to produce lesions (in turkey breast in-vitro (n = 37), and rabbit liver (n = 4) and thigh muscle in-vivo (n = 11)). Tissue temperature was measured at the tissue-air interface using a thermal (infrared) camera. Ultrasound imaging was used to detect bubbles at the HIFU focus, appearing as a hyperechoic region. In-vitro results showed that when no bubbles were present at the focus (at lower intensities of 460-850 W/cm2), the temperature at the interface increased continuously, up to 7.3 +/- 4.0 degrees C above the baseline by the end of treatment. When bubbles formed immediately after the start of HIFU treatment (at the high intensity of 3360 W/cm2), the temperature increased briefly for 3.5 s to 7.4 +/- 3.6 degrees C above the baseline temperature and then decreased to 4.0 +/- 1.4 degrees C above the baseline by the end of treatment. Similar results were obtained in in-vivo experiments with the temperature increases (above the baseline temperature) at the muscle-air and liver-air interfaces at the end of the high intensity treatment lower by 7.1 degrees C and 6.0 degrees C, respectively, as compared to the low intensity treatment. Thermal effects of HIFU at post-focal tissue-air interfaces, such as in bowels, could result in clinically significant increases in temperature. Bubble formation at the HIFU focus may provide a method for shielding the post-focal region from potential thermal damage.

Mathematical Modeling of Thermal Ablation in Tissue Surrounding a Large Vessel

Thermal ablation of a solid tumor in a tissue with radio-frequency (rf) energy can be accomplished by using a probe inserted into the tissue under the guidance of magnetic resonance imaging. The extent of the ablation can be significantly reduced by heat loss from capillary perfusion and by blood flow in a large vessel in the tissue. A mathematical model is presented of the thermal processes that occur during rf ablation of a tissue near a large blood vessel, which should not be damaged. Temperature distribution dynamics are described by the combination of a 3D bioheat transport in tissue together with a 1D model of convective-dispersive heat transport in the blood vessel. The objective was to determine how much of the tissue can be ablated without damaging the blood vessel. This was achieved by simulating the tissue temperature distribution dynamics and by determining the optimal power inputs so that a maximum temperature increase in the tissue was achieved without inducing tissue damage at the edge of the large vessel. The main contribution of this study is to provide a model analysis for pretreatment and, eventually, for intra-operative application to thermal ablation of a tumor located near a large blood vessel.

Minimizing abdominal wall damage during high-intensity focused ultrasound ablation by inducing artificial ascites

High-intensity focused ultrasound (HIFU) is becoming an important tool for tumor treatment [especially hepatocellular carcinoma (HCC)] in Asian countries. A HIFU system provides unique advantages of low invasiveness and absence of nonradiation. However, if the target HCC is close to the proximal surface of the liver, HIFU may overheat diaphragm, abdominal wall or skin. To avoid this complication, a method using artificial ascites in the abdominal cavity to separate the liver from the peritoneum, and to serve as a heat sink to cool overlying structures and thereby avoid inducing permanent damage was proposed. Target tissue that was 10 mm below the liver surface was ablated in 12 New Zealand white rabbits: 6 in the experimental group and 6 in the control group. Artificial ascites was established in the experimental group by injecting normal saline into the abdominal cavity until the pressure reached 150 mm H2O. Artificial ascites not only reduced the probability and extent of thermal damage to intervening structures, but also had no adverse affect on the efficacy of HIFU ablation.