HIFU Applications Research Articles

Comparative Characterization of Nonlinear Ultrasound Fields Generated by Sonalleve V1 and V2 MR-HIFU Systems.

A Sonalleve magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) clinical system (Profound Medical, Mississauga, ON, Canada) has been shown to generate nonlinear ultrasound fields with shocks up to 100 MPa at the focus as required for HIFU applications such as boiling histotripsy of hepatic and renal tumors. The Sonalleve system has two versions V1 and V2 of the therapeutic array, with differences in focusing angle, focus depth, arrangement of elements, and the size of a central opening that is twice larger in the V2 system compared to the V1. The goal of this study was to compare the performance of the V1 and V2 transducers for generating high-amplitude shock-wave fields and to reveal the impact of different array geometries on shock amplitudes at the focus. Nonlinear modeling of the field in water using boundary conditions reconstructed from holography measurements shows that at the same power output, the V2 array generates 10-15-MPa lower shock amplitudes at the focus. Consequently, substantially higher power levels are required for the V2 system to reach the same shock-wave exposure conditions in histotripsy-type treatments. Although this difference is mainly caused by the smaller focusing angle of the V2 array, the larger central opening of the V2 array has a nontrivial impact. By excluding coherently interacting weakly focused waves coming from the central part of the source, the presence of the central opening results in a somewhat higher effective focusing angle and thus higher shock amplitudes at the focus. Axisymmetric equivalent source models were constructed for both arrays, and the importance of including the central opening was demonstrated. These models can be used in the "HIFU beam" software for simulating nonlinear fields of the Sonalleve V1 and V2 systems in water and flat-layered biological tissues.

Cornea opacity, uveitis with iris atrophy and lens damage following cosmetic high-intensity ultrasound of the eyelid: a case report

BackgroundHigh-intensity focused ultrasound (HIFU) is a cosmetic procedure that aims to tone the skin through thermal collagen coagulation. The energy is delivered in the deep layers of the skin, and because of these characteristics, the risks of severe damage to adjacent tissue and the ocular surface may be underestimated. Previous reports have demonstrated superficial corneal opacities, cataracts, increased intraocular pressure, or ocular refractive changes in different patients following HIFU. In this case, we report deep stromal opacities associated with anterior uveitis, iris atrophy and lens opacity formation following a single HIFU superior eyelid application.Case presentationA 47-year-old female presented to the ophthalmic emergency department complaining of pain, hyperemia and photophobia in the right eye following a HIFU application to the superior right eyelid. A slit lamp examination showed three temporal-inferior corneal infiltrates with edema and severe anterior uveitis. The patient was treated with topical corticosteroids, and six months later, there was residual corneal opacity, iris atrophy and peripherical cataract formation. No surgical procedure was needed, and the final vision was Snellen 20/20 (1.0).ConclusionThe risk of severe impairment to the ocular surface and ocular tissues may be underestimated. Cosmetic surgeons and ophthalmologists must be aware of the complications, and the long-term follow-up of these changes needs further investigation and discussion. Safety protocols of the HIFU intensity threshold for thermal lesions in the eye and the use of protective eye devices should be better evaluated.

Phase-Changeable Nanoparticle-Mediated Energy Conversion Promotes Highly Efficient High-Intensity Focused Ultrasound Ablation.

This review describes how phase-changeable nanoparticles enable highly-efficient high-intensity focused ultrasound ablation (HIFU). HIFU is effective in the clinical treatment of solid malignant tumors; however, it has intrinsic disadvantages for treating some deep lesions, such as damage to surrounding normal tissues. When phase-changeable nanoparticles are used in HIFU treatment, they could serve as good synergistic agents because they are transported in the blood and permeated and accumulated effectively in tissues. HIFU's thermal effects can trigger nanoparticles to undergo a special phase transition, thus enhancing HIFU ablation efficiency. Nanoparticles can also carry anticancer agents and release them in the targeted area to achieve chemo-synergistic therapy response. Although the formation of nanoparticles is complicated and HIFU applications are still in an early stage, the potential for their use in synergy with HIFU treatment shows promising results.

An Introduction to High Intensity Focused Ultrasound: Systematic Review on Principles, Devices, and Clinical Applications.

Ultrasound can penetrate deep into tissues and interact with human tissue via thermal and mechanical mechanisms. The ability to focus an ultrasound beam and its energy onto millimeter-size targets was a significant milestone in the development of therapeutic applications of focused ultrasound. Focused ultrasound can be used as a non-invasive thermal ablation technique for tumor treatment and is being developed as an option to standard oncologic therapies. High-intensity focused ultrasound has now been used for clinical treatment of a variety of solid malignant tumors, including those in the pancreas, liver, kidney, bone, prostate, and breast, as well as uterine fibroids and soft-tissue sarcomas. Magnetic resonance imaging and Ultrasound imaging can be combined with high intensity focused ultrasound to provide real-time imaging during ablation. Magnetic resonance guided focused ultrasound represents a novel non-invasive method of treatment that may play an important role as an alternative to open neurosurgical procedures for treatment of a number of brain disorders. This paper briefly reviews the underlying principles of HIFU and presents current applications, outcomes, and complications after treatment. Recent applications of Focused ultrasound for tumor treatment, drug delivery, vessel occlusion, histotripsy, movement disorders, and vascular, oncologic, and psychiatric applications are reviewed, along with clinical challenges and potential future clinical applications of HIFU.

Tissue-mimicking thermochromic phantom for characterization of HIFU devices and applications

Purpose: Tissue-mimicking phantoms (TMPs) are synthetic materials designed to replicate properties of biological tissues. There is a need to quantify temperature changes following ultrasound or magnetic resonance imaging-guided high intensity focused ultrasound (MR-HIFU). This work describes development, characterization and evaluation of tissue-mimicking thermochromic phantom (TMTCP) for direct visualization and quantification of HIFU heating. The objectives were to (1) develop an MR-imageable, HIFU-compatible TMTCP that reports absolute temperatures, (2) characterize TMTCP physical properties and (3) examine TMTCP color change after HIFU.Methods and materials: A TMTCP was prepared to contain thermochromic ink, silicon dioxide and bovine serum albumin (BSA) and its properties were quantified. A clinical MRI-guided and a preclinical US-guided HIFU system were used to perform sonications in TMTCP. MRI thermometry was performed during HIFU, followed by T2-weighted MRI post-HIFU. Locations of color and signal intensity change were compared to the sonication plan and to MRI temperature maps.Results: TMTCP properties were comparable to those in human soft tissues. Upon heating, the TMTCP exhibited an incremental but permanent color change for temperatures between 45 and 70 °C. For HIFU sonications the TMTCP revealed spatially sharp regions of color change at the target locations, correlating with MRI thermometry and hypointense regions on T2-weighted MRI. TMTCP-based assessment of various HIFU applications was also demonstrated.Conclusions: We developed a novel MR-imageable and HIFU-compatible TMTCP to characterize HIFU heating without MRI or thermocouples. The HIFU-optimized TMTCP reports absolute temperatures and ablation zone geometry with high spatial resolution. Consequently, the TMTCP can be used to evaluate HIFU heating and may provide an in vitro tool for peak temperature assessment, and reduce preclinical in vivo requirements for clinical translation.

Bubble dynamics in viscoelastic soft tissue in high-intensity focal ultrasound thermal therapy

The present study is aimed to investigate bubble dynamics in a soft tissue, to which HIFU’s continuous harmonic pulse is applied by introducing a viscoelastic cavitation model. After a comparison of some existing cavitation models, we decided to employ Gilmore-Akulichev model. This chosen cavitation model should be coupled with the Zener viscoelastic model in order to be able to simulate soft tissue features such as elasticity and relaxation time. The proposed Gilmore-Akulichev-Zener model was investigated for exploring cavitation dynamics. The parametric study led us to the conclusion that the elasticity and viscosity both damp bubble oscillations, whereas the relaxation effect depends mainly on the period of the ultrasound wave. The similar influence of elasticity, viscosity and relaxation time on the temperature inside the bubble can be observed. Cavitation heat source terms (corresponding to viscous damping and pressure wave radiated by bubble collapse) were obtained based on the proposed model to examine the cavitation significance during the treatment process. Their maximum values both overdominate the acoustic ultrasound term in HIFU applications. Elasticity was revealed to damp a certain amount of deposited heat for both cavitation terms.

MO-DE-202-00: Image-Guided Interventions: Advances in Intraoperative Imaging, Guidance, and An Emerging Role for Medical Physics in Surgery

At least three major trends in surgical intervention have emerged over the last decade: a move toward more minimally invasive (or non-invasive) approach to the surgical target; the development of high-precision treatment delivery techniques; and the increasing role of multi-modality intraoperative imaging in support of such procedures. This symposium includes invited presentations on recent advances in each of these areas and the emerging role for medical physics research in the development and translation of high-precision interventional techniques. The four speakers are: (1)Keyvan Farahani, “Image-guided focused ultrasound surgery and therapy” (2)Jeffrey H. Siewerdsen, “Advances in image registration and reconstruction for image-guided neurosurgery” (3)Tina Kapur, “Image-guided surgery and interventions in the advanced multimodality image-guided operating (AMIGO) suite” (4)Raj Shekhar, “Multimodality image-guided interventions: Multimodality for the rest of us” Learning Objectives: 1.Understand the principles and applications of HIFU in surgical ablation. 2.Learn about recent advances in 3D–2D and 3D deformable image registration in support of surgical safety and precision. 3.Learn about recent advances in model-based 3D image reconstruction in application to intraoperative 3D imaging. 4.Understand the multi-modality imaging technologies and clinical applications investigated in the AMIGO suite. 5.Understand the emerging need and techniques to implement multi-modality image guidance in surgical applications such as neurosurgery, orthopaedic surgery, vascular surgery, and interventional radiology. Research supported by the NIH and Siemens Healthcare.; J. Siewerdsen; Grant Support - National Institutes of Health; Grant Support - Siemens Healthcare; Grant Support - Carestream Health; Advisory Board - Carestream Health; Licensing Agreement - Carestream Health; Licensing Agreement - Elekta Oncology.; T. Kapur, P41EB015898; R. Shekhar, Funding: R42CA137886 and R41CA192504 Disclosure and CoI: IGI Technologies, small-business partner on the grants

MO-DE-202-03: Image-Guided Surgery and Interventions in the Advanced Multimodality Image-Guided Operating (AMIGO) Suite

At least three major trends in surgical intervention have emerged over the last decade: a move toward more minimally invasive (or non-invasive) approach to the surgical target; the development of high-precision treatment delivery techniques; and the increasing role of multi-modality intraoperative imaging in support of such procedures. This symposium includes invited presentations on recent advances in each of these areas and the emerging role for medical physics research in the development and translation of high-precision interventional techniques. The four speakers are: (1)Keyvan Farahani, “Image-guided focused ultrasound surgery and therapy” (2)Jeffrey H. Siewerdsen, “Advances in image registration and reconstruction for image-guided neurosurgery” (3)Tina Kapur, “Image-guided surgery and interventions in the advanced multimodality image-guided operating (AMIGO) suite” (4)Raj Shekhar, “Multimodality image-guided interventions: Multimodality for the rest of us” Learning Objectives: 1.Understand the principles and applications of HIFU in surgical ablation. 2.Learn about recent advances in 3D–2D and 3D deformable image registration in support of surgical safety and precision. 3.Learn about recent advances in model-based 3D image reconstruction in application to intraoperative 3D imaging. 4.Understand the multi-modality imaging technologies and clinical applications investigated in the AMIGO suite. 5.Understand the emerging need and techniques to implement multi-modality image guidance in surgical applications such as neurosurgery, orthopaedic surgery, vascular surgery, and interventional radiology. Research supported by the NIH and Siemens Healthcare.; J. Siewerdsen; Grant Support - National Institutes of Health; Grant Support - Siemens Healthcare; Grant Support - Carestream Health; Advisory Board - Carestream Health; Licensing Agreement - Carestream Health; Licensing Agreement - Elekta Oncology.; T. Kapur, P41EB015898; R. Shekhar, Funding: R42CA137886 and R41CA192504 Disclosure and CoI: IGI Technologies, small-business partner on the grants

MO-DE-202-04: Multimodality Image-Guided Surgery and Intervention: For the Rest of Us

At least three major trends in surgical intervention have emerged over the last decade: a move toward more minimally invasive (or non-invasive) approach to the surgical target; the development of high-precision treatment delivery techniques; and the increasing role of multi-modality intraoperative imaging in support of such procedures. This symposium includes invited presentations on recent advances in each of these areas and the emerging role for medical physics research in the development and translation of high-precision interventional techniques. The four speakers are: (1)Keyvan Farahani, “Image-guided focused ultrasound surgery and therapy” (2)Jeffrey H. Siewerdsen, “Advances in image registration and reconstruction for image-guided neurosurgery” (3)Tina Kapur, “Image-guided surgery and interventions in the advanced multimodality image-guided operating (AMIGO) suite” (4)Raj Shekhar, “Multimodality image-guided interventions: Multimodality for the rest of us” Learning Objectives: 1.Understand the principles and applications of HIFU in surgical ablation. 2.Learn about recent advances in 3D–2D and 3D deformable image registration in support of surgical safety and precision. 3.Learn about recent advances in model-based 3D image reconstruction in application to intraoperative 3D imaging. 4.Understand the multi-modality imaging technologies and clinical applications investigated in the AMIGO suite. 5.Understand the emerging need and techniques to implement multi-modality image guidance in surgical applications such as neurosurgery, orthopaedic surgery, vascular surgery, and interventional radiology. Research supported by the NIH and Siemens Healthcare.; J. Siewerdsen; Grant Support - National Institutes of Health; Grant Support - Siemens Healthcare; Grant Support - Carestream Health; Advisory Board - Carestream Health; Licensing Agreement - Carestream Health; Licensing Agreement - Elekta Oncology.; T. Kapur, P41EB015898; R. Shekhar, Funding: R42CA137886 and R41CA192504 Disclosure and CoI: IGI Technologies, small-business partner on the grants

MO-DE-202-01: Image-Guided Focused Ultrasound Surgery and Therapy

At least three major trends in surgical intervention have emerged over the last decade: a move toward more minimally invasive (or non-invasive) approach to the surgical target; the development of high-precision treatment delivery techniques; and the increasing role of multi-modality intraoperative imaging in support of such procedures. This symposium includes invited presentations on recent advances in each of these areas and the emerging role for medical physics research in the development and translation of high-precision interventional techniques. The four speakers are: (1) Keyvan Farahani, “Image-guided focused ultrasound surgery and therapy” (2) Jeffrey H. Siewerdsen, “Advances in image registration and reconstruction for image-guided neurosurgery” (3) Tina Kapur, “Image-guided surgery and interventions in the advanced multimodality image-guided operating (AMIGO) suite” (4) Raj Shekhar, “Multimodality image-guided interventions: Multimodality for the rest of us” Learning Objectives: 1. Understand the principles and applications of HIFU in surgical ablation. 2. Learn about recent advances in 3D–2D and 3D deformable image registration in support of surgical safety and precision. 3. Learn about recent advances in model-based 3D image reconstruction in application to intraoperative 3D imaging. 4. Understand the multi-modality imaging technologies and clinical applications investigated in the AMIGO suite. 5. Understand the emerging need and techniques to implement multi-modality image guidance in surgical applications such as neurosurgery, orthopaedic surgery, vascular surgery, and interventional radiology. Research supported by the NIH and Siemens Healthcare.; J. Siewerdsen; Grant Support - National Institutes of Health; Grant Support - Siemens Healthcare; Grant Support - Carestream Health; Advisory Board - Carestream Health; Licensing Agreement - Carestream Health; Licensing Agreement - Elekta Oncology.; T. Kapur, P41EB015898; R. Shekhar, Funding: R42CA137886 and R41CA192504 Disclosure and CoI: IGI Technologies, small-business partner on the grants

MO-DE-202-02: Advances in Image Registration and Reconstruction for Image-Guided Neurosurgery

At least three major trends in surgical intervention have emerged over the last decade: a move toward more minimally invasive (or non-invasive) approach to the surgical target; the development of high-precision treatment delivery techniques; and the increasing role of multi-modality intraoperative imaging in support of such procedures. This symposium includes invited presentations on recent advances in each of these areas and the emerging role for medical physics research in the development and translation of high-precision interventional techniques. The four speakers are: (1) Keyvan Farahani, “Image-guided focused ultrasound surgery and therapy” (2) Jeffrey H. Siewerdsen, “Advances in image registration and reconstruction for image-guided neurosurgery” (3) Tina Kapur, “Image-guided surgery and interventions in the advanced multimodality image-guided operating (AMIGO) suite” (4) Raj Shekhar, “Multimodality image-guided interventions: Multimodality for the rest of us” Learning Objectives: 1. Understand the principles and applications of HIFU in surgical ablation. 2. Learn about recent advances in 3D–2D and 3D deformable image registration in support of surgical safety and precision. 3. Learn about recent advances in model-based 3D image reconstruction in application to intraoperative 3D imaging. 4. Understand the multi-modality imaging technologies and clinical applications investigated in the AMIGO suite. 5. Understand the emerging need and techniques to implement multi-modality image guidance in surgical applications such as neurosurgery, orthopaedic surgery, vascular surgery, and interventional radiology. Research supported by the NIH and Siemens Healthcare.; J. Siewerdsen; Grant Support - National Institutes of Health; Grant Support - Siemens Healthcare; Grant Support - Carestream Health; Advisory Board - Carestream Health; Licensing Agreement - Carestream Health; Licensing Agreement - Elekta Oncology.; T. Kapur, P41EB015898; R. Shekhar, Funding: R42CA137886 and R41CA192504 Disclosure and CoI: IGI Technologies, small-business partner on the grants

Magnetic resonance-guided high-intensity focused ultrasound combined with radiotherapy for palliation of head and neck cancer-a pilot study.

BackgroundRadiotherapy is a critical component of the multidisciplinary management of cancers of the head and neck. It may comprise the primary curative treatment modality or is used in an adjuvant setting to improve local control and survival by preventing seeding and reseeding of distant metastases from persistent reservoirs of locoregional disease. Although considerable advances have been made recently in the fields of radiotherapy, systemic treatment and surgery for head and neck tumours, locoregional recurrence rates remain high and treatment side effects may have severe impact on patients’ quality of life.Magnetic resonance-guided high-intensity focused ultrasound (MRg-HIFU) is a novel technique in the treatment of cancer that has the potential to improve tumour cure rates and decrease treatment-related toxicity. Clinical applications of HIFU are being used increasingly for the treatment of several tumour sites, for example uterine leiomyomas and prostate cancer.Methods/DesignThe pilot study presented here is an initial step toward utilizing MRg-HIFU for head and neck cancer treatment. The rationale for novel treatment options in head and neck cancer is reviewed as well as emerging evidence that support the increasing clinical utilization of MRg-HIFU.DiscussionThis pilot study aims to assess safety, toxicity and feasibility of MRg-HIFU treatments to the head and neck region and to evaluate changes caused by MRg-HIFU within the treated tumour regions based on post-treatment MRI.

Design of a low power hybrid HIFU applicator for haemostasis based on acoustic propagation modelling

Purpose: The aim of this study was to design an applicator for haemostasis usage needing lower acoustic intensities (<880 W/cm2) than in previous devices intended for it, which is based on ultrasound propagation FEM modelling using a 2-MHz HIFU transducer. Materials and methods: Acoustic field characterisation and numerical simulations in water were performed with and without the proposed applicator. Parameters such as form factor, ellipsoidal shape ratio, and Euclidean distance were used (among others) to compare simulated data with transducer measurements without applicator. A low density polyethylene cone was manufactured from geometries validated from acoustic field modelling. The hollow cone was filled with 10% polyacrylamide gel as a coupling medium with liver phantom or chicken liver. Focal temperature was measured with a thermocouple embedded in the phantom for 1–20 W driving powers for 120 s. Standing wave ratios (SWR) were used as coupling indexes. Ex vivo experimentation in chicken liver was made at 10–20 W. Results: Simulated acoustic patterns showed good concordance with measurements. Experimental focal distance was 20.72 ± 0.24 mm, while the simulated was 19.79 mm (≈4% error). SWR at low power were: 2.01 with transducer emitting in air, 1.53 at applicator tip, and 1.35 after phantom placement. Average SWR at high power was 1.31. Similarity of percentages for data comparison in focal plane was over 60%. Maximum temperature measured at focus was 88.7 °C with 20 W after 85 s. Conclusions: Temperatures reached at focus suggest that this applicator has good efficiency, which notably reduces the power typically needed for haemostasis effect.

Design of dual mode high intensity focused ultrasound transducers for prostate therapy.

In HIFU applications, either MRI or ultrasound is used for treatment guidance and monitoring. When ultrasound imaging is used, a separate imaging transducer can be confocally aligned with the therapy probe itself. Yet, in endocavitary applications, such as transrectal prostate HIFU treatment, there are significant constraints on the size and geometry of the final probe, and the integratation of the imaging transducer with the therapy probe is a significant challenge. One solution to this problem is to use the same transducer elements for both imaging and therapy. Dual‐mode transducers meet the specifications for therapy and imaging by taking into account the constraints linked to the features of the therapy transducer different from those of the imaging probe. This presentation discusses the results of simulations undertaken to determine an optimal geometry for a dual‐mode probe that can be used both for thermal ablation of the prostate as well as for obtaining ultrasound images of sufficient quality for ...

Focusing of High-Intensity Ultrasound Through the Rib Cage Using a Therapeutic Random Phased Array

A method for focusing high-intensity ultrasound (HIFU) through a rib cage that aims to minimize heating of the ribs while maintaining high intensities at the focus (or foci) was proposed and tested theoretically and experimentally. Two approaches, one based on geometric acoustics and the other accounting for diffraction effects associated with propagation through the rib cage, were investigated theoretically for idealized source conditions. It is shown that for an idealized radiator, the diffraction approach provides a 23% gain in peak intensity and results in significantly less power losses on the ribs (1% vs. 7.5% of the irradiated power) compared with the geometric one. A 2-D 1-MHz phased array with 254 randomly distributed elements, tissue-mimicking phantoms and samples of porcine rib cages are used in experiments; the geometric approach is used to configure how the array is driven. Intensity distributions are measured in the plane of the ribs and in the focal plane using an infrared camera. Theoretical and experimental results show that it is possible to provide adequate focusing through the ribs without overheating them for a single focus and several foci, including steering at ± 10–15 mm off and ± 20 mm along the array axis. Focus splitting caused by the periodic spatial structure of ribs is demonstrated both in simulations and experiments; the parameters of splitting are quantified. The ability to produce thermal lesions with a split focal pattern in ex vivo porcine tissue placed beyond the rib phantom is also demonstrated. The results suggest that the method is potentially useful for clinical applications of HIFU, for which the rib cage lies between the transducer(s) and the targeted tissue. (E-mail: vera@apl.washington.edu)

WE‐E‐304A‐05: Acoustic Droplet Vaporization for Enhancement of High Intensity Focused Ultrasound Thermal Ablation

Purpose: Acoustic droplet vaporization (ADV) shows promise for the spatial control and acceleration of thermal lesion production. In this study, we investigated the hypothesis that ADV bubbles can substantially enhance HIFU ablation by increasing and controlling local energy absorption. Method and Materials: HIFU/ADV phantoms were made of polyacrylamide, egg white, degassed water and lipid‐coated perfluoropentane droplets (1.8 μm mean dia.). These egg white‐based phantoms (acoustic attenuation of 0.3 dB/cm at 1.44 MHz) were optically transparent, which allowed direct visualization of lesion formation during HIFU application. Phantoms were exposed in degassed water at 37°C to a focused ultrasound (1.44 MHz) with a focal intensity of 2000 W/cm2. Lesion volumes were measured by fluid displacement or T2 weighted MR imaging. Results: For 2‐s HIFU exposure on droplet‐containing phantoms (n = 5), apparent opacity changes were observed consistently, while no lesions were found in phantoms without droplets (n = 8). It was noted that the volume of the lesion with 2‐second HIFU exposure in droplet‐containing phantoms was comparable to that made by 5‐s exposure in the phantom without droplets. Lesion volumes (5‐s exposure) measured by MR imaging were found to be 45.5±5.5 (n=8), 102.5±13.5 (n=5), and 288.0±50.1μL (n=8) in phantoms of 0, 104, 105 droplets/mL, respectively. Conclusion: The presence of ADV bubbles reduced the acoustic energy for comparably sized lesion and increasing the speed of HIFU lesion formation. Our findings may address the pre‐focal heating problem in traditional HIFU ablation.

Standardisation and metrology for High Intensity focused ultrasound: a clinical perspective

The clinical application of HIFU is a young and rapidly expanding field, and it is essential that validated methods for measurement and testing are made available as soon as possible. These issues have not been addressed to date in any systematic fashion. There is, therefore, an urgent need to produce standard registration, testing equipment and methodology to allow users to characterize clinical HIFU systems for checking safety and reproducibility of a machine's output, comparing different devices or commissioning new systems. HIFU is delivered to the target volume via a number of routes. Extra‐corporeal and trans‐rectal probes are the most commonly used, although preliminary testing of intra‐cavitary, catheter based devices is also underway19. These different devices present a wide range of different transducer geometries, f‐numbers (∼0.8‐1.8), operating frequencies (∼0.8 ‐ 5 MHz), and focal peak intensities (∼7.5 102 ‐ 3 104 W cm−2). Clinical HIFU systems are currently assessed on an ad hoc basis by individual clinical departments and manufacturers, using methods, many of which are unpublished. The main requirements from a clinical perspective are for the quantification of parameters which relate directly to individual HIFU treatments, allow reproducible treatments between different patients, and allow clinical trial data to be compared.

Dynamical focusing method for high-intensity ultrasound applications

New method for direct synthesis of dynamically focused acoustic field suitable for HIFU applications is proposed. The method is based on the continuous cyclic generation of different focal patterns at optimal repetition frequency by simultaneous applying of "M" different frequency signals to "N" sectors of spherically shaped sectored transducer array. The HIFU transducer arrays comprising a spherical piezoceramic cap with back electrode divided circumferentially into "N" regular or specially shaped sectors were designed and tested. The sectors were simultaneously powered by "M" (M ⩽ N) sinus or burst drive signals with different frequencies from 6 dB bandwidth of the transducer. Calculation and modeling of acoustic field patterns for different array configurations and frequencies sets were performed. Acoustic pressure in focal planes was measured in water using calibrated hydrophone and 3D acoustic scanning system. In vitro experiments on different tissues confirming the advantages of dynamical focusing method were performed. The benefits of the method are creating bigger treated tissue volumes and enhancement of the cavitational, mechanical or thermal influences due to coincidence of the repetition frequency with a specific resonance/relaxation times for tissue and/or for cavitation "cloud". Applications of the method for ultrasonic therapy, hyperthermia, and body aesthetics were considered.

Blood mimicking fluid for high-intensity focused ultrasound applications

A blood-mimicking fluid (BMF) having viscosity, attenuation, and backscatter similar to that of human blood has been developed for the acoustic and thermal characterization of HIFU ablation devices. The BMF consists of gellan gum, low density polyethelene particles, and nylon particles dispersed in water, glycerin, and alcohol. The BMF was characterized for attenuation coefficient, speed of sound, backscatter coefficient, viscosity, thermal diffusivity, and thermal conductivity. The low attenuation as well as the sound speed and the bloodlike backscatter indicate the usefulness of the BMF for ultrasound imaging and flow applications. These properties, along with thermal conductivity and diffusivity also within the range found in human tissue and the ability to withstand temperature increases above 70&amp;lt;th&amp;gt;°C with no significant change in its properties, make this material appropriate for HIFU applications. This fluid also exhibits viscosity varying as a function of velocity (or shear rate). [This research was supported by the Defense Advanced Research Projects Agency (DARPA) through IAG No. 224-05-6016.]

Initial Experience Using a Forward Directed, High‐Intensity Focused Ultrasound Balloon Catheter for Pulmonary Vein Antrum Isolation in Patients with Atrial Fibrillation

A high-intensity-focused ultrasound balloon catheter (HIFU-BC) is designed to isolate pulmonary veins (PV) outside the ostia (PV antrum). This catheter uses a parabolic CO2 balloon (behind water balloon) to focus a 20-, 25-, or 30-mm diameter ring of ultrasound forward of the balloon (parallel to catheter shaft). The purpose of this study is to test the safety and efficacy of the HIFU-BC for PV antrum isolation in patients with atrial fibrillation (AF). Twenty-seven patients with paroxysmal (19 patients) or persistent (8 patients) AF were studied. Double transseptal puncture was performed for left atrial deployment of a Lasso catheter (for PV mapping) and the 14 Fr HIFU-BC. The HIFU-BC was positioned outside the PV orifice over a guidewire. HIFU energy (acoustic power 45 watts) was applied for 40 seconds with a 20-mm sonicating ring and 40 or 60 seconds with a 25-mm or 30-mm sonicating ring. No other ablation system was utilized. PV antrum isolation was attempted using HIFU-BC in 78 of 104 PVs (25/27 RSPVs, all 23 LSPVs, all 23 LIPVs, all four left common trunks and 3/27 RIPVs). HIFU-BC successfully isolated 68 (87%) of the 78PV antra with 1-26 (median 3) HIFU applications. The complications include transient bleeding from a distal branch of the left superior PV resulting from guidewire manipulation in one patient and right phrenic nerve injury in another patient. No PV stenosis (>50% narrowing) and no LA-esophageal fistula occurred. At the 12-month follow-up, 16 (59%) of the 27 patients were free of symptomatic episodes of AF (only 3 of the 16 patients were receiving antiarrhythmic medications). Forward-focused HIFU applications isolated PVs outside the PV ostium with elimination of AF in 16 (59%) of the 27 patients at 12 months following the single ablation procedure.