Characterization of a multi-element clinical HIFU system using acoustic holography and nonlinear modeling

Volumetric hyperthermia delivery using the ExAblate Body MR-guided focused ultrasound system

OBJECTIVES

To investigate image-guided volumetric hyperthermia strategies using the ExAblate Body MR-guided focused ultrasound ablation system, involving mechanical transducer movement and sector-vortex beamforming.

MATERIALS AND METHODS

Acoustic and thermal simulations were performed to investigate volumetric hyperthermia using mechanical transducer movement combined with sector-vortex beamforming, specifically for the ExAblate Body transducer. The system control in the ExAblate Body system was modified to achieve fast transducer movement and MR thermometry-based hyperthermia control, mechanical transducer movements and electronic sector-vortex beamforming were combined to optimize hyperthermia delivery. The experimental validation was performed using a tissue-mimicking phantom.

RESULTS

The developed simulation framework allowed for a parametric study with varying numbers of heating spots, sonication durations, and transducer movement times to evaluate the hyperthermia characteristics for mechanical transducer movement and sector-vortex beamforming. Hyperthermic patterns involving 2-4 sequential focal spots were analyzed. To demonstrate the feasibility of volumetric hyperthermia in the system, a tissue-mimicking phantom was sonicated with two distinct spots through mechanical transducer movement and sector-vortex beamforming. During hyperthermia, the average values of Tmax, T10, Tavg, T90, and Tmin over 200 s were measured within a circular ROI with a diameter of 10 pixels. These values were found to be 8.6, 7.9, 6.6, 5.2, and 4.5 °C, respectively, compared to the baseline temperature.

CONCLUSIONS

This study demonstrated the volumetric hyperthermia capabilities of the ExAblate Body system. The simulation framework developed in this study allowed for the evaluation of hyperthermia characteristics that could be implemented with the ExAblate MRgFUS system.

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.

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

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

Hydrophone Measurements for Biomedical Ultrasound Applications: A Review

This article presents basic principles of hydrophone measurements, including mechanisms of action for various hydrophone designs, sensitivity and directivity calibration procedures, practical considerations for performing measurements, signal processing methods to correct for both frequency-dependent sensitivity and spatial averaging across the hydrophone sensitive element, uncertainty in hydrophone measurements, special considerations for high-intensity therapeutic ultrasound, and advice for choosing an appropriate hydrophone for a particular measurement task. Recommendations are made for information to be included in hydrophone measurement reporting.

A Modular, Kerf-Minimizing Approach for Therapeutic Ultrasound Phased Array Construction

A novel method for fabricating a modular, kerf-minimizing histotripsy phased array was developed and tested. The method utilizes arbitrarily shaped elements, 3-D printing, water jet cutting, and a thin, 125- [Formula: see text] electrically insulating epoxy coating to maximize aperture utilization while allowing for replacement of individual transducer modules. The method was used to fabricate a 750-kHz truncated circular aperture array (165 mm ×234 mm) transducer with a focal length of 142 mm. The aperture was segmented into 260 arc-shaped modular elements, each approximately 11.5 mm ×11.5 mm, arranged in concentric rings. The resulting aperture utilization was 92%. The full-width-half-maximum (FWHM) focal zone of the array was measured to be 1.6 mm ×1.1 mm ×4.5 mm, and the FWHM electrical steering range was measured to be 38.5 mm ×33 mm 40 mm. The array was estimated to be capable of generating approximately 120-MPa peak negative pressure at the geometric focus. In addition, the array was used to ablate a 5-cm3 volume of tissue with electric focal steering.

Theoretical Zero-Thickness Broadband Holograms Based on Acoustic Sieve Metasurfaces

Acoustic holography is an essential tool for controlling sound waves, generating highly complex and customizable sound fields, and enabling the visualization of sound fields. Based on acoustic sieve metasurfaces (ASMs), this paper proposes a theoretical design approach for zero-thickness broadband holograms. The ASM is a zero-thickness rigid screen with a large number of small holes that allow sound waves to pass through and produce the desired real image in the target plane. The hole arrangement rules are determined using a genetic algorithm and the Rayleigh–Sommerfeld theory. Because the wave from a hole has no extra phase or amplitude modulation, the intractable modulation dispersion can be physically avoided, allowing the proposed ASM-based hologram to potentially function in any frequency band as long as the condition of paraxial approximation is satisfied. Using a numerical simulation based on the combination of the finite element method (FEM) and the boundary element method (BEM), this research achieves broadband holographic imaging with a good effect. The proposed theoretical zero-thickness broadband hologram may provide new possibilities for acoustic holography applications.

Spatiotemporal Deconvolution of Hydrophone Response for Linear and Nonlinear Beams-Part I: Theory, Spatial-Averaging Correction Formulas, and Criteria for Sensitive Element Size

This article reports spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. Readers who are uninterested in hydrophone theory may proceed directly to Appendix A for an easy method to estimate spatial-averaging correction factors. Hydrophones were modeled as angular spectrum filters. Simulations modeled nine circular transducers (1-10 MHz; F/1.4-F/3.2) driven at six power levels and measured with eight hydrophones (432 beam/hydrophone combinations). For example, the model predicts that if a 200- [Formula: see text] membrane hydrophone measures a moderately nonlinear 5-MHz beam from an F/1 transducer, spatial-averaging correction factors are 33% (peak compressional pressure or p_c ), 18% (peak rarefactional pressure or p ), and 18% (full width half maximum or FWHM). Theoretical and experimental estimates of spatial-averaging correction factors to were in good agreement (within 5%) for linear and moderately nonlinear signals. Criteria for maximum appropriate hydrophone sensitive element size as functions of experimental parameters were derived. Unlike the oft-cited International Electrotechnical Commission (IEC) criterion, the new criteria were derived for focusing rather than planar transducers and can accommodate nonlinear signals in addition to linear signals. Responsible reporting of hydrophone-based pressure and beamwidth measurements should always acknowledge spatial-averaging considerations.

Spatiotemporal Deconvolution of Hydrophone Response for Linear and Nonlinear Beams-Part II: Experimental Validation

This article reports experimental validation for spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. The method was validated using linear and nonlinear beams transmitted by seven single-element spherically focusing transducers (2-10 MHz; F /#: 1-3) and measured with five hydrophones (sensitive element diameters d_g : 85-1000 [Formula: see text]), resulting in 35 transducer/hydrophone combinations. Exponential functions, exp( -αx ), where x = d_g /( λ₁F /#) and λ₁ is the fundamental wavelength, were used to model focal pressure ratios p'/p (where p' is the measured value subjected to spatial averaging and p is the true axial value that would be obtained with a hypothetical point hydrophone). Spatiotemporal deconvolution reduced α (followed by root mean squared difference between data and fit) from 0.29-0.30 (7%) to 0.01 (8%) (linear signals) and from 0.29-0.40 (8%) to 0.04 (14%) (nonlinear signals), indicating successful spatial averaging correction. Linear functions, Cx + 1, were used to model FWHM'/FWHM, where FWHM is full-width half-maximum. Spatiotemporal deconvolution reduced C from 9% (4%) to -0.6% (1%) (linear signals) and from 30% (10%) to 6% (5%) (nonlinear signals), indicating successful spatial averaging correction. Spatiotemporal deconvolution resulted in significant improvement in accuracy even when the hydrophone geometrical sensitive element diameter exceeded the beam FWHM. Responsible reporting of hydrophone-based pressure measurements should always acknowledge spatial averaging considerations.

Design and Characterization of an Ultrasound Transducer for Combined Histotripsy-Thrombolytic Therapy

Chronic thrombi of the deep veins of the leg are resistant to dissolution or removal by current interventions and can act as thrombogenic sources. Histotripsy, a focused ultrasound therapy, uses the mechanical activity of bubble clouds to liquefy target tissues. In vitro experiments have shown that histotripsy enhances thrombolytic agent recombinant tissue plasminogen activator in a highly retracted clot model resistant to lytic therapy alone. Although these results are promising, further refinement of the acoustic source is necessary for in vivo studies and clinical translation. The source parameters for use in vivo were defined, and a transducer was fabricated for transcutaneous exposure of porcine and human iliofemoral deep-vein thrombosis (DVT) as the target. Based on the design criteria, a 1.5-MHz elliptical source with a 6-cm focal length and a focal gain of 60 was selected. The source was characterized by fiber-optic hydrophone and holography. High-speed photography showed that the cavitation cloud could be confined to dimensions smaller than the specified vessel lumen. The source was also demonstrated in vitro to create confined lesions within clots. The results support that this design offers an appropriate clinical prototype for combined histotripsy-thrombolytic therapy.

"HIFU Beam:" A Simulator for Predicting Axially Symmetric Nonlinear Acoustic Fields Generated by Focused Transducers in a Layered Medium

"HIFU beam" is a freely available software tool that comprises a MATLAB toolbox combined with a user-friendly interface and binary executable compiled from FORTRAN source code (HIFU beam. (2021). Available: http://limu.msu.ru/node/3555?language=en). It is designed for simulating high-intensity focused ultrasound (HIFU) fields generated by single-element transducers and annular arrays with propagation in flat-layered media that mimic biological tissues. Numerical models incorporated in the simulator include evolution-type equations, either the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation or one-way Westervelt equation, for radially symmetric ultrasound beams in homogeneous and layered media with thermoviscous or power-law acoustic absorption. The software uses shock-capturing methods that allow for simulating strongly nonlinear acoustic fields with high-amplitude shocks. In this article, a general description of the software is given along with three representative simulation cases of ultrasound transducers and focusing conditions typical for therapeutic applications. The examples illustrate major nonlinear wave effects in HIFU fields including shock formation. Two examples simulate propagation in water, involving a single-element source (1-MHz frequency, 100-mm diameter, 90-mm radius of curvature) and a 16-element annular array (3-MHz frequency, 48-mm diameter, and 35-mm radius of curvature). The third example mimics the scenario of a HIFU treatment in a "water-muscle-kidney" layered medium using a source typical for abdominal HIFU applications (1.2-MHz frequency, 120-mm diameter, and radius of curvature). Linear, quasi-linear, and shock-wave exposure protocols are considered. It is intended that "HIFU beam" can be useful in teaching nonlinear acoustics; designing and characterizing high-power transducers; and developing exposure protocols for a wide range of therapeutic applications such as shock-based HIFU, boiling histotripsy, drug delivery, immunotherapy, and others.

Dual-Use Transducer for Ultrasound Imaging and Pulsed Focused Ultrasound Therapy

Pulsed focused ultrasound (pFUS) uses short acoustic pulses delivered at low duty cycle and moderate intensity to noninvasively apply mechanical stress or introduce disruption to tissue. Ultrasound-guided pFUS has primarily been used for inducing cavitation at the focus, with or without contrast agents, to promote drug delivery to tumors. When applied in tandem with contrast agents, pFUS is often administered using an ultrasound imaging probe, which has a small footprint and does not require a large acoustic window. The use of nonlinear pFUS without contrast agents was recently shown to be beneficial for localized tissue disruption, but required higher ultrasound pressure levels than a conventional ultrasound imaging probe could produce. In this work, we present the design of a compact dual-use 1-MHz transducer for ultrasound-guided pFUS without contrast agents. Nonlinear pressure fields that could be generated by the probe, under realistic power input, were simulated using the Westervelt equation. In water, fully developed shocks of 42-MPa amplitude and peak negative pressure of 8 MPa were predicted to form at the focus at 458-W acoustic power or 35% of the maximum reachable power of the transducer. In absorptive soft tissue, fully developed shocks formed at higher power (760 W or 58% of the maximum reachable power) with the shock amplitude of 33 MPa and peak negative pressure of 7.5 MPa. The electronic focus-steering capabilities of the array were evaluated and found to be sufficient to cover a target with dimensions of 19 mm in axial direction and 44 mm in transversal direction.

Dimension Estimate of Uniform Attractor for a Model of High Intensity Focussed Ultrasound-Induced Thermotherapy

High intensity focussed ultrasound (HIFU) has emerged as a novel therapeutic modality, for the treatment of various cancers, that is gaining significant traction in clinical oncology. It is a cancer therapy that avoids many of the associated negative side effects of other more well-established therapies (such as surgery, chemotherapy and radiotherapy) and does not lead to the longer recuperation times necessary in these cases. The increasing interest in HIFU from biomedical researchers and clinicians has led to the development of a number of mathematical models to capture the effects of HIFU energy deposition in biological tissue. In this paper, we study the simplest such model that has been utilized by researchers to study temperature evolution under HIFU therapy. Although the model poses significant theoretical challenges, in earlier work, we were able to establish existence and uniqueness of solutions to this system of PDEs (see Efendiev et al. Adv Appl Math Sci 29(1):231-246, 2020). In the current work, we take the next natural step of studying the long-time dynamics of solutions to this model, in the case where the external forcing is quasi-periodic. In this case, we are able to prove the existence of uniform attractors to the corresponding evolutionary processes generated by our model and to estimate the Hausdorff dimension of the attractors, in terms of the physical parameters of the system.

AAPM Task Group 241: A medical physicist's guide to MRI-guided focused ultrasound body systems

Magnetic resonance-guided focused ultrasound (MRgFUS) is a completely non-invasive technology that has been approved by FDA to treat several diseases. This report, prepared by the American Association of Physicist in Medicine (AAPM) Task Group 241, provides background on MRgFUS technology with a focus on clinical body MRgFUS systems. The report addresses the issues of interest to the medical physics community, specific to the body MRgFUS system configuration, and provides recommendations on how to successfully implement and maintain a clinical MRgFUS program. The following sections describe the key features of typical MRgFUS systems and clinical workflow and provide key points and best practices for the medical physicist. Commonly used terms, metrics and physics are defined and sources of uncertainty that affect MRgFUS procedures are described. Finally, safety and quality assurance procedures are explained, the recommended role of the medical physicist in MRgFUS procedures is described, and regulatory requirements for planning clinical trials are detailed. Although this report is limited in scope to clinical body MRgFUS systems that are approved or currently undergoing clinical trials in the United States, much of the material presented is also applicable to systems designed for other applications.

Accelerating frequency-domain numerical methods for weakly nonlinear focused ultrasound using nested meshes

The numerical simulation of weakly nonlinear ultrasound is important in treatment planning for focused ultrasound (FUS) therapies. However, the large domain sizes and generation of higher harmonics at the focus make these problems extremely computationally demanding. Numerical methods typically employ a uniform mesh fine enough to resolve the highest harmonic present in the problem, leading to a very large number of degrees of freedom. This paper proposes a more efficient strategy in which each harmonic is approximated on a separate mesh, the size of which is proportional to the wavelength of the harmonic. The increase in resolution required to resolve a smaller wavelength is balanced by a reduction in the domain size. This nested meshing is feasible owing to the increasingly localised nature of higher harmonics near the focus. Numerical experiments are performed for FUS transducers in homogeneous media to determine the size of the meshes required to accurately represent the harmonics. In particular, a fast volume potential approach is proposed and employed to perform convergence experiments as the computation domain size is modified. This approach allows each harmonic to be computed via the evaluation of an integral over the domain. Discretising this integral using the midpoint rule allows the computations to be performed rapidly with the FFT. It is shown that at least an order of magnitude reduction in memory consumption and computation time can be achieved with nested meshing. Finally, it is demonstrated how to generalise this approach to inhomogeneous propagation domains.

Acoustic hologram optimisation using automatic differentiation

Acoustic holograms are the keystone of modern acoustics. They encode three-dimensional acoustic fields in two dimensions, and their quality determines the performance of acoustic systems. Optimisation methods that control only the phase of an acoustic wave are considered inferior to methods that control both the amplitude and phase of the wave. In this paper, we present Diff-PAT, an acoustic hologram optimisation platform with automatic differentiation. We show that in the most fundamental case of optimizing the output amplitude to match the target amplitude; our method with only phase modulation achieves better performance than conventional algorithm with both amplitude and phase modulation. The performance of Diff-PAT was evaluated by randomly generating 1000 sets of up to 32 control points for single-sided arrays and single-axis arrays. This optimisation platform for acoustic hologram can be used in a wide range of applications of PATs without introducing any changes to existing systems that control the PATs. In addition, we applied Diff-PAT to a phase plate and achieved an increase of > 8 dB in the peak noise-to-signal ratio of the acoustic hologram.

A Prototype Therapy System for Boiling Histotripsy in Abdominal Targets Based on a 256-Element Spiral Array

Boiling histotripsy (BH) uses millisecond-long ultrasound (US) pulses with high-amplitude shocks to mechanically fractionate tissue with potential for real-time lesion monitoring by US imaging. For BH treatments of abdominal organs, a high-power multielement phased array system capable of electronic focus steering and aberration correction for body wall inhomogeneities is needed. In this work, a preclinical BH system was built comprising a custom 256-element 1.5-MHz phased array (Imasonic, Besançon, France) with a central opening for mounting an imaging probe. The array was electronically matched to a Verasonics research US system with a 1.2-kW external power source. Driving electronics and software of the system were modified to provide a pulse average acoustic power of 2.2 kW sustained for 10 ms with a 1-2-Hz repetition rate for delivering BH exposures. System performance was characterized by hydrophone measurements in water combined with nonlinear wave simulations based on the Westervelt equation. Fully developed shocks of 100-MPa amplitude are formed at the focus at 275-W acoustic power. Electronic steering capabilities of the array were evaluated for shock-producing conditions to determine power compensation strategies that equalize BH exposures at multiple focal locations across the planned treatment volume. The system was used to produce continuous volumetric BH lesions in ex vivo bovine liver with 1-mm focus spacing, 10-ms pulselength, five pulses/focus, and 1% duty cycle.

Modelling and measurement of laser-generated focused ultrasound: Can interventional transducers achieve therapeutic effects?

Laser-generated focused ultrasound (LGFU) transducers used for ultrasound therapy commonly have large diameters (6-15 mm), but smaller lateral dimensions (<4 mm) are required for interventional applications. To address the question of whether miniaturized LGFU transducers could generate sufficient pressure at the focus to enable therapeutic effects, a modelling and measurement study is performed. Measurements are carried out for both linear and nonlinear propagation for various illumination schemes and compared with the model. The model comprises several innovations. First, the model allows for radially varying acoustic input distributions on the surface of the LGFU transducer, which arise from the excitation light impinging on the curved transducer surfaces. This realistic representation of the source prevents the overestimation of the achievable pressures (shown here to be as high as 1.8 times). Second, an alternative inverse Gaussian illumination paradigm is proposed to achieve higher pressures; a 35% increase is observed in the measurements. Simulations show that LGFU transducers as small as 3.5 mm could generate sufficient peak negative pressures at the focus to exceed the cavitation threshold in water and blood. Transducers of this scale could be integrated with interventional devices, thereby opening new opportunities for therapeutic applications from inside the body.

Measurement and simulation of steered acoustic fields generated by a multielement array for therapeutic ultrasound

Modelling of fields generated by therapeutic ultrasound arrays can be prone to errors arising from differences from nominal transducer parameters, and variations in relative outputs of array elements when driven under different conditions, especially when simulating steered fields. Here, the effect of element size, element positions, relative source pressure variations, and electrical crosstalk on the accuracy of modelling pressure fields generated by a 555 kHz 32-element ultrasonic array were investigated. For this transducer, errors in pressure amplitude and focal position were respectively reduced from 20% to 4% and 3.3 mm to 1.5 mm using crosstalk prediction, and experimentally determined positions.

Quantitative prediction of the extent of pelvic tumour ablation by magnetic resonance-guided high intensity focused ultrasound

BACKGROUND

Patient suitability for magnetic resonance-guided high intensity focused ultrasound (MRgHIFU) therapy of pelvic tumors is currently assessed by visual estimation of the proportion of tumor that can be reached by the device's focus (coverage). Since it is important to assess whether enough energy reaches the tumor to achieve ablation, a methodology for estimating the proportion of the tumor that can be ablated (treatability) was developed. Predicted treatability was compared against clinically achieved thermal ablation.

METHODS

MR Dixon sequence images of five patients with recurrent gynecological tumors were acquired during their treatment. Acousto-thermal simulations were performed using k-Wave for three exposure points (the deepest and shallowest reachable focal points within the tumor, identified from tumor coverage analysis, and a point halfway in-between) per patient. Interpolation between the resulting simulated ablated tissue volumes was used to estimate the maximum treatable depth and hence, tumor treatability. Predicted treatability was compared both to predicted tumor coverage and to the clinically treated tumor volume. The intended and simulated volumes and positions of ablated tissues were compared.

RESULTS

Predicted treatability was less than coverage by 52% (range: 31-78%) of the tumor volume. Predicted and clinical treatability differed by 9% (range: 1-25%) of tumor volume. Ablated tissue volume and position varied with beam path length through tissue.

CONCLUSION

Tumor coverage overestimated patient suitability for MRgHIFU therapy. Employing patient-specific simulations improved treatability assessment. Patient treatability assessment using simulations is feasible.

Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications

Aberrations induced by soft tissue inhomogeneities often complicate high-intensity focused ultrasound (HIFU) therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel and ballistic gel was built to mimic alternating layers of water-based and lipid tissues characteristic of an abdominal body wall and to reproducibly distort HIFU fields. The density, sound speed, and attenuation coefficient of each material were measured using a homogeneous gel layer. A surface with random topographical features was designed as an interface between gel layers using a 2D Fourier spectrum approach and replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for linear and nonlinear beam focusing and compared to the corresponding distortion induced by an ex vivo porcine body wall of the same thickness. Both spatial shift and widening of the focal lobe were observed, as well as dramatic reduction in focal pressures caused by aberrations. The results suggest that the phantom produced levels of aberration that are similar to a real body wall and can serve as a research tool for studying HIFU effects as well as for developing algorithms for aberration correction.

Correction for Hydrophone Spatial Averaging Artifacts for Circular Sources

This article reports an investigation of an inverse-filter method to correct for experimental underestimation of pressure due to spatial averaging across a hydrophone sensitive element. The spatial averaging filter (SAF) depends on hydrophone type (membrane, needle, or fiber-optic), hydrophone geometrical sensitive element diameter, transducer driving frequency, and transducer F number (ratio of focal length to diameter). The absolute difference between theoretical and experimental SAFs for 25 transducer/hydrophone pairs was 7% ± 3% (mean ± standard deviation). Empirical formulas based on SAFs are provided to enable researchers to easily correct for hydrophone spatial averaging errors in peak compressional pressure ( p_c ), peak rarefactional pressure ( p_r ), and pulse intensity integral. The empirical formulas show, for example, that if a 3-MHz, F /2 transducer is driven to moderate nonlinear distortion and measured at the focal point with a 500- [Formula: see text] membrane hydrophone, then spatial averaging errors are approximately 16% ( p_c ), 12% ( p_r ), and 24% (pulse intensity integral). The formulas are based on circular transducers but also provide plausible upper bounds for spatial averaging errors for transducers with rectangular-transmit apertures, such as linear and phased arrays.

Holography applications toward medical field: An overview

Purpose:

3D Holography is a commercially available, disruptive innovation, which can be customised as per the requirements and is supporting Industry 4.0. The purpose of this paper is to study the potential applications of 3D holography in the medical field. This paper explores the concept of holography and its significant benefits in the medical field.

Methods:

The paper is derived through the study of various research papers on Holography and its applications in the medical field. The study tries to identify the direction of research &development and see how this innovative technology can be used effectively for better treatment of patients.

Results:

Holography uses digital imaging inputs and provides an extensive visualisation of the data for training doctors, surgeons and students. Holography converts information about the body into a digital format and has the potential to inform, promote and entertain the medical students and doctors. However, it needs a large amount of space for data storage and extensive software support for analysis and skills for customising. This technology seems good to solve a variety of medical issues by storing and using patient data in developing 3D holograms, which are useful to assist successful treatment and surgery. It seems useful in providing flexible solutions in the area of medical research. Finally, the paper identifies 13 significant applications of this technology in the medical field and discusses them appropriately.

Conclusion:

The paper explores holographic applications in medical research due to its extensive capability of image processing. Holographic images are non-contact 3D images having a large field of depth. A physician can now zoom the holographic image for a better view of the medical part. This innovative technology can create advancements in the diagnosis and treatment process, which can improve medical practice. It helps in quick detection of problems in various organs like brain, heart, liver, kidney etc. By using this technology, medical practitioners can see colourful organs at multiple angles with better accuracy. It opens up an innovative way of planning, testing of procedures and diagnosis. With technological developments, compact hardware and software are now available to help medical research and related applications.

Rapid quantitative imaging of high intensity ultrasonic pressure fields

High intensity focused ultrasound (FUS) is a noninvasive technique for treatment of tissues that can lie deep within the body. There is a need for methods to rapidly and quantitatively map FUS pressure beams for quality assurance and accelerate development of FUS systems and techniques. However, conventional ultrasound pressure beam mapping instruments, including hydrophones and optical techniques, are slow, not portable, and expensive, and most cannot map beams at actual therapeutic pressure levels. Here, a rapid projection imaging method to quantitatively map FUS pressure beams based on continuous-wave background-oriented schlieren (CW-BOS) imaging is reported. The method requires only a water tank, a background pattern, and a camera and uses a multi-layer deep neural network to reconstruct two-dimensional root-mean-square (RMS) projected pressure maps that resolve the ultrasound propagation dimension and one lateral dimension. In this work, the method was applied to collect beam maps over a 3 × 1 cm2 field-of-view with 0.425 mm resolution for focal pressures up to 9 MPa. Results at two frequencies and comparisons to hydrophone measurements show that CW-BOS imaging produces high-resolution quantitative RMS projected FUS pressure maps in under 10 s, the technique is linear and robust to beam rotations and translations, and it can map aberrated beams.

Theoretically Estimating the Acoustic Intensity of High-Intensity Focused Ultrasound (HIFU) Using Infrared Thermography

High-intensity focused ultrasound (HIFU) has been used successfully in clinics for the treatment of a variety of cancerous and benign tumors. Characterization of the focused acoustic field is of importance in the planning of the ablation procedure and further development of HIFU technology. Quantitative estimation of acoustic intensity is feasible using the infrared (IR) thermography on an absorber. However, the current approach is limited to low power output. In this study, a theoretical model was established to describe the acoustic field and absorbed energy in the absorber with the presence of harmonics in the HIFU pressure waveform at the focus and then to calculate the temperature elevations during the HIFU heating, from which the acoustic intensities could be derived. The absolute difference between the derived and incident acoustic intensities in a 2-mm absorber at the varied acoustic power output (up to 80 W) and attenuation of the absorber (up to 346 Np/m/MHz) is <6%. In addition, the proposed approach was found to also work well for the absorber with a varied thickness because of the monotonic decrease of the temperature elevation. The effects of pulse duration and duty cycle of pulsed HIFU ablation on the estimation accuracy were also investigated. It is found that the estimation accuracy is good for short pulse duration using the equivalent acoustic intensity. Overall, the proposed approach could estimate the acoustic intensity and harmonic distribution of HIFU at the high power output using IR thermography for a large parametric range of absorber and HIFU output in the numerical investigation.

Simulation of nonlinear trans-skull focusing and formation of shocks in brain using a fully populated ultrasound array with aberration correction

Multi-element high-intensity focused ultrasound phased arrays in the shape of hemispheres are currently used in clinics for thermal lesioning in deep brain structures. Certain side effects of overheating non-targeted tissues and skull bones have been revealed. Here, an approach is developed to mitigate these effects. A specific design of a fully populated 256-element 1-MHz array shaped as a spherical segment (F-number, F_# = 1) and filled by randomly distributed equal-area polygonal elements is proposed. Capability of the array to generate high-amplitude shock fronts at the focus is tested in simulations by combining three numerical algorithms for linear and nonlinear field modeling and aberration correction. The algorithms are based on the combination of the Rayleigh integral, a linear pseudo-spectral time domain Kelvin-Voigt model, and nonlinear Westervelt model to account for the effects of inhomogeneities, aberrations, reflections, absorption, nonlinearity, and shear waves in the skull. It is shown that the proposed array can generate nonlinear waveforms with shock amplitudes >60 MPa at the focus deep inside the brain without exceeding the existing technical limitation on the intensity of 40 W/cm² at the array elements. Such shock amplitudes are sufficient for mechanical ablation of brain tissues using the boiling histotripsy approach and implementation of other shock-based therapies.

Correction for Spatial Averaging Artifacts in Hydrophone Measurements of High-Intensity Therapeutic Ultrasound: An Inverse Filter Approach

High-intensity therapeutic ultrasound (HITU) pressure is often measured using a hydrophone. HITU pressure waves typically contain multiple harmonics due to nonlinear propagation. As harmonic frequency increases, harmonic beamwidth decreases. For sufficiently high harmonic frequency, beamwidth may become comparable to the hydrophone effective sensitive element diameter, resulting in signal reduction due to spatial averaging. An analytic formula for a hydrophone spatial averaging filter for beams with Gaussian harmonic radial profiles was tested on HITU pressure signals generated by three transducers (1.45 MHz, F/1; 1.53 MHz, F/1.5; 3.91 MHz, F/1) with focal pressures up to 48 MPa. The HITU signals were measured using fiber-optic and needle hydrophones (nominal geometrical sensitive element diameters: 100 and [Formula: see text]). Harmonic radial profiles were measured with transverse scans in the focal plane using the fiber-optic hydrophone. Harmonic radial profiles were accurately approximated by Gaussian functions with root-mean-square (rms) differences between transverse scans and Gaussian fits less than 9% for frequencies up to approximately 50 MHz. The Gaussian harmonic beamwidth parameter σ_n varied with harmonic number n according to a power law, σ_n = σ₁/n^q where . RMS differences between experimental and theoretical spatial averaging filters were 11% ± 1% (1.45 MHz), 8% ± 1% (1.53 MHz), and 4% ± 1% (3.91 MHz). For the two more highly focused (F/1) transducers, the effect of spatial averaging was to underestimate peak compressional pressure (pcp), peak rarefactional pressure (prp), and pulse intensity integral (pii) by (mean ± standard deviation) (pcp: 4.9% ± 0.5%, prp: 0.4% ± 0.2%, pii: 2.9% ± 1%) and (pcp: 28.3% ± 9.6%, prp: 6% ± 2.4%, pii: 24.3% ± 6.7%) for the 100- and 400- [Formula: see text]-diameter hydrophones, respectively. These errors can be suppressed by the application of the inverse spatial averaging filter.

Mechanical damage induced by the appearance of rectified bubble growth in a viscoelastic medium during boiling histotripsy exposure

In boiling histotripsy, the presence of a boiling vapour bubble and understanding of its dynamic behaviour are crucially important for the initiation of the tissue fractionation process and for the control of the size of a lesion produced. Whilst many in vivo studies have shown the feasibility of using boiling histotripsy in mechanical fractionation of solid tumours, not much is known about the evolution of a boiling vapour bubble in soft tissue induced by boiling histotripsy. The main objective of this present study is therefore to investigate the formation and dynamic behaviour of a boiling vapour bubble which occurs under boiling histotripsy insonation. Numerical and experimental studies on the bubble dynamics induced in optically transparent tissue-mimicking gel phantoms exposed to the field of a 2.0 MHz High Intensity Focused Ultrasound (HIFU) transducer were performed with a high speed camera. The Gilmore-Zener bubble model coupled with the Khokhlov-Zabolotskaya-Kuznetsov and the Bio-heat Transfer equations was used to simulate bubble dynamics driven by boiling histotripsy waveforms (nonlinear-shocked wave excitation) in a viscoelastic medium as functions of surrounding temperature and of tissue elasticity variations. In vivo animal experiments were also conducted to examine cellular structures around a freshly created lesion in the liver resulting from boiling histotripsy. To the best of our knowledge, this is the first study reporting the numerical and experimental evidence of the appearance of rectified bubble growth in a viscoelastic medium. Accounting for tissue phantom elasticity adds a mechanical constraint on vapour bubble growth, which improves the agreement between the simulation and the experimental results. In addition the numerical calculations showed that the asymmetry in a shockwave and water vapour transport can result in rectified bubble growth which could be responsible for HIFU-induced tissue decellularisation. Strain on liver tissue induced by this radial motion can damage liver tissue while preserving blood vessels.

Investigation of the repeatability and reproducibility of hydrophone measurements of medical ultrasound fields

Accurate measurements of acoustic pressure are required for characterisation of ultrasonic transducers and for experimental validation of models of ultrasound propagation. Errors in measured pressure can arise from a variety of sources, including variations in the properties of the source and measurement equipment, calibration uncertainty, and processing of measured data. In this study, the repeatability of measurements made with four probe and membrane hydrophones was examined. The pressures measured by these hydrophones in three different ultrasound fields, with both linear and nonlinear, pulsed and steady state driving conditions, were compared to assess the reproducibility of measurements. The coefficient of variation of the focal peak positive pressure was less than 2% for all hydrophones across five repeated measurements. When comparing hydrophones, pressures measured in a spherically focused 1.1 MHz field were within 7% for all except 1 case, and within 10% for a broadband 5 MHz pulse from a diagnostic linear array. Larger differences of up to 55% were observed between measurements of a tightly focused 3.3 MHz field, which were reduced for some hydrophones by the application of spatial averaging corrections. Overall, the major source of these differences was spatial averaging and uncertainty in the complex frequency response of the hydrophones.

Acoustic Field Characterization of Medical Array Transducers Based on Unfocused Transmits and Single-Plane Hydrophone Measurements

Medical ultrasonic arrays are typically characterized in controlled water baths using measurements by a hydrophone, which can be translated with a positioning stage. Characterization of 3D acoustic fields conventionally requires measurements at each spatial location, which is tedious and time-consuming, and may be prohibitive given limitations of experimental setup (e.g., the bath and stage) and measurement equipment (i.e., the hydrophone). Moreover, with the development of new ultrasound sequences and modalities, multiple measurements are often required to characterize each imaging mode to ensure performance and clinical safety. Acoustic holography allows efficient characterization of source transducer fields based on single plane measurements. In this work, we explore the applicability of a re-radiation method based on the Rayleigh–Sommerfeld integral to medical imaging array characterization. We show that source fields can be reconstructed at single crystal level at wavelength resolution, based on far-field measurements. This is herein presented for three practical application scenarios: for identifying faulty transducer elements; for characterizing acoustic safety parameters in focused ultrasound sequences from 2D planar measurements; and for estimating arbitrary focused fields based on calibration from an unfocused sound field and software beamforming. The results experimentally show that the acquired pressure fields closely match those estimated using our technique.

Considerations for Choosing Sensitive Element Size for Needle and Fiber-Optic Hydrophones-Part I: Spatiotemporal Transfer Function and Graphical Guide

The spatiotemporal transfer function for a needle or reflectance-based fiber-optic hydrophone is modeled as separable into the product of two filters corresponding to frequency-dependent sensitivity and spatial averaging. The separable hydrophone transfer function model is verified numerically by comparison to a more general rigid piston spatiotemporal response model that does not assume separability. Spatial averaging effects are characterized by frequency-dependent "effective" sensitive element diameter, which can be more than double the geometrical sensitive element diameter. The transfer function is tested in simulation using a nonlinear focused pressure wave model based on Gaussian harmonic radial pressure distributions. The pressure wave model is validated by comparing to experimental hydrophone scans of nonlinear beams produced by three source transducers. An analytic form for the spatial averaging filter, applicable to Gaussian harmonic beams, is derived. A second analytic form for the spatial averaging filter, applicable to quadratic harmonic beams, is derived by extending the spatial averaging correction recommended by IEC 62127-1 Annex E to nonlinear signals with multiple harmonics. Both forms are applicable to all hydrophones (not just needle and fiber-optic hydrophones). Simulation analysis performed for a wide variety of transducer geometries indicates that the Gaussian spatial averaging filter formula is more accurate than the quadratic formula over a wider range of harmonics. Additional experimental validation is provided in Part II. Readers who are uninterested in hydrophone theory may skip the theoretical and experimental sections of this paper and proceed to the graphical guide for practical information to inform and support selection of hydrophone sensitive element size (but might be well advised to read the Introduction).

Considerations for Choosing Sensitive Element Size for Needle and Fiber-Optic Hydrophones-Part II: Experimental Validation of Spatial Averaging Model

Acoustic pressure can be measured with a hydrophone. Hydrophone measurements can underestimate incident acoustic pressure due to spatial averaging effects across the hydrophone sensitive element. The spatial averaging filter for a nonlinear focused beam is a low-pass filter that decreases monotonically from 1 to 0 as frequency increases from 0 to infinity. Experiments were performed to test an analytic model for the spatial averaging filter. Nonlinear pressure tone bursts were generated by three source transducers with driving frequencies ranging from 2.5 to 6 MHz, diameters ranging from 19 to 64 mm, and focal lengths ranging from 38 to 89 mm. The nonlinear pressure fields were measured using four needle hydrophones with nominal geometrical sensitive element diameters of 200, 400, 600, and [Formula: see text]. The average root-mean-square difference between theoretical and experimental spatial averaging filters was 5.8% ± 2.6%.

Mechanical decellularization of tissue volumes using boiling histotripsy

High intensity focused ultrasound (HIFU) is rapidly advancing as an alternative therapy for non-invasively treating specific cancers and other pathological tissues through thermal ablation. A new type of HIFU therapy-boiling histotripsy (BH)-aims at mechanical fractionation of into subcellular fragments, with a range of accompanying thermal effects that can be tuned from none to substantial depending on the requirements of the application. The degree of mechanical tissue damage induced by BH has been shown to depend on the tissue type, with collagenous structures being most resistant, and cellular structures being most sensitive. This has been reported for single BH lesions, but has not been replicated in large volumes. Such tissue selectivity effect has potential uses involving tissue decellularization for biofabrication technologies as well as mechanical ablation by BH while sparing critical structures. The goal of this study was to investigate tissue decellularization effect in larger, clinically relevant liquefied volumes of tissue, and to evaluate the accumulated thermal effect in the volumetric lesions under different exposure parameters. All BH exposures were performed with a 256-element 1.2 MHz array of a magnetic resonance imaging-guided HIFU (MR-HIFU) clinical system (Sonalleve V1, Profound Medical Inc, Mississauga, Canada). The volumetric BH lesions were produced in degassed ex vivo bovine liver using 1-10 ms long pulses with in situ shock amplitudes of 75-100 MPa at the focus and pulse repetition frequencies (PRFs) of 1-10 Hz covering a range of effects from pure mechanical homogenization to thermal ablation. Multimodal analysis of the lesions was then performed, including microstructure (histological), ultrastructure (electron microscopy), and molecular (biochemistry) methods. Results show a range of tissue effects in terms of the degree of tissue selectivity and the amount of heat generated in large BH lesions, thereby demonstrating potential for treatments tailored to different clinical applications.

Directivity and Frequency-Dependent Effective Sensitive Element Size of a Reflectance-Based Fiber-Optic Hydrophone: Predictions From Theoretical Models Compared With Measurements

The goal of this work was to measure the directivity of a reflectance-based fiber-optic hydrophone at multiple frequencies and to compare it to four theoretical models: rigid baffle (RB), rigid piston (RP), unbaffled (UB), and soft baffle (SB). The fiber had a nominal 105- [Formula: see text] diameter core and a 125- [Formula: see text] overall diameter (core + cladding). Directivity measurements were performed at 2.25, 3.5, 5, 7.5, 10, and 15 MHz from ±90° in two orthogonal planes. Effective hydrophone sensitive element radius was estimated by least-squares fitting the four models to the directivity measurements using the sensitive element radius as an adjustable parameter. Over the range from 2.25 to 15 MHz, the average magnitudes of differences between the effective and nominal sensitive element radii were 59% ± 49% (RB), 10% ± 5% (RP), 46% ± 38% (UB), and 71% ± 19% (SB). Therefore, the directivity of a reflectance-based fiber-optic hydrophone may be best estimated by the RP model.

Coalescence of residual histotripsy cavitation nuclei using low-gain regions of the therapy beam during electronic focal steering

Following collapse of a histotripsy cloud, residual microbubbles may persist for seconds, distributed throughout the focus. Their presence can attenuate and scatter subsequent pulses, hindering treatment speed and homogeneity. Previous studies have demonstrated use of separate low-amplitude (~1 MPa) pulses interleaved with histotripsy pulses to drive bubble coalescence (BC), significantly improving treatment speed without sacrificing homogeneity. We propose that by using electronic focal steering (EFS) to direct the therapy focus throughout specially-designed EFS sequences, it is possible to use low-gain regions of the therapy beam to accomplish BC during EFS without any additional acoustic sequence. First, to establish proof of principle for an isolated focus, a 50-foci EFS sequence was constructed with the first position isolated near the geometric focus and remaining positions distributed post-focally. EFS sequences were evaluated in tissue-mimicking phantoms with gas concentrations of 20% and 100% with respect to saturation. Results using an isolated focus demonstrated that at 20% gas concentration, 49 EFS pulses were sufficient to achieve BC in all samples for pulse repetition frequency (PRF)  ⩽  800 Hz and 84.1%  ±  3.0% of samples at 5 kHz PRF. For phantoms prepared with 100% gas concentration, BC was achieved by 49 EFS pulses in 39.2%  ±  4.7% of samples at 50 Hz PRF and 63.4%  ±  15.3% of samples at 5 kHz. To show feasibility of using the EFS-BC method to ablate a large volume quickly, a 1000-foci EFS sequence covering a volume of approximately 27 ml was tested. Results indicate that the BC effect was similarly present. A treatment rate of 27  ±  6 ml min^-1 was achieved, which is signficantly faster than standard histotripsy and ultrasound thermal ablation. This study demonstrates that histotripsy with EFS can achieve BC without employing a separate acoustic sequence which has the potential to accelerate large-volume ablation while minimizing energy deposition.

Validation of hybrid angular spectrum acoustic and thermal modelling in phantoms

In focused ultrasound (FUS) thermal ablation of diseased tissue, acoustic beam and thermal simulations enable treatment planning and optimization. In this study, a treatment-planning methodology that uses the hybrid angular spectrum (HAS) method and the Pennes' bioheat equation (PBHE) is experimentally validated in homogeneous tissue-mimicking phantoms. Simulated three-dimensional temperature profiles are compared to volumetric MR thermometry imaging (MRTI) of FUS sonications in the phantoms, whose acoustic and thermal properties are independently measured. Additionally, Monte Carlo (MC) uncertainty analysis is performed to quantify the effect of tissue property uncertainties on simulation results. The mean error between simulated and experimental spatiotemporal peak temperature rise was +0.33°C (+6.9%). Despite this error, the experimental temperature rise fell within the expected uncertainty of the simulation, as determined by the MC analysis. The average errors of the simulated transverse and longitudinal full width half maximum (FWHM) of the profiles were -1.9% and 7.5%, respectively. A linear regression and local sensitivity analysis revealed that simulated temperature amplitude is more sensitive to uncertainties in simulation inputs than in the profile width and shape. Acoustic power, acoustic attenuation and thermal conductivity had the greatest impact on peak temperature rise uncertainty; thermal conductivity and volumetric heat capacity had the greatest impact on FWHM uncertainty. This study validates that using the HAS and PBHE method can adequately predict temperature profiles from single sonications in homogeneous media. Further, it informs the need to accurately measure or predict patient-specific properties for improved treatment planning of ablative FUS surgeries.

A convenient, reliable, and fast acoustic pressure field measurement method for magnetic resonance-guided high-intensity focused ultrasound systems with phased array transducers

BACKGROUND

With the expanding applications of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), there is an urgent need for a convenient, reliable, and fast acoustic pressure field measurement method to aid treatment protocol design, ensure consistent and safe operation of the transducer, and facilitate regulatory approval of new techniques. Herein, we report a method for acoustic pressure field characterization of MR-HIFU systems with multi-element phased array transducers. This method integrates fiber-optic hydrophone measurements and electronic steering of the ultrasound beam with MRI-assisted HIFU focus alignment to the fiber tip.

METHODS

A clinical MR-HIFU system (Sonalleve V2, Profound Medical Inc., Mississauga, Canada) was used to assess the proposed method. A fiber-optic hydrophone was submerged in a degassed water bath, and the fiber tip location was traced using MRI. Subsequently, the nominal transducer focal point indicated on the MR-HIFU therapy planning software was positioned at the fiber tip, and the HIFU focus was electronically steered around the fiber tip within a 3D volume for 3D pressure field mapping, eliminating the need for an additional, expensive, and MRI-compatible 3D positioning stage. The peak positive and negative pressures were measured at the focus and validated using a standard hydrophone measurement setup outside the MRI magnet room.

RESULTS

We found that the initial MRI-assisted HIFU focus alignment had an average offset of 2.23 ± 1.33 mm from the fiber tip as identified by the 3D pressure field mapping. MRI guidance and electronic beam steering allowed 3D focus localization within ~ 1 h, i.e., faster than the typical time required using the standard laboratory setup (~ 3-4 h). Acoustic pressures measured using the proposed method were not significantly different from those obtained with the standard laboratory hydrophone measurements.

CONCLUSIONS

In conclusion, our method offers a convenient, reliable, and fast acoustic pressure field characterization tool for MR-HIFU systems with phased array transducers.

Numerical Modeling of Ultrasound Propagation in Weakly Heterogeneous Media Using a Mixed-Domain Method

A mixed-domain method (MDM) is presented in this paper for modeling one-way linear/nonlinear wave propagation in biological tissue with arbitrary heterogeneities, in which sound speed, density, attenuation coefficients, and nonlinear coefficients are all spatial varying functions. The present method is based on solving an integral equation derived from a Westervelt-like equation. One-dimensional problems are first studied to verify the MDM and to reveal its limitations. It is shown that this method is accurate for cases with small variation of sound speed. A 2-D case is further studied with focused ultrasound beams to validate the application of the method in the medical field. Results from the MATLAB toolbox k-Wave are used as the benchmark. Normalized root-mean-square (rms) error estimated at the focus of the transducer is 0.0133 when the coarsest mesh (1/3 of the wavelength) is used in the MDM. Fundamental and second-harmonic fields throughout the considered computational domains are compared and good agreement is observed. Overall, this paper demonstrates that the MDM is a computationally efficient and accurate method when used to model wave propagation in biological tissue with relatively weak heterogeneities.

Broadband characterization of plastic and high intensity therapeutic ultrasound phantoms using time delay spectrometry-With validation using Kramers-Kronig relations

Time delay spectrometry (TDS) is extended for broadband characterization of plastics (low-density polyethylene, LDPE) and tissue-mimicking material (TMM). The results suggest that TDS and the conventional broadband pulse method give comparable measurements for frequency-dependent attenuation coefficient and phase velocity near the center frequency, where signal-to-noise ratio is high. However, TDS measurements show enhanced bandwidth for attenuation coefficient of 30%-40% (LDPE) and 89%-100% (TMM) and for phase velocity of 43% (LDPE) and 36% (TMM) for a single transmitter/receiver pair. In addition, TDS provides measurements of dispersion that are consistent with predictions based on the Kramers-Kronig relations to within 5 m/s over the band from 2 to 12 MHz in LDPE and to within 1 m/s in TMM over the band from 0.5 to 29 MHz.

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

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

Experimental study of beam distortion due to fiducial markers during salvage HIFU in the prostate

Background

Prostate cancer is frequently treated using external beam radiation therapy (EBRT). Prior to therapy, the prostate is commonly implanted with a small number of permanent fiducial markers used to monitor the position of the prostate during therapy. In the case of local cancer recurrence, high-intensity focused ultrasound (HIFU) provides a non-invasive salvage treatment option. However, the impact of the fiducial markers on HIFU treatment has not been thoroughly studied to date. The objective of this study was to experimentally investigate the effect of a single EBRT fiducial marker on the efficacy of HIFU treatment delivery using a tissue-mimicking material (TMM).

Methods

A TMM with the acoustic properties of the prostate was developed based on a polyacrylamide hydrogel containing bovine serum albumin. Each phantom was implanted with a cylindrical fiducial marker and then sonicated using a 3.3 MHz focused bowl HIFU transducer. Two sets of experiments were performed. In the first, a single lesion was created at different positions along either the anteroposterior or left-right axes relative to the marker. In the second, a larger ablation volume was created by raster scanning. The size and position of the ablated volume were assessed using a millimetre grid overlaid on the phantom.

Results

The impact of the marker on the position and size of the HIFU lesion was significant when the transducer focus was positioned within 7 mm anteriorly, 18 mm posteriorly or within 3 mm laterally of the marker. Beyond this, the generated lesion was not affected. When the focus was anterior to the marker, the lesion increased in size due to reflections. When the focus was posterior, the lesion decreased in size or was not present due to shadowing.

Conclusions

The presence of an EBRT fiducial marker may result in an undertreated region beyond the marker due to reduced energy arriving at the focus, and an overtreated region in front of the marker due to reflections. Depending on the position of the targeted regions and the distribution of the markers, both effects may be undesirable and reduce treatment efficacy. Further work is necessary to investigate whether these results indicate the necessity to reconsider patient selection and treatment planning for prostate salvage HIFU after failed EBRT.

Mechanical fractionation of tissues using microsecond-long HIFU pulses on a clinical MR-HIFU system

PURPOSE

High intensity focussed ultrasound (HIFU) can non-invasively treat tumours with minimal or no damage to intervening tissues. While continuous-wave HIFU thermally ablates target tissue, the effect of hundreds of microsecond-long pulsed sonications is examined in this work. The objective of this study was to characterise sonication parameter-dependent thermomechanical bioeffects to provide the foundation for future preclinical studies and facilitate clinical translation.

METHODS AND MATERIALS

Acoustic power, number of cycles/pulse, sonication time and pulse repetition frequency (PRF) were varied on a clinical magnetic resonance imaging (MRI)-guided HIFU (MR-HIFU) system. Ex vivo porcine liver, kidney and cardiac muscle tissue samples were sonicated (3 × 3 grid pattern, 1 mm spacing). Temperature, thermal dose and T2 relaxation times were quantified using MRI. Lesions were histologically analysed using H&E and vimentin stains for lesion structure and viability.

RESULTS

Thermomechanical HIFU bioeffects produced distinct types of fractionated tissue lesions: solid/thermal, paste-like and vacuolated. Sonications at 20 or 60 Hz PRF generated substantial tissue damage beyond the focal region, with reduced viability on vimentin staining, whereas H&E staining indicated intact tissue. Same sonication parameters produced dissimilar lesions in different tissue types, while significant differences in temperature, thermal dose and T2 were observed between the parameter sets.

CONCLUSION

Clinical MR-HIFU system was utilised to generate distinct types of lesions and to produce targeted thermomechanical bioeffects in ex vivo tissues. The results guide HIFU research on thermomechanical tissue bioeffects, inform future studies and advice sonication parameter selection for direct tumour ablation or immunomodulation using a clinical MR-HIFU system.

Determination of Acoustic Cavitation Probabilities and Thresholds Using a Single Focusing Transducer to Induce and Detect Acoustic Cavitation Events: I. Method and Terminology

A method to determine acoustic cavitation probabilities in tissue-mimicking materials (TMMs) is described that uses a high-intensity focused ultrasound (HIFU) transducer for both inducing and detecting the acoustic cavitation events. The method was evaluated by studying acoustic cavitation probabilities in agar-based TMMs with and without scatterers and for different sonication modes like continuous wave, single pulses (microseconds to milliseconds) and repeated burst signals. Acoustic cavitation thresholds (defined here as the peak rarefactional in situ pressure at which the acoustic cavitation probability reaches 50%) at a frequency of 1.06 MHz were observed between 1.1 MPa (for 1 s of continuous wave sonication) and 4.6 MPa (for 1 s of a repeated burst signal with 25-cycle burst length and 10-ms burst period) in a 3% (by weight) agar phantom without scatterers. The method and its evaluation are described, and general terminology useful for standardizing the description of insonation conditions and comparing results is provided. In the accompanying second part, the presented method is used to systematically study the acoustic cavitation thresholds in the same material for a range of sonication modes.

Pressure Pulse Distortion by Needle and Fiber-Optic Hydrophones due to Nonuniform Sensitivity

Needle and fiber-optic hydrophones have frequency-dependent sensitivity, which can result in substantial distortion of nonlinear or broadband pressure pulses. A rigid cylinder model for needle and fiber-optic hydrophones was used to predict this distortion. The model was compared with measurements of complex sensitivity for a fiber-optic hydrophone and three needle hydrophones with sensitive element sizes ( ) of 100, 200, 400, and . Theoretical and experimental sensitivities agreed to within 12 ± 3% [root-mean-square (RMS) normalized magnitude ratio] and 8° ± 3° (RMS phase difference) for the four hydrophones over the range from 1 to 10 MHz. The model predicts that distortions in peak positive pressure can exceed 20% when and spectral index (SI) >7% and can exceed 40% when and SI >14%, where is the wavelength of the fundamental component and SI is the fraction of power spectral density contained in harmonics. The model predicts that distortions in peak negative pressure can exceed 15% when . Measurements of pulse distortion using a 2.25 MHz source and needle hydrophones with , 400, and agreed with the model to within a few percent on the average for SI values up to 14%. This paper 1) identifies conditions for which needle and fiber-optic hydrophones produce substantial distortions in acoustic pressure pulse measurements and 2) offers a practical deconvolution method to suppress these distortions.

Reconstruction of nonlinear ultrasound field of an annular therapeutic array from acoustic holograms of its individual elements

Acoustic holography method has been shown to provide accurate reconstruction of 3D ultrasound fields generated by various medical transducers including multi-element arrays as well as to set a boundary condition for nonlinear field modeling at high pressure levels. Here an approach of measuring holograms of individual array elements for modeling of an entire array field is proposed and tested for a 3 MHz 16-element annular array (48 mm diameter and 35 mm radius of curvature). The array is a part of a high intensity focused ultrasound system with magnetic resonance guidance used for developing thermal and mechanical methods of tissue ablation in mouse tumors. The holograms measured separately for each array element were combined together to obtain a boundary condition for the array with all operating elements. Modeling results were compared to low-amplitude beam scans and good agreement was demonstrated. Then, nonlinear field simulations were performed at increasing power outputs based on the 3D Westervelt equation. It was shown that the transducer is capable to produce focal waveforms with 140 MPa shock amplitude at 110 W acoustic power and thus is well suited for evaluating shock-based ablation therapies in small animals.

Rapid Spatial Mapping of Focused Ultrasound Fields Using a Planar Fabry-Pérot Sensor

Measurement of high acoustic pressures is necessary in order to fully characterize clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing measurement devices are unable to withstand the extreme pressures generated in these fields, and those that can often exhibit low sensitivity. Here, a planar Fabry-Pérot interferometer with hard dielectric mirrors and spacer was designed, fabricated, and characterized, and its suitability for measurement of nonlinear focused ultrasound fields was investigated. The noise equivalent pressure (NEP) of the scanning system scaled with the adjustable pressure detection range between 49 kPa for pressures up to 8 MPa and 152 kPa for measurements up to 25 MPa, over a 125 MHz measurement bandwidth. Measurements of the frequency response of the sensor showed that it varied by less than 3 dB in the range 1-62 MHz. The effective element size of the sensor was 65 and waveforms were acquired at a rate of 200 Hz. The device was used to measure the acoustic pressure in the field of a 1.1 MHz single-element spherically focused bowl transducer. Measurements of the acoustic field at low pressures compared well with measurements made using a Polyvinylidene difluoride needle hydrophone. At high pressures, the measured peak focal pressures agreed well with the focal pressure modeled using the Khokhlov-Zabolotskaya-Kuznetsov equation. Maximum peak positive pressures of 25 MPa and peak negative pressures of 12 MPa were measured, and planar field scans were acquired in scan times on the order of 1 min. The properties of the sensor and scanning system are well suited to measurement of nonlinear focused ultrasound fields, in both the focal region and the low-pressure peripheral regions. The fast acquisition speed of the system and its low NEP are advantageous, and with further development of the sensor, it has potential in application to HIFU metrology.

A Prototype Therapy System for Transcutaneous Application of Boiling Histotripsy

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

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

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

Acoustic field characterization of a clinical magnetic resonance-guided high-intensity focused ultrasound system inside the magnet bore

PURPOSE

With the expanding clinical application of magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU), acoustic field characterization of MR-HIFU systems is needed for facilitating regulatory approval and ensuring consistent and safe power output of HIFU transducers. However, the established acoustic field measurement techniques typically use equipment that cannot be used in a magnetic resonance imaging (MRI) suite, thus posing a challenge to the development and execution of HIFU acoustic field characterization techniques. In this study, we developed and characterized a technique for HIFU acoustic field calibration within the MRI magnet bore, and validated the technique with standard hydrophone measurements outside of the MRI suite.

METHODS

A clinical Philips MR-HIFU system (Sonalleve V2, Philips, Vantaa, Finland) was used to assess the proposed technique. A fiber-optic hydrophone with a long fiber was inserted through a 24-gauge angiocatheter and fixed inside a water tank that was placed on the HIFU patient table above the acoustic window. The long fiber allowed the hydrophone control unit to be placed outside of the magnet room. The location of the fiber tip was traced on MR images, and the HIFU focal point was positioned at the fiber tip using the MR-HIFU therapy planning software. To perform acoustic field mapping inside the magnet, the HIFU focus was positioned relative to the fiber tip using an MRI-compatible 5-axis robotic transducer positioning system embedded in the HIFU patient table. To perform validation measurements of the acoustic fields, the HIFU table was moved out of the MRI suite, and a standard laboratory hydrophone measurement setup was used to perform acoustic field measurements outside the magnetic field.

RESULTS

The pressure field scans along and across the acoustic beam path obtained inside the MRI bore were in good agreement with those obtained outside of the MRI suite. At the HIFU focus with varying nominal acoustic powers of 10-500 W, the peak positive pressure and peak negative pressure measured inside the magnet bore were 3.87-68.67 MPa and 3.56-12.06 MPa, respectively, while outside the MRI suite the corresponding pressures were 3.27-67.32 MPa and 3.06-12.39 MPa, respectively. There was no statistically significant difference (P > 0.05) between measurements inside the magnet bore and outside the MRI suite for the p⁺ and p^- at any acoustic power level. The spatial-peak pulse-average intensities (I_SPPA ) for these powers were 312-17816 W/cm² and 220-15698 W/cm² for measurements inside and outside the magnet room, respectively. In addition, when the scanning step size of the HIFU focus was increased from 100 μm to 500 μm, the execution time for scanning a 4 × 4 mm² area decreased from 210 min to 10 min, the peak positive pressure decreased by 14%, the peak negative pressure decreased by 5%, and the lateral full width at half maximum dimension of pressure profiles increased from 1.15 mm to 1.55 mm.

CONCLUSIONS

The proposed hydrophone measurement technique offers a convenient and reliable method for characterizing the acoustic fields of clinical MR-HIFU systems inside the magnet bore. The technique was validated for use by measurements outside the MRI suite using a standard hydrophone calibration technique. This technique can be a useful tool in MR-HIFU quality assurance and acoustic field assessment.

Extending the Utility of the Parabolic Approximation in Medical Ultrasound Using Wide-Angle Diffraction Modeling

Wide-angle parabolic models are commonly used in geophysics and underwater acoustics but have seen little application in medical ultrasound. Here, a wide-angle model for continuous-wave high-intensity ultrasound beams is derived, which approximates the diffraction process more accurately than the commonly used Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation without increasing implementation complexity or computing time. A method for preventing the high spatial frequencies often present in source boundary conditions from corrupting the solution is presented. Simulations of shallowly focused axisymmetric beams using both the wide-angle and standard parabolic models are compared to assess the accuracy with which they model diffraction effects. The wide-angle model proposed here offers improved focusing accuracy and less error throughout the computational domain than the standard parabolic model, offering a facile method for extending the utility of existing KZK codes.