Acoustic Holography Measurements Research Articles

Electronic Focus Steering Capabilities of a Diagnostic-Type Linear Ultrasound Array Designed for High Power Therapy and Its Visualization

The focus steering capabilities of a 1 MHz linear phased array transducer (64 rectangular elements, 14.8 × 51.2 mm aperture) intended for drug delivery applications in abdominal organs were assessed and compared with its design-stage computer model. Acoustic fields generated by the transducer and predicted by the models of an ideal array with uniformly vibrating elements and either a plane or a cylindrically focused surface were simulated using the Rayleigh integral and angular spectrum methods. The boundary conditions for the transducer were reconstructed from acoustic holography measurements performed for selected focusing configurations of the array and also synthesized from holography data measured for each of its individual elements. It was shown that the transducer field with electronic focus steering can be accurately synthesized based on the holography data of its elements, which significantly simplified acoustic field characterization. Variability of the power and directivity patterns of the array elements were analyzed. A twofold smaller range of electronic steering in the transverse direction for the transducer compared to its computer model is discussed.

Large-aperture 256-element fully populated random array for therapeutic ultrasound applications: Experimental study of its functionality and characterization of the radiated field

Recently, a new method has been proposed for designing random high-intensity focused ultrasound (HIFU) arrays with the maximum possible filling factor and polygonal elements of equal area, based on capacity constrained tessellation [Rosnitskiy et al., IEEE Trans. UFFC (2018)]. Following this approach, a 256-element piezocomposite array with a radius of curvature of 150 mm, a diameter of 200 mm, and an operating frequency of 1.2 MHz was designed at the Laboratory for Industrial and Medical Ultrasound (LIMU) of Moscow State University. The array was subsequently manufactured by Imasonic (France). It can be driven using a Verasonics ultrasound engine. Electrical matching was performed for each element using matching network created by Image Guided Therapy (France). This report presents the system design procedure and experimental results for array characterization using acoustic holography and acoustic power measurements using the radiation force balance. In addition, a theoretical study was carried out using the “HIFU beam” simulator developed at LIMU (freely available at www.limu.msu.ru) to assess the array's ability to achieve highly nonlinear regimes with shocked pressure waveforms. The possibility of using the array for HIFU applications such as acoustic levitation and manipulation, histotripsy and transcostal HIFU therapy is discussed. [Work supported by RSF 19-12-00148.]

Experimental characterization of basic violin structural acoustics

Considering its simple construction the complex material-shape traditional violin, fashioned from nominally orthotropic materials to form a ported, compliant-wall cavity, enjoys a notable scientific complexity, e.g., four separate radiation mechanisms, coupling between the two lowest cavity modes A0 (Helmholtz resonance, ∼275 Hz) and A1 (1st longitudinal “slosh” mode, ∼470 Hz), and between A0 and two 1st corpus bending (B1) modes near 500 Hz. Comprehensive EMA (velocity/force), radiativity (pressure/force) and near-field acoustical holography ( f -hole volume flows/force) measurements plus bridge tuning systematics were combined to establish a dual-region structural acoustics model of the violin. In the “deterministic” region the large-volume-change B1 modes drive large, in-phase f-hole volume flows accounting for >50% of B1 radiation and excite A0; oppositely directed upper-bout/lower-bout cavity volume flows excite A1. A wall-driven, dual-Helmholtz resonator network successfully modeled A0 and B1 radiation. In the “statistical” region where the major bridge and plate tuning effects occur, radiation efficiency (averaged-over-sphere radiativity divided by averaged-over-corpus mobility) led to radiation damping and critical frequency. With total damping from mobility fits the radiation/total damping ratio (fraction of vibrational energy radiated) was combined with a parametric model of mobility. Splicing the regions created the “dynamic filter” model of violin radiation.Considering its simple construction the complex material-shape traditional violin, fashioned from nominally orthotropic materials to form a ported, compliant-wall cavity, enjoys a notable scientific complexity, e.g., four separate radiation mechanisms, coupling between the two lowest cavity modes A0 (Helmholtz resonance, ∼275 Hz) and A1 (1st longitudinal “slosh” mode, ∼470 Hz), and between A0 and two 1st corpus bending (B1) modes near 500 Hz. Comprehensive EMA (velocity/force), radiativity (pressure/force) and near-field acoustical holography ( f -hole volume flows/force) measurements plus bridge tuning systematics were combined to establish a dual-region structural acoustics model of the violin. In the “deterministic” region the large-volume-change B1 modes drive large, in-phase f-hole volume flows accounting for >50% of B1 radiation and excite A0; oppositely directed upper-bout/lower-bout cavity volume flows excite A1. A wall-driven, dual-Helmholtz resonator network successfully modeled A0 and B1 radiat...

Holographic characterization of therapeutic array transducers with assessment of measurement limitations and robustness

Acoustic holography measurements and corresponding numerical projection methods have recently gained acceptance for characterizing the fields generated by high intensity therapeutic ultrasound (HITU) applications. To facilitate the standardization of such measurement-based simulation methods, it is important to understand how practical measurement challenges and limitations impact the results. Toward this end, holography measurements were acquired and analyzed for characterization of two therapeutic array transducers, each with 256 elements. For different focusing configurations, measurements comprised 2D holography scans as well as 1D scans used to independently quantify the focal lobe. The fields projected from holograms were compared to independent 1D scans to evaluate the impacts of hydrophone directivity, non-orthogonality of the axes of the positioner used to move the hydrophone, and overall mechanical stability of measurements. Results demonstrate that directivity effects exceeded those expected based on hydrophone size and altered focal pressures on the order of 10%. Non-orthogonality of the positioner shifted apparent focal locations by measurable distances, which could be confused with small shifts in device fixturing. However, holographic reconstruction of the ultrasound field structure near the focal lobe was robust with respect to directivity and non-orthogonality, which enables compensation by relative calibration of source output levels. [Funding support by NIH R01-EB025187, R01-EB007643, and P01-DK043881.]

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.

Non-negative intensity and back-calculated non-negative intensity for analysis of directional structure-borne sound.

Non-negative intensity (NNI) is an approach to identify the surface areas of a structure that contribute to sound power. NNI is evaluated in terms of the acoustic impedance matrix obtained directly at the structural surface and as such can only identify surface contributions to sound power at a far-field receiver surface that fully circumscribes the structure. In contrast, back-calculated NNI is evaluated in terms of the acoustic impedance matrix obtained at a far-field receiver surface, and hence can identify surface contributions to sound power at a far-field receiver surface that does not fully circumscribe the structure. In this work, NNI and acoustic intensity obtained numerically using the boundary element method and experimentally from near-field acoustic holography measurements are compared for different modes. Back-calculated NNI evaluated for full and partial receiver surfaces is also compared with acoustic intensity for the different modes. Results for back-calculated NNI show that different regions on the plate surface contribute sound to different receiver locations.

Two reduced-order approaches for characterizing the acoustic output of high-power medical ultrasound transducers

An approach that combines numerical modeling with measurements is gaining acceptance for field characterization in medical ultrasound. The general method is suitable for accurate simulation of the fields radiated by therapeutic transducers in both water and tissue. Here, three characterization methods are compared. Simulations based on the 3D Westervelt equation with a boundary condition determined from acoustic holography measurements are used as the most accurate benchmark method. Two simplified methods are based on an axially symmetric nonlinear parabolic formulation, either the KZK equation or its wide-angle extension. Various approaches for setting a boundary condition to the parabolic models are presented and discussed. Simulation results obtained with the proposed methods are compared for a typical therapeutic array and a strongly focused single-element transducer, with validation measurements recorded by a fiber optic hydrophone at the focus at increasing acoustic outputs. It is shown that the wide-angle parabolic model is more accurate than the KZK model in governing diffraction effects in the nearfields of the focused beams. However, both methods give accurate results in the focal zone, even at very high outputs when shocks are present. [Work supported by and RSF 14-12-00974, P01 DK043881, NIH EB7643, and NSBRI through NASA 9-58.]

Near field acoustic holography measurements of carbon nanotube thin film speakers.

Carbon nanotube (CNT) thin film speakers produce sound with the thermoacoustic effect. Better understanding of the physical acoustic properties of these speakers will drive future design improvements. Measuring acoustic properties at the surface of the CNT thin film is difficult because the films, themselves, do not vibrate, are fragile and have a high surface temperature. In order to measure the surface particle velocity and sound pressure level (SPL), near field acoustic holography (NAH) has been used by employing probe microphones. NAH images the acoustic quantities of the source system using the set of acoustic pressure measurements on a hologram parallel to the source surface. It is shown that the particle velocity at the surface of an open-air, double-sided speaker is nominally zero, as expected. However, the SPL distribution is not uniform on the source surface, contrary to common lumped parameter model assumptions. Also, particle velocity and sound intensity distributions on the hologram have been obtained in this study. Finally, measured directivity patterns of the planar CNT speaker are reported.

Characterization of medical ultrasound fields using modeling with a boundary condition obtained from measurements

Numerical modeling is becoming an important metrological tool for accurate characterization of ultrasound fields generated by medical transducers in water and in situ. While acoustic propagation equations are well established, setting a boundary condition relevant to the experiment still remains a critical problem. Two methods differing in complexity are described to address this problem. The first method comprises 3D simulations of the Westervelt equation with a boundary condition determined from acoustic holography measurements. The second, simpler method utilizes simulations based either on the KZK or Westervelt equation with an equivalent source boundary condition obtained by matching linear simulations to low-amplitude beam profiles along and transverse to the axis of the source. Calculations with both methods are compared to fiber optic hydrophone measurements of 2D phased therapeutic arrays, a 128-element diagnostic probe, and strongly focused single-element transducers. It is shown that the 3D Westervelt model with holographic boundary condition can accurately simulate the entire nonlinear ultrasound field. The simplified methods based on an equivalent source are shown to give accurate results in the focal zone of the transducers, even when shocks are present in the focal waveform. [Work supported by NIH EB7643, EB016118, DK043881, and RSF 14-12-00974.]

Equivalent sources method for supersonic intensity of arbitrarily shaped geometries

Supersonic acoustic intensity is utilized to locate radiating regions on a complex vibrating structure. The supersonic intensity is obtained by a special process that removes the evanescent waves from the near-field acoustical holography measurement. The filtering process is well understood for separable geometries, but unfortunately, there are few results for arbitrarily shaped objects. This work proposes a methodology based on a stable invertible representation of the radiated power operator. The power operator is approximated numerically by the equivalent source formulation and the appropriate complete spectral basis is employed to form the stable invertible operator. The operator is formed with the most efficient radiation modes and these modes are utilized to obtain the supersonic solution for the near-field holographic problem. This concept is tested using numerically generated data in a spherical geometry and the results are validated with the spherical harmonic, supersonic filter. Finally, a vibrating ship-hull structure provides a physical example for application and validation of the proposed methodology in a more complex geometry.

Equivalent sources method for supersonic intensity of arbitrarily shaped geometries

Supersonic acoustic intensity is utilized to locate radiating regions on a complex vibrating structure. The supersonic intensity is obtained by a special process that removes the subsonic waves from the near-field acoustical holography measurement. The filtering process is well understood for separable geometries, but unfortunately, there are few results for arbitrarily shaped objects. This work proposes a methodology based on a stable invertible representation of the radiated power operator. The power operator is approximated numerically by the equivalent source formulation and the appropriate complete spectral basis is employed to form the stable invertible operator. The operator is formed with the most efficient radiation modes and these modes are utilized to obtain the supersonic solution for the near-field holographic problem. This concept is tested using numerically generated data in a spherical geometry and the results are validated with the spherical harmonic, supersonic filter. Finally, a vibrating ship-hull structure provides a physical example for application and validation of the proposed methodology in a more complex geometry. [This work was supported by the Office of Naval Research.]

Addressing nonlinear propagation effects in characterization of high intensity focused ultrasound fields and prediction of thermal and mechanical bioeffects in tissue

Nonlinear propagation effects are present in most fields generated by high intensity focused ultrasound (HIFU) sources. In some newer HIFU applications, these effects are strong enough to result in the formation of high amplitude shocks that actually determine the therapy and provide a means for imaging. However, there is no standard approach yet accepted to address these effects. Here, a set of combined measurement and modeling methods to characterize nonlinear HIFU fields in water and predict acoustic pressures in tissue is presented. A characterization method includes linear acoustic holography measurements to set a boundary condition to the model and nonlinear acoustic simulations in water for increasing pressure levels at the source. A derating method to determine nonlinear focal fields with shocks in situ is based on the scaling of the source pressure for data obtained in water to compensate for attenuation losses in tissue. The accuracy of the methods is verified by comparing the results with hydrophone and time-to-boil measurements. Major effects associated with the formation of shocks are overviewed. A set of metrics for determining thermal and mechanical bioeffects is introduced and application of the proposed tools to strongly nonlinear HIFU applications is discussed. [Work supported by NIH EB007643, T32 DK007779, and RFBR 13-02-00183.]

References per coherence length: A figure of merit for multireference acoustical holography

Multireference partial field decomposition (PFD) can be used to generate coherent holograms for scan-based near-field acoustical holography measurements. PFD is successful when the reference array completely senses all independent subsources, but meeting this requirement is not straightforward when the number of subsources and their locations are ambiguous (such as in aeroacoustic sources). A figure of merit based on spatial coherence lengths, called references per coherence length (RPLc), is a useful metric to guide inter-reference spacing in the array design so that the source is spanned. Coherence length is defined as the axial distance over which the ordinary coherence drops from unity to some desired value. Numerical experiments involving an extended, partially correlated source show that sufficiency of the reference array for different source conditions may be simply expressed in terms of RPLc. For sources of varying spatial coherence and over a large range of frequencies, one reference per coherence length is equivalent to sensing all independent subsources.

Coherence length as a figure of merit in multireference near-field acoustical holography

Multireference partial field decomposition (PFD) can be used to generate coherent holograms for near-field acoustical holography measurements. PFD is most successful when the reference array completely senses all independent subsources, but meeting this requirement is not straightforward when the number of subsources and their locations are ambiguous (such as in aeroacoustic sources). A figure of merit based on spatial coherence lengths, called references per coherence length (RPL(C)), is a useful metric to guide inter-reference spacing in the array design. For numerical, extended, arbitrarily coherent sources one reference per coherence length results in a sufficient reference array.

Comprehensive approach for noise reduction in helicopter cabins

The work presented here settles down in the general framework of a “Comfortable Helicopter” Research program, and particularly focuses on acoustical comfort improvement in helicoptersʼ cabins. To optimize internal noise levels at given added sound-proofing weight, space and cost constraints, one needs both to have a good knowledge of noise sources and path contributions to the interior noise and to develop some efficient and innovative sound-proofing concepts adapted to the sources frequency content and localization in the helicopter. This paper presents the comprehensive approach chosen by Eurocopter including a diagnosis and modelling methodology based upon the coupling of Nearfield Acoustical Holography measurements and geometrical acoustics simulations. Innovative noise reductions concepts such as optimized sound-proofing panels and windows and Main Gear Box suspension devices have been developed. All these solutions have been tested in flight. Finally, the achieved overall performances obtained on the demonstrator are highlighted, insisting on the significant comfort improvement obtained in flight and on Eurocopter expectations for further noise reductions.

Prestress effects on the eigenfrequencies of the soundboards: Experimental results on a simplified string instrument

This paper presents an experimental study of the effects of prestresses on the vibration behavior of string instruments. These prestresses are created by gluing ribs (crowning) and tensioning string (downbearing). The effects of these prestresses were previously studied numerically for a piano soundboard by Mamou-Mani et al. [J. Acoust. Soc. Am. 123, 2401-2406 (2008)] and analytically for simplified models by Mamou-Mani et al. [Acta Acust. Unit. Acust. 95, 915-926 (2009)]. In the present study, a specified test bench is designed, including a simplified soundboard (a rectangular plate), a bridge, and a single string. The plate is subjected to in-plane and transverse loads. Vibrational eigenmodes are identified using nearfield acoustical holography (NAH) measurement. The evolution of eigenfrequencies with these specific prestresses is studied. The results show the effectiveness of NAH for this purpose and a very good qualitative concordance with previous numerical and analytical calculus.

Comparison of modal analysis results of laser vibrometry and nearfield acoustical holography measurements of an aluminum plate.

Two methods for non-contact vibration measurements are compared using modal analysis. Laser vibrometry and nearfield acoustical holography are used to measure mode shapes of an aluminum plate excited by a mechanical shaker with random noise. Laser vibrometry provides a direct measurement of the panel surface velocity, while the nearfield acoustical holography measurements are used to calculate the surface velocity indirectly. Both data sets are compared in frequency, damping, and mode shapes. The results are also compared to finite element and theoretical analyzes. Results are presented from all four approaches.

Application of near-field acoustical holography to high-performance jet aircraft noise.

Near-field acoustical holography (NAH) measurements were made to visualize the sound field in the jet exhaust region of an F-22 Raptor. This is one of the largest-scale applications of NAH since its development in the 1980s. A scan-based holographic measurement was made using a 90-microphone array with 15-cm regular grid spacing for four engine power settings. The array was scanned through 93 measurement positions along three different measurement planes in a region near 7 m from the jet centerline and 23 m downstream. In addition, 50 fixed reference microphones were placed along the ground 11.6 m from the jet centerline, spanning 30.8 m. The reference microphones have been used to perform virtual coherence on the measurement planes. Statistically optimized NAH has been used to backpropagate the sound field to the source region for low frequencies and to identify jet noise characteristics. Ground reflection interference and other non-ideal measurement conditions must be dealt with. Details relating to jet coherence-lengths and their relation to reference microphone requirements will be discussed. Preliminary results of this ongoing work will be presented. [Work supported by Air Force SBIR.]

Aircraft jet source noise measurements of a Lockheed Martin F-22 fighter jet using a prototype near-field acoustical holography measurement system.

Military jet aircraft generate high levels of noise which require innovative measurement and analysis methods to characterize the jet noise. A near-field acoustic holography (NAH) system is being developed to provide model refinement and benchmarking, evaluate performance of noise control devices, and predict ground maintenance personnel and community noise exposure. A prototype NAH system was used to perform jet source noise measurements of an F-22 at Holloman AFB, NM. This NAH system utilizes a patch and scan measurement approach using a 90 channel microphone array and over 50 reference microphones. The microphone array was used to make pressure measurements on a planar surface extending from the jet nozzle to 75 ft along the plume and over 6 ft in height. These measurements were made parallel either to the shear layer or to the nozzle center line of the jet plume at three different offset distances. Measurements were made every 6 in. on the surface which results in over 1800 measurements per offset distance. The ground based fixed reference microphones were located parallel to the nozzle center line. The measurement approach and pressure measurements for four power conditions detailing the source size, frequency content, and ground effect will be detailed. [Work supported by AFRL SBIR.]

Acoustic holography measurements in comparison with a three‐dimensional finite element method modulation of the body of a classical guitar.

The body of a classical guitar (consiting of the rib, back plate, top plate with bridge and fan bracing, soundhole and wooden block) which had been performed by 3‐D finite element method (FEM) was presented at the 151st meeting of the ASA (session 3aMU3 in Providence, RI). As a submittal, a real guitar was used whose geometrical parameters had precisely been modeled using a computer. The resulting computation has subsequently been matched to the measurements of an acoustical holography taken from the same guitar body. This time the aim was to demonstrate how closely the FEM model resembles the “real” sound radiation of the guitar. Acoustic holography graphically displays the characteristics of sound radiation from a given body. Again the mode‐shapes are of interest, but this time the characteristics of the sound in relation to the frequencies are of greater importance. They demonstrate how realistic the modulation is compared to the actual acoustics of the guitar.