Radiation Force Balance Research Articles

Acoustic streaming-based calibration of ultrasound transducers

The accurate determination of the transfer function of ultrasound transducers is important for their design and operational performance. However, conventional methods for quantifying the transfer function, such as hydrophone measurements, radiation force balance, and pulse-echo measurements, are costly and complex due to specialized equipment required. In this study, we introduce a novel approach to estimate the transfer function of ultrasound transducers by measuring the acoustic streaming velocity generated by the transducer. We utilize an experimental setup consisting of a water tank with a millimeter scale, an ink-filled syringe, and a camera for recording the streaming phenomenon. Through streaming velocity measurements in the frequency range from 2 to 8 MHz, we determined the transfer function of an unfocused circular transducer with a center frequency of 5 MHz and a radius of 5.6 mm. We compared the performance of our method with hydrophone and pulse-echo measurements. At the center frequency, we measured a transmit efficiency of 1.9 kPa/V using the streaming approach, while hydrophone and pulse-echo measurements yielded transmit efficiencies of 2.1 kPa/V and 1.8 kPa/V, respectively. These findings demonstrate that the proposed method for estimating the transfer function of ultrasound transducers achieves a sufficient level of accuracy comparable to pulse-echo and hydrophone measurements.

Head-to-Head Comparison of Acoustic Properties of Lead-Free and PZT-Based HIFU Transducers Operating at 12 MHz.

A direct comparison of performance and acoustic properties of high-intensity focused ultrasonic transducers utilizing lead-free (sodium bismuth titanate-NBT) and lead-based (lead zirconate titanate-PZT) piezoceramics is discussed. All transducers operate at 12 MHz at third harmonic frequency, having an outer diameter of 20 mm, a central hole of 5 mm in diameter, and a radius of curvature of 15 mm. The electroacoustic efficiency determined by a radiation force balance is evaluated in a range of input power levels up to 15 W. Schlieren tomography as well as hydrophone measurements are used for evaluation of the acoustic field distribution. It is found that the average electroacoustic efficiency of NBT-based transducers is approximately 40%, while it is around 80% in the PZT-based devices. NBT devices show significantly higher inhomogeneity of the acoustic field under schlieren tomography compared to PZT devices. From pressure measurements in the prefocal plane, it was found that the inhomogeneity could be attributed to depoling of significant areas of the NBT piezo-component during the fabrication process. In conclusion, PZT-based devices performed significantly better than those using lead-free material. However, the NBT devices show promise for this application and their electroacoustic efficiency as well as the uniformity of the acoustic field could be improved by employing a low-temperature fabrication process or repoling after processing.

Dynamic ultrasound focusing and centimeter-scale ex vivo tissue ablations with a CMUT probe developed for endocavitary HIFU therapies.

Thermal ablation of localized prostate tumors via endocavitary Ultrasound-guided High Intensity Focused Ultrasound (USgHIFU) faces challenges that could be alleviated by better integration of dual modalities (imaging/therapy). Capacitive Micromachined Ultrasound Transducers (CMUTs) may provide an alternative to existing piezoelectric technologies by exhibiting advanced integration capability through miniaturization, broad frequency bandwidth and potential for high electro-acoustic efficiency. An endocavitary dual-mode USgHIFU probe was built to investigate the potential of using CMUT technologies for transrectal prostate cancer ablative therapy. The USgHIFU probe included a planar 64-element annular HIFU CMUT array (fHIFU = 3 MHz) surrounding a 256-element linear imaging CMUT array. Acoustic characterization of the HIFU array included 3D pressure field mapping and radiation force balance measurements. Ex vivo proof-of-concept experiments consisted in generating HIFU thermal ablations with the CMUT probe on porcine liver tissues. The planar CMUT probe enabled HIFU dynamic focusing (distance range: 32 - 72 mm) while providing acoustic surface intensities of 1 W/cm2 that allowed producing elementary ex vivo ablations in depth of liver tissue (L×W ≈ 10 mm × 5 mm). Combinations of dynamic focusing, along with probe rotation and translation produced larger thermal ablations (L×W ≈ 20 mm × 20 mm) by juxtaposing multiple elementary ablations, consistent with expected results obtained through numerical modeling. The technical feasibility of using a USgHIFU probe, fully-developed using CMUTs for tissue ablation purposes, was demonstrated. The HIFU-CMUT array showed tissue ablation capabilities with volumes compatible with localized cancer targeting thus providing assets for further development of focal therapies.

How reliable are the ultrasonic output power values emitted by therapeutic ultrasound devices in physiotherapy?

Therapeutic ultrasound is one of the most used electrophysical agents in physiotherapy. Despite this important and widespread application, therapeutic ultrasound devices (TUSD) may operate outside the range recommended by the International Standard IEC 61689:2013, and, thus, may not contribute to the treatment (if the ultrasonic output power, UOP, used in clinical practice are lower than the nominal power, NP) or even aggravate the pre-existing injury (if the UOP used is higher than the NP). The aim of this study was to verify whether the UOP values emitted by three TUSD from the Physiotherapy Section of the Esquadrão de Saúde de Pirassununga (named as TUSD1, TUSD2 and TUSD3) were in accordance with the IEC 61689:2013. Additionally, statistical tests were employed to assess the repeatability of UOP measurements made with the TUSD on three different days. An acoustic radiation force balance was used to measure the UOP values emitted by TUSD in three days of testing by each equipment, which were configured to operate in continuous mode. In addition, 10 UOP measurements were performed for each NP value with all available frequencies and effective radiation area. TUSD, even with up-to-date maintenance, presented several values of UOP with relative error greater than that preconized by IEC 61689:2013 (81.39% of 1800 UOP measurements with TUSD1, 68.61% of 1800 UOP measurements with TUSD2 and 75.33% of 1200 UOP measurements with TUSD3). So, It is important that the professional responsible for monitoring the maintenance/calibration of this equipment is trained to interpret the reports prepared by companies that provide maintenance/calibration services.

Image-Guided Measurement of Radiation Force Induced by Focused Ultrasound Beams.

The radiation force balance (RFB) is a widely used method for measuring acoustic power output of ultrasonic transducers. The reflecting cone target is attractive due to its simplicity and long-term stability, at a reasonable cost. However, accurate measurements using this method depend on the alignment between the ultrasound beam and cone axes, especially for highly focused beams utilized in therapeutic applications. With the advent of dual-mode ultrasound arrays (DMUAs) for imaging and therapy, image-guided measurements of acoustic output using the RFB method can be used to improve measurement accuracy. In this article, we describe an image-guided RFB measurement of focused DMUA beams using a widely used commercial instrument. DMUA imaging is used to optimize the alignment between the acoustic beam and reflecting cone axes. In addition to image-guided alignment, DMUA echo data is used to track the displacement of the cone, which provides an auxiliary measurement of acoustic power. Experimental results using a DMUA prototype with [Formula: see text] shows that 1-2 mm of misalignment can result in 5%-14% error in the measured acoustic power. In addition to the use of B-mode image guidance for improving measurement accuracy, we present preliminary results demonstrating the benefit of displacement tracking using real-time DMUA imaging during the application of (sub)therapeutic focused beams. Displacement tracking provides a direct measurement of the radiation force with high sensitivity and follows the expected dependence on changes in amplitude and duty cycle (DC) of the focused ultrasound (FUS) beam. This could lead to simpler, more reliable methods for measuring acoustic power based on the radiation force principle. Combined with appropriate computational modeling, the direct measurement of acoustic radiation force could lead to reliable dosimetry in situ in emerging applications such as transcranial FUS (tFUS) therapies.

Acoustic Radiation Forces at the Crossroads of Ultrasound Exposimetry, HIFU, and Elastography.

The applications of the acoustic radiation force (ARF) continue to multiply and extend from elastography into high-intensity focused ultrasound (HIFU), diagnostic imaging, lithotripsy, sonochemistry, levitation, and microsonics yet fundamental principles remain shrouded in mystery. A well-known and popular equation often used for calculating ARF in elastography is in conflict with the equation commonly employed for calculating ARF for determining acoustic power in radiation force balances (RFBs). Controversies have sparked debate for over a century concerning the physical mechanisms underlying ARFs. For over four decades, the science of ultrasound exposimetry has steadily improved and has provided clues in terms of accumulated data about the characteristics of transmitted ultrasound fields. Concurrently, the availability and capability of predicting these fields have improved significantly. The author draws on these sources to re-examine the physical principles behind ARFs. Conflicts are shown to stem from idealized configurations and simplistic assumptions. By more fully accounting for the pulse shape and spectrum, the effect of frequency power law attenuation, diffraction, and nonlinearity, more accurate equations are developed for ARFs for practical applications which are more consistent with exposimetry observations. Simulations compare well to corrected 1.5 MHz RFB data. While some questions await resolution, the approach presented here settles several longstanding conflicts and provides a new broadband framework for future ARF work.

Titanium ultrasonic reactor tuned to 500 kHz

This study describes the design considerations, principles, and performance of a water-filled ultrasonic reactor formed by a 125 mm size titanium cylinder covered with 67 piezoelectric transducers, tuned as a system for peak emissions at 500 kHz. The total acoustic power measured by a radiation force balance is 107 W. The sound intensity is amplified by the cavity and focusing attributes of the cylindrical wall. The reactor can generate ZnO nanoparticles from ZnAc2 solution, and the nanoparticle are found fixated to an epoxy substrate as observed under a scanning transmission electron microscope. These indications are similar to a sonochemical reaction reported at 20 kHz, which validates that inertial cavitation has been reached. The titanium wall has a transmission efficiency of 51% compared to a well-matched POCO graphite-resin layer. The efficiency exceeds the value of 17% expected from a naïve calculation based on the impedance-translation theorem. The problem of optimal emission from a piezoelectric source is more complex than a simple reduction of reflections at the transducer boundary. COMSOL simulations show that the condition for optimal transmission requires consideration of elasticity and piezoelectric charge matrices instead of acoustic impedance. Approximated analytical calculation is suggested as a preliminary guidance for design of an optimal matching layer.

Low-Cost Thermochromic Quality Assurance Phantom for Therapeutic Ultrasound Devices: A Proof of Concept.

High-intensity focused ultrasound (HIFU) transducer acoustic output can vary over time as a result of an inconsistent power supply, damage to the transducer or deterioration over time. Therefore, easy implementation of a daily quality assurance (DQA) method is of great importance for pre-clinical research and clinical applications. We present here a thermochromic material-based phantom validated by thermal simulations and found to provide repeatable visual power output assessments in fewer than 15 s that are accurate to within 10%. Whereas current available methods such as radiation force balance measurements provide an estimate of the total acoustic power, we explain here that the thermochromic phantom is sensitive to the shape of the acoustic field at focus by changing the aperture of a multi-element transducer with a fixed acoustic power. The proposed phantom allows the end user to visually assess the transducer's functionality without resorting to expensive, time-consuming hydrophone measurements or image analysis.

My contributions to ultrasound metrology

My work in ultrasound metrology has centered around developing pre-clinical testing methods for therapeutic ultrasound devices. These test methods include radiation force balance systems and methods specifically for high-intensity therapeutic ultrasound, time-delay spectrometry methods for measuring hydrophone sensitivity, acoustic characterization of tissue-mimicking material that has been approved as a medical device development tool and a comparison protocol of computational and experimental acoustic characterization for therapeutic ultrasound devices. The purpose of this work at the FDA is to develop bench testing methods to ensure proper review of medical ultrasound devices to evaluate device safety and performance. The test instruments and methods developed have helped promote the development and recognition of voluntary consensus standards, as well as FDA industry guidance, that can provide the basis for least-burdensome data collection requirements by manufacturers and allow the FDA to assess device safety and effectiveness in a more consistent, methodical and scientifically rigorous manner.

Radiation Force Balance Method for Measuring Ultrasonic Power of an Ultrasonic Cleaning Device at Low Operating Frequencies (≤1 Mhz)

Radiation Force Balance Method for Measuring Ultrasonic Power of an Ultrasonic Cleaning Device at Low Operating Frequencies (≤1 Mhz)

1Pb2-4 Ultrasonic power measurement using radiation force balance method with absorbing target for high ultrasonic power

1Pb2-4 Ultrasonic power measurement using radiation force balance method with absorbing target for high ultrasonic power

Research on sound power measurement of multi-element ultrasonic focusing transducer based on cross-spectrum method.

In this paper, aiming at the problem that a multi-array focused ultrasonic transducer working in a high power range has a nonlinear influence on the measurement accuracy, a study of acoustic power measurement based on the near field cross-spectrum method was carried out. The focused acoustic field of a multi-array transducer was derived theoretically by combining the cross-spectrum method and the Westervelt nonlinear acoustic propagation equation. Finite element simulation was used to establish the model of the focused sound field of the multi-array transducer under different excitation conditions, and the influence law of each harmonic on the total sound power under specific excitation conditions was obtained. A cross-spectrum measurement system was built to scan the two near focusing planes under different excitation conditions. The total energy, each harmonic energy, and their proportion in the focusing region under the corresponding excitation were obtained through calculation and processing. The theoretical and simulation results were verified, and the harmonic energies were compensated in the calculation of the total ultrasonic power. The measurement results were compared with those obtained by radiation force balances. It was found that the maximum measurement deviation of compensated ultrasonic power was 7.89%, which met the requirements of acoustic measurement. The accuracy of the method and conclusion was verified under the power range of 10-60W.

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.]

Characterization and Ex Vivo evaluation of an extracorporeal high-intensity focused ultrasound (HIFU) system.

BackgroundHigh‐intensity focused ultrasound (HIFU) has been in clinical use for a variety of solid tumors and cancers. Accurate and reliable calibration is in a great need for clinical applications. An extracorporeal clinical HIFU system applied for the investigational device exemption (IDE) to the Food and Drug Administration (FDA) so that evaluation of its characteristics, performance, and safety was required.MethodsThe acoustic pressure and power output was characterized by a fiber optic probe and a radiation force balance, respectively, with the electrical power up to 2000 W. An in situ acoustic energy was established as the clinical protocol at the electrical power up to 500 W. Temperature elevation inside the tissue sample was measured by a thermocouple array. Generated lesion volume at different in situ acoustic energies and pathological examination of the lesions was evaluated ex vivo.ResultsAcoustic pressure mapping showed the insignificant presence of side/grating lobes and pre‐ or post‐focal peaks (≤−12 dB). Although distorted acoustic pressure waveform was found in the free field, the nonlinearity was reduced significantly after the beam propagating through tissue samples (i.e., the second harmonic of −11.8 dB at 500 W). Temperature elevation was <10°C at a distance of 10 mm away from a 20‐mm target, which suggests the well‐controlled HIFU energy deposition and no damage to the surrounding tissue. An acoustic energy in the range of 750–1250 J resulted in discrete lesions with an interval space of 5 mm between the treatment spots. Histology confirmed that the lesions represented a region of permanently damaged cells by heat fixation, without causing cell lysis by either cavitation or boiling.ConclusionsOur characterization and ex vivo evaluation protocol met the IDE requirement. The in‐situ acoustic energy model will be used in clinical trials to deliver almost consistent energy to the various targets.

Measurement of the temperature-dependent output of lead zirconate titanate transducers

The effect of temperature and electrical drive conditions on the output of lead zirconate titanate (PZT) transducers is of particular interest in ultrasound metrology and medical ultrasound applications. In this work, the temperature-dependent output of two single-element PZT transducers was measured between 22 °C and 46 °C. Two independent measurement methods were used, namely radiation force balance measurements and laser vibrometry. When driven at constant voltage using a 50 Ω matched signal generator and amplifier using continuous wave (CW) or quasi-CW excitation, the output of the two transducers increased on average by 0.6 % per degree, largely due to an increase in transducer efficiency with temperature. The two measurement methods showed close agreement. Similar trends were observed when using single cycle excitation with the same signal chain. However, when driven using a pulser (which is not electrically matched), the two transducers exhibited different behaviour depending on their electrical impedance. Accounting for the temperature-dependent output of PZT transducers could have implications for many areas of ultrasound metrology, for example, in therapeutic ultrasound where a coupling fluid at an increased or decreased temperature is often used.

2Pa2-4 Study on measurement technique of ultrasonic power for low frequency by radiation force balance method

2Pa2-4 Study on measurement technique of ultrasonic power for low frequency by radiation force balance method

Improved characterization of ultrasound sources by radiation force balance using acoustic holography in place of plane-wave and geometric approximations

The radiation force balance (RFB) method is a widely accepted approach to characterize ultrasound sources used both in therapy and imaging. Measurement of the radiation force F acting on an absorbing or reflecting target provides the acoustic power W. The RFB method is based on the linear relationship W = Y · c · F, where c is the speed of sound in the propagation medium and Y is a dimensionless coefficient that depends on the wave structure and reflective properties of the target. The widely accepted expression for a plane wave and an ideal absorber is Y = 1. For a spherically focused source the geometric approximation factor Y = 2/(1 + cos α) is used, where α is the half-aperture angle. The exactness of these and similar approximate relations cannot be checked experimentally because no real sources emit such idealized waves. In the current talk, a more precise approach is presented to determine Y for a given source. To account for the spatial field structure, the acoustic holography method was employed using a 2-D scan with a calibrated hydrophone. In this way, the ultrasound beam's explicitly measured angular spectrum can be used to relate radiation force and acoustic power. [Work supported by NIH R01EB025187 and R01EB007643, and RSF 19-12-00148.]

Comparison between experimental and computational methods for the acoustic and thermal characterization of therapeutic ultrasound fields

For high intensity therapeutic ultrasound (HITU) devices, pre-clinical testing can include measurement of power, pressure/intensity and temperature distribution, acoustic and thermal simulations, and assessment of targeting accuracy and treatment monitoring. Though relevant standards are available, technical challenges remain because of the often focused, large amplitude pressure fields encountered. Measurement and modeling issues include using hydrophones and radiation force balances at HITU power levels, validation of simulation models, and a suitable tissue-mimicking material (TMM) for temperature measurements. To better understand these issues, a comparison study was undertaken between simulations and measurements of the HITU acoustic field distribution in water and TMM, and temperature rise in TMM. For the specific conditions of this study, the following results were obtained. In water, the simulated values for p + and p- were 3% lower and 10% higher, respectively, than those measured by hydrophone. In TMM, the simulated values for p + and p- were 2% and 10% higher than those measured by hydrophone, respectively. The simulated spatial-peak temporal-average intensity values in water and TMM were greater than those obtained by hydrophone by 3%. Simulated and measured end-of-sonication temperatures agreed to within their respective uncertainties (coefficients of variation of approximately 20% and 10%, respectively).

Continuous Wave Ultrasonic Interferometers with Relatively Higher Excitation are Inappropriate for Liquid Characterization

Ultrasonic velocity is an important property for various physicochemical investigations in the liquids. The continuous wave ultrasonic interferometers are widely used as a cost-effective instrument for the estimation of ultrasonic propagation velocity. One of the critical aspects of this instrument is the level of radio frequency excitation supplied to piezoelectric transducer, which can cause heating of the sample. The error in the ultrasonic velocity measurement due to heating by the ultrasonic interferometer is described. In this article, the distilled water was used as experimental liquid to study the heating effect. Firstly, the ultrasonic transducer has been used to generate ultrasound in the sample and was calibrated for its radiation conductance by using primary radiation force balance facility of the laboratory. Primary calibration of the ultrasonic transducer in a commercially available system used in the liquid cell showed 68 mW power for excitation level. In a laboratory interferometer designed for low-power operation in the same test cell, the measured power was only 85 µW. In order to study the heating effect, the ultrasonic velocity was measured with both the systems without controlling the temperature of liquid cell. The measurements by commercial system showed significant deviation (5.6 ms−1) in ultrasonic velocity as compared to that measured under low power excitation, which remains almost undeviated. The Marczak equation was used to calculate the deviation in velocity due to ultrasound heating.

A Radiation Force Balance Target Material for Applications below 0.5 MHz

Acoustic output power is an important safety-related parameter whose standardised measurement method involves use of a radiation force balance in conjunction with a special target that is typically designed to be totally absorbing to ultrasound. International Standard International Electrotechnical Commission (IEC) 61161 specifies important performance criteria for such an absorber, such as transmission loss and reflection loss. Currently, there is a lack of acoustic absorbers meeting these requirements at low frequencies (<0.5 MHz). This is unsatisfactory given emerging clinical applications, particularly therapeutic. Described here is an acoustic absorber appropriate for application below 0.5 MHz. Through use of two National Physical Laboratory measurement facilities, the absorber transmission loss and reflection loss have been derived over the frequency range 50–500 kHz. Results are presented and compared with performance requirements specified in IEC 61161, revealing the efficacy of the new material as an absorbing radiation force balance target down to a frequency of approximately 120 kHz.