Membrane Hydrophone Research Articles

A Miniature Ultrasound Source for Neural Modulation.

This work describes a unique ultrasound (US) exposure system designed to create very localized ( [Formula: see text]) sound fields at operating frequencies that are currently being used for preclinical US neuromodulation. This system can expose small clusters of neuronal tissue, such as cell cultures or intact brain structures in target animal models, opening up opportunities to examine possible mechanisms of action. We modified a dental descaler and drove it at a resonance frequency of 96 kHz, well above its nominal operating point of 28 kHz. A ceramic microtip from an ultrasonic wire bonder was attached to the end of the applicator, creating a 100- [Formula: see text] point source. The device was calibrated with a polyvinylidene difluoride (PVDF) membrane hydrophone, in a novel, air-backed, configuration. The experimental results were confirmed by simulation using a monopole model. The results show a consistent decaying sound field from the tip, well-suited to neural stimulation. The system was tested on an existing neurological model, Drosophila melanogaster, which has not previously been used for US neuromodulation experiments. The results show brain-directed US stimulation induces or suppresses motor actions, demonstrated through synchronized tracking of fly limb movements. These results provide the basis for ongoing and future studies of US interaction with neuronal tissue, both at the level of single neurons and intact organisms.

Characterisation of hydrophone sensitivity with temperature using a broadband laser-generated ultrasound source

In this work, we present a novel method for characterising the relative variation in hydrophone sensitivity with temperature, addressing a key aspect of measurements in the field of ultrasound metrology. Our study focused on a selection of miniature ultrasonic hydrophones commonly used in medical applications. The method is based on using water as a temperature-sensitive laser-generated ultrasound (LGUS) source for calibration, allowing for flexible characterisation across a wide temperature range. The measurements were performed using both the LGUS method and the established self-reciprocity method. Our results demonstrate good agreement within 5% between the two methods, validating the effectiveness of the LGUS approach. We found that the sensitivity of the tested hydrophones exhibited low temperature dependence less than −0.2% per ∘C within the studied temperature range from 17 ∘C up to 50 ∘C. The presented LGUS method offers greater flexibility than current approaches as it allows for characterisation of membrane hydrophones with small element sizes and non-electrical transducers. By combining the relative sensitivity variation obtained through the LGUS method with the standard calibration at room temperature, absolute values of hydrophone sensitivity can be determined. The expanded uncertainty of our measurements, which was evaluated at temperature intervals of 8 ∘C, was determined to be on average 10%. Our work provides valuable insights into the temperature dependence of hydrophone sensitivity and lays the foundation for further investigations in this area. The LGUS method holds promise for future enhancements, such as increased bandwidth of the LGUS source and frequency domain analysis, to explore the frequency dependency of sensitivity variation with temperature.

Peculiarities in the Transformation of the Profile of Acoustic Nonlinear Waves Reflected from a Stepped Structure

The article presents the results of studying the reflection of an intense acoustic beam from stepped structures. The case of high Reynolds numbers is considered, when a sawtooth wave profile formed in the incident beam, and an obstacle in the form of a step was located normal to the acoustic axis and behind the discontinuity coordinate. When reflected from the obstacle, the acoustic beam incident on it splits into two parts, between which there is a path difference, specified by the stepped structure itself. The experiment focused on obstacles that create a path difference between the two parts of the beam during reflection, equal to 0, λ/3, λ/2, λ, etc. A broadband membrane hydrophone was used to record the shape of the profile of nonlinear waves of the incident and reflected acoustic beams at an arbitrary spatial point. This made it possible to analyze the evolution of the beam shape with increasing distance from the obstacle, as well as to study its transverse structure. The effect of doubling the characteristic frequency of a signal as an acoustic beam is reflected from a stepped structure is shown, which creates a path difference of λ/2 between the two parts of the reflected beam. It is demonstrated that the dynamics of nonlinear effects in intense beams reflected from stepped structures is similar to the transformation of the profile of acoustic waves generated by dual-frequency pump signals.

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 ag . 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 ag 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 ag .

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 pc ), 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.

Hydrophone Spatial Averaging Correction for Acoustic Exposure Measurements From Arrays-Part II: Validation for ARFI and Pulsed Doppler Waveforms.

This article reports the experimental validation of a method for correcting underestimates of peak compressional pressure ( pc) , peak rarefactional pressure ( pr) , and pulse intensity integral (pii) due to hydrophone spatial averaging effects that occur during output measurement of clinical linear and phased arrays. Pressure parameters ( pc , pr , and pii), which are used to compute acoustic exposure safety indexes, such as mechanical index (MI) and thermal index (TI), are often not corrected for spatial averaging because a standardized method for doing so does not exist for linear and phased arrays. In a companion article (Part I), a novel, analytic, inverse-filter method was derived to correct for spatial averaging for linear or nonlinear pressure waves from linear and phased arrays. In the present article (Part II), the inverse filter is validated on measurements of acoustic radiation force impulse (ARFI) and pulsed Doppler waveforms. Empirical formulas are provided to enable researchers to predict and correct hydrophone spatial averaging errors for membrane-hydrophone-based acoustic output measurements. For example, for a 400- [Formula: see text] membrane hydrophone, inverse filtering reduced errors (means ± standard errors for 15 linear array/hydrophone pairs) from about 34% ( pc) , 22% ( pr) , and 45% (pii) down to within 5% for all three parameters. Inverse filtering for spatial averaging effects significantly improves the accuracy of estimates of acoustic pressure parameters for ARFI and pulsed Doppler signals.

A Comparison of Different Calibration Techniques for Hydrophones Used in Medical Ultrasonic Field Measurement.

The acoustic output characterization of medical ultrasonic equipment requires regular calibration of the hydrophones used to ensure the reliability of measurements. Such hydrophone calibration is offered as a service by several institutions. Various calibration techniques using a variety of ultrasonic excitation pressure waveforms comprising different pressure amplitude ranges and frequency compositions as well as different reference measurement systems have been proposed and applied over the past decades. Currently, four different setups for hydrophone calibration are available at the Physikalisch-Technische Bundesanstalt (PTB). This internal comparison study addresses the consistency of all four methods, including direct primary calibration and substitution calibration using reference hydrophones. The methods apply single-frequency tonebursts and swept tonebursts in the kPa amplitude range of quasi-linear acoustics as well as impulse excitation including nonlinear propagation. In recent years, a new primary calibration setup using a high-frequency vibrometer has been implemented at PTB, enabling the characterization of hydrophone frequency responses in modulus and phase and extending the upper frequency limit to up to 100 MHz. For the comparison in the frequency range from 0.5 MHz to 60 MHz, two passive membrane hydrophones with well-known characteristics gained from many years of measurements were used. Another membrane hydrophone with a nominal diameter of 0.2 mm and an integrated preamplifier was applied to address the frequency range up to 100 MHz. The results obtained with the different setups showed good agreement with average root-mean-square (rms) deviations of 3% (primary calibrations, 1-60 MHz) and 4% (1-100 MHz). The consistency of the implementations was thus verified in this comparison.

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 ( pc ), peak rarefactional pressure ( pr ), 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% ( pc ), 12% ( pr ), 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.

Inverse-filter method to suppress pressure measurement distortion due to spatial averaging across a membrane hydrophone sensitive element

This paper 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) [Wear, IEEE Trans UFFC 66, 318–339 (2019)]. The absolute difference between theoretical and experimental SAFs for 25 transducer / hydrophone pairs was 7% ± 3% (mean ± standard deviation). Empirical formulas based on SAFs allow for correction for hydrophone spatial averaging errors in peak compressional pressure (pc), peak rarefactional pressure (pr), pulse intensity integral (pii), mechanical index (MI), and spatial-peak-temporal-average intensity (Istpa). The 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-μm membrane hydrophone, then spatial averaging errors are approximately 16% (pc), 12% (pr and MI), and 24% (pii and Istpa). 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.

Correction for Spatial Averaging Artifacts for Circularly-Symmetric Pressure Beams Measured with Membrane Hydrophones.

This paper investigates experimental underestimation of pressure measurements due to spatial averaging across a hydrophone sensitive element. Empirical relationships are measured to enable correction for hydrophone spatial averaging errors in peak compressional pressure (p c ), peak rarefactional pressure (p r ), and pulse intensity integral (pii). The empirical relationships 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-μm membrane hydrophone, then spatial averaging errors are approximately 16% (p c ), 12% (p r ), and 24% (pii).

Hydrophone Spatial Averaging Artifacts for ARFI Beams from Array Transducers.

This paper reports underestimation of peak compressional pressure (p c), peak rarefactional pressure (p r ), and pulse intensity integral (pii) due to hydrophone spatial averaging of acoustic radiation force impulse (ARFI) beams generated by clinical linear and phased arrays. Although a method exists for correcting for hydrophone spatial averaging for circularly-symmetric beams, there is presently no analogous method for rectangularly-symmetric beams generated by linear and phased arrays. Consequently, pressure parameters (p c, p r , and pii) from clinical arrays are often not corrected for spatial averaging. This can lead to errors in Mechanical Index (MI) and Thermal Index (TI), which are derived from pressure measurements and are displayed in real-time during clinical ultrasound scans. ARFI beams were generated using three clinical linear array transducers. Output pressure waveforms for all three transducers were measured using five hydrophones with geometrical sensitive element sizes (dg) ranging from 85 to 1000 μm. Spatial averaging errors were found to increase with hydrophone sensitive element size. For example, if dg = 500 μm (typical membrane hydrophone), frequency = 2.25 MHz and F/# = 1.5, then average errors are approximately -20% (pc), -10% (pr), and -25% (pii). Therefore, due to hydrophone spatial averaging, typical membrane hydrophones can exhibit significant underestimation of ARFI pressure measurements, which likely compromises exposure safety indexes.

Directivity and Frequency-Dependent Effective Sensitive Element Size of Membrane Hydrophones: Theory Versus Experiment

It is important to know hydrophone frequency-dependent effective sensitive element size in order to account for spatial averaging artifacts in acoustic output measurements. Frequency-dependent effective sensitive element size may be obtained from hydrophone directivity measurements. Directivity was measured at 1, 2, 3, 4, 6, 8, and 10 MHz from ±60° in two orthogonal planes for eight membrane hydrophones with nominal geometrical sensitive element radii ( ag ) ranging from 100 to [Formula: see text]. The mean precision of directivity measurements (obtained from four repeated measurements at each frequency and angle) averaged over all frequencies, angles, and hydrophones was 5.8%. Frequency-dependent effective hydrophone sensitive element radii aeff(f) were estimated by fitting the theoretical directional response for a disk receiver to directivity measurements using the sensitive element radius ( a ) as an adjustable parameter. For the eight hydrophones in aggregate, the relative difference between effective and geometrical sensitive element radii, ( aeff - ag)/ag , was fit to C /( kag)n , where k = 2π/λ and λ = wavelength. The functional fit yielded C = 1.89 and n = 1.36 . The root mean square difference between the data and the model was 34%. It was shown that for a given value for ag , [Formula: see text] for membrane hydrophones far exceeds that for needle hydrophones at low frequencies (e.g., < 4 MHz when [Formula: see text]). This empirical model for [Formula: see text] provides information required for the compensation of spatial averaging artifacts in acoustic output measurements and is useful for choosing an appropriate sensitive element size for a given experiment.

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.

Investigations on the sound field between waveguide and counterpart induced by high-intensity focused ultrasound in thin polymer films

The use of high-intensity focused ultrasound (HIFU) to heat polymer films is getting more in focus of interest in the industry due to some advantages e.g. low damages of thin polymer films during film processing or cost reduction of the processing tool. The capability to heat thin polymer films using a solid waveguide transducer was already reported in the literature. However, some significant hurdles persist: the physical principles responsible for the heating of thin polymer films are not fully understood. This study aims to investigate the sound field occurring in thin polymer films between a solid waveguide transducer (having a resonance frequency of 1.1 MHz) and a solid counterpart and compared it to the sound field in mm-thick polymer sample. An analogy between the sound field measured in water and the sound field inside polymer films during the heating process is done. The sound field between counterpart and solid waveguide is analyzed by means of membrane hydrophone sound field measurements and finite element simulations. The impact of the transducer-counterpart gap size is studied. This investigation reveals that the behavior and characteristics of the sound field in water in a transducer-counterpart gap differ considerably from each other depending on the gap size. When the gap is smaller than a wavelength, the sound cannot be focused within the waveguide-counterpart gap and complex interferences occur inside the small gap. Furthermore, the investigation showed that similarities occur between the sound field measured in water and the sound field calculated in polymer. These findings are relevant for the understanding of HIFU-based processes involving thin polymer films.

Application of optical interferometry in focused acoustic field measurement

Optical interferometry has been successfully applied in measuring acoustic pressures in plane-wave fields and spherical-wave fields. In this paper, the “effective” refractive index for focused acoustic fields was developed, through numerical simulation and experiments, the feasibility of the optical method in measuring acoustic fields of focused transducers was proved. Compared with the results from a membrane hydrophone, it was concluded that the optical method has good spatial resolution and is suitable for detecting focused fields with fluctuant distributions. The influences of a few factors (the generated lamb wave, laser beam directivity, etc.) were analyzed, and corresponding suggestions were proposed for effective application of this technology.

Validation of a wide-angle parabolic model for shallow-focus ultrasound transducers

Recently, a novel numerical method was developed for a wide-angle parabolic equation which accommodates steep gradients and discontinuities in the pressure distribution of the source boundary condition (Soneson, IEEE Trans Ultrason., Ferroelectr., Freq. Control 64, pp. 679–687, 2017). The method allows rapid computation of acoustic fields with improved diffraction modeling capability over the standard parabolic approximation of the Helmholtz equation with no additional computational overhead, and is free from oscillatory artifacts which previously precluded the use of wide-angle models with discontinuous sources. In this work the wide-angle model, using the expression of a converging spherical wave (with a sharp cutoff at the edge of the active area) as the source boundary condition, is validated against measurements obtained using a 200 μm membrane hydrophone. The ultrasound field was produced by a single-element transducer with f-number 0.7 at 1.5 MHz, which was selected to test the wide angle model’s a...

Using a heterodyne vibrometer in combination with pulse excitation for primary calibration of ultrasonic hydrophones in amplitude and phase

A high-frequency vibrometer was used with ultrasonic pulse excitation in order to perform a primary hydrophone calibration. This approach enables the simultaneous characterization of the amplitude and phase transfer characteristic of ultrasonic hydrophones. The method allows a high frequency resolution in a considerably short time for the measurement. Furthermore, the uncertainty contributions of this approach were investigated and quantified.A membrane hydrophone was calibrated and the uncertainty budget for this measurement was determined. The calibration results are presented up to . The measurement results show good agreement with the results obtained by sinusoidal burst excitation through the use of the vibrometer and by a homodyne laser interferometer, with RMS deviation of approximately –4% in the frequency range from to . Further hydrophones were characterized up to with this procedure to demonstrate the suitability for very high frequency calibration.

Evaluation of frequency-dependent ultrasound attenuation in transparent medium using focused shadowgraph technique

Acoustic fields of a short-pulsed ultrasound propagating through a transparent medium with ultrasound attenuation were visualized by the focused shadowgraph technique. A brightness waveform and its spatial integrations were derived from a visualized field image and compared with a pressure waveform measured by a membrane hydrophone. The experimental results showed that first-order integration of the brightness wave has good agreement with the pressure waveforms. Frequency-dependent attenuation of the pulse propagating through castor oil was derived from brightness and pressure waveforms, and attenuation coefficients determined from focused shadowgraphy and hydrophone techniques showed good agreement. The results suggest the usefulness of the shadowgraph technique not only for the visualization of ultrasound fields but also for noncontact estimation of rough pressure waveforms and correct ultrasound attenuation.

Development of disposable membrane hydrophones for a frequency range from 1 MHz to 10 MHz

A method for fabricating disposable membrane hydrophones is presented. The disposable hydrophones are intended for onetime use in such damaging environments as chemically contaminating fluids and high-amplitude (peak amplitude ∼100MPa) shock wave fields, where the use of commercial membrane hydrophones is not recommended. Fabrication of a hydrophone is done using only off-the-shelf components and hand tools, which translates into ease of fabrication and orders-of-magnitude reduction in unit cost. In particular, poling and sputtering, the two processes that are chiefly responsible for the cost and difficulty associated with the conventional fabrication method, are replaced with the use of pre-poled polyvinylidene fluoride (PVDF) films and polyethylene terephthalate (PET)-coated aluminum foils, respectively. Despite the seemingly crude construction, these disposable hydrophones can exhibit voltage sensitivity response that compares favorably with that of commercial hydrophones. For example, one prototype having a 2mm×2mm active element shows the end-of-cable voltage sensitivity of −270 (±1.9) dB re 1V/μPa over the frequency range of 1–10MHz.

Visualization of ultrasound fields in a transparent medium with ultrasound attenuation using the focused shadowgraphy technique

Knowledge of ultrasound pressure fields of ultrasound inside the human body is essential to confirm the safety of medical ultrasound equipment. Typically, pressure waveforms in the body are estimated by correction of those measured in water considering typical attenuation in biological tissues. However, the correction of frequency-dependent attenuation of a wide band pulse is difficult because biological tissues have a non-linear transmission characteristic. In this study, acoustic fields of pulsed ultrasound propagating in a transparent medium with ultrasound attenuation were visualized using focused shadowgraphy (Kudo, Ultrasound Med. Biol. 41, 2071-2081, 2015). Pressure waveforms of the pulsed ultrasound were also measured using a membrane hydrophone. Brightness curves and their 1st-order integrations derived from the visualized field images were compared with the pressure waves. Experimental results obtained by using castor oil demonstrated that the 1st-order integrations of brightness curves are in b...