Low Megahertz Frequency Range Research Articles

A PVDF Receiver for Acoustic Monitoring of Microbubble-Mediated Ultrasound Brain Therapy.

The real-time monitoring of spectral characteristics of microbubble (MB) acoustic emissions permits the prediction of increases in blood-brain barrier (BBB) permeability and of tissue damage in MB-mediated focused ultrasound (FUS) brain therapy. Single-element passive cavitation detectors provide limited spatial information regarding MB activity, greatly affecting the performance of acoustic control. However, an array of receivers can be used to spatially map cavitation events and thus improve treatment control. The spectral content of the acoustic emissions provides additional information that can be correlated with the bio-effects, and wideband receivers can thus provide the most complete spectral information. Here, we develop a miniature polyvinylidene fluoride (PVDF thickness = 110 μm, active area = 1.2 mm2) broadband receiver for the acoustic monitoring of MBs. The receiver has superior sensitivity (2.36-3.87 V/MPa) to those of a commercial fibre-optic hydrophone in the low megahertz frequency range (0.51-5.4 MHz). The receiver also has a wide -6 dB acceptance angle (54 degrees at 1.1 MHz and 13 degrees at 5.4 MHz) and the ability to detect subharmonic and higher harmonic MB emissions in phantoms. The overall acoustic performance of this low-cost receiver indicates its suitability for the eventual use within an array for MB monitoring and mapping in preclinical studies.

Laser-generated focused ultrasound transmitters with frequency-tuned outputs over sub-10-MHz range

Previous laser-generated focused ultrasound (LGFU) systems have been operated with >15 MHz frequency, allowing for high spatial precision (<100 μm). However, they have been limited only to proximal biomedical applications ex vivo with treatment depths smaller than 10 mm from the lens surface. Although the low-megahertz frequency operation has the advantage of a longer range of therapy, this requires a proper photoacoustic lens made of a nanocomposite coating over a spherically curved substrate whose transmission layer is physically designed for frequency-tuned outputs. This demands a fabrication method that can provide such a nanocomposite structure. We demonstrate photoacoustic lenses operated in an unexplored frequency range of 1–10 MHz, which can simultaneously produce high-amplitude pressure outputs sufficient for pulsed acoustic cavitation. We physically design a spatially elongated photoacoustic output and then fabricate a transmitter by controlling the density of light-absorbing nanoscale elements in a solution form and by using a replica mold to shape the lens curvature. Our approach is validated by fabricating and characterizing planar transmitters and then applied to focal configurations. This offers various possibilities for LGFU-based treatments (e.g., pulsed cavitational therapy such as histotripsy) over the low-megahertz frequency range, which has not been realized by conventional LGFU systems.

A novel 6 × 6 element MEMS capacitive ultrasonic transducer with multiple moving membranes for high performance imaging applications

A 6×6 element multiple moving membrane capacitive micromachined ultrasonic transducer (M3-CMUT) array has been developed for air-coupled detection purposes in the low megahertz frequency range. Unlike conventional capacitive MEMS transducers, the developed transducer array employs a novel cell configuration where more than one flexible membrane contributes in the transducer signal transmission and detection. In this structure, a stack of two vibrating membranes is suspended over a fixed grounded bottom electrode. The transducer array has been fabricated through employing a MEMS sacrificial technique, with the top and middle membrane radii of 65 and 42μm, respectively. The electrical, optical and acoustic evaluation results of this transducer array are presented in this paper. It is shown that using this configuration, the amplitude of membrane displacement is increased which enhances the transducer power output and sensitivity for a given bias condition. The experimental results demonstrate that this transducer array structure can be highly beneficial in high performance imaging application systems.

Plane wave source with minimal harmonic distortion for investigating nonlinear acoustic properties

The objective of this investigation is to introduce and validate a practical ultrasound source to be used in the investigation of the nonlinear material properties of liquids and soft tissues studied in vitro. Methods based on the progressive distortion of finite amplitude ultrasonic waves in the low megahertz frequency range are most easily implemented under the assumption of plane wave propagation. However, achieving an approximately planar ultrasonic field over substantial propagation distances can be challenging. Furthermore, undesired harmonic distortion of the ultrasonic field prior to insonification of the specified region of interest represents another serious limitation. This paper introduces an approach based on the use of the ultrasonic field emanating from a stainless-steel delay line. Both simulation and direct experimental measurement demonstrate that such a field exhibits relatively planar wave fronts to a good approximation (such that a 3-mm-diam receiver would be exposed to no more than 3 dB of loss across its face) and is free from the significant harmonic distortion that would occur in a conventional water path.

High-temperature piezoelectric film ultrasonic transducers by a sol-gel spray technique and their application to process monitoring of polymer injection molding

Thick-film (90 /spl mu/m) piezoelectric ceramic high-temperature ultrasonic transducers (HTUTs) have been successfully deposited on metallic substrates by a sol-gel spray technique. The gel is composed of fine powders of bismuth titanate dispersed in a lead-zirconate-titanate solution. The films with desired thickness have been obtained through multilayer coating approach. Piezoelectricity is achieved using the corona discharge poling method. The center frequencies of ultrasonic signals generated by these HTUTs are around 10 MHz and their signal-to-noise ratio (SNR) is more than 30 dB in pulse-echo mode at 500/spl deg/C. The main advantages of these new HTUTs are that they 1) are applicable at temperatures higher than 500/spl deg/C, 2) are miniature, 3) can be coated on flat and curved surfaces, 4) do not need ultrasonic couplant, 5) can be operated at low and medium megahertz frequency range with sufficient frequency bandwidth, and 6) have sufficient piezoelectric strength and SNR. The ability of the HTUTs to monitor the polymer injection molding process in real-time at the mold insert of the machine is demonstrated.

Vertically Laminated Magnetic Cores by Electroplating Ni-Fe Into Micromachined Si

The fabrication and characterization of vertically laminated, electrodeposited Ni-Fe magnetic cores in micromachined silicon for the low megahertz frequency range is presented. Laminated cores were fabricated by etching vertical trenches (60-180 /spl mu/m wide and 525 /spl mu/m deep) through silicon wafers and directly plating Ni-Fe onto the etched sidewalls. The Ni-Fe was plated over a range of thicknesses (3-53 /spl mu/m) in order to study the influence of eddy currents. The measured impedances in the range 10 kHz-40 MHz confirm the reduction of eddy current losses. This work demonstrates a simple fabrication method for achieving high-aspect-ratio, vertically laminated magnetic cores where the geometry of the structures can be easily tailored for various applications.

Acoustic impedance matching of piezoelectric transducers to the air

The purpose of this work is threefold: to investigate material requirements to produce impedance matching layers for air-coupled piezoelectric transducers, to identify materials that meet these requirements, and to propose the best solution to produce air-coupled piezoelectric transducers for the low megahertz frequency range. Toward this end, design criteria for the matching layers and possible configurations are reviewed. Among the several factors that affect the efficiency of the matching layer, the importance of attenuation is pointed out. A standard characterization procedure is applied to a wide collection of candidate materials to produce matching layers. In particular, some types of filtration membranes are studied. From these results, the best materials are identified, and the better matching configuration is proposed. Four pairs of air-coupled piezoelectric transducers also are produced to illustrate the performance of the proposed solution. The lowest two-way insertion loss figure is -24 dB obtained at 0.45 MHz. This increases for higher frequency transducers up to -42 dB at 1.8 MHz and -50 at 2.25 MHz. Typical bandwidth is about 15-20%.

Microwave Dielectric Relaxation, Electrical Conductance, and Ultrasonic Relaxation of LiPF6 in Poly(ethylene oxide) Dimethyl Ether-500

Microwave dielectric complex permittivity ε* = ε‘ − Jε‘ ‘ in the frequency range ∼1−130 GHz and at temperatures of 25, 40, and 55 °C for the system LiPF6 dissolved in poly(ethylene oxide) dimethyl ether of average molar mass 500 (PEO-500) are reported. The dielectric spectra are interpreted by a Cole−Davidson distribution function for both the solvent and the solutions. The activation parameters ΔH⧧ and ΔS⧧ for the dielectric process have been determined for both solvent and solutions. Audio frequency electrical conductance of solutions of LiPF6 in PEO-500, in the concentration range ∼5 × 10-4 to 1.0 M (M = mol/dm3) and temperatures 25, 40, and 55 °C, have been determined. The conductance data in the diluted range have been analyzed by two forms of the Fuoss-Onsager conductance theory. Despite the presence of a minimum in the conductance vs concentration functions, the presence of conducting triple ions has not been postulated, because the minimum is predicted by the functional form of the conductance theory at low solvent permittivities, for moderate ion-pair associations. Ultrasonic relaxation spectra in the frequency range ∼1 to ∼400 MHz and LiPF6 concentration range 0.1−0.5 M at 25 °C are reported and analyzed in terms of the sum of two debye relaxation processes. The upper one centered around 150−200 MHz and common to the solvent PEO-500 is attributed to polymer-chain rearrangements, also influenced by the presence of the electrolyte. The lower relaxation process in the low megahertz frequency range is interpreted in terms of formation of dimers-ion pairs according to the process 2LiPF6 ⇌ (LiPF6)2, thus rationalizing the presence of a maximum in the static dielectric permitivity vs concentration.

Silicon-based ultrasonic microsensors and micropumps

Abstract With the addition of a thin piezoelectric zinc oxide film and transducing electrodes, a thin membrane of low-stress silicon nitride formed in a silicon die can be made into a versatile ultrasonic sensor and actuator. We will describe the fabrication and application of this device, which typically operates in the low megahertz frequency range and can perform many functions: measuring the viscosity of microliter volumes of liquids; measuring the densities of gases and liquids; serving, with the addition of a thin polymer film, as a sensor for measuring the concentrations of organic gases; determining, when immersed in a liquid, the mass per unit membrane area of adsorbed protein molecules, cells and bacteria; pumping air at velocitities of centimerters per second and pumping water at hundreds of microns per second; and stirring microliter volumes of fluids.

Applicators for generating ultrasound-induced hyperthermia in neoplastic tumours and for use in ultrasound physiotherapy

Ultrasound in the low megahertz frequency range is used by physiotherapists to treat a variety of conditions. The treatments reduce swelling, reduce pain and increase movement at joints. Ultrasound is now also beginning to be used to treat malignant tumours. The tumours are heated to temperatures between 42 degrees and 45 degrees C, producing cellular damage, the extent of which is determined by the duration and number of treatments and 'concurrent' treatments by chemo and X-ray therapy. In the light of the results of computer simulation, the authors discuss: (a) The properties of a five-element divergent transducer array to treat large superficial neoplastic tumours. The field distribution measured by the Sarvazyan method is described and also compared with that of computer simulation. The benefits of frequency wobbling are also discussed. (b) The possibility of employing a multiple field applicator for use by physiotherapists to obviate the need for continuous movement of a small transducer of area approximately 5 cm2 over the affected area is considered.

A multisensor employing an ultrasonic Lamb-wave oscillator

Initial experimental, analytical, and numerical evaluations of a microsensor that uses ultrasonic Lamb waves propagating in a thin plate supported by a silicon die are presented. Changes of oscillator frequency indicate magnitudes of the variables sensed. Because it is sensitive to many measurands, the device could operate as a microphone, biosensor, chemical vapor or gas detector, scale, pressure sensor, densitometer, radiometer, or thermometer. Because it is based on the use of Lamb waves, the sensor has selective response and sensitive operation in the low-megahertz frequency range in vacuum, in a gas, or while immersed in a liquid. >

The practical design of ultrasonic test tanks

A systematic approach to the design of ultrasonic test tanks is outlined in which criteria are given first for the choice of tank length and subsequently for the choice of tank width and height. Calculations are presented for typical transducer beam measurements in the low megahertz frequency range, indicating the individual and combined effects of side lobe inclusion and excitation pulse length. The methods used may be easily extended to determine, for a given situation, either the minimum tank size that needs to be used, or the limits imposed by an existing tank on the experiments that can be performed in it.

Evidence for free radical production by ultrasonic cavitation in biological media

The statement is often made that free radicals will be produced during the violent collapse of transient cavities in liquids, that may result from the passage of ultrasonic waves in the low megahertz frequency range. Evidence for such free radicals is mostly indirect. We have insonated some argon-saturated aqueous solutions of biological molecules (trypsin, Dulbecco's Minimum Essential Medium, fetal bovine serum, and human blood plasma) and whole human blood, all containing a spin trapping compound, at 1 MHz, continuous wave, 6.6 W/cm 2 (SPTA) for 15 min at 4–8°C. Electron spin resonance spectra of the samples were recorded 10 and 30 min. after cessation of insonation. The spectra included those of spin adducts formed with ·OH and ·H radicals, which were absent in the cases of the sham treated controls. Thus, we have demonstrated the formation of the highly reactive ·OH and ·H radicals in the insonated media, even in the presence of natural radical scavengers in the mammalian derived products.

An ultrasonic light diffraction technique for studying acoustic wave propagation in air above 1 MHz

Studies of ultrasonic light diffraction above 1 MHz in bulk media have normally been conducted in liquids and solids due to the difficulty of generating and propagating ultrasonic waves in gases. A sensitive broadband optical diffraction technique has been used to study the propagation of ultrasonic waves in air in the low megahertz frequency range. An ultrasonic beam at 1.4 MHz was produced in air by driving a medical ultrasound transducer (circular face, unfocused, 13-mm diameter) with a narrowband rf pulse having a peak potential amplitude of 300 V. The radiated waveform and power of approximately 30 mW was detected with a 1-mm-diam laser beam from approximately 2–30 mm from the transducer face. The fundamental frequency was found to experience an attenuation of −4.1 dB/cm over this range. A significant amount of the second harmonic was observed to be generated over this path length demonstrating the presence of nonlinear acoustical phenomena. This technique could be useful for studying the fundamental propagation characteristics of ultrasound in gases above 1 MHz and for the analysis of broadband acoustic pulses having significant frequency content above 200 kHz. In practical application, it should be possible to place an upper limit on the acoustic power levels radiated into air by medical diagnostic pulse-echo systems with this approach. [Work supported by NIGMS 27755.]

Linear dynamic range of an optical nearfield diffraction technique for studying ultrasonic waves

An optical nearfield diffraction technique was recently described for studying ultrasonic waveforms in the low megahertz frequency range [W. A. Riley, J. Acoust. Soc. Am. 67, 1386–1388 (1980)]. The theoretical optical amplitude response of this technique is linear with acoustic pressure under only a limited range of experimental conditions. The objective of this work was to experimentally determine the linear dynamic range of this optical system assembled from inexpensive components and used in conjunction with a good laboratory oscilloscope. A linear dynamic range of approximately 20 dB was determined from these results which corresponds to acoustical powers between 0.1 and 10 mW in distilled water. This 20‐dB window can be lowered approximately 10 dB by selecting an acousto‐optic medium more efficient than water. It can be raised by 40 dB by examining normal reflections from the boundary between liquids having small acoustic impedance differences. With this approach, one can effectively observe narrow‐band acoustic pulse waveforms over a 70‐dB dynamic range from approximately 10 μW to 100 W with a precision of approximately 1 dB.

The linear dynamic range of a simple optical nearfield diffraction technique for studying ultrasonic waves

An optical nearfield diffraction technique was recently described for studying ultrasonic waveforms in the low megahertz frequency range [W. A. Riley, J. Acoust. Soc. Am. 67, 1386–1388 (1980)]. The theoretical optical amplitude response of this technique is linear with acoustical pressure under only a limited range of experimental conditions. Higher harmonics of the fundamental ultrasonic frequency are also generated in the optical signal above this range. Application of this method to the calibration of transducers used in a variety of medical and industrial applications requires that careful consideration be given to the linear dynamic range of this approach. The objective of this work was to experimentally determine the practical linear dynamic range of an optical probe assembled from inexpensive components and used in conjunction with a good laboratory oscilloscope. Measurements were made on narrowband pulses transmitted by circular unfocussed transducers having fundamental frequencies of 2.5 MHz, 5.0 MHz, and 7.5 MHz. A linear dynamic range of approximately 20 dB was determined from these results. This corresponds to acoustical powers between approximately 0.1 and 10 mW in distilled water. The existence of random intensity fluctuations in the megahertz frequency range in the output of the helium-neon laser source was the limiting factor at the lower end of this range. The nonlinearity of the optical nearfield diffraction pattern produced the upper limit. This 20-dB window may be lowered approximately 10 dB by selecting a more efficient acousto-optic medium than water. It may effectively be raised by at least 40 dB by examining normal reflections from the boundary of liquids having small acoustic impedance differences. Consequently, one can effectively observe acoustic pulses over a 70-dB range with this approach. [Work supported by grant GM27755.]

Ultrasonics in medical diagnosis

Beginning with the wave equation, the characteristics of ultrasound are described in terms of propagation, reflexion, beam formation, scattering and attenuation. In medical diagnosis, ultrasound in the low megahertz frequency range is both generated and detected by piezoelectric transducers. At these frequencies the wavelength is in the order of 1 m m ; this is one of the fundamental limitations of attainable resolution. Because attenuation increases with frequency, it is necessary to compromise between resolution and required penetration. The time delays between pulse transmission and echo reception are proportional to the distances between the transducer and the reflectors along the ultrasonic beam. The echo amplitudes are controlled both by the characteristics of the reflectors, and by attenuation in the intervening media. The pulse-echo method is used to produce A-scan, B-scan and C-scan displays. At present the B-scan is the most important. It is used for the study of structure motion by time-position recording, and for tomography by two-dimensional scanning. Grey-scale display gives clues about tissue characteristics. Manually operated, automatic, and very rapid (real-time) two-dimensional scanners are in clinical use, especially in obstetrics and gynaecology, internal medicine and cardiology. The Doppler frequency-shift method is used to detect the movements of structures (especially the foetal heart) and to measure blood flow velocities. Analysis of Doppler blood-flow signals gives data on vessel characteristics. Pulsed Doppler systems can be used to estimate both velocity and position. Newer methods include phase compensation for the distortion otherwise introduced by the skull in brain scanning, tissue characterization, computerized tomography, Doppler blood flow transfer function analysis and tumour detection.

Effect of phase cancellation artifact on frequency dependence of ultrasonic attenuation measurements

It is well known that the ultrasonic attenuation coefficient of mammalian tissues, in the low megahertz frequency range, varies approximately linearly with frequency for all measurement methods (recent work has shown that the absorption coefficient of these tissues also varies nearly linearly with frequency). However, the magnitude of the measured attenuation may vary by as much as a factor of three with the measurement technique. In particular it has been shown that measurements with phase sensitive receivers, which are subject to phase cancellation due to an inhomogeneous velocity distribution within the tissue yield higher values than methods utilizing phase insensitive receivers. The frequency dependence of the phase cancellation artifact depends upon the frequency range examined, the range of velocity inhomogeneity, the sample thickness and the receiver aperture.

The observation of pulsed ultrasonic gratings in the optical farfield

An optical method is described for studying acoustical pulses as short as two or three cycles in duration in the low-megahertz frequency range. A collimated light beam having a width of one ultrasound wavelength is observed in the optical farfield of the ultrasonic grating midway between the maxima of the central and first diffraction orders. A close approximation to both the amplitude and phase of the acoustical pressure along the light path is obtained. This method is proposed as a simple calibration procedure for recording the transmit pulse characteristics of medical ultrasound systems.

The swept frequency acoustic resonant interferometer: measurement of acoustic dispersion parameters in the low megahertz frequency range

The general principles of operation of the swept frequency acoustic resonant interferometer are outlined and its use for measurements of energy loss and velocity dispersion in the low megahertz frequency range discussed. A number of different variations on the design of the resonator are described. Data on a variety of chemically different systems are presented illustrating the range and versatility of the technique for measurement of samples with low acoustic losses.