Low Megahertz Frequency Research Articles

Synthesis of a digitally controlled impedance element.

A practical design for the synthesis of a digitally controlled electrical impedance element is presented. The impedance element comprises a real impedance element in series with a voltage source whose magnitude is determined by the applied voltage multiplied by a factor k. The value of k is shown strongly to affect the circuit's performance. Results are presented which demonstrate the correspondence between circuit models and practical measurements. When negative values for k were employed the circuit element offered a controlled impedance range of 1:1000 and was stable to at least 1.5 MHz, providing that low source impedance values were used. With a positive k, a restricted range of impedance values could be obtained and the value of source impedance was less critical, though the circuit's performance was acceptable only to about 100 kHz. Consideration is given to the specification of a multiplier that would permit the circuit's range of application to be extended to low megahertz frequencies.

Ultrasonic characterisation of inhomogeneous materials: some problems with measurements of forward scattered fields.

Measurements of the complex pressure fields transmitted by scattering specimens consisting of glass beads in silicone rubber and of in vitro samples of ox myocardium, liver and kidneys have been carried out by point-by-point mapping of the amplitude and phase of the fields, using a new design of PVDF needle hydrophone in a high precision scanning tank. The results show that disruption of the phase fronts by the specimens is small and the measured values depend on the choice of measurement distance and frequency, and spatial sampling interval. As a result, there is little difference between phase sensitive and phase insensitive measurements at frequencies below 5 MHz. It is therefore unlikely that the proposed method could be applied successfully to the characterisation of such materials at low megahertz frequencies.

A conceptual model for acoustic microcavitation

A conceptual model is developed to explain microcavitation events at low megahertz frequencies and low duty cycles, facilitated by smooth, spherical, submicrometer sized particles in clean water. The extremely high levels of accelerations associated with high-frequency acoustic fields, the active detector field, can, according to this model, ‘‘coax’’ such smooth particles to nucleate cavitation events at reduced thresholds. This conceptual model should help further experimentation exploring the phenomenon of ‘‘acoustic coaxing’’ for microcavitation.

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.

Quadrupolar spectroscopy of nematic liquid crystals.

Quadrupolar spectroscopy of weakly polar nematic liquid crystals shows temperature-dependent Debye relaxation at low megahertz frequencies. The quadrupolar technique, which is similar to dielectric spectroscopy but which probes the motions of electric quadrupoles, reveals details of the single-particle reorientation potential which complement information from dielectric studies. In particular, the results are consistent with a nematic potential which, unlike the simplest mean-field theory of the nematic state, includes higher-order terms and a strong density dependence.

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.

Linear thermocouple arrays for in vivo observation of ultrasonic hyperthermia fields.

Interest in localised hyperthermia for cancer therapy and other treatments continues to increase (10th L. H. Gray Conference, 1981). Ultrasound, operated in the low megahertz frequency region and at exposure intensities in the neighbourhood of 1 W/cm2, has emerged as an accurate, easily managed way of producing elevated temperatures both superficially and at appreciable depths in human tissues and organs (Hunt, 1982). The ubiquitous thermocouple remains the most useful thermal detector in this field and, as it can be made very small in comparison with an acoustic wavelength (viz. about 1.5 mm at 1 MHz), may be employed without significantly altering the incident radiation field. We describe in this note the manufacture of thermocouple arrays useful for measuring temperature fields to appreciable depths in tissue in large mammalian subjects. The probe arrays we have used to date have involved up to five junctions, inserted to a depth of 4 cm in the thigh muscle of 300 kg pigs (ter Haar & Hopewell, 1982). T...

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.

Ultrasonic attenuation and absorption in liver tissue.

A large range of values for ultrasonic attenuation and absorption coefficients of tissues are reported in the literature. An important distinction both practically and theoretically is the magnitude of the true absorption, which characterizes the rate of conversion of ultrasonic to thermal energy, as compared with the total attenuation of the ultrasonic signal as it propagates through tissue. The magnitudes of these quantities were studied in bovine liver. Total attenuation was measured, in the range of 1-6 MHz, by both phase sensitive and phase insensitive insertion loss techniques. Ultrasonic absorption was determined by two thermal techniques. The standard "transient thermoelectric" or rate-of-heating method, and a new measurement technique based on the temperature decay following a short ultrasonic pulse were employed for the determination of the ultrasonic absorption coefficient. The results demonstrate that the ultrasonic amplitude attenuation and absorption coefficients at low megahertz frequencies are not significantly different in liver. The mean values cluster around 0.05 nepers/cm/MHz (0.4dB/cm/MHz). The sample-to-sample variation is indicated by the standard deviation in the measurements of 0.01 nepers/cm/MHz (0.09dB/cm/MHz) or less. The results show that in liver tissue, absorption is the dominant feature of attenuation over this frequency range.

Two miniature ceramic ultrasonic probes

The design, construction and performance of two types of miniature ceramic hydrophones are described, which are intended for acoustic pressure measurements in liquids at low megahertz frequencies. One of the devices has a 1.6 mm diameter cylindrical element with good directional characteristics and a sensitivity of approximately -224 dB re 1 V mu Pa-1 up to 0.7 MHz. The other probe uses a 0.5 mm disc showing a sensitivity of -245 dB re 1 V mu Pa-1 from 0.5 to 6 MHz and diminishing uniformity of directional response with increasing frequency. The stability of the probes is also discussed.

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.]

Acoustic elliptic lenses: An historical note

The early employment of acoustic elliptic lenses in the low megahertz frequency region is recounted briefly.

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.

Ultrasonic field parameters measured by optical interferometry

The use of optical interferometry for the measurement of ultrasonic field parameters was demonstrated by Mezrich et al. [RCA Rev. 35, (December 1974)] and applications have been reported by others [M. E. Haran and H. F. Stewart “Optical interferometric measurements of ultrasonic radiation and its applications to medicine,” NBS SP456 (1976)]. This system, known as the ultrasonovision, has recently been interfaced to a Hewlett—Packard 21MX minicomputer. The result is a decrease by a factor of twenty in the time required to analyze each frame of data. In addition, a method has been developed to make the system self-correcting in that its performance is unaffected by variations in the light level, slight optical misalignment, or variations in reflectivity of the pellicle membrane. Details of the interface and self-correction method are given. Present studies with this system include mapping the field emitted by plane and focused transducers of low megahertz frequencies as a function of distance from the source.