Gerald R. Harris

Review of transient field theory for a baffled planar piston

The theoretical approaches which have been used to study the velocity potential and pressure fields radiated by a planar piston source vibrating in an infinite rigid baffle are broadly reviewed and discussed with emphasis on the basic mathematical methods employed. Attention is focused on those aspects of piston theory directly related to the point or spatially averaged transient field of a pulsed piston radiator. Studies of the nonuniformly vibrating piston are also reviewed. Much of this material has not been previously compared, so where possible common origins and general relationships are stressed.

Transient field of a baffled planar piston having an arbitrary vibration amplitude distribution

A theoretical model is presented for evaluating the transient velocity potential and pressure fields radiated by a pulsed planar piston set in an infinite rigid baffle and having an arbitrary spatial velocity distribution. The method is based on the development of a generalized spatial impulse response function which describes the velocity potential, either at a point or averaged over a finite receiving surface, resulting from the velocity impulse excitation of a uniformly or nonuniformly vibrating piston source. For the specific case of a circular source and receiver this function is plotted using several source distributions and receiver sizes. Then corresponding plots are presented for two piston velocity waveforms: a Gaussian pulse and a Gaussian‐modulated sinusoidal burst. These plots are compared and the effects of a nonuniformly vibrating source and finite receiver are discussed. Lastly, a previous solution by Schoch, in which the field of a rigid piston is separated into geometrical and boundary d...

American Institute of Ultrasound in Medicine consensus report on potential bioeffects of diagnostic ultrasound: Executive summary

The continued examination of potential biological effects of ultrasound and their relationship to clinical practice is a key element in evaluating the safety of diagnostic ultrasound. Periodically, the American Institute of Ultrasound in Medicine (AIUM) sponsors conferences bringing experts together to examine the literature on ultrasound bioeffects and to develop conclusions and recommendations related to diagnostic ultrasound. The most recent effort included the examination of effects whose origins were thermal or nonthermal, with separate evaluations for potential effects related to fetal ultrasound. In addition, potential effects due to the introduction of ultrasound contrast agents were summarized. This information can be used to assess risks in comparison to the benefits of diagnostic ultrasound. The conclusions and recommendations are organized into 5 broad categories, with a comprehensive background and evaluation of each topic provided in the corresponding articles in this issue. The following summary is not meant as a substitute for the detailed examination of issues presented in each of the articles but rather as a means to facilitate further study of this consensus report and implementation of its recommendations. The conclusions and recommendations are the result of several rounds of deliberations at the consensus conference, subsequent review by the Bioeffects Committee of the AIUM, and approval by the AIUM Board of Governors.

Acoustic power calibration of high-intensity focused ultrasound transducers using a radiation force technique

To address the challenges associated with measuring the ultrasonic power from high-intensity focused ultrasound transducers via radiation force, a technique based on pulsed measurements was developed and analyzed. Two focused ultrasound transducers were characterized in terms of an effective duty factor, which was then used to calculate the power during the pulse at high applied power levels. Two absorbing target designs were used, and both gave comparable results and displayed no damage and minimal temperature rise if placed near the transducer and away from the focus. The method yielded reproducible results up to the maximum pulse power generated of approximately 230 W, thus allowing the radiated power to be calibrated in terms of the peak-to-peak voltage applied to the transducer.

Angular response of miniature ultrasonic hydrophones

The voltage response of ceramic and polyvinylidene fluoride (PVDF) hydrophones was measured in the receive mode for angles of incidence ranging from 0 degrees to 90 degrees. The measurements were performed at 2.5, 3.5, 5.0, and 8.0 MHz; these frequencies are typical of those used in medical diagnosis. The results are compared to three theoretical models based on diffraction theory; correlation between the measured response and theoretical models is evident for some PVDF hydrophones but not for others, and not for any ceramic hydrophone. The effective radius, as defined in the AIUM-NEMA standard for diagnosis ultrasound, is calculated and compared to the test criteria established in that standard. All of the ceramic hydrophones and two of the five PVDF hydrophones failed to meet the criteria.

The Risk of Exposure to Diagnostic Ultrasound in Postnatal Subjects Thermal Effects

This review evaluates the thermal mechanism for ultrasound‐induced biological effects in postnatal subjects. The focus is the evaluation of damage versus temperature increase. A view of ultrasound‐induced temperature increase is presented, based on thermodynamic Arrhenius analyses. The hyperthermia and other literature revealed data that allowed for an estimate of a temperature increase threshold of tissue damage for very short exposure times. This evaluation yielded an exposure time extension of the 1997 American Institute of Ultrasound in Medicine Conclusions Regarding Heat statement (American Institute of Ultrasound in Medicine, Laurel, MD) to 0.1 second for nonfetal tissue, where, at this exposure time, the temperature increase threshold of tissue damage was estimated to be about 18°C. The output display standard was also evaluated for soft tissue and bone cases, and it was concluded that the current thermal indices could be improved to reduce the deviations and scatter of computed maximum temperature rises.

Calibration of miniature ultrasonic receivers using a planar scanning technique

This paper discusses a method currently used to calibrate miniature ultrasonic receivers in the 1‐ to 10‐MHz frequency range. The method compares the power radiated by a source transducer to the integrated intensity obtained by scanning the receiver across the ultrasound field in a series of linear parallel lines. Sources of error are presented and theoretical models and experimental data are analyzed to estimate the maximum uncertainty associated with this technique, as used by the Bureau of Radiological Health. Our calculations estimate the accuracy of the intensity calibration constants determined to be within +17%, −23% of the true value. With refinement of this technique an uncertainty within +14%, −13% should be achievable.

Progress in medical ultrasound exposimetry

Biomedical applications of ultrasound have experienced tremendous growth over the past 50 years. Early work in thermal therapy and surgery soon was followed by diagnostic imaging and Doppler. Because patient safety was an important issue from the beginning, the study of methods for measuring exposure levels, and their relationship to possible biological effects, paralleled the growth of the various therapeutic and diagnostic techniques. The diverse conditions of use have presented a range of exposure measurement challenges, and the sensors and techniques used to evaluate ultrasound fields have had to evolve as new or expanded clinical applications have emerged. In this paper some of the more notable of these developments are presented and discussed. Topics covered include devices and techniques, methods of calibration, progress in standardization, and current problem areas, including the effects of nonlinear propagation. Some early methods are described, but emphasis is given to more recent work applicable to present and future uses of ultrasound in medicine and biology.

Acoustically transparent hydrophone probe

An acoustically transparent hydrophone probe consisting of a rigid hoop structure in which is secured an assembly of very thin piezoelectric polymer sheet material, such as polyvinylidene fluoride, with one or more very small central sensitive portions. In its simplest form it consists of a single sheet with a small central poled piezoelectric area and with very thin metallic electrodes deposited on the sheet on opposite sides of the piezoelectric area and having fine conductive leads extending from the electrodes and adapted to be connected to a suitable amplifier or transmission line. The sheet is of biaxially stretched material, and is held taut in the hoop structure. Other embodiments may employ multiple sheets. With two sheets, the outermost surfaces are metallized and are at common ground potential, and the inner surfaces have superimposed deposited metallic electrodes with poled piezoelectric areas adjacent thereto. The electrodes have superimposed deposited metallic leads which have a common electrical connection to a transmission line or to an amplifier. With four sheets, the two innermost sheets form a bilaminate subassembly similar to the 2-sheet embodiment. The outer sheets have metallized outer surfaces which are grounded. Between said outer sheets and the bilaminate subassembly, guard rings coaxial with the piezoelectric active areas are provided. The guard rings can be driven electrically in a manner to eliminate the effects of the capacitance of the electrical leads.

Comparative study of temperature measurements in ex vivo swine muscle and a tissue-mimicking material during high intensity focused ultrasound exposures

Tissue-mimicking materials (TMMs) can provide a convenient, stable, and reproducible means for testing high intensity focused ultrasound (HIFU) devices. When TMMs containing thermal sensors are used to measure ultrasound-induced temperature rise, it is important that measurement results reasonably represent those that occur in biological tissue. Therefore the aim of this paper is to compare the thermal behavior of the TMM under HIFU exposure to that of ex vivo tissue. This was accomplished using both a previously developed TMM and fresh ex vivo swine muscle that were instrumented with bare 50 µm thin wire thermocouples. HIFU at 825 kHz was focused at the thermocouple junction. 30 s exposures of increasing peak negative pressure (1 to 5 MPa) were applied and the temperature profile during and after sonication was recorded. B-mode imaging was used to monitor bubble activity during sonication. If bubble formation was noted during the sonication, the sonication was repeated at the same pressure levels two more times at 20 min intervals. Temperature traces obtained at various pressure levels demonstrated similar types of heating profiles in both the tissue and TMM, the exact nature of which depended on whether bubbles formed during the HIFU exposure. The onset of bubble activity occurred at lower ultrasonic pressures in the TMM, but the basic temperature rise features due to HIFU exposure were essentially the same for both materials.