Bruce A. Herman

Models and regulatory considerations for transient temperature rise during diagnostic ultrasound pulses

A new diagnostic ultrasound (US) technique, sometimes called radiation force imaging, produces and detects motion in solid tissue or acoustic streaming in fluids via a high-intensity beam. Current models for estimating temperature rise during US exposure calculate the steady-state rise, using time-averaged acoustic output, as the worst case for safety consideration. Although valid for very short pulses, this analysis might not correspond to a worst-case scenario for the longer pulses or pulse bursts, up to hundreds of ms, used by this newer method. Models are presented to calculate the transient temperature rise from these pulse bursts for both the bone at focus and soft tissue situation. It is shown, based on accepted time-temperature dose criteria, that, for the bone at focus case and pulse lengths and intensities utilized by these methods, temperature may increase to levels that raise safety concerns. Also, regulatory aspects of this modality are analyzed in terms of the current FDA acoustic output limits for diagnostic US devices.

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.

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.

Interlaboratory comparison of ultrasonic attenuation and speed measurements

A set of test samples, all containing ultrasonically equivalent tissue‐mimicking material, was produced and measurements of ultrasonic speed and ultrasonic attenuation coefficients were made at seven laboratories using various techniques. The ultrasonic speed values agree well with one another, having a spread of about 0.3 per cent; thus, speed values for tissue parenchyma appearing in the literature are likely to be accurate. Values of ultrasonic attenuation coefficients agree fairly well with one another, with differences between individual values and the group mean of generally less than 20 per cent of the group mean.

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.

The Thermal Index Its Strengths, Weaknesses, and Proposed Improvements

The thermal index (TI) has been used as a relative indicator of thermal risk during diagnostic ultrasound examinations for many years. It is useful in providing feedback to the clinician or sonographer, allowing assessment of relative, potential risks to the patient of an adverse effect due to a thermal mechanism. Recently, several shortcomings of the TI formulations in quantifying the risk to the patient have been identified by members of the basic scientific community, and possible improvements to address these shortcomings have been proposed. For this reason, the Output Standards Subcommittee of the American Institute of Ultrasound in Medicine convened a subcommittee to review the strengths of the TI formulations as well as their weaknesses and proposed improvements. This article summarizes the findings of this subcommittee. After a careful review of the literature and an assessment of the cost of updating the TI formulations while maximizing the quality of patient care, the Output Standards Subcommittee makes the following recommendations: (1) some inconsistencies in the current TI formulations should be resolved, and the break point distance should be redefined to take focusing into consideration; (2) an entirely new indicator of thermal risk that incorporates the time dependence not be implemented at this time but be included in continuing efforts toward standards or consensus documents; (3) the exponential dependence of risk on temperature not be incorporated into a new definition of the TI formulations at this time but be included in continuing efforts toward standards or consensus documents; (4) the TI formulations not be altered to include nonlinear propagation at this time but be included in continuing efforts toward standards or consensus documents; and (5) a new indicator for risk from thermal mechanisms should be developed, distinct from the traditional TI formulations, for new imaging modalities such as acoustic radiation force impulse imaging, which have more complicated pulsing sequences than traditional imaging.

Development of a HIFU Phantom

The field of high intensity focused ultrasound (HIFU) is developing rapidly. For basic research, quality control, and regulatory assessment a reusable phantom that has both thermal and acoustic properties close to that of soft tissue is critical. A hydrogel‐based tissue mimicking material (TMM) has been developed that shows promise for such a phantom. The acoustic attenuation, speed of sound, B/A, thermal diffusivity and conductivity, as well as the cavitation threshold, were measured and found to mimic published values for soft tissue. The attenuation of 0.53f1.04 from 1 MHz to 8 MHz, as well as the sound speed of 1565 m/s and the tissue‐like image quality, indicate the usefulness of the TMM for ultrasound imaging applications. These properties along with the thermal conductivity of 0.58 W/m‐ °C, diffusivity of 0.15 (mm2)/s, and the ability to withstand temperatures above 95 °C make this material appropriate for HIFU applications. The TMM also allows for the embedding of thermocouples and the formation o...

Theoretical study of steady-state temperature rise within the eye due to ultrasound insonation

The soft tissue thermal index (TIS), as defined in the AIUM/NEMA Output Display Standard, may not be relevant with respect to eye exposure, primarily because of differences in actual vs. assumed acoustic and thermal properties. Therefore, a theoretical study of temperature rise within the eye due to ultrasound insonation was undertaken to compare the TIS with more exact calculations. At each plane in the direction of propagation, the focused ultrasound beam was modeled as a disc of uniform intensity. Each disc becomes a heat source, and integration over all discs provides the total temperature rise at any axial position. Calculations were done assuming the ultrasound beam intersects the lens of the eye as well as for the case in which the beam does not intersect the lens. Results were found for frequencies of 7.0 MHZ to 40 MHZ, transducer diameters of 0.2 cm to 1.0 cm, and focal lengths ranging from 0.2 cm to 3.0 cm. Perfusion was assumed negligible and thermal and acoustic parameters were taken from reported studies. For every case, the ratio of maximum temperature rise to the TIS (assuming constant output power) was calculated. For the lens case, the ratio varied from 7.35 to 0.8. For the no-lens case, the ratio varied from 4.1 to 0.4. These results indicate that the TIS is not adequate to represent the temperature rise occurring within the eye upon insonation.