Ultrasonic biophysics is the study of mechanisms responsible for how ultrasound and biological materials interact. Ultrasound-induced bioeffect or risk studies focus on issues related to the effects of ultrasound on biological materials. On the other hand, when biological materials affect the ultrasonic wave, this can be viewed as the basis for diagnostic ultrasound. Thus, an understanding of the interaction of ultrasound with tissue provides the scientific basis for image production and risk assessment. Relative to the bioeffect or risk studies, that is, the biophysical mechanisms by which ultrasound affects biological materials, ultrasound-induced bioeffects are generally separated into thermal and non-thermal mechanisms. Ultrasonic dosimetry is concerned with the quantitative determination of ultrasonic energy interaction with biological materials. Whenever ultrasonic energy is propagated into an attenuating material such as tissue, the amplitude of the wave decreases with distance. This attenuation is due to either absorption or scattering. Absorption is a mechanism that represents that portion of ultrasonic wave that is converted into heat, and scattering can be thought of as that portion of the wave, which changes direction. Because the medium can absorb energy to produce heat, a temperature rise may occur as long as the rate of heat production is greater than the rate of heat removal. Current interest with thermally mediated ultrasound-induced bioeffects has focused on the thermal isoeffect concept. The non-thermal mechanism that has received the most attention is acoustically generated cavitation wherein ultrasonic energy by cavitation bubbles is concentrated. Acoustic cavitation, in a broad sense, refers to ultrasonically induced bubble activity occurring in a biological material that contains pre-existing gaseous inclusions. Cavitation-related mechanisms include radiation force, microstreaming, shock waves, free radicals, microjets and strain. It is more challenging to deduce the causes of mechanical effects in tissues that do not contain gas bodies. These ultrasonic biophysics mechanisms will be discussed in the context of diagnostic ultrasound exposure risk concerns.

Ultrasound—biophysics mechanisms

This review article provides a summary rationale and derivation for the mechanical index (MI) and thermal index (TI) formulas presented in the body of the "Standard for real-time display of thermal and mechanical acoustic output indices on diagnostic ultrasound equipment" (AIUM/NEMA 1998). For purposes of simplicity, this standard is referred to in this article by its colloquial "nickname," the "output display standard" or ODS. This review expands on the summary information provided in Appendix A of the ODS. Numerous references are made to the root publications from which the formulas were derived. As will be discussed in the derivation notes that follow, key parts of the MI and TI models rely heavily on experimental data. This document does not attempt to do any more than describe the relevant results of the experiments. A general overview is provided in the Introduction and Rationale for the nontechnical reader and the more rigorous Models and Derivation Notes provides a detailed mathematical review of the MI and TI models. A complete set of defined terms is provided in Appendix 1.

Rationale and derivation of MI and TI--a review.

Several laboratories are investigating the use of acoustic radiation force to image the mechanical properties of tissue. Acoustic Radiation Force Impulse (ARFI) imaging is one approach that rises brief, high-intensity, focused ultrasound pulses to generate radiation force in tissue. This radiation force generates tissue displacements that are tracked using conventional correlation-based ultrasound methods. The tissue response provides a mechanism to discern mechanical properties of the tissue. The acoustic energy that is absorbed by tissue generates radiation force and tissue heating. A finite element methods model of acoustic heating has been developed that models the thermal response of different tissues during short duration radiation force application. The beam sequences and focal configurations used during ARFI imaging are modeled herein; the results of these thermal models can be extended to the heating due to absorption associated with other radiation force-based imaging modalities. ARFI-induced thermal diffusivity patterns are functions of the transducer f-number, the tissue absorption, and the temporal and spatial spacing of adjacent ARFI interrogations. Cooling time constants are on the order of several seconds. Tissue displacement due to thermal expansion is negligible for ARFI imaging. Changes in sound speed due to temperature changes call be appreciable. These thermal models demonstrate that ARFI imaging of soft tissue is safe, although thermal response must be monitored when ARFI beam sequences are being developed.

On the thermal effects associated with radiation force imaging of soft tissue

Protection from inappropriate or hazardous exposure to ultrasound is controlled through international standards and national regulations IEC standard 60601 part 1 establishes requirements for the mechanical, electrical, chemical and thermal safety for all electro-medical equipment The associated part 2 standard for diagnostic medical ultrasonic equipment sets no upper limits on ultrasonic exposure Instead, safety indices are defined that are intended to advise users on the degree of thermal and mechanical hazard At present the display of these safety indices satisfies regulatory requirements in both the USA and Europe Nevertheless there are reservations about the effectiveness of this approach to protection management In the USA, there are national regulatory limits on diagnostic exposure, based on acoustic output from clinical equipment in use over 20 years ago The IEC 60601 part 2 standard for therapeutic equipment sets 3 W cm−2 as the limit on acoustic intensity Transducer surface temperature is controlled for both diagnostic and therapy devices For airborne ultrasound, interim guidelines on limits of human exposure published by the IRPA are now 2 decades old A limit on sound pressure level of 100 dB for the general population is recommended The absence of protection standards for infrasound relates to difficulties in measurement at these low frequencies

Medical and non-medical protection standards for ultrasound and infrasound

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.

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

An acoustic-thermal model was developed for scanned diagnostic ultrasound in soft tissue. An adiabatic surface between the transducer and the skin was justified, and the model accounted for attenuation and focusing. The temperature along the central plane of the temporally averaged acoustic field was calculated by integration of line sources of heat that result from the tissue's absorption of ultrasound. The temperature profiles were calculated for 1400 transducers. The results show that current diagnostic transducers heat more significantly at the transducer-tissue interface than at the focus. The temperature rise in the focal region is typically less than 25% of that at the surface. The acoustic power per scan length that results in a 1 degrees C temperature rise at the surface is calculated as (210 mW-MHz/cm)/f. These results apply to both linear arrays and sectorlike scan formats. The temperature rises for simultaneous multimode scanned beams are additive as the peak temperatures of each mode will occur on the surface. Consideration was given to the surface boundary condition for such models. This boundary is considered adiabatic for calculation of heating due to acoustic absorption alone. Additional heating or cooling resulting from the transducer can then be superimposed on this solution. >

M.G. Curley

Papers

Soft tissue temperature rise caused by scanned, diagnostic ultrasound