Because bioeffects, some of which are harmful, may be caused by ultrasound under certain exposure conditions, there is a hypothetical possibility that ultrasonic imaging may not be completely safe (Wells 1986). Moreover, the ultrasonic exposure levels used by commercially-available scanners have been steadily increasing, in order to obtain more information (Duck and Martin 1991). Consequently, both regulatory authorities and prudent clinicians take the subject seriously.

The World Federation for Ultrasound in Medicine and Biology (WFUMB 1992) has published the following statements on thermal effects in clinical applications.

B-mode imaging: known diagnostic ultrasound equipment as used today for simple B-mode imaging operates at acoustic outputs that are not capable of producing harmful temperature rises. Its use in medicine is therefore not contraindicated on thermal grounds. This includes endoscopic, transvaginal and transcutaneous applications.

Doppler: it has been demonstrated in experiments with unperfused tissue that some Doppler diagnostic equipment has the potential to produce biologically significant temperature rises, specifically at bone-soft tissue interfaces. The effects of elevated temperatures may be minimized by keeping the time for which the beam passes through any one point in tissue as short as possible. Where output power can be controlled, the lowest available power level consistent with obtaining the desired diagnostic information should be used.

Although the data on humans are sparse, it is clear from animal studies that exposures resulting in temperatures less than 38.5 ° C can be used without reservation on thermal grounds. This includes obstetric applications.

Transducer heating: a substantial source of heating may be the transducer itself.

Tissue heating from this source is localized to the volume in contact with the transducer.

The possibility that nonthermal effects of ultrasound may be hazardous in some situations is more contentious (Barnett et al 1994). Cavitation, defined as the formation or activity of gas- or vapour-filled cavities (bubbles) in a medium exposed to an ultrasonic field, is the phenomenon of most concern. Other possible nonthermal mechanisms include radiation force, acoustic torque and acoustic streaming.

Many questions relating to the safety of the ultrasonic exposures used for imaging remain to be answered. For example, is there a linear relationship between the quantity of image information and the ultrasonic energy needed to obtain it? The thermal effect of a given ultrasonic power may be independent of the exposure duty cycle, but what are the nonthermal effects under different regimes? Pragmatically, users of ultrasound for diagnosis should apply the ALARA (‘as low as reasonably achievable’) principle to the exposures to which they subject their patients. The exposure levels should be at the lowest intensities and for the briefest times necessary to obtain diagnostically-adequate images. To assist in achieving this goal, a predictor of cavitation known as the mechanical index (MI) has been developed (AIUM/NEMA 1992), given by the expression MI = Pr ¦ 3(zsp)/c1/2, (23)

where Pr ¦ 3(zsp) is the peak rarefactional pressure (MPa) derated by 0.3 dB cm-1 MHz-1 to the point on the beam axis (zsp) where the pulse intensity integral is maximum, and /c is the centre frequency (MHz). Some modern scanners display the MI value on the screen, so the operator can aim to minimize it.

Although it is right to be concerned about exposure conditions, it is the consequences of misdiagnosis that are likely to be the greatest hazard of an ultrasonic investigation (Wells 1986). Other risks must not be ignored, but they must be viewed in the proper perspective.