The acoustic safety of new ultrasound technologies

Abstract

There is evidence from surveys carried out in the 1990s that improvements in ultrasound technology have been associated with increases in acoustic output levels. Current medical devices regulations in Europe and the USA require manufacturers to declare the acoustic output levels from their equipment. This paper analyses such declarations to identify recent trends in acoustic output levels. Comparison of declared values with the earlier surveys showed a 50% increase in mean rarefaction pressures in B-mode and colour Doppler mode and a 75% increase in pulsed Doppler mode. In B-mode, the mean temporal average intensity level has approximately doubled, but approximately halved in pulsed Doppler mode. Analysis of mechanical and thermal safety index values (MI and TI) suggested that some manufacturers are designing their ultrasound systems to be capable of using the highest MI values permitted by the Food and Drugs Administration (FDA). This results in the use of higher acoustic pressures at higher frequencies. Most maximum values of TI declared to the FDA were much lower than the normal maximum value of 6.0. Increases in acoustic pressure and intensity levels would be expected to occur as transducer efficiency and beam-forming methods are improved and new techniques introduced. It is likely that future developments will continue to push typical output levels towards the regulatory limits. Users must maintain awareness of acoustic safety issues and make use of on-screen safety indices to ensure appropriate use of imaging modes that use the highest output levels.

Ultraschall Emissionen: Thermische (TI) und mechanische (MI) Indizes

“…The trend towards increasing values of mean pressure and intensity (Fig.2a,b) has continued for modern diagnostic equipment [4], which means that MI and TI values are also rising to the upper end within the FDA output limits. The potential risk of causing thermal effects in sensitive organs and other tissue, especially during fetal scanning, increases linearly with exposure time, but exponentially with temperature.…”

“…The TI has thus been deliberately defined to be without units and has a maximum allowed value of 6 [2,4,5]. The equations have evolved from calculations giving estimates of worst-case average values and should not be interpreted as the numerical value of heating in degrees Centigrade within insonated tissues.…”

Ultrasound in the First Trimester and Earlier: How to Keep It Safe

“…Based on these considerations, some recommend extreme caution when employing Doppler in the first trimester [149]. Furthermore, acoustic outputs, as published by the various ultrasound instruments manufacturers may not always be adequate [150] and an additional cause for concern is the increase in instruments outputs over the years [151]. Despite this, as detailed above, in recent years there has been a major recrudescence in the usage of Doppler in very early pregnancy.…”

Ultrasonic imaging: safety considerations

“…In a recent study, Martin [3] conducted a survey of five major ultrasound manufacturers, asking them for output data from current ultrasound systems. [24].…”

“…None of these were colour flow or pulsed Doppler examinations, and B-mode outputs have increased by more than an order of magnitude over the intervening years [3]. We therefore cannot be complacent, and must make sure that good practice is continued to maintain ultrasound imaging's, to date, excellent safety record.…”