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.