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Non-linear wave propagation and safety standards for diagnostic ultrasound devices

Overview This review paper discusses the safety standards for clinical diagnostic ultrasound devices developed for scanners which predominantly operate in 1-10MHz. In neglecting the effects caused by nonlinear propagation the authors suggest that significant underestimation in MI values are possible. Additionally at even higher frequencies (up to 50MHz), greater errors for thermal indices may be observed as the indices do not take into consideration energy loss caused by absorption of higher harmonics.


Authors Jafarzadeh E and Sinclair AN


Journal Ultrasound in Medicine and Biology 2019: 45(1):11-20
Recommendation/Comment Relevant for high frequency applications.
Clinical implication At higher frequencies, the thermal and mechanical indices may underestimate the true thermal and mechanical effects.
Link (DOI) https://doi.org/10.1016/j.ultrasmedbio.2018.08.021
Ultrasound speciality Physics and US equipment; Quality and Safety issues


Safety standards for clinical diagnostic ultrasonic devices were developed for use in relatively low-frequency systems (1–10 MHz), under the assumption that non-linear effects would be negligible. This article reviews ways in which neglecting non-linear wave propagation affects the measurements and calculations required to comply with safety standards and U.S. Food and Drug Administration guidance that recognizes these standards. An attempt is made to evaluate whether ignoring non-linear effects could result in significant error in the exposure quantities defined in these standards at either low or high frequencies, based on published literature. This article maintains that although non-linear effects have been considered in some parts of safety standards related to hydrophone requirements, the coverage is inadequate, especially for modern equipment with high working frequencies. A new approach is required to assess the magnitude of thermal heating for recently developed high-frequency systems to incorporate non-linear effects. In contrast, the current approach for evaluating the risk of cavitation can be used after appropriate modifications.


Ultrasound imaging, Safety standards, Non-linear acoustics, Thermal index, Mechanical index.