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Acoustic saturation and output regulation.

¹Medical Physics Department, Royal United Hospital, Bath, UK. f.duck@bath.ac.uk

Ultrasound in Medicine & Biology

|August 26, 1999

Summary

Acoustic saturation in water can prevent diagnostic ultrasound equipment from exceeding U.S. Food and Drug Administration (FDA) limits for mechanical index (MI) and intensity, especially at higher frequencies and deeper focal depths.

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Area of Science:

Acoustics and Ultrasound Physics
Biomedical Engineering
Medical Imaging Safety

Background:

Diagnostic ultrasound devices utilize acoustic energy, necessitating safety regulations to prevent potential bioeffects.
Regulatory bodies like the FDA and IEC establish output limits for ultrasound equipment.
Acoustic saturation is a phenomenon that can limit the maximum achievable acoustic pressure in a medium.

Purpose of the Study:

To predict acoustic saturation pressures for ultrasonic beams across various frequencies and focal depths.
To calculate saturation values for mechanical index (MI) and derated intensities.
To compare these saturation values with existing regulatory thresholds set by the FDA and IEC.

Main Methods:

Approximation methods were used to predict acoustic saturation.
Calculations were performed for mechanical index (MI) and derated spatial-peak, time-average and pulse-average intensities.
Computed saturation values were compared against FDA and IEC regulatory thresholds.

Main Results:

Acoustic saturation in water limits MI and regulated intensities, preventing them from exceeding FDA thresholds under specific conditions.
These limiting conditions are more prevalent at higher ultrasound frequencies and deeper focal lengths.
IEC 61157 action thresholds are low enough that acoustic saturation does not pose a similar limitation.

Conclusions:

Current regulations may not fully limit the acoustic output of all diagnostic ultrasound equipment.
Acoustic saturation in water can create conditions where output is not subject to control, particularly at higher frequencies and depths.
There is a need to re-evaluate regulatory effectiveness in light of acoustic saturation phenomena.

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