Key point: Authors relate that limit not based on safety considerations.

The Risk of Exposure to Diagnostic Ultrasound in Postnatal Subjects – Thermal Effects

William D. O’Brien, Jr, PhD, Cheri X. Deng, PhD, Gerald R. Harris, PhD, Bruce A. Herman, MS, Christopher R. Merritt, MD, Naren Sanghvi, MS, and James F. Zachary, DVM, PhD

Journal of Ultrasound Medicine. 2008 April; 27(4): 517–540.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2724319/

Excerpt referring to the 8-fold increase in ultrasound intensity limit:

In the mid-1980s, the US Food and Drug Administration (FDA) initiated the regulatory process for diagnostic ultrasound equipment and set application-specific intensity limits that could not be exceeded.65,66 Unfortunately, these limits were not based on safety considerations. Rather, they were based on relative risk for regulatory decision-making purposes and were obtained from measurements of the acoustic output of diagnostic ultrasound systems that had been entered into interstate commerce before May 28, 1976, the date when the Medical Devices Amendments were enacted.67 In the early 1990s, the FDA implemented the Standard for Real-Time Display of Thermal and Mechanical Indices on Diagnostic Ultrasound Equipment (commonly called the ODS).12,65,68,69 While the ODS12 did not specify upper limits, the FDA’s implementation65 of the ODS stipulated regulatory upper limits of 720 mW/cm2 for the derated (0.3 dB/cm-MHz) ISPTA.3 and either 1.9 for the MI or 190 W/cm2 for the derated (0.3 dB/cm-MHz) ISPPA.3. There is, however, an exception for ophthalmic applications, for which ISPTA.3 less than or equal to 50 mW/cm2 and MI less than or equal to 0.23 are specified.65 In addition, the FDA65 requires the manufacturer to justify thermal indices (TIs) greater than 6. The ODS has been revised,13,14 but the FDA’s regulatory upper limits have not changed.

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