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A Little Physics

Diagnostic ultrasound is emitted in short pulses lasting in the order of a microsecond (millionth of a second).  The system then waits a fraction of a millisecond (one thousandth of a second) for the returning echo.  Pulse intensities are measured in W/cm2 (Watts per square centimetre) and time-averaged intensities in mW/cm2 (milliwatts per square centimetre).  The time-averaged (formally expressed as Ispta) intensity quoted for the ADR 2130 used in the Norwegian handedness outcome study around 1980 was 0.11 mW/cm2 (1).  The ADR was a widely used unit at that time, and the only one I had access to for about 2 years.  Results of this study raised the question of effects of prenatal ultrasound on subsequent handedness.  A Mayo Clinic demonstration of fetal stimulation by diagnostic ultrasound (combined standard imaging and pulsed Doppler) using an Acuson 128 XP gave the standard imaging (B-mode) intensity as144 mW/cm2, quoted from the 1995 service manual (2).  The authors of this article describe the output value as typical of modern scanners.  There are technical arguments about how comparable the measurements related to these two studies are, but there is as much reason to believe that the more than 1000-fold increase is an underestimate as an overestimate (31)

The current regulatory upper limit for Ispta is 720 mW/cm2, except for ophthalmic ultrasound where the limit is 50 mW/cm2   (9c, 15a).  Although not of direct relevance, a recognized authority in ultrasound physics has noted that the threshold for pain in the audible sound range, usually given as 130 dB, represents an intensity of 1 mW/cm2 (3).  (Ultrasound at diagnostic frequencies is completely blocked by air and therefore is not a hearing hazard to operators or patients.)