Statement on Mammalian Biological Effects in Tissues with Naturally Occurring Gas Bodies

Biologically significant adverse nonthermal effects have been identified in tissues containing stable bodies of gas for diagnostically relevant exposure conditions. Gas bodies occur naturally in postnatal lungs and in the folds of the intestinal mucosa. This statement concerns naturally occurring gas bodies encountered in pulmonary and abdominal ultrasound, whereas a separate statement deals with the use of gas body contrast agents.
1. The outputs of some currently available diagnostic ultrasound devices can generate levels that produce capillary hemorrhage in the lungs1 and intestines2 of laboratory animals.
2. Thresholds for adverse nonthermal effects associated with naturally occurring gas bodies depend on tissue characteristics and the physiologic status, including anesthesia,3 and on physical ultrasound parameters, including attenuation by intervening tissue, exposure duration, ultrasonic output, frequency, pulse duration, and pulse repetition frequency.2
3. A mechanical index (MI)* has been formulated to assist users in evaluating the likelihood of mechanical (nonthermal) adverse biological effects for diagnostically relevant exposures, and its value is displayed on screen in accordance with output display specifications.
4. The minimum threshold value of the experimental MI (the in situ value of the peak rarefactional pressure amplitude divided by the square root of the frequency) for pulmonary capillary hemorrhage in laboratory mammals is approximately 0.4. The corresponding threshold for the intestine is MI = 1.4.
5. The implications of these observations for human exposure during thoracic or abdominal ultrasound examinations are yet to be determined.
*The MI is equal to the derated peak rarefactional pressure (in megapascals) at the point of the maximum derated pulse intensity integral divided by the square root of the ultrasonic center frequency (in megahertz).

Approved: 10/07/1987; Reapproved: 03/18/1993, 11/08/2008, 03/25/2015

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