Statement on Mammalian Biological Effects in Tissues Without Gas Bodies

Biologically significant adverse nonthermal effects have been observed in tissues not known to contain stabilized gas bodies when exposed to pulsed ultrasound in the low-megahertz frequency range. This statement concerns the conditions under which small gas bodies, ie, bubbles, may be nucleated and driven to undergo inertial cavitation by an ultrasonic field.
1. Thresholds for adverse nonthermal effects depend on tissue characteristics, the depth of target tissue, the exposure duration, and ultrasound parameters such as frequency, pulse duration, and pulse repetition frequency. The relationship of these parameters with the threshold for adverse nonthermal effects is complex.
2. A mechanical index (MI) has been formulated to assist users in evaluating the likelihood of cavitation-related adverse biological effects for diagnostically relevant exposures. 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).1 Currently available diagnostic ultrasound equipment follows the Food and Drug Administration guidance2 on output, which allows the MI to range up to 1.9.
3. The lowest theoretical threshold for bubble nucleation and subsequent inertial cavitation is 3.9 MPa.3 This corresponds to MI = 3.9 at 1 MHz, approximately the lower bound for diagnostic ultrasound.
4. Experimental values for the inertial cavitation threshold in vivo lie at or above approximately 3.6 MPa, the value at 0.56 MHz. This is equivalent to an effective MI of 4.8.4
5. The lowest thresholds for induction of adverse nonthermal effects in tissues correspond to MI > 4 for extravasation of blood cells in mouse kidneys, and MI > 5 for hind limb paralysis in the mouse neonate.3
6. For mammalian biological effects research relating to diagnostic ultrasound, including experiments with pulsed ultrasound3,5,6 and studies of contrast-enhanced diagnostic ultrasound with negative control tests without gas bodies,7 no adverse nonthermal bioeffects have been observed for MI values less than the upper limit for diagnostic ultrasound, MI = 1.9, in tissues without gas bodies.


Approved: 03/25/2015;

© American Institute of Ultrasound in Medicine
14750 Sweitzer Lane, Suite 100 · Laurel, MD 20707-5906 USA
Phone: 301-498-4100 · Fax: 301-498-4450 · Website: