## 2aPAa3. Finite amplitude distortion in derating of ultrasound fields.

### Session: Tuesday Morning, May 14

**Author: T. Christopher**

**Author: E. L. Carstensen**

**Location: Rochester Ctr. for Biomed. Ultrasound, Univ. of Rochester, Rochester, NY 14627**

**Abstract:**

All ultrasound exposure indices that have been formulated up to the present
time assume that the propagation of ultrasound is linear. In fact, under most
exposure conditions for which biological effects may be a concern, sound
propagation is highly nonlinear. A nonlinear propagation model has been used to
evaluate the nature of the effects that occur under realistic exposure
conditions encountered in diagnostic procedures. Because of the way that the
thermal index is defined, it turns out that ignoring nonlinear propagation leads
to underestimates of tissue temperature increments that typically are less than
40%. As currently implemented, the mechanical index may be underestimated by
more than a factor of 2 because it ignores the saturation of the sound fields
that result from nonlinear propagation. For large propagation distances in soft
tissues (e.g., 10 cm at 3 MHz in liver), however, it is physically difficult to
exceed tissue pressures corresponding to MI>2 because of these same saturation
phenomena.

from ASA 131st Meeting, Indianapolis, May 1996