Nonlinear Acoustics in Medical Application of Ultrasound

                      Shin-ichiro Umemura, Ph. D.

                       Chief Research Scientist
                     Central Research Laboratory
                           Hitachi Ltd., 
              Kokubunji, Tokyo 185-8601, Japan
                  sumemura@crl.hitachi.co.jp

Nonlinear phenomena of ultrasound in biological tissues have been
known to exist for a long time, but they have started being utilized
in medical applications only very recently. Two kinds of them are
being utilized in diagnostic applications and have potential to be
utilized in therapeutic applications. One is the nonlinear propagation
in tissues and the other is the nonlinear response of
microbubbles. The former is being utilized in "tissue harmonic
imaging". The ultrasonic pulse echo image is constructed on the
assumption that the ultrasonic pulse is reflected only once in the
tissues, but the effect of multiple reflections to the image cannot be
ignored in reality. Nonlinear components are being accumulated during
the propagation of ultrasonic pulse after they have been formed in
phase, but they become out of phase after the first reflection. Thus,
the intensity of nonlinear components by multiple reflection is much
lower than that by single reflection. This is the primary reason for
the high acoustical S/N ratio in the tissue harmonic images, higher
than the fundamental images by orders of magnitude. Microbubbles show
much higher nonlinear response than surrounding liquid or biological
tissues. Thus, highly contrasted echo images can be constructed from
the harmonic components in the reflected echo when a microbubble
contrast agent is used. This kind of imaging with microbubbles can be
understood and optimized by numerically analyzing the nonlinear
behavior of microbubbles under acoustic pressure. We have found
sonodynamically active cavitation can be induced at much lower
ultrasonic intensity when the second harmonic is superimposed onto the
fundamental. This effect can also be explained based on such analysis.