ABSTRACT
Biological effects of ultrasound In earlier years, ultrasound propagation in biological tissues was assumed to be a linear phenomenon for the sake of simplicity since it was believed that the imparted power by a diagnostic instrument was so low that nonlinear effects could be ignored. As the requirement for better sensitivity (signal-to-noise ratio) and image quality grew, both the peak and average intensity used in diagnostic instruments increased. At sufficiently high ultrasonic pressure levels and intensities, it is inevitable that nonlinear effects occur. So do new acoustic phenomena. Among the most important are heating, wave distortion, cavitation, radiation force, and streaming (NCRP, 1983; Hamilton and Blackstock, 1998). Table 10.1 lists typical acoustic output values for ultrasonic diagnostic instruments (AIUM, 1992a; Patton et al., 1994; Zagzebski, 1996).
