In addition to heat, scientists have begun to learn more about the various types of mechanical
effects that ultrasound can have on the body. They divide these effects into two categories. The first category is called acoustic cavitation. Cavitation can occur when sound passes through an area that
contains a cavity, such as a gas bubble or other air pocket. Some tissues, most notably adult lung and
intestine, do contain air bubbles, and are therefore more vulnerable to these cavitation effects. The fetal
lung and intestine do not contain obvious air bubbles, because the fetus does not breathe air yet—it gets
oxygen from the mother’s blood stream. However, researchers believe that tiny bubbles could potentially
form in parts of the body other than the lung and intestine. More research is needed in this area.