In cavitation, the sound waves can cause the bubbles or air pockets to expand and contract
rhythmically: in other words, to pulsate, or resonate. When they pulsate, the bubbles send secondary sound
waves off in all directions. These secondary sound waves can actually improve ultrasound images because
the secondary waves also reflect back to the transducer, and provide more information. Thus, doctors now
sometimes inject artificial bubbles known as contrast agents into the body before taking ultrasound images,
for instance, of the circulatory system. However, these contrast agents are not used to image the fetus.