Explainer: What is ultrasound?

Light, heat and sound are all forms of energy that travel as waves. The waves created by strumming a guitar string, for example, vibrate molecules in the air — and in our ears. That’s why we hear the guitar. But not all sounds are audible. For instance, ultrasound is not. The difference between the guitar’s sound and ultrasound is the frequency at which the wave vibrates. Musicians call that frequency the pitch.

Scientists measure a wave’s frequency in cycles per second, or hertz. Human ears can hear anything between 20 and 20,000 hertz. Waves with a higher frequency are known as ultrasound. Those higher pitches are beyond human hearing. (Sounds below human hearing are known as infrasound.)

In air, sound travels at a constant speed. If the wave’s frequency changes, so does its wavelength. That wavelength is the distance from the peak of one wave to the peak of the next. Long waves sound low and short waves sound high.

Waves of the same frequency can differ in the amount of energy they carry. That energy is measured by the height of a wave’s peak. Sound waves with higher peaks, or bigger vibrations, carry more energy than those with lower peaks. (Within the range of human hearing, for instance, we perceive sound waves with higher peaks as louder.)

Medical applications of ultrasound

For many decades, medicine has relied on ultrasound to picture soft tissues inside the body. This type of diagnostic ultrasound uses low-energy waves. That makes it safe for checking the health of unborn babies in the womb. Doctors also use it to scan for diseases in children and adults. More recently, researchers have been studying how ultrasound with higher energy might be used to treat certain diseases. This is called therapeutic ultrasound.

Ultrasound imaging with low-energy waves uses a transducer. This device converts one form of energy into another. One part of the transducer converts electrical energy into short ultrasound pulses. It sends those pulses into the body. Another part of the transducer receives the echoes that bounce back from the tissues those pulses hit inside the body. A computer analyzes those echoes to create an image. The echoes from different tissue types show up as darker and brighter parts in the image.

Doctors already use therapeutic ultrasound to treat kidney stones. These are small, hard mineral deposits in the kidneys that are painful to pass out along with urine. The high-energy sound waves break kidney stones into tiny pieces. That makes it easier for the body to flush them out.

Researchers are studying other possible therapeutic uses for ultrasound, too. For example, they would like to destroy cancer cells without harming neighboring cells. They also have shown that ultrasound can trigger brain cells to release signaling chemicals. This might one day lead to treatments for brain diseases and mood disorders.

Some researchers have even shown that ultrasound can make certain cells release insulin. That’s a hormone that keeps blood sugar at healthy levels. One day, it might be possible to use pulses of ultrasound to manage diabetes.