Union University
Union University Department of Physics

The Science Guys



Science Guys > March 2004

March 2004

How is an ultrasound image of an unborn baby generated?

Ultrasound refers to very high frequency sound waves. Sound waves are generated anytime an object disturbs a fluid, such as air - like a guitar string oscillating or a drumhead moving up and down. These motions cause some regions of air to be more bunched up than normal (compressions) or more spread out than normal (rarefactions).

With medical ultrasound equipment, a probe called a transducer emits the sound. The transducer contains tiny quartz crystals called piezoelectric (pz) crystals. When electricity flows through these crystals, they vibrate, moving back and forth quickly. The vibrations create sound waves in whatever material the transducer contacts, for example the abdomen of a pregnant woman. The pz crystals in medical ultrasound instrument vibrate with a frequency from 1 million to 7 million vibrations per second. Thus, the transducer sends out millions of sound waves into the abdomen every second.

Have you ever noticed that when light encounters a piece of glass, some of the light transmits through the glass, while some reflects? Ultrasound waves act similar to light waves. Therefore, when sound waves encounter a boundary between two different tissues, some of the sound energy reflects from the boundary and some energy passes through the boundary into other tissue.

The reflected waves travel back to the transducer and affect the pz crystals. Not only do the crystals vibrate when given an electrical pulse, they also can produce electricity when they are vibrated. The returning sound waves vibrate the pz crystals and produce an electric signal. This returning signal contains the data needed to produce an image. Both the intensity of the returning sound and the time it takes the sound wave to return are used to generate an image.

Sound waves move in human tissue about 1540 m/s. That is 5000 ft/s or 3400 mph. Thus, the time at which sound waves are received relative to when they were emitted yields the distance the sound wave has traveled. This information allows the position of the tissue boundary to be determined. Adding information about the intensity of the returning sound wave allows an image to be produced.

Ultrasound is best known in obstetrics, but it is also useful in cardiology to image the heart, blood vessels, and flow through the heart. It is also used in urology to locate kidney stones, to monitor blood flow in the kidneys, and to detect prostate cancer.

The ultrasound device is an amazing tool made possible by careful engineering applications of the fundamental laws of physics. Today ultrasound signals are swept across the mother’s abdomen in order to produce a real-time image that can be recorded - mom’s first baby videos!

So, Mom, the next time the doctor images your precious little one in utero, take a moment to thank the physicists, engineers, and technicians that made the ultrasound picture possible!