Ultrasound is a popular option for medical imaging because, unlike x-rays, it doesn't involve radiation or "contrast agents" ingested to improve visibility. It's a simple, relatively inexpensive procedure that requires little patient preparation and is typically not invasive.

Ultrasound is comprised of sound waves that have a frequency (typically 2 to 5 megahertz) higher than humans can hear. The waves are emitted from the transducer probe of an ultrasound machine and sent through the patient's body around the area being examined (such as a pregnant woman's abdomen). As the waves bounce off of internal organs, tissue and other dense areas, they create "echoes" that reflect back to the transducer. The transducer translates the vibrations into electrical pulses that are processed by the machine, calculating the strength of the echoes and how long they take to return and eventually transforming the data into an image that appears on a monitor.

An ultrasound picture can be 2-D, 3-D or 4-D (which includes motion). Doppler measurements that utilize the Doppler effect can be used to aid in ultrasound exams to measure objects in motion, such as blood flow or moving muscles.