How images are obtained  
 

An ultrasound transducer, shown on the right, is placed onto the patients skin with the gray area in full contact with the skin. Gel is placed between the skin and transducer to prevent any air from intervening between the two.

The transducer emits a narrow beam of sound waves of a specific high frequency. Depending on the frequency, the sound penetrates the patient's tissues to a variable extent. The sound waves are refracted, scattered, absorbed or reflected. The reflected waves bounce off tissues and are reflected back to the transducer, which also acts as a sensor, or detector.

The sound waves that are reflected are called an "echo" and are used to generate the image.