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Ultrasound imaging is based on the pulse-echo principle, which is also the basis of radar, and came into use as a medical imaging technique after WW2. Ultrasound imaging uses ultra-high-frequency sound waves (3-10 MHz). Human hearing is much lower frequency (in the range of 20 to 20 000 Hz) and therefore you cannot hear the sound waves being given off by the ultrasound imaging machine. A transducer (a device that converts one form of energy to another) is placed on the skin. Ultrasonic waves are emitted by the transducer and they travel through human tissues at a velocity of approximately
When the wave reaches an object or surface with a different texture or acoustic nature, a wave is reflected back. These echoes are received by the apparatus, changed into electric current and a 2-D image is produced.