Ultrasound (US) consists of high-frequency sound waves that are above the range of human hearing, at frequencies higher than 20 kHz. Medical ultrasound imaging is performed at much higher frequencies, typically in the MHz range. Ultrasound differs from other conventional imaging methods in important ways. First, unlike electromagnetic radiation, ultrasound waves are non-ionizing pressure waves. Second, the ultrasound signal is recorded in the reflection mode rather than the transmission mode used for x-ray and CT imaging. In ultrasound imaging, the imaged structures are not the sources that emit radiation. Instead, the sample is imaged by applying external acoustic energy to it. A “pulse echo” technique is used to create an image from longitudinal mechanical waves that interact with tissues of the body. The applied energy is reflected to the source by tissue inhomogeneities. The resulting signals carry information about their source as well as about the sample. Decoding these signals into an image requires separating the detected signal components due to the external source from those due to the sample.

Medical ultrasound imaging systems typically incorporate a piezoelectric crystal as the external signal source. This crystal vibrates in response to an oscillating electric current, producing longitudinal mechanical waves. The ultrasound signal propagates linearly through various media, including water and soft tissue, at an average speed of 1540 m/s, but does not propagate satisfactorily through bone or air. As a result, ultrasound is most suited for imaging soft tissues.