The idea of using ultrasonic waves in medical and engineering fields came from military applications of sound waves in sonar systems. Sokolov, Firestone, Simons, and several other researchers are among the ones who studied this high frequency kind of wave in the first half of twentieth century. Ultrasonic waves are defined as those with frequencies higher than 20 kHz. The constant development of data acquisition systems and probes seems to work, removing the upper limit of the wave frequency that can be acquired. The phenomenal improvement of this technique, allied with its simplicity and the low cost of the required equipment transformed the initially focused technique to one.
