Transducers spend over 99% of the time “listening” for returning waves.Once the ultrasound wave is generated and travels through the medium, the crystal switches from ‘sending’ into ‘listening’ mode and awaits returning ultrasound echoes.These crystals have the ability to transform an electrical current into mechanical pressure waves (ultrasound waves) and vice versa.The source of the ultrasound wave is the piezoelectric crystal, which is housed in the transducer.The production and interpretation of ultrasound waves is based on the so-called ‘ pulse-echo-principle’.While most of the original wave continues to travel in its original path, a small portion of the sound waves are scattered in random directions.Scatter occurs when ultrasound waves encounter a medium with a heterogeneous surface.The amount of deflection is proportional to the difference in the two tissues ‘stiffness’.The angle of incidence will be different from the angle of transmission.If the two mediums have different “stiffness” the resulting change in propagation speeds will cause the wave to be “bent” from its original path ( refraction).The remaining sound wave travels through the second medium (or tissue).The angle of approach (incidence) is identical to the angle of the reflection.Some of the waves bounce back towards the source as an echo ( reflection).These occur as sound waves encounter a boundary between two different media.Hearing range in various animals and humans. Other causes of attenuation are reflection, refraction and scatter.The major source of attenuation in soft tissue is absorption, the conversion of acoustic energy into heat.Attenuation is the loss of intensity and amplitude as sound waves travel through a medium.Sound waves travel through human soft tissue at approximately 1540 m/s (about one mile per second).This means that sound waves travel faster in solids than liquids or gases The greater the stiffness, the faster the wave will travel.The propagation speed of an acoustic wave traveling through a specific medium is determined by the stiffness of that medium.Diagnostic ultrasound typically uses frequencies between 2 and 20 million Hertz (Megahertz - MHz).Ultrasound refers to any sound waves with frequencies greater than 20kHz. ![]() For most humans audible sound ranges between 20 Hz and 20,000 Hz (20 kHz).The illustration shows a schematic drawing of wave length, pressure and amplitude. The frequency of the wave is measured in cycles per second or Hertz (cycles/s, Hz) (Illustration 1).The wavelength is the distance traveled during one cycle. ![]()
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