Post Number 1: The Basics of Ultrasounds

          

 [Image credit: Jacaranda Physics 1 2nd Edition © John Wiley & Sons, Inc.]

            Acoustics, echoes and sound waves have been a focal point of fascination for many centuries, dating back to the time of Aristotle. However, It isn’t until 1877, when Lord Rayleigh describes sound in terms of a mathematical equation in  ‘The Theory of Sound’ that the study of sound waves really takes off. A few years later, Jacques and Pierre Curie will revolutionize the study of sound through their discovery of the ‘Piezo-Electric’ effect. ‘The Piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress”[1]. When the Piezoelectric effect occurs, energy is converted by applying pressure to the material which results in a shifting between positive and negative charge centers in the material causing an external electric field. The reversal of the piezoelectric converts the energy back to its original form. The reversal of the piezoelectric effect happens when the outer electric field either compresses or stretches the piezoelectric material. Essentially, the piezoelectric effect is this conversion between mechanical energy (sound waves) to electrical energy and visa versa, which is significant because these piezoelectric crystals can then send signals as well as receive signals.

          Because these crystals are where the conversion between electrical energy and mechanical energy occur, the type of crystal that can emit the right frequency to produce a sound wave is critical. Quartz used to be the most common crystal used; however, a man made ceramic, lead zirconate titanate, has replaced quartz in modern medicine.

          With ultrasounds, an electrical current is applied to these crystals in the transducer probe where they begin to rapidly change shape which results in vibrations that produce sound-waves traveling outwards.  These sound-waves then bounce off of the surfaces of our body and travel back as sound or pressure waves to hit the crystals again and are converted back to electrical energy.

          Using the ‘pulse-echo’ principle, when electricity is converted into mechanical energy (sound waves), it is read as a pulse by the ultrasound transducer; the reverse of this effect is when the sound energy is converted back to electricity, and the transducer reads the signal as an echo. These pulses are sent to the soft tissues of our body where they then interact with these tissues are converted to echoes. These echoes then travel back to the ultrasound machine. An ultrasound machine will spend only 1% of its time sending out pulses and 99% of its time listening for echoes. These echoes are then interpreted by measuring the time between when the sound was sent and received the pitch of the sound and the amplitude of the sound. When traveling through various media in the body, the intensity and amplitude of these sound waves diminishes. This is why echoes from structures deeper in the body are weaker than those coming from superficial surface areas of the body.

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[1] "The Piezoelectric Effect - Piezoelectric Motors & Motion Systems." Nanomotion. Johnson Electric, 1 Mar. 2008. Web. 13 Feb. 2015. <http://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/>.