Ultrasound Imaging 2

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Piezoelectric Devices

To produce ultrasound, we have to produce vibrations at a very high frequency. A piezoelectric crystal can do this.

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  • When a voltage is applied across a piezoelectric crystal, the crystal begins to contract and expand at a high frequency, this produces very high-frequency vibrations (which are sound waves).
  • Similarly, if a pressure is applied to the crystal (in the form of a wave reflected off tissue layers), a voltage is produced which can be recorded.
  • The crystal acts as both a transmitter and a receiver of sound.
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Recording readings

  • Ultrasound is partially absorbed by tissue on its path, both on its journey away from the transducer and on its return journey.
  • We can find the time between when the original signal is sent and when the reflections from various boundaries between media are received.
  • From this information, we can work out the nature and the position of the boundary
  • The loss of intensity can measure the density of the medium.

Acoustic Impedance Equations

Acoustic impedance defines how much sound can travel through a boundary.

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Impedance calculations

  • Acoustic impedance is the factor defining how much sound is reflected at a boundary. If the gap is too large then ultrasound imaging cannot be used.
  • The equation for acoustic impedance is:
    • Acoustic impedance = the density of the material x the speed of sound in the material
    • Z=ρcZ={\rho}c
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Reflection calculations

  • The intensity of the reflected wave can be found as a fraction of the intensity of the incident wave. For ultrasound to work the fraction reflected should be low.
  • The equation for the reflected fraction is given by:
    • IrIi=((Z2Z1)(Z2+Z1))2\frac{{I_r}}{{I_i}}={(\frac{({Z_2}-{Z_1})}{({Z_2}+{Z_1})})}^2
      • Where I is the intensity of the wave and Z is the acoustic impedance.

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