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Ultrasound Imaging 2

Physics: AQA A Level, topic 10.4.2

Piezoelectric Devices

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

Crystals

Crystals

  • 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.
Recording readings

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.

Impedance calculations

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
Reflection calculations

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=((Z2−Z1)(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.

Test yourself on Ultrasound Imaging 2

  • Which of these statements are true about piezoelectric devices?

  • How can ultrasound be used to find the distance between boundaries of tissue?

  • What is the equation for acoustic impedance?

  • What is the equation for the fraction of sound reflected by a boundary?

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