10.4.2

# Ultrasound Imaging 2

Test yourself

## Piezoelectric Devices

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

### 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.

• 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

• 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
• $Z={\rho}c$

### 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:
• $\frac{{I_r}}{{I_i}}={(\frac{({Z_2}-{Z_1})}{({Z_2}+{Z_1})})}^2$
• Where I is the intensity of the wave and Z is the acoustic impedance.