Generating & Using Ultrasound (CIE A Level Physics)

• An ultrasound is defined as:

A high frequency sound above the range of human hearing

• This is above 20 kHz, although in medical applications the frequencies can be up to the MHz range
• An ultrasound transducer is made up of a piezoelectric crystal and electrodes which produce an alternating p.d.
• The crystal is heavily damped, usually with epoxy resin, to stop the crystal from vibrating too much
  • This produces short pulses and increases the resolution of the ultrasound device

An ultrasound transducer

The structure of an ultrasound transducer

• A piezoelectric crystal can act as both a receiver or transmitter of ultrasound
  • When it is receiving ultrasound, it converts the sound waves into an alternating p.d.
  • When it is transmitting ultrasound, it converts an alternating p.d. into sound waves

An ultrasound machine with a piezoelectric crystal

A piezoelectric crystal can act as both a receiver or transmitter of ultrasound

• In an ultrasound scanner, the transducer sends out a beam of sound waves into the body
• The sound waves are reflected back to the transducer by boundaries between tissues in the path of the beam
  • For example, the boundary between fluid and soft tissue or tissue and bone

• When these echoes hit the transducer, the transducer vibrates, which generates electrical signals that are sent to the ultrasound scanner
  • Using the speed of sound and the time of each echo’s return, the scanner calculates the distance from the transducer to the tissue boundary
  • These distances can be used to generate two-dimensional images of tissues and organs

• The ultrasound gives two main pieces of information about the boundary:
  • Depth: the time between transmission and receipt of the pulse (the time delay)
  • Nature: the amount of transmitted intensity received (will vary depending on the type of tissue)

• The frequency of the ultrasound is important because:
  • The higher the frequency of the ultrasound, the higher the resolution
  • The higher the resolution, the smaller the details of structures that can be distinguished

Ultrasound scan

Using ultrasound to obtain information about an unborn child