CIE A Level Physics复习笔记24.1.2 Generating & Using Ultrasound

• 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

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

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

Generation:

• • An alternating p.d. is applied across a piezo-electric crystal, causing it to change shape
  • The alternating p.d. causes the crystal to vibrate and produce ultrasound waves
  • The crystal vibrates at the frequency of the alternating p.d., so, the crystal must be cut to a specific size in order to produce resonance

Detection:

• • When the ultrasound wave returns, the crystal vibrates which produces an alternating p.d. across the crystal
  • This received signal can then be processed and used for medical diagnosis

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

Using ultrasound to obtain information about an unborn child

• 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: amount of transmitted intensity received (will vary depending on the type of tissue)

ANSWER:

• • A pulse of ultrasound is emitted by the piezo-electric crystal
  • This is reflected from the boundaries between media
  • The reflected pulse is detected by the ultrasound transmitter
  • The signal is then processed and displayed on the screen for the healthcare worker to analyse and use for medical diagnosis
  • The intensity of the reflection gives information about the nature of the boundary
  • The time between transmission and receipt of the pulse (the time delay) gives information about the depth of the boundary