Syllabus Edition
First teaching 2023
First exams 2025
Generating & Using Ultrasound (Cambridge (CIE) A Level Physics)
Revision Note
Generating 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
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
Worked Example
Explain the principles of the generation and detection of ultrasound waves.
Answer:
Step 1: Explain the principles of the generation of ultrasound
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
Step 2: Explain the principles of detection of ultrasound
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
Using ultrasound in medical imaging
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
Worked Example
Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures.
Answer:
Step 1: Describe the journey of an ultrasound pulse
A pulse of ultrasound is emitted by the piezo-electric crystal in the transducer
The pulse is reflected by the boundary between two media
The reflected pulse is detected by the ultrasound transmitter
Step 2: Explain how the reflected ultrasound pulse is used
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
Examiner Tips and Tricks
6 mark exam questions about this topic are very common, make sure you practice writing about using and detecting ultrasounds in full, coherent sentences with correct spelling and grammar. Writing short or vague answers could lose you marks, as well as misspelling words!
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