Ultrasound Imaging
Physics: AQA A Level, topic 10.4.1
Acoustic Impedance and Attenuation
Reflection at a boundary between materials depends upon the difference in Z. The reflected intensity increases with increasing difference in acoustic impedance.
Acoustic impedance
- Acoustic impedance (more accurately specific acoustic impedance) of a material (Z) = density x velocity of sound in the material.
Comparative attenuation
- Bone has approximately 4x the impedance of muscle, and 10 000x the impedance of air.
- Ultrasound will also be attenuated by the material it is propagating through.
- The attenuation depends upon the frequency of the sound.
A and B Scans
There are two types of ultrasound scans:
A scans
- A-scans are similar to traditional echo-sounding techniques.
- Distances between layers in the body can be easily measured.
B scans
- For a B-scan, the position and orientation of the transducer are used to provide an image of a section of the body.
- The transducer is moved by the operator to provide information from these different positions.
Advantages and Disadvantages of Ultrasound
Ultrasound can be a very helpful medical imaging device however, it cannot be used effectively in all situations.
Advantages of ultrasound
- Ultrasound gives accurate information about the distance of layers of tissue from the time taken for reflection to happen.
- Ultrasound is non-ionising so is safe for soft tissue.
- Ultrasound can give information on the density of material from how intense the reflection is.
- Ultrasound is safe for use on pregnant mothers.
Disadvantages of ultrasound
- The resolution is not as high as when an X-ray is used.
- Due to the difference in acoustic impedance ultrasound will not produce an image from organs with large amounts of gas, like the lungs.
- We are not aware of the long-term effects of ultrasound on soft tissue or on a fetus.
Test yourself on Ultrasound Imaging
What is the equation for the acoustic impedance?
Which of these statements are true about ultrasound?
Which type of ultrasound scan uses the position and orientation of a transducer?
Why is ultrasound NOT as good as an X-ray?