Ultrasound Imaging

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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.

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Acoustic impedance

  • Acoustic impedance (more accurately specific acoustic impedance) of a material (Z) = density x velocity of sound in the material.
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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:

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A scans

  • A-scans are similar to traditional echo-sounding techniques.
    • Distances between layers in the body can be easily measured.
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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.

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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.
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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.

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