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Medical Physics

Ultrasound imaging

  • 5 things to know
  • Paper 3, section B

Overview

Acoustic impedance, the reflection coefficient, coupling gel, and A-scans and B-scans.

What you need to know

Answer each one before you reveal it.

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Numbers

How is acoustic impedance defined, and what unit does it take?

Numbers

What determines how much ultrasound reflects at a boundary?

Numbers

How is depth found in an A-scan, and what is a B-scan?

Evaluation

State three practical advantages of ultrasound imaging equipment

Key term

Why is ultrasound imaging unsuitable for examining the lungs or structures behind bone?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How is acoustic impedance Z defined in terms of the properties of a medium?

    Model answer

    Acoustic impedance is the product of the density of the medium (ρ) and the speed of sound in that medium (c): Z = ρc

  2. 2

    What is the SI unit of acoustic impedance?

    Model answer

    kg m⁻² s⁻¹

  3. 3

    What happens to an ultrasound wave when it encounters a boundary between two media with different acoustic impedances?

    Model answer

    Part of the wave is reflected and part is transmitted

  4. 4

    What property of two adjacent media determines the fraction of ultrasound intensity that is reflected at their boundary?

    Model answer

    The difference in acoustic impedances of the two media

  5. 5

    State the equation for the intensity reflection coefficient at a tissue boundary in terms of the acoustic impedances Z₁ and Z₂.

    Model answer

    Iᵣ/Iᵢ = [(Z₂ − Z₁)/(Z₂ + Z₁)]²

  6. 6

    Define each term in the intensity reflection coefficient equation Iᵣ/Iᵢ = [(Z₂ − Z₁)/(Z₂ + Z₁)]².

    Model answer

    Iᵣ is the reflected intensity, Iᵢ is the incident intensity, Z₁ is the acoustic impedance of the first medium, and Z₂ is the acoustic impedance of the second medium

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