Explain why ultrasound can be used to distinguish between two different soft tissues that are next to each other inside the body.

AQA A-Level Physics (7408) — 3.10.4 Non-ionising Imaging · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Ultrasound is used in medical imaging to produce pictures of internal structures without exposing patients to ionising radiation.

Model answer (5 marks)

Ultrasound waves are partially reflected at the interface between two tissues because the tissues have different acoustic impedances (density × speed of sound). The reflected pulse travels back to the transducer, where it is detected. By measuring the time taken for the echo to return and knowing the speed of sound in tissue, the depth of the interface is calculated. Each distinct interface produces a separate echo, so the transducer records a series of echoes that can be plotted to distinguish adjacent soft tissues and form an image.

Examiner tips

  • Use the term ‘acoustic impedance’ to show understanding of the reflection mechanism.
  • Explain how the time‑of‑flight gives depth – include the speed of sound in tissue.
  • Show that multiple echoes give a picture – emphasise ‘separate echoes’.
  • Mention that the transducer both emits and receives the pulse.

Mark scheme (5 marks)

  1. Ultrasound is partially reflected at a boundary between two different tissues
  2. Reflection occurs because the two tissues have different densities / acoustic properties
  3. The reflected pulse travels back to the detector / transducer
  4. The time taken for the pulse to return is used to calculate the depth / position of the boundary
  5. Different boundaries produce separate echoes / signals, allowing the two tissues to be distinguished / an image to be built up

Key terms in this question

ultrasound

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