Explain how an ultrasound scanner produces an image of the inside of a patient's body from the time taken for ultrasound pulses to return to the probe.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An ultrasound probe is placed on a patient's skin with a layer of gel between the probe and the skin. The probe emits short pulses of ultrasound and detects the echoes that return.
Model answer (5 marks)
1. The probe emits short pulses of ultrasound that travel into the body.
2. When the pulse encounters a boundary between tissues of different acoustic impedance, part of the sound is reflected back as an echo.
3. The time taken for the echo to return to the probe is measured.
4. The depth of the reflecting boundary is calculated from the time and the speed of sound in tissue (depth = speed × time ÷ 2).
5. A computer processes many such echoes from different points to construct a two‑dimensional image displayed on the screen.
2. When the pulse encounters a boundary between tissues of different acoustic impedance, part of the sound is reflected back as an echo.
3. The time taken for the echo to return to the probe is measured.
4. The depth of the reflecting boundary is calculated from the time and the speed of sound in tissue (depth = speed × time ÷ 2).
5. A computer processes many such echoes from different points to construct a two‑dimensional image displayed on the screen.
Examiner tips
- Use the word ‘reflected’ and ‘echo’ to show understanding of acoustic impedance. Show the formula depth = speed × time ÷ 2 or explain distance ∝ time. Mention the gel’s role in preventing reflection at the skin‑air interface. Explain that the computer assembles multiple echoes into a 2‑D image.
Common mistakes
- Forgetting to divide the time by 2 when calculating depth. Not recognising that the gel prevents reflection at the skin‑air boundary. Using the wrong speed of sound value or not specifying units.
Mark scheme (5 marks)
- Ultrasound pulses are reflected (at boundaries) between different tissues / media
- The time taken for each echo to return to the probe is measured
- The depth / distance of the boundary is calculated using the speed of ultrasound in the tissue and the time (depth = speed × time / 2 or equivalent statement that distance is proportional to time)
- The gel / coupling medium prevents (ultrasound) reflection at the skin–air boundary / ensures ultrasound enters the body
- Information from multiple echoes / reflections is processed (by a computer) to build up a two-dimensional image on a screen
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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