A doctor uses ultrasound to carry out foetal scanning. Explain how ultrasound waves are used to produce an image of a foetus inside the womb.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Foetal scanning is a non-invasive imaging technique used routinely during pregnancy. Ultrasound waves have a frequency higher than 20 kHz, above the upper limit of human hearing.
Model answer (5 marks)
Ultrasound waves are emitted from a transducer that is placed on the mother’s abdomen. The waves travel through the abdominal tissues and reach the boundary between the foetus and the surrounding fluid or tissue. At this interface part of the wave is reflected back to the transducer while the rest is transmitted into the foetus. The transducer also acts as a receiver, detecting the reflected waves and recording the time taken for the echo to return. By measuring this time and using the speed of sound in tissue, the distance to each reflecting boundary is calculated. Repeating this for many points builds up a two‑dimensional image of the foetus.
Examiner tips
- Use the word ‘transducer’ for the source and detector; examiners look for this term. Include the speed of sound calculation step. Show that the image is formed by many reflected echoes.
- Remember to mention that the waves are high‑frequency sound, not visible light.
Mark scheme (5 marks)
- Ultrasound waves are emitted from a source/transmitter directed at the womb
- When the ultrasound waves reach a boundary between two different media (e.g. between the foetus and surrounding fluid/tissue), they are partially reflected back
- The remainder of the waves continue to pass through / are transmitted into the next medium
- A receiver/detector next to the emitter detects the reflected waves and measures the time between emission and detection
- The time measurements are used to determine the position/distance of boundaries, building up an image of the foetus
Key terms in this question
ultrasound waves · foetal scanning
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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