A medical team uses ultrasound waves to carry out a foetal scan on a pregnant patient. Explain how ultrasound waves are used to produce an image of the foetus, and why ultrasound is chosen for this purpose rather than X-rays.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ultrasound waves have a frequency above 20 kHz, which is beyond the upper limit of human hearing. When ultrasound reaches a boundary between two different media, the waves are partially reflected back towards the source.
Model answer (5 marks)
Ultrasound waves are emitted from a transducer placed against the patient’s abdomen and travel into the uterus.
At each interface between different tissues (e.g. fluid–muscle, muscle–bone) part of the wave is reflected back to the transducer.
The transducer also acts as a receiver, detecting the reflected echoes and recording the time interval between emission and detection.
Because the speed of sound in soft tissue is essentially constant (~1540 m s⁻¹), the time delay can be converted into a distance to each reflecting boundary, and by scanning across the abdomen a two‑dimensional image of the foetus is constructed.
Ultrasound is preferred to X‑rays because it is non‑ionising; it does not deposit ionising radiation that can damage cells or cause mutations, making it safe for the developing foetus.
At each interface between different tissues (e.g. fluid–muscle, muscle–bone) part of the wave is reflected back to the transducer.
The transducer also acts as a receiver, detecting the reflected echoes and recording the time interval between emission and detection.
Because the speed of sound in soft tissue is essentially constant (~1540 m s⁻¹), the time delay can be converted into a distance to each reflecting boundary, and by scanning across the abdomen a two‑dimensional image of the foetus is constructed.
Ultrasound is preferred to X‑rays because it is non‑ionising; it does not deposit ionising radiation that can damage cells or cause mutations, making it safe for the developing foetus.
Examiner tips
- Use the word ‘transducer’ and ‘echo’ to show understanding of the device and waves.
- Explain the time‑distance conversion and mention the constant speed of sound.
- State clearly that ultrasound is non‑ionising and X‑rays are ionising, linking safety to the choice.
Common mistakes
- Confusing ultrasound with visible light or radio waves.
- Failing to mention the constant speed of sound or the time‑distance calculation.
- Claiming ultrasound is ionising or that X‑rays are safe for foetuses.
Mark scheme (5 marks)
- Ultrasound waves are emitted from a device placed against the patient's body and travel into the womb
- At each boundary between different media / tissues, some of the ultrasound is partially reflected back
- A receiver / detector detects the reflected ultrasound waves and measures the time between emission and detection
- Because the speed of ultrasound is constant, the time taken allows the distance to each boundary to be calculated, building up an image
- Ultrasound is chosen instead of X-rays because ultrasound is non-ionising / does not damage cells, whereas X-rays can cause cell damage or mutations
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
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