A hospital uses ultrasound waves to produce images of structures inside a patient's body. Explain how ultrasound waves are used to produce an image of an internal structure, and explain why ultrasound is preferred over X-rays for repeated or routine scanning of soft tissue.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ultrasound scanners emit high-frequency sound waves into the body. These waves travel through different tissues and organs before returning to the scanner's detector.
Model answer (5 marks)
Ultrasound scanners emit high‑frequency sound waves that travel through the body. At each interface where the acoustic impedance of two tissues differs, part of the wave is reflected back to the transducer and part is transmitted further. The reflected echoes are detected and the time taken for the echo to return is measured. Using the speed of sound in tissue, the distance to each reflecting interface is calculated, and successive echoes are combined to build up a two‑dimensional image of the internal structure.
X‑rays are ionising radiation that can damage DNA and increase the risk of mutation and cancer. Ultrasound uses non‑ionising mechanical waves, so it does not cause cellular damage. Because it is safe, non‑invasive and inexpensive, ultrasound is preferred for routine or repeated scans of soft tissue, such as foetal imaging.
X‑rays are ionising radiation that can damage DNA and increase the risk of mutation and cancer. Ultrasound uses non‑ionising mechanical waves, so it does not cause cellular damage. Because it is safe, non‑invasive and inexpensive, ultrasound is preferred for routine or repeated scans of soft tissue, such as foetal imaging.
Examiner tips
- Use the word "reflected" and "transmitted" when describing wave behaviour; include the time‑of‑flight calculation; mention ionising vs non‑ionising radiation; note safety and repeatability for soft tissue
Common mistakes
- Confusing the speed of sound with the speed of light; forgetting to mention the time‑of‑flight distance calculation; stating that X‑rays are safe for routine scans
Mark scheme (5 marks)
- Ultrasound waves are partially reflected at the boundary between two different media (e.g. between different tissues or organs)
- The remainder of the waves continue to travel through (are transmitted) and can reflect at further boundaries
- The time between emitting the ultrasound and detecting the reflected waves is used to determine the distance to each boundary / build up an image
- X-rays can cause cell damage or mutations (and potentially cancer), whereas ultrasound does not cause cell damage
- Ultrasound is a non-invasive method that is safe for repeated scanning, making it preferable for routine or repeated examinations (e.g. foetal scanning)
Key terms in this question
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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