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GCSE Questions: Medical Physics

- scanning and therapy

Q7. X-rays and ultrasound can both be used for scanning internal organs.

(a) Ultrasound is used to scan unborn babies but X-rays are not used to scan unborn babies.

Explain why.

Ultrasound is non-ionising radiation but X-rays are ionising radiation so X-rays increase the health risks (cancer, stunted growth, impaired brain function etc.) to the (unborn) baby.

Saying 'X-Rays are dangerous' is too vague to get you the mark.

[3 marks]

(b) The behaviour of ultrasound waves when they meet a boundary between two different materials is used to produce an image.

Describe how.

Ultrasound/waves are partially reflected when they meet a boundary between two different media/substances/tissues. The time taken for the pulse to travel to the boundary and back to the probe is measured and is used to the determine distances travelled.

You had to show that you knew only part of the ultrasound energy was reflected to get the mark. If you cave the impression it was all reflected they did not award you the mark.

[2 marks]

(c) The diagram below shows two pulses from a scan of an unborn baby.

The emitted pulse is labelled A.

The returning pulse picked up by the receiver is labelled B.

The closest distance between the unborn baby and the mother's skin is 4.0 cm.

Use information in the diagram above to calculate the average speed of the pulse.

Time between pulses = 50 =μs = 50 x 10-6 s

That is the time the ultrasound travelled a distance of 2 x 4.0 cm (from the probe to the baby then back to the probe again)

Distance travelled = 8.0 cm = 8.0 x 10-2 m

Speed = distance/time

Speed = 8.0 x 10-2/ 50 x 10-6

Speed = 1600 m/s

[3 marks]

(Total 8 marks)