Question

Ultrasound can be used to measure the diameter of the head of a baby in the womb.

Ultrasound measuring size of baby's head

When ultrasound reaches the baby’s head at A, some ultrasound is reflected back to the detector and produces pulse A on the CRO.

Some ultrasound passes through the head to point B, and is reflected back to the detector.

Some ultrasound passes through the head to point B, and is reflected back to the detector. This reflection produces pulse B on the CRO.

This reflection produces pulse B on the CRO.

  1. What name is given to the reflection of a sound wave?
  2. The CRO is adjusted so that each horizontal division on the diagram above corresponds to a time of 40 microseconds. How long is the time interval between the arrival of pulse A and the arrival of pulse B at the detector? Give your answer in microseconds.
  3. How long does it take for the ultrasound to travel from one side of the baby’s head to the other? Give your answer in microseconds.
  4. Ultrasound travels at a speed of 1500 m/s in a baby’s head. Use your answer to question 3 to calculate the width of the baby’s head from A to B. Give your answer in cm. (1 microsecond = 1 × 10−6 s). You are advised to show clearly how you get your answer.
  5. Explain why it is better to use ultrasound rather than X-rays when monitoring the development of a baby in the womb.
  1. The reflection of a sound wave is called an echo.
    • Pulse A is 9 squares across the screen. Each square represents 40 microseconds or 40 μs. 9 squares = 9 x 40 μs = 360 μs. Pulse A arrives at the detector after 360 μs.
    • Pulse B is 12 squares across the screen. 12 squares = 12 x 40 μs = 480 μs. Pulse B arrives at the detector after 480 μs.
    • The time interval between the arrival of pulse A and the arrival of pulse B at the detector = 480 μs - 360 μs = 120 μs.
  2. 120 μs is the time taken for the ultrasound to travel across the head and back again. Hence, the time taken for the ultrasound to travel across the baby’s head is 120 μs ÷ 2 = 60 μs.
    • Distance = speed x time
    • Speed = 1,500 m/s
    • Time = 60 μs = 60 x 10-6 s
    • Distance = 1 500 m/s x 60 x 10-6 s
    • Distance = 0.09 m = 9 cm
    • The width of the baby’s head from A to B is 9 cm.
  3. X-rays are ionising and can kill or damage cells and so are harmful to the baby. A change to cell DNA can lead to cancer. Ultrasound does not kill cells or harm the baby.
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