Medical Physics - Ultrasound

This GCSE medical Physics quiz looks at ultrasound. Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans (you should know that the range of human hearing is about 20 Hz to 20,000 Hz). Ultrasound can be used as a useful tool in medical imaging; the scientific word for this is ultrasonography but if you don't remember that, don't worry, it won't lose you any exam marks. Despite us not being able to hear it with the human ear, we can create detectors which allow us to map out an image of ultrasound waves which can show us the internal structure of objects. It is possible to use ultrasound in this way because the sound waves have a small amplitude and so they are low energy and therefore less harmful to living tissue.

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An ultrasound scan works by sending high frequency sound waves into the body. A special gel is applied to the skin to make sure that the sound waves are transferred efficiently. Without the gel, the big density difference between the skin and the air would mean most of the ultrasound would be reflected and would never get into the body. Inside the body, some of the sound reflects off different tissue surfaces and bounces back to the detector. The rest of the sound continues on. When it reaches another layer, a little more is reflected and so on.

The reflected sound is then either presented as a graph or converted into an image by a computer. The image is displayed on a screen and can also be printed. To an untrained eye, an ultrasound scan image looks very blurred and out of focus. Medical staff who are trained to read ultrasound scans can see a lot more detail. When compared with an image produced from X-rays, the latter is much sharper and clearer because the wavelength of X-rays is a lot shorter than ultrasound. An advantage of ultrasound scanners over X-rays is that sound waves are non-ionising and so no damage is done to cells, making it safer for the patient than X-rays.

Its most commonly known use is to examine the foetus inside the womb during a pregnancy but it is also used to examine organs such as the liver, kidneys, gall bladder and heart. It is also possible to use it to measure the speed at which the blood is flowing round a patient's body (using a Doppler ultrasound scanner) and to break up kidney stones - they are vibrated by the ultrasound waves until they shake themselves into smaller fragments that can pass through the urethra and out of the body. Away from the body, ultrasound can be used to clean industrial components and medical instruments.

The computer in an ultrasound scanner calculates the depth of each layer of tissue from which the ultrasound has been reflected. The time taken for the reflections (echoes) to reach the detector is measured and the distance calculated from that using an equation that you should already know - distance = velocity x time (s = v x t).

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  1. The speed of ultrasound in air is 330 m/s. How far away is a car if an ultrasound wave is sent to it along a road and takes 0.1 s to return?
    distance = speed x time so the answer to this calculation is 330 x 0.05 m. When dealing with echoes, remember that you will often need to half the time otherwise you will be calculating the distance of the journey of the sound there and back
  2. Ultrasound waves travel approximately 1,500 m/s in water. A boat sends out an ultrasound wave to the bottom of the ocean. It takes 0.5 seconds for the wave to hit the bottom of the floor and reflect back to the detector. How deep is the ocean floor below the boat?
    This is an example of a non-medical use that you need to know for the GCSE. We can use ultrasound to determine depths and distances to underwater obstructions or to detect other vessels
  3. Which of the following can be a use of ultrasound waves?
    Ultrasound is not just useful in medical applications
  4. Which of the following is/are advantages of ultrasound used in medical scanning?
    It is safe enough to use to scan a foetus in the womb to check on its progress
  5. Which formula correctly relates speed, distance and time?
    This equation is used by medical ultrasound scanners to build up an image
  6. Which of the following statements is correct?
    If the ultrasound waves were fully reflected, the method would be of no use as you would only be able to observe the first boundary it hits
  7. Which of the following is/are disadvantages of ultrasound?
    Whilst ultrasound has some disadvantages such as those listed above, the advantages far outweigh them for the applications for which they are used. Using a non-invasive imaging technique for delicate medical issues such as foetal scanning is much more preferred than an intrusive procedure which could cause unnecessary complications
  8. What is the frequency above which a sound wave is known as ultrasound?
    This frequency is above the limit at which the human ear can hear, however many other animals can hear it easily
  9. Which of the following are medical uses of ultrasound?
    Pre-natal (also called ante-natal) means 'before the birth', the opposite is post-natal which refers to the time after a birth
  10. The speed of ultrasound in metal is 5,000 m/s. How far away is a crack in the metal if an ultrasound wave is sent through the metal and is reflected back from the crack to the detector in 0.01 ms?
    Such short time periods can only be measured electronically

Author: Martin Moore

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