P3 1.1-1.4 (X-rays, CT Scans and Ultrasound)

These are some notes on the first part of the P3 Unit. Physics in medical uses. Hope it helps you.

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  • Created by: Iswariya
  • Created on: 21-05-13 17:46

Electromagnetic Spectrum

X-arys are part of the electromagnetic spectrum. All the waves in the electromagentic spectrum are transverse waves that travel at the same speed in a vacuum. However, the properties of the waves vary with their wavelength and so we give different names to different parts of the spectrum.

(http://amazing-space.stsci.edu/resources/qa/graphics/qa_emchart.gif)Electromagnetic Waves transfer energy - the shorter wavelengths transferring the most energy. Some rays can be ionising incluging X-rays meaning that they have enogh energy to remove electrons from atoms or molecules and form ions. This radiation can be harful for the body and can even cause cancer.

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X-Ray Images

X-rays affect the chemicals in photographic film in a similar way to light. When it is exposed to x-rays it will turn black, but x-rays are transmitted through some materials that are opaque to visible light, such as healthy human tissue. Other materials such as metal and bones will absorb the x-rays. This means that when the image is produced only these material show as white on a black background. For example, the bones would be white and the skin and muscle tissues that don't absorb the xrays will be black.

Modern x-ray machines use charge-coupled devices(CCD's) instead of photographic film. A CCD converts light to electrical signals, which can be recorded in a computer's memory. This device is also used in Digital Camera's.


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CT Scans

Computerised Tomography(CT) is a method used to bulid up a three-dimensional image of the inside of the object.

To make a CT scan of an object, or part of an object, X-ray images are taken from a number of differnt position and angles. All the results are processed in a computer to build a three-dimensional x-ray image. The image can be processed in a number of ways, depending on the nature of the investigation being carried out.


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Sound waves are longitudinal waves that can travel through solids, liquids and gases.

The human hearing range goes from 20Hz to 20 000Hz (or 20kHz).

Sounds higher than this frequency (20kHz) are known as ULTRASOUND. Ultra sound is used in medicine, but also has othee uses in industry and at sea.


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How an Ultrasound Works

  • When a wave passes through one medium to another, some of it is reflected off the boundary betwwn the two media, and some is transmitted (and refracted). This is partial reflection.
  • What this means is that you can point a pulse of ultrasound at an object, and wherevre there are some boundaries between one substance and another, some of the ultrasound gets refelected back.
  • The time it takes for the reflection to reach a detector can be used to measure how far the boundary is.
  • This is how ultrasound imaging works.


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The Formula

  • An oscilloscope trace can show an ultrasound pulse reflecting off seperate boundaries.
  • Give the "seconds per division" setting of the oscilloscope, you can qwork out the time between the pulse by measuring on the screen.
  • If you know the speed of sound in the medium you can work out the distance between the boundaries using this formula:

 s = v x t 

  s   is distance in metres, m

  v   is speed in meters per second, m/s


  t   is time in seconds, s

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Medical Uses Of Ultrasound

Pre-Natal Scanning

Since ultrasound can be partially reflected when it hits boundaries (between the fluid in the womb and the skin of the foetus) , the waves are detected. The exact timing and distribution of these echoes are processed by a computer to produce a video of the foetus.

Breaking Down Of Kidney Stones

Kidney Stones are hard masses that can block the urinary tract. An ultrasound beam concentrates high-energy waves at the kidney stone and turns it into sand-like particles. These particles then pass out of the body in the urine. It's a good method because the patient doesn't need surgery and it is relatively painless.


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Ad&Disad of Ultrasound

Advantages of Ultrasound Scanning

  • Ultrasound is a non-invasive technique i.e. it does not need to go beneath the skin or enter any orifice making it painless.
  • Ultrasound does not produce any harmful radiations thus making it safe for monitoring unborn babies.
  • It is a low cost and easy procedure therefore reducing the demand of hospital resources.
  • It can display real time images of the movement and function of body organs and blood vessels.

Disadvantage of Ultrasound Scanning

  • Ultrasound does not reflect clearly from bone or air thus giving it limitations. For images of the lungs and bones other techniques are clearer and therefore preferred
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Summary of ad&disad

CT scans use more radiation than X-rays so are not used as often

Ultrasound waves are non-ionising so are safe.

X-rays are ionising - can cause cancer.

Ultrasound images are typically fuzzy and not as clear - harder to diagnose conditionsusing these images.

X-ray photographs produce good images of bones and metals but, nothing else.

CT scans produce detailed images that can also be 3D so you can look around the object or patient.

X-rays can be aimed at a tumor to destroy it but, the rays have to be coming from different directions and crossing at the damaged cell or else they will damage more cells.

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