OCR A Level Physics

Revision Notes

6.11.4 X-ray Imaging

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Using X-rays in Medical Imaging

  • X-rays have been highly developed to provide detailed images of soft tissue and even blood vessels
  • When treating patients, the aims are to:
    • Reduce the exposure to radiation as much as possible
    • Improve the contrast of the image

Reducing Exposure

  • X-rays are ionising, meaning they can cause damage to living tissue and can potentially lead to cancerous mutations
  • Therefore, healthcare professionals must ensure patients receive the minimum dosage possible
  • In order to do this, aluminium filters are used
    • This is because many wavelengths of X-ray are emitted
    • Longer wavelengths of X-ray are more penetrating, therefore, they are more likely to be absorbed by the body
    • This means they do not contribute to the image and pose more of a health hazard
    • The aluminium sheet absorbs these long wavelength X-rays making them safer

Contrast & Sharpness

  • Contrast is defined as:

The difference in degree of blackening between structures

  • Contrast allows a clear difference between tissues to be seen
  • Image contrast can be improved by:
    • Using the correct level of X-ray hardness: hard X-rays for bones, soft X-rays for tissue
    • Using a contrast media

  • Sharpness is defined as:

How well defined the edges of structures are

  • Image sharpness can be improved by:
    • Using a narrower X-ray beam
    • Reducing X-ray scattering by using a collimator or lead grid
    • Smaller pixel size

Contrast Media

  • Contrast media is defined as:

A substance, such as barium or iodine, which is a good absorber of X-rays. A patient is given this so it can give a better contrast on an X-ray image

  • These are sometimes used because:
    • Some soft tissue organs do not show up on X-rays when the organ has a similar attenuation coefficient to other tissues in the same area
    • Contrast media are good absorbers of X-rays as they have a large attenuation coefficient
    • Hence when contrast media enter an organ, the image of the organ is enhanced when imaged using X-rays
  • Barium and iodine are used depending on the organ being imaged
    • Iodine is used as a contrast medium in liquids i.e. to observe blood flow - this is usually injected into the patient
    • Barium sulphate is used as a contrast medium in the digestive system - this is usually ingested by mouth and is known as a barium meal
  • The large attenuation coefficient of contrast materials is due to the large atomic number of these elements
    • Barium has an atomic number of 56, while iodine has an atomic number of 53



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