X-ray Imaging (OCR A Level Physics) : Revision Note

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 less 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