Using X-rays in Medical Imaging (AQA A Level Physics)

• X-ray imaging has become a highly developed technique which enables physicians to produce detailed images of bones, soft tissues and even blood vessels
• When treating patients, the main aims of X-ray imaging are to:
  • Reduce the patient's 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 radiation dosage possible
• The X-ray dose given to a patient depends on
  • The exposure time
  • The intensity of the beam

• X-ray equipment is designed to reduce exposure and minimise the risk to the patient by
  • Controlling the intensity of the X-ray beam
  • Using a beam definer
  • Using a metal filter
  • Using sensitive detection methods

Controlling the intensity of the X-ray beam

• The anode p.d. controls the maximum energy of the X-ray photons from an X-ray tube
  • The higher the anode p.d., the shorter the wavelength and hence, the higher the energy of the X-ray photons
  • Shorter wavelengths of X-ray (high energy photons) are more penetrating, therefore, they are less likely to be absorbed by the body
• The cathode current controls the intensity of the X-ray beam
  • The higher the cathode current, the more electrons that are emitted by thermionic emission
  • If more electrons reach the anode each second, then more X-ray photons are emitted per second

• To minimise the exposure to the patient, the beam intensity should be reduced by lowering the cathode current
• This minimises the risk to the patient by reducing the number of ionising photons passing through the patient each second

Beam definers

• A beam definer, or lead diaphragm plate, consists of two pairs of lead sheets with a narrow aperture in the centre which is placed close to the X-ray tube (where the X-rays are emitted)

• Lead diaphragm plates minimise the exposure to the patient by producing a focused (collimated) beam
• This is necessary because:
  • Photons are emitted by the X-ray tube in many directions
  • The lead plates absorb the scattered photons and the aperture allows X-rays travelling in a specific direction to pass through

• This minimises the risk to the patient because the narrow beam is used to investigate a specific area of the body only 
• Therefore, the areas of the body not being scanned are not exposed to ionising photons 

Metal filters

• A metal filter is a thin sheet of metal, usually aluminium, which is placed in the path of the beam between the X-ray tube and the patient 

• Aluminium filters minimise the exposure to the patient by reducing the intensity of low-energy X-rays
• This is necessary because:
  
  • Many wavelengths of X-ray are emitted by the X-ray tube
  • Longer wavelengths of X-ray (low energy photons) are less penetrating, therefore, they are more likely to be absorbed by the body

• As a result, an aluminium filter minimises the risk to the patient because it reduces the amount of ionising photons which the body could absorb
• This happens because the aluminium sheet:
  • Absorbs a large percentage of the low-energy photons (which are not needed to produce an image)
  • Allows the high-energy photons to pass straight through

Sensitive detection methods

• The exposure time can be reduced by using a more sensitive X-ray detector, by
  • Using an electronic detector instead of photographic detection
  • Intensifying the image

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

• A contrast medium is defined as:

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

• The use of a contrast medium is sometimes required because:
  • Some soft tissue organs do not show up on X-rays when the organs have similar attenuation coefficients
  • Contrast media are good absorbers of X-rays as they have a large attenuation coefficient
  • Hence when contrast media enter an organ, the X-ray image is enhanced as the substance is opaque to 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

Using iodine as a contrast medium

Iodine makes liquids, such as blood, opaque to X-rays and improves the contrast of the X-ray image

Using a barium meal as a contrast medium

Barium makes intestines opaque to X-rays and improves the contrast of the X-ray image