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

Using X-rays in Medical Imaging

• 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

Contrast Enhancement

• 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