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
