Radiographic Physics

Elizabeth Stokell MA VetMB CertVR MRCVS
Department of Clinical Veterinary Medicine
University of Cambridge
Madingley Road

1. Introduction

The production of a radiograph involves the use of complicated apparatus and a sequence of complex physical processes. A basic understanding of radiological physics will allow the radiographer to make the best use of the equipment available.

Radiography - the processes involved in getting a radiograph
Radiology - the procedure of looking at a radiograph

2. The Electromagnetic spectrum and ionising radiation

  1. X-Rays form only one part of the electromagnetic spectrum, which includes radio waves, microwaves (radar, heating etc.), infra-red, visible light, ultraviolet light, X-Rays and gamma rays.
  2. These radiations all have a few common properties:
  3. The velocity (300,000,000 m/s in a vacuum) is equal to the wavelength multiplied by the frequency
  4. The energy of a photon is equal to the frequency multiplied by Planck's constant

3. Properties of X-Rays

  1. Fluorescence
  2. Photographic effect
  3. Penetration
  4. Excitation and ionisation
  5. Biological effect

4. The production of X-Rays

  1. X-Rays are produced whenever charged particles are slowed down or stopped
  2. Continuous spectrum
  3. Characteristic spectrum

5. The X-Ray tube

  1. Since electrons are produced when fast-moving electrons are slowed down or stopped, there are three basic requirements for X-Ray production in an X-Ray tube:
  2. The cathode:
  3. The anode

6. Other parts of an X-Ray machine

  1. The filament circuit to heat the filament
  2. The high voltage circuit to provide the high potential difference across the tube
  3. A timer to control the duration of the exposure

7. Factors affecting the quality and intensity of beam

  1. Quality is a measurement of the penetrating power of the X-Ray photons. The quality of the beam increases as the proportion of high energy photons increases
  2. Intensity is a measure of the amount of radiation produced
  3. Various factors affect the quality and intensity of the beam

8. Interaction of X-Rays with matter

  1. When a beam of X-Rays passes through matter, its intensity is reduced as the energy is absorbed or scattered
  2. The degree of attenuation depends on the atomic number and physical density of the tissue, and the energy of the X-Rays
  3. The differential absorption of X-Rays by the various tissues of the patient allows a radiographic image to be made

9. Properties of radiographic film and formation of the radiographic image

  1. X-Rays will produce a pattern of varying depths of blackness on the film
  2. The incident photons are attenuated to varying degrees by the patient's tissues
  3. The emerging pattern of photons of differing energies affect the film emulsion to produce the latent image
  4. Developing the film allows this image to become visible
  5. The film emulsion consists of gelatin containing finely dispersed grains of silver bromide
  6. The absorption of X-Rays by the silver bromide grains renders them developable
  7. Silver bromide grains which have been exposed to X-Rays will be turned black by the developer
  8. Unexposed grains are dissolved and removed and leave a white area on the film
Consider the possibilities when making a radiograph of a patient's thorax:
  1. Some rays does not penetrate the chest at all, and are only slightly attenuated by air.
    Therefore the maximum number of photons will pass through to the film.
    This causes maximum exposure of that part of the film, causing blackening of the film
  2. Some rays pass through the body wall and the lung.
    The body wall will absorb a number of photons, but few photons are absorbed by the aerated lung.
    Therefore the film will receive a medium exposure
  3. Some rays pass through the body wall, mediastinum and heart.
    Most of the photons will be absorbed, and only a few pass on to the film.
    This results in a low exposure of that part of the film, causing a relatively white area to be seen on the film

10. Effect of the exposure factors on the film

  1. kV: governs the penetrating power of the photons
  2. mAs: governs the amount of X-Rays reaching the film
  3. Distance: affects the amount of X-Rays reaching the film by the inverse square law
  4. In essence, the kV controls the number of shades of grey, and the mAs controls how dark they are

11. Cassettes and intensifying screens

  1. A cassette is a light-tight, robust container which will:
  2. Two intensifying screens are present within the cassette, either side of the film

12. Qualities of a good diagnostic radiograph

  1. An accurate portrayal of the structures under examination, which requires good positioning with the minimum of geometric distortion (the region of interest should be under the central ray, parallel to and close to the cassette)
  2. Easy perception of the relevant structures with good contrast and detail
  3. No misleading artefacts

13. Objective qualities of a good radiograph

  1. Contrast
  2. Detail and definition

14. Identification of radiographs

  1. All films must be adequately labelled and show
  2. Various methods are available
  3. Additional information may need to be incorporated

15. Radiation safety

  1. Protection of the radiographer
  2. Protection of the patient
  3. Protection of the public