1. Optimisation of Patient Protection for Radiography
Colin Martin and David Sutton

2. Aspects of Optimisation
1 Recognition of the lowest level of image quality required to permit diagnosis
2 Finding the set of imaging conditions to give this image quality with the lowest dose
3 Ensuring that any reduction in image quality does not jeopardise diagnosis
Linked to type of examination
This includes adequacy of equipment

3. Optimisation Optimisation depends on:
Understanding the advantages and disadvantages of available options
Optimum utilisation of the options available
Use of the lowest dose to give the required level of image quality
Close co-operation between practitioners, operators and physicists

4. Factors which affect Patient Dose & Image Quality
Photon fluence - No of photons
Speed index of film/screen combination
Factors chosen for Computed Radiography
Radiation quality – Energy distribution of photons
Tube potential
Filtration

5. Film Radiography – Speed Index The most likely cause of higher doses in an X-ray department
Speed of film/screen system
Reciprocal dose to film in mGy
Speed depends on phosphor thickness and crystal size
Need to match application to film / screen combination
Recommended speed index
400 use of 200 will double dose
Film processing also affects result

6. Factors Affecting Dose and Image Quality Techniques for scatter reduction
Grid (grid factors)
Use of a grid typically doubles the dose
Grid parameters should be chosen for application
air gap – high kV chest
No grid for
paediatric examinations on younger children
some fluoroscopic examinations
Too short a focus to film distance will increase skin dose

7. Factors Affecting Dose Attenuation of components between patient and image receptor
Table
Modern equipment uses low attenuation materials.
New radiographic tables transmissions  95%
Some older tables  70%
Grid interspaces
New grid – fibre interspaces
Old grids – aluminium transmits < 50%
These factors should be considered in equipment selection

8. Radiation Quality
Contrast results from the removal of photons from the primary beam
Photoelectric Effect
Probability  Z3
Good contrast, but higher dose as photons
Compton Scattering
Low contrast (Tissue density alone)
Scattered photons add to noise
More Photoelectric, greater contrast; less noise

9. Mass attenuation coefficients for photoelectric and Compton interactions
Good contrast at lower energies
Mainly Compton scattering, so poor contrast at high energies

10. Radiation Quality
Use of low tube potentials requires a greater fluence to achieve desired exposure level
Higher patient dose
Beam quality must achieve balance between these elements.

11. Tube potential: Effective dose decreases as kVp is increased
Lower dose
Poor contrast
Higher dose Good contrast
Exposure adjusted to give similar dose at the image receptor

12. Copper filtration: Removes low energy photons
Spectra of 80 kV X-ray beams with and without 0.2 mm copper filter.
X-ray photons
removed
Reduces skin dose
Increase in mAs required
Exposure adjusted to give similar dose at the image receptor

13. Optimisation following dose surveys
Survey patient doses
Compare results with Diagnostic Reference Levels
Identify departments where doses are high
Analyse performance
Determine why doses are high
Determine whether action could be taken to optimise
Make recommendations
Make recommendations on technique
Follow up with further dose surveys to monitor changes
Highlight priority for replacement of equipment

14. EC European Guidelines for radiographic images
Provide recommendations for:
speed index of film screen system
filtration
tube potential
anti-scatter grid
AEC
exposure time

15. EC European Guidelines for radiographic images
Quantitative assessment of clinical image quality
EC European Guidelines for radiographic images
Visually sharp reproduction of features, e.g. chest: Vascular pattern in lung, trachea, borders of heart and aorta
Visibility of image details in lung, e.g. high contrast 0.7 mm, low contrast 2 mm.

16. EC image quality criteria
A quantitative measure of clinical image quality
Relatively simple method for evaluation of individual clinical images
Should be used to assess changes in technique - e.g. lower dose methods
Important in achieving balance between dose and image quality

17. Optimisation of performance
Efficient dose management depends on
Understanding the advantages and disadvantages of the dose / imaging options available
Correct film/screen speed and combination
Appropriate tube potential for each examination
Consider removal of grid for paediatrics and low kV chest
Physicist can advise on dose consequences
The radiologist must decide whether the clinical outcome may be compromised