Download How do we achieve Optimization?

Optimisation of Patient
Protection for Radiography
Colin Martin and David Sutton
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
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
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
Film Radiography – Speed Index
The most likely cause of higher doses in an
X-ray department

Speed of film/screen system
 Reciprocal
 Speed
dose to film in mGy
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
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
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
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
Good contrast at
lower energies
Mainly Compton
scattering, so poor
contrast at high
energies
Tissue Photoelectric
Tissue Compton sc.
Mass attenuation coefficient
Mass attenuation
coefficients for
photoelectric and
Compton
interactions
1
Bone Photoelectric
0.1
Bone Compton sc.
0.01
0.001
0.0001
0
50
100
Photon energy (keV)
150
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.
Tube potential:
Effective dose decreases as kVp is increased
2.5
Abdomen AP view
Dose (mSv)
2
1.5
Lower dose
1
Higher dose
Good contrast
0.5
Poor contrast
0
50
60
70
80
90
100
110
120
130
Tube Potential kVp
Exposure adjusted to give similar dose at the image receptor
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
1.00E+00
Reduces skin dose
Al 3 mm
Relative air kerma
8.00E- 01
Al 3 mm, Cu 0.2 mm
Increase in mAs
required
6.00E- 01
4.00E- 01
2.00E- 01
0.00E+00
0
10
20
30
40
50
60
70
80
Photon energy (keV)
Exposure adjusted to give similar dose at the image receptor
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
EC European Guidelines for
radiographic images
Provide recommendations for:
 speed index of film screen system
 filtration
 tube potential
 anti-scatter grid
 AEC
 exposure time
Quantitative assessment of clinical image quality
EC European
Guidelines for
radiographic
images

Visually sharp reproduction of features,

Visibility of image details in lung,
e.g. chest:
Vascular pattern in lung, trachea, borders of heart and aorta
e.g. high contrast 0.7 mm, low contrast 2 mm.
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
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