Flashcards in Optimisation of X-ray imaging Deck (18)
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1
What are the goals of image optimisation?
Image quality should be adequate for the task not as
good as the system is capable of producing.
Aim for ALARP dose
Consider NDRLs and DRls
2
Impact of scatter
Degrades Image quality
3
Impact of a grid
- Improves image quality
- Reduces scatter
- Inc patient dose up to factor of 5
4
When might the use of a grid be avoided ?
Extremities: low kV and thin tissue
Paediatric patients: greater radio-sensitivity
5
Give an example of a general Optimisation Strategy
- Use phantom to simulation clinical examination
- Take exposures over a range of mAs values
- ID lowest dose with acceptable Im quality
6
Advantages of DR
- Constant contrast
- Post-processing
- Can optimise for CNR
But difficult to detect over/under exposure
7
Impact of Inc tube voltage
- inc output per mAs
- dec the rad contrast
- Inc the penetration of the beam
- Inc detector dose for given entrance dose
- Reduce patient dose for same det dose
8
Impact of Inc mAs
• Increase detector dose
• Increase patient dose
• Maintain radiation contrast
• Reduce noise
9
Impact of Inc filtration
• Decrease output per mAs
• Increase penetration
• Decrease radiation contrast
• Reduce entrance surface dose for given detector dose
• Increase detector dose for given input dose
10
Effect of CNR with dose and kV
CNR inc with ESD and inc kV
11
Effect of increasing tube voltage on contrast and ESKA
Relative contrast and ESKA decrease with inc Voltage
12
Impact of Cu thickness
Inc Cu thickness:
- inc rel det AK
- dec rel ESAK
13
Figure of merit
FOM = (CNR)^2 / Dose
Dose dep is removed from analysis
Take exposures over a range of beam qualities for a range of phantom thicknesses / compositions.
Highest FOM is optimal
14
Mammo optimisation
Large drop in CNR with thickness
Dose for thin breast is remedial level
15
Fluoro optimisation
• Maximise FSD
• Minimise patient-detector distance
• Minimise fluoro time
• Used pulsed fluoro
• Avoid exposing same area
• Larger patients
• Oblique projections
• Avoid mag views in fluoroscopy
• Minimise number of frames of acquisitions
• Use collimation
16
Fluoro - impact of reducing field size
- Inc entrance surface dose/doserate
- Inc limiting spatial resolution
17
Cardiology - Fluoro Optimisation
- Im quality sacrificed to reduce patient dose
- Im quality adequate to show easily visualised wires.
18
