Optimisation of X-ray imaging Flashcards Preview

Medical Physics 2: Radiology > Optimisation of X-ray imaging > Flashcards

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

CT Optimisation

- Auto mA - AEC
- Auto kV - some auto sys
- Lower mA over sensitive organs - inc mA to maintain im quality