Optimisation of X-ray imaging

Question 1

What are the goals of image optimisation?

Answer 1

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

Question 2

Impact of scatter

Answer 2

Degrades Image quality

Question 3

Impact of a grid

Answer 3

• Improves image quality
• Reduces scatter
• Inc patient dose up to factor of 5

Question 4

When might the use of a grid be avoided ?

Answer 4

Extremities: low kV and thin tissue

Paediatric patients: greater radio-sensitivity

Question 5

Give an example of a general Optimisation Strategy

Answer 5

• Use phantom to simulation clinical examination
• Take exposures over a range of mAs values
• ID lowest dose with acceptable Im quality

Question 6

Advantages of DR

Answer 6

• Constant contrast
• Post-processing
• Can optimise for CNR

But difficult to detect over/under exposure

Question 7

Impact of Inc tube voltage

Answer 7

• 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

Question 8

Impact of Inc mAs

Answer 8

• Increase detector dose
• Increase patient dose
• Maintain radiation contrast
• Reduce noise

Question 9

Impact of Inc filtration

Answer 9

• Decrease output per mAs
• Increase penetration
• Decrease radiation contrast
• Reduce entrance surface dose for given detector dose
• Increase detector dose for given input dose

Question 10

Effect of CNR with dose and kV

Answer 10

CNR inc with ESD and inc kV

Question 11

Effect of increasing tube voltage on contrast and ESKA

Answer 11

Relative contrast and ESKA decrease with inc Voltage

Question 12

Impact of Cu thickness

Answer 12

Inc Cu thickness:

• inc rel det AK
• dec rel ESAK

Question 13

Figure of merit

Answer 13

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

Question 14

Mammo optimisation

Answer 14

Large drop in CNR with thickness

Dose for thin breast is remedial level

Question 15

Fluoro optimisation

Answer 15

• 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

Question 16

Fluoro - impact of reducing field size

Answer 16

• Inc entrance surface dose/doserate

- Inc limiting spatial resolution

Question 17

Cardiology - Fluoro optimisation

Answer 17

Image quality sacrificed to reduce patient dose

Image quality adequate to show easily visualised wires

Question 18

CT optimisation

Answer 18

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