1 to 1- Exposure and dose OCSE style questions

Question 1

For 2 marks, explain the difference in DRL between an AP pelvis and an AP lumbar spine despite the same exposure factors being used
Pelvis kV 80, mAs 15 DRL 3
AP lumbar spine kV 80, mAs 15 DRL 1.6

Answer 1

An AP lumbar spine has a smaller field of view/collimation than an AP pelvis
This reduces the area exposed and the DAP leading to a lower DRL

Question 2

For 2 marks: Based on the exposure chart, give an mAs for a lateral lumbar spine on an average patient. Justify your answer

Pelvis kV 80, mAs 15 DRL 3
AP lumbar spine kV 80, mAs 15 DRL 1.6
Lat lumbar kV 85, mAs ??? DRL 3

Answer 2

30mAs (or thereabouts)
There is approximately twice as much tissue depth for a lateral L-spine as there is for an AP pelvis therefore twice as many photons are needed therefore mAs is doubled

Question 3

For 1 mark: Based on the exposure chart, state the kV and mAs you would set to undertake an AP pelvis, ?#, on this 32 week pregnant lady – the exposure has been justified by a radiologist.

For a further 2 marks, justify your choice of exposures

Pelvis kV 80, mAs 15 DRL 3

Answer 3

100kV; 3.75mAs
110kV; 1.8mAs

A high kV technique is needed to reduce absorbed dose and thus dose to the foetus
Explanation of exposure manipulation
Image contrast is reduced but overall brightness is maintained so ALARP is achieved. Change in risk/benefit ratio
Medico-legally life of the mother take precedence

For every 10kV added, half the mAs

90kV, 7.5mAs (not high enough kV to significantly affect absorbed dose)
100kV, 3.75mAs
110kV, 1.8mAs

Question 4

You are asked to undertake AP and lateral weight bearing projections of both ankles on a 14 year old boy.

You undertake the first AP projection, using the standard exposure factors. You note that the deviation index is shown as amber with a figure of +2.7.

For one mark, what action do you now take and for a two further marks, justify your answer?

AP ankle kV 60, mAs 3.2 DRL 0.08

Answer 4

For the remaining projections, exposure should be reduced
This image is over exposed

+2 deviation = 60% too much exposure
Therefore, need to reduce by 60%
Stay at 60kVp (skin dose)
Reduce mAs by 60%
= approx. 1.5mAs

Question 5

For 2 marks what does DRL stand for and what is it used to indicate?

Answer 5

Diagnostic reference level
To identify unusually high/low exposure levels

Question 6

For 1 mark what exposure factors would you use for an AP ankle on a 2 year old child?
For a further mark, justify your choice of exposures

AP ankle kV 60, mAs 3.2 DRL 0.08

Answer 6

60kV 0.8mAs @ 100cm
50kV 1.6mAs @ 100cm

2 year old has about half the volume of tissue in ankle than an adult

Question 7

You image a standard AP ankle, the DAP is recorded at 1.2µGy.m2 For 4 marks what action would you now take?

AP ankle kV 60, mAs 3.2 DRL 0.08

Answer 7

Check the settings to ensure the correct exposure has been set
Review the image to assess exposure index:
If EI/DI is within normal range, send image, undertake QA and report incident
If EI implies over exposure, may need to repeat image.
Report to RPS (in either case)
Complete datex form
Do not use room again until situation resolved, and QA checks undertaken
If patient has received too much dose, they must be informed (duty of candor)
Any other reasonable response

Question 8

For 1 mark, what exposure would you use when undertaking a chest x-ray on a patient with previously diagnosed COPD? For a further mark, justify your answers

PA chest kV 125, mAs 1, DRL 0.12

Answer 8

115kVp, 0.5mAs

COPD leads to emphysema and thus very low density lungs
Halving the exposure will ensure ALARP is achieved and take cumulative dose into account

Question 9

For 2 marks what does DRL stand for and what is it used to indicate?

Answer 9

Diagnostic reference level
To identify unusually high/low exposure levels

Question 10

For 1 mark what exposure factors would you use for an AP ankle on the 2 year old child shown?
For a further mark, justify your choice of exposures

Answer 10

60kV 0.8mAs @ 100cm
50kV 1.6mAs @ 100cm

The 2 year old has about half the volume of tissue in ankle than an adult

2b) further explained (not needed to gain marks)
Halving the volume of tissue requires half the exposure
To half an exposure – use the “rule of 10”
Minus 10kVp and ½mAs

Therefore 60kVp and 3.2mAs becomes 50kVp and 1.6mAs

50kVp may be too low (is there an increased skin dose??)

Though good contrast to noise ration…
Manipulate to increase kV to at least 60kVp (why? – there is some debate over the need to stay above 60Kv in paeds)

Increasing kVp requires a reduction in mAs to maintain the same overall image density

A different “rule of 10”…
For every 10kVp increase – half the mAs

Therefore manipulate to 60kVp and 0.8mAs
In practice 60kVp and anything between 0.5 & 1mAs will work

Question 11

You image a standard AP ankle, the DAP is recorded at 1.2µGy.m2 For 4 marks what action would you now take?

normal DAP 0.08

Answer 11

Check the settings to ensure the correct exposure has been set

Review the image to assess exposure index:
If EI/DI is within normal range, send image, undertake QA and report incident

If EI implies over exposure, may need to repeat image.

Report to RPS (in either case)

Complete datex form

Do not use room again until situation resolved, and QA checks undertaken

If patient has received too much dose, they must be informed (duty of candor)

Any other reasonable response

Question 12

For 1 mark, what exposure would you use when undertaking a chest x-ray on the pictured patient with previously diagnosed COPD? For a further mark, justify your answers

Patient is standing and able bodied

Answer 12

115kVp, 0.5mAs

COPD leads to emphysema and thus hyperinflated, low density lungs

Halving the exposure will ensure ALARP is achieved and take cumulative dose into account