dose and image quality

Question 1

what 3 factors affect patient dose in CT

Answer 1

• kV
• mA
• rotating time

Question 2

mA directly proportional to dose
rotation time proportional to dose

kV also has an effect on contrast

Question 3

more photons are produced at higher kVp

Question 4

what is dose modulation

Answer 4

• adjust dose to maintain image quality

(if pt diameter increases by 4cm, 2x dose)

• centering is important

Question 5

what are 3 checks done for CT quality control

Answer 5

• image quality tests
• xray dose measurements
• mechanical checks

Question 6

what are you checking for during the quality checks

Answer 6

• baseline values (established either at acceptance testing or during first few ,months of scanners lifetime)
• during routine testing, you are checking that measurements do not differ significantly from these baseline

Question 7

why would you measure noise in relation to quality control checks

Answer 7

changes in noise is relative to the baseline values which can be caused by changes in:

• tube output
• beam width collimation
• focal spot size
• beam filtration
• detector problems
• reconstruction filter
• image filter

Question 8

what is used to check noise levels

Answer 8

scanners QA phantom
- produces noise image with region of interest and shows standard deviation (mean CT number)

Question 9

how often is the scanner QA phantom used to check noise levels

Answer 9

daily

Question 10

what does it mean if you see the same noise in each slice taken by the phantom

Answer 10

even distribution of dose to each detector

Question 11

what does it mean if you see different amount of noise in each slice image taken by phantom

Answer 11

collimation width not matched to detectors so non even dose given to detectors

Question 12

why would you test CT number values

Answer 12

• CT number depends on the xray energy spectrum of the beam
• any changes in spectrum may have more effect at one end of the CT number range than at other
• particular issue in quantitiative work and radiotherapy planning

Question 13

how is CT number uniformly checked/tested

Answer 13

• using head and body phantoms
• image taken and the difference between the centre and peripheral regions are compared
• ring artefacts are checked first using narrow window

Question 14

how often is CT number uniformity checked

Answer 14

annually

Question 15

how is high contrast spatial resolution checked

Answer 15

• using bar and point phantoms
• look for smaller distinguishable pattern you can see
• quote line pairs/cm

Question 16

what determines the slice thickness in an axial scan

Answer 16

detector groups

(post patient collimation which covers the detectors leaving only the required detectors open for the beam to reach)

Question 17

what determines the slice thickness in helical scannign

Answer 17

depends on helical interpolation algorithm

simultaneous circular movement in the X and Y axes combined with an axial feed (Z axis) at a defined pitch.

Question 18

what is used to test slice thickness in axial scans

Answer 18

• 0.6mm aluminium inclined plate at 30 degrees

Question 19

how is the aluminium plate used to analyse slice thickness

Answer 19

• plat passes diagonally through image slice length wise
• as you already know the sample/focal aperture distance and the angle in which the plate passes through the slice
• use SOHCAHTOA (tan specifically) to calculate the width/thickness of the slice

Question 20

what is used to calculate slice width in helical scans

Answer 20

0.05mm tungsten thin disc in a perspex rod

Question 21

why measure CTDI

Answer 21

• most sensitive and valuable parameter
• changes in CTDI can be due to : tube output, beam width collimation, focal spot, beam filtration

Question 22

how do you measure irradiated beam thickness

Answer 22

• use therapy xray film (Kodak XV-2) placed in gantry isocentre
• scan with half clinical mAs
• analyse with scanning micro-densitometer
• or use ruler for quick check

Question 23

how often is a scan place localisation with alignment light check done

Answer 23

1-3 month check

• test alignment of light to xray beam

Question 24

how / when /why is a alignment light test done

Answer 24

• done at start of quality control session
• helps subsequent set up of phantoms
• important clinically to have accurate alignment

Question 25

what are the 2 equipment/ways to test alignment of lights

Answer 25

- radio-opaque wire place on phantom or table - pinholes made in film at position of laser light

Question 26

how do you assess the quality of table travel measurement

Answer 26

- mark placed on couch top and couch base (patient table and below it) - gantry display shows the distance the table moves - if you move the table 1m (measured with a ruler) - the gantry display should also show a distance of 1m

Question 27

how can you check tube to detector alignment

Answer 27

after image is taken, you should see an image with central darkness, the fading from grey to white going out IF the film shows asymmetry then there is poor alignment of focal spot with collimation

Question 28

IRR = staff safety - scanning behind lead glass - never in room during scan - contingency plans in place IRMER = patient safety - justified exam - trained operator - pregnancy - alarp - dose optimisation