Topic 14: CT quality assurance + dose reduction Flashcards
(30 cards)
Why is dose determination important for CT?
1) CT = largest medical contribution of collective dose to population
2) Irradiation of image receptor = sufficient to get usable image
- CT invariability = non-uniform irradiation of many organs with different sensitivities
Give the contribution of CT to radiation exposure
- Total = 24%
- Medical = 50%
Define overall dose
- Closest thing = effective dose
How to estimate overall risk from CT exposure?
Risk = 0.05 (Sv−1) × ED (Sv)
How is effective dose found?
- Using 3 part computation
1) Calculate CT dose index for given technique factors of patient = CTDI
2) Multiply number by length of region irradiated = dose-length product = DLP
3) Conversion factor via Monte Carlo calculation = transforms DLP > ED
Define CTDI
- Patient independent measure = of machine output for set of technique factors
- Index of X-ray output of CT machine
- Invented to give average dose for single axial slice
Describe the structure of CTDI phantom
- Starting point for dose profile = narrow study of CTDI phantom
- Material = acrylic cylinder = 15 cm long
- Head phantom = 16 cm diameter
- Body phantom = 32 cm diameter
- Have 5 longitudinal holes = to insert calibrated 100 mm long pencil ion chamber = find dose profiles
Describe dose distribution in phantom
- Depends on beam angle
- Dose profile = dose averaged along central axis in full rotation of gantry
Factors causing CT profile to be bell-shaped
1) Focal spot not perfect tiny size = finite size = small amount of shadow blurring edges of primary X-ray beam creating image
2) Compton scatter within phantom/patient
Describe the dose along ion chamber
- 360° scan beam = certain width
- Dose differs along chamber depending on position
Describe CTDI100
- Total dose recorded over 100 mm by chamber
- Measured by ion chamber in central hole + peripheral
Describe how CTDIw is calculated
- Average of center + peripharal chambers
𝑪𝑻𝑫𝑰𝒘 =𝟏/𝟑 𝑪𝑻𝑫𝑰𝟏𝟎𝟎𝒄𝒆𝒏 + 𝟐/𝟑 𝑪𝑻𝑫𝑰𝟏𝟎𝟎𝒑𝒆𝒓
Describe pitch in spiral CT scanners
- Pitch = ratio of how far table moves in 1 rotation + width of X-ray
- Width of X-ray depending on number of detectors + width
- Pitch > 1 → the table moves faster → less overlap between slices → lower image quality = lower dose
- Pitch < 1 → the table moves slower → more overlap → better image quality = higher radiation dose
How is CTDIvol calculated?
𝑪𝑻𝑫𝑰𝒗𝒐𝒍 =𝑪𝑻𝑫𝑰𝒘/𝒑𝒊𝒕𝒄h
Describe CTDIvol
- CTDIvol = radiation dose per unit length
- Doesn’t account for how long the scan is = small area/large area = CTDIvol stays same
- Doesn’t reflect reality = total radiation patient receives depends on how much of their body is scanned
- To estimate total radiation exposure + risk = calculate dose-length product
How is DLP calculated?
DLP = CTDIvol × length
How is DLP transformed into effective dose?
- Monte Carlo calculations
- Method of convenience + simplicity = NOT accurate
- Multiply by either constant:
1) 0.002 = head
2) 0.015 = abdomen
Give typical values of effective doses of CT
- 1-2 mSv = head examination
- 5-7 mSv = chest
- 8-11 mSv = abdomen + pelvis
Describe the accuracy of CTDIvol
- Doesn’t give exact patient dose = general value
- Measure of radiation CT gives off = machine output = useful to compare between machines + settings
What method gives patient specific exact dose?
- Patient Air Kerma in Tomography methodology = PAKT
What programs bring awareness to dose reduction?
- Image gently
- ESR Eurosafe imaging
What are the reports about children’s CT examinations?
- 30% examinations are:
> Medically unnecessary
> Can be done using safer alternatives = US/MRI
Give effective ways for facilities to reduce dose
- Comparison of own protocols
- Caparison with other facilities with same machine type
How much dose reduction still has adequate image quality?
- Up to 50% reduction
- Due to change in kVp
- Due to reducing mA-s/rotation by 10%
