1. Radiographic Dosimetry
IAEA DRLs KAMPALA
Radiographic Dosimetry
David Sutton / Colin Martin
Dundee
Kampala

2. Reminder…. Projection radiography (2D)
Image receptor may be screen-film / CR / DDR
Examinations such as chest, abdomen, limbs, skull ……
Fixed position on body
Relatively low doses
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3. Dosimetric quantities
Incident air kerma IAK (Ki)
measured for phantoms
calculated for patients
Entrance surface air kerma ESAK (Ke)
measured or calculated for patients
Air kerma-area product KAP (PKA)
measured for patients
Kampala

4. Incident Air Kerma
Measured Free in Air on the central beam axis at the focal spot to surface distance.
Only primary beam is considered, that is, no scatter contribution.
Unit: joule/kg or gray (Gy)
Kampala

5. Entrance Surface Air Kerma (ESAK)
ESAK measured on the surface of the patient or phantom where X-ray beam enters the patient or phantom.
Includes a contribution from photons scattered back from deeper tissues, which is not included in free in air measurements.
Also known as Entrance Surface Dose (ESD)
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6. Entrance Surface Air Kerma (ESAK)
If measurements are made at other distances than the true focus - to - skin distance, doses must be corrected by the inverse square law and backscatter factor incorporated into the calculation.
References:
Dosimetry in Diagnostic Radiology: An International code of practice, TRS 457, IAEA, 2007
Phys. Med. Biol. 43 (1998)
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7. Air kerma-area product
The air kerma-area product, PKA or KAP, is the integral of the air kerma over the area of the X ray beam in a plane perpendicular to the beam axis, thus
Unit: Gy m2
KAP has the useful property that it is approximately invariant with distance from the X ray tube focus (when interactions in air and extra-focal radiation can be neglected), as long as the planes of measurement do not include a significant contribution from backscattered radiation from the patient or phantom.
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8. Kerma-Area Product: KAP
The kerma - area product (KAP) is defined as the kerma in air in a plane perpendicular to the incident beam axis, integrated over the area of interest.
This is the dose related quantity measured and displayed on all modern X-ray equipment excluding CT (in Europe).
KAP meter
Kampala

9. Kerma-Area Product: KAP
Area = 1 Dose = 1
Area = 4 Dose = 1/4
d1=1
d2=2
KAP = K x Area
the SI unit of KAP is the Gy·cm2
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10. Kerma-Area Product: KAP
KAP is independent of distance from the X-ray source, as:
Air Kerma decreases with the inverse square law.
Area increase with the square distance
KAP is usually measured at the level of the tube diaphragms
Area = 1 Dose = 1
Area = 4 Dose = 1/4
d1=1
d2=2
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11. Dosimetry using Phantoms
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12. Phantoms Dosimetry with phantoms only makes sense if AEC is used
With manual setting of mAs phantom is not needed (will only be used as holding device for dosimeter) – Ki measurement can be made free in air without phantom
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13. Phantoms for general radiography measurements
CDRH Chest & Abdomen/L Spine phantoms
Correspond to average US citizen in PA/AP projection
Incorporate holders for ionization chambers (avoiding back scatter)
Constructed from PMMA & Aluminium (plus air for chest phantom)
Obtainable commercially or can be manufactured
Kampala

14. Kampala

15. Alternative phantoms ICRU phantoms ANSI phantoms
PMMA walls filled with water
ANSI phantoms
PMMA + Al
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16. Equipment for phantom measurements
Diagnostic dosimeter calibrated for general radiography beam qualities
CDRH chest phantom
CDRH abdomen/lumbar spine phantom
Set of Al attenuators and lead diaphragm for HVL measurements
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17. Methodology for phantom measurements
Set up equipment for chosen exam of normal adult patient
AEC
tube voltage (kV)
grid / air gap
focus-skin distance (FSD)
collimation
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18. Kampala

19. Calculation of incident air kerma
dFTD : measured tube focus-to-patient support distance in mm dm : distance from the table top (or a wall Bucky) to the reference point of the chamber at the measurement position tP : thickness of a standard chest (or abdomen/lumbar spine) patient
Kampala

20. BSF ~ 1.35, but there are tables
Calculation of ESAK
Determine appropriate backscatter factor (B) for clinical beam HVL & field-size
ESAK =IAK*BSF
BSF ~ 1.35, but there are tables
Kampala

21. Patient dosimetry IAK calculated from measured tube output
ESAK calculated from measured tube output
ESAK measured using TLD
KAP measured using KAP meter on x-ray unit.
Kampala

22. Calculation
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23. First – know tube output
Equipment for measuring tube output:
Calibrated diagnostic dosimeter
Chamber support stand
Tape measure or ruler
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24. Calculation of patient incident air kerma
Record technique parameters for examination
tube voltage
tube loading - mAs
focus-skin-distance or focus-film distance (dFTD) & patient thickness (tp)
Where Y(d) is the X-ray tube output (mGy/mAs) at distance d from tube
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25. BSF ~ 1.35, but there are tables
Calculation of ESAK
Determine appropriate backscatter factor (B) for clinical beam HVL & field-size
ESAK =IAK*BSF
BSF ~ 1.35, but there are tables
Kampala

26. Alternatively Measure ESAK to 20 cm perspex at 100cm FSD
Use inverse square law
But note
IAK is measured as part part of QA mesurement
Kampala

27. When can’t you calculate?
When is this approach not possible?
AEC used on a system with no post exposure mAs display
Still possible: TLD or KAP
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28. Determination of patient doses from measurements on patients (TLDs)
Direct determination of patient exposure
Patient exposure rather than dose because PKA is not a dose
Kampala

29. Equipment for direct measurement of ESAK
Thermoluminescence reader (or access to external TLD service)
Well calibrated TLD in sachets
Worksheet for recording data
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30. Methodology for direct measurement of entrance air kerma
Retain 1 TLD sachet for assessment of background correction
When patient positioned, attach 3 TLD sachets to skin at centre of entrance beam
Record patient & technique data with TLD identification
Remove TLD after exposure & attach to worksheet
Read TLDs to obtain dose readings & background correction
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31. Entrance air kerma from TLD measurements
: mean value of dosimeter readings with background correction kf : correction factor for fading of TL signal NK,Qo : dosimeter calibration coefficient kQ : factor which corrects for differences in the response of the dosimeter at the calibration quality Q0, and at the quality Q of the clinical X- ray beam
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32. Sources of uncertainty
Measurement scenario
Precision of reading
Uncertainty in measurement position
Uncertainty in back scatter factors
Uncertainty in TLD correction factors
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33. Determination of patient doses from KAP measurements
Direct determination of patient exposure
Patient exposure rather than dose because PKA is not a dose
Kampala

34. KAP
Transmission ionization chamber
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35. Kerma-Area Product: KAP
It is always necessary to calibrate and to check the transmission chamber for the X-ray installation in use
In some European countries, it is compulsory that new equipment is equipped with an integrated ionization transmission chamber or with automatic calculation methods
Radiation Protection in Paediatric Radiology L02. Understanding radiation units

36. Typical uncertainties
Phantom measurement of IAK: 6- 12%
Calculation of IAK : %
Calculation of ESAK : %
TLD measurement of ESAK : 12% minimum , but probably a lot more
KAP : up to 25%
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37.
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