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Radiographic Dosimetry
David Sutton / Colin Martin
Dundee
Kampala
IAEA DRLs KAMPALA
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
Kampala
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
Kampala
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)
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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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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) 2237-2250.
Air kerma-area product
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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.
Kampala
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
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KAP = K x Area
the SI unit of KAP is the Gy·cm2
Kerma-Area Product: KAP
Area = 1Dose = 1
Area = 4Dose = 1/4
d1=1
d2=2
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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 = 1Dose = 1
Area = 4Dose = 1/4
d1=1
d2=2
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Kerma-Area Product: KAP
Dosimetry using Phantoms
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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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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
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Kampala
Alternative phantoms
• ICRU phantoms– PMMA walls filled with water
• ANSI phantoms– PMMA + Al
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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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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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Kampala
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
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Calculation of ESAK
Determine appropriate backscatter factor (B) for clinical beam HVL & field-size
ESAK =IAK*BSF
BSF ~ 1.35, but there are tables
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Patient dosimetry
1. IAK calculated from measured tube output
2. ESAK calculated from measured tube output
3. ESAK measured using TLD
4. KAP measured using KAP meter on x-ray unit.
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Calculation
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First – know tube output
• Equipment for measuring tube output:
– Calibrated diagnostic dosimeter
– Chamber support stand
– Tape measure or ruler
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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)
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Where Y(d) is the X-ray tube output (mGy/mAs) at distance d from tube
Calculation of ESAK
Determine appropriate backscatter factor (B) for clinical beam HVL & field-size
ESAK =IAK*BSF
BSF ~ 1.35, but there are tables
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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
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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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Determination of patient doses from measurements on patients
(TLDs)
Direct determination of patient exposure
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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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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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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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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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Determination of patient doses from KAP measurements
Direct determination of patient exposure
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KAP
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Transmission ionization chamber
Radiation Protection in Paediatric Radiology L02. Understanding radiation units
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
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Typical uncertainties
• Phantom measurement of IAK: 6- 12%
• Calculation of IAK : 5.5 - 12.5%
• Calculation of ESAK : 6 - 13%
• TLD measurement of ESAK : 12% minimum , but probably a lot more
• KAP : up to 25%
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https://dl.dropbox.com/u/76170928/DRL%20%20Practical%20Exercise%20G03.xls
https://dl.dropbox.com/u/76170928/DRL%20Practical%20Exercise%20G02.xls