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

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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

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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.

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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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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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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