Radiographic Dosimetry IAEA DRLs KAMPALA Radiographic Dosimetry David Sutton / Colin Martin Dundee 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

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

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) Kampala

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. Kampala

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. 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 Kampala

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 Kampala 9 9 9

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 Kampala 10 10 10

Dosimetry using Phantoms Kampala

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 Kampala

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

Kampala

Alternative phantoms ICRU phantoms ANSI phantoms PMMA walls filled with water ANSI phantoms PMMA + Al Kampala

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 Kampala

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 Kampala

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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 Kampala

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

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

Calculation Kampala

First – know tube output Equipment for measuring tube output: Calibrated diagnostic dosimeter Chamber support stand Tape measure or ruler Kampala

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 Kampala

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

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

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 Kampala

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

Equipment for direct measurement of ESAK Thermoluminescence reader (or access to external TLD service) Well calibrated TLD in sachets Worksheet for recording data Kampala

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 Kampala

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 Kampala

Sources of uncertainty Measurement scenario Precision of reading Uncertainty in measurement position Uncertainty in back scatter factors Uncertainty in TLD correction factors Kampala

Determination of patient doses from KAP measurements Direct determination of patient exposure Patient exposure rather than dose because PKA is not a dose Kampala

KAP Transmission ionization chamber Kampala

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

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% Kampala

https://dl.dropbox.com/u/76170928/DRL%20%20Practical%20Exercise%20G03.xls https://dl.dropbox.com/u/76170928/DRL%20Practical%20Exercise%20G02.xls Kampala