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Calibration of the DAP-meter

Step-by-Step

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DAP-meter must be calibrated

• Comparing the readings of the DAP-meter being calibrated and those of the reference meter results in the calibration factor

• Real Dose can be aquired from the readings on the DAP-meter with the help of the calibration factor

• Real Dose can be compared with national and international patient dose recommendations

• The reference meter has to be calibrated beforehand by a reliable facility (e.g. STUK in Finland)

• The reference meter needs to have its own calibration factor

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Calibration Meters

• Either Air Kerma meter or another DAP-meter can be used for calibration of DAP-meter

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Air Kerma Meter

• Displays the dose absorbed in air

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Air Kerma Meter

• A precise real field size is needed

• Remember to check the distance. You will need to know it when calibrating

• Area exposed can be calculated from the field size

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Verifying the Field Size

• The field size can be adjusted with a film, an imaging plate (CR) or a direct digital imaging (DR)

• Remember to measure the distance perpendicularly

• The field size should be relatively large e.g. 15cmx15cm with 1m distance

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Verifying the Field Size

• The real field size is measured with the accuracy of 1mm either from the film or the display

• Please do notice that the image on the display can be in the wrong scale and must be adjusted with a proper measurement technique included in the computer program

• It is also possible to place the scale on the film/image plate to make the measuring easier

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Air Kerma Meter

• The air kerma meter should be exposed to radiation with the same distance and field size as in the verification of the field size

• If you do use a different distance for some reason, remember to calculate the correct dose from the reading on the air kerma meter accordingly

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Radiation of the Air Kerma Meter

• The film is replaced by the air kerma meter

• The exposure parameters should stay the same as before

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Air Kerma Meter• The exposure parameters and

filtering should be the same as commonly used with patients

• The exposure parameters should be realtively high for example those used for thorax, hip or abdomen

• Please do notice that with lower exposure parameters the accuracy of the DAP-meter might decrease

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Calibration Factor• The calibration factor can be calculated

by dividing the reading on the DAP-meter with the multiplication of the area exposed and the air kerma

• DAP/(Air Kermaxcm2)

• If the calibration factor is below 1, the reading on the DAP-meter is too high. In other words the DAP-meter is showing higher dose than it should

• If the calibration factor is above 1, the reading on the DAP-meter is lower than the real dose.

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How to Use the Calibration Factor

• The calibration factor is multiplied with the reading on the DAP-meter

• The result is called Real Dose, which can be compared with both national and international reference values

• For the comparison purposes it is necessary to gather the DAP-values of several similar examinations. For example the thorax-examinations of 10 patients weighing about 70kg

• Please do notice that the patient dose given by the DAP-meter is not adequate to determine the real dose received by an individual patient

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Tandem Method

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Tandem Method

• In the tandem method the DAP-meter readings are to be compared with those of a reference DAP-meter

• The reference meter should be positioned in the air at appropriate distance (e.g. 30cm) of the DAP-meter to be calibrated. This is done to minimize the effect of the electromagnetic scattering

• The field size should be fitted so that it is completely within the ionisation chamber of the DAP-meter e.g. 8cmx8cm

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Tandem Method

• Both of the DAP-meters are to be exposed to radiation simultaneously

• The exposure parameters and filtering should be the same as used commonly with real patients

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Calibration Factor

• The reading on the meter to be calibrated is divided by the reading on the reference meter. The result is called calibration factor

• The calibration factor is used to determine the real dose

• Real dose can be used for comparison with other doses measured

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Calibration of the Calibration Meter

• It is very important that the reference meter used in the tandem method has been calibrated by a reliable facility

• List of such facilities in the Nordic countries is presented at the end of this slide show

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Reference Values

• DAP-values cannot be used to determine the real dose received by an individual patient

• DAP-meter is best suited for valuation of quality in an x-

ray unit

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DAP and ESD

• ESD =entrance surface dose

• ESD can be determined with help of DAP-meter

• For determening the ESD you need the DAP value and the size of the radiation field on the patient’s skin level

• It is necessary to change DAP values to ESD values if the reference values are ESD values

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ESD

• ESD = (DAP/A)·BSF

DAP = Reading on the DAP-meter

A = Field size on the patients skin

BSF = Back scatter factor

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Reference Values

• Council Directive 97/43/EURATOM

• Authorities Responsible for Radiation Safety in the Nordic Countries

• Calculation of backscatter factors for diagnostic radiology using Monte Carlo methods – N Petoussi-Henss et al 1998 Phys. Med. Biol.

43 2237-2250

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Back Scatter Factor• Council Directive 97/43/EURATOM

• Authorities Responsible for Radiation Safety in the Nordic Countries

• Patient and staff doses in interventional X-ray procedures in Sweden. – Jan Persliden1,2

1 Department of Medical Physics, Örebro University Hospital, Örebro University, SE-701 85 Örebro, Sweden2 Department of Radiation Physics, IMV, Linköping University, SE-581 85 Linköping, Sweden

• Dose-image optimisation in digital radiology with a direct digital detector: an example applied to pelvic examinations

– Jan Persliden1, , K.-W. Beckman1, H. Geijer2 and T. Andersson2´(1) Department of Medical Physics, Örebro University Hospital, 701 85 Örebro, Sweden

(2) Department of Radiology, Örebro University Hospital, 701 85 Örebro, Sweden

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Authorities Responsible for Radiation Safety in the Nordic Countries

• Sweden: Strål säkerhets myndigheten/ Swedish Radiation Safety Authority/ SKI

• Norway: Statens strålevern/ Norwegian Radiation Protection Authority/ NRPA

• Denmark: Statens institut for strålbeskyttelse/ National Institute of Radiation Hygiene/ SIS

• Iceland: Geislavarnir Rikisins/ Icelandic Radiation Safety Authority/ GE

• Finland: Säteilyturvakeskus/ Finnish Radiation and Nuclear Safety Authority/ STUK