Calibration of the DAP-meter Step-by-Step

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

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

Air Kerma Meter Displays the dose absorbed in air

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

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

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

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

Radiation of the Air Kerma Meter The film is replaced by the air kerma meter The exposure parameters should stay the same as before

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

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.

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

Tandem Method

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

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

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

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

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

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

ESD ESD = (DAP/A)·BSF DAP = Reading on the DAP-meter A = Field size on the patients skin BSF = Back scatter factor

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

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, Sweden 2 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

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