Transcript calibration factor

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 DAPmeter 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 DAPmeter
• 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 DAPvalues of several similar examinations. For example the thoraxexaminations 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, SE701 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
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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
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