Download calibration factor

Calibration of the DAP-meter
Step-by-Step
1
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
2
Calibration Meters
• Either Air Kerma meter
or another DAP-meter
can be used for
calibration of DAP-meter
3
Air Kerma Meter
• Displays the dose
absorbed in air
4
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
5
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
6
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
7
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
8
Radiation of the Air Kerma Meter
• The film is replaced by
the air kerma meter
• The exposure parameters
should stay the same as
before
9
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
10
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.
11
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
12
Tandem Method
13
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
14
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
15
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
16
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
17
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
18
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
19
ESD
• ESD = (DAP/A)·BSF
DAP = Reading on the DAP-meter
A = Field size on the patients skin
BSF = Back scatter factor
20
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
21
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
22
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
23