Transcript DAP-mittareiden kalibrointi ja mittaukset

The DAP Meter
Teuvo Parviainen
STUK
Translation
Eija Grönroos, Elina Allenius, Heidi Varonen,
Metropolia University of Applied Sciences
STUK • SÄTEILYTURVAKESKUS
STRÅLSÄKERHETSCENTRALEN
RADIATION AND NUCLEAR SAFETY AUTHORITY
STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 1
What is DAP?
Dose-area product, unit Gy·m2
• Product of primary beam area and absorbed
dose in air
• In x-ray examinations air kerma and area
product is KAP ≈ DAP
• DAP doesn’t include scattered radiation from
patient, phantom or environment
• The distance between focus and the measuring
level doesn’t affect the DAP value
STUK • SÄTEILYTURVAKESKUS
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STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 2
Recording the information on procedures
• Information on procedures involving exposure to
radiation shall be recorded, on the basis of which the
radiation dose from the procedure to a person examined
or treated may be determined if necessary
•
A summary of the number of examinations and of
radiation doses shall be prepared according to
instructions to be separately issued, on the basis of
which national appraisals of the radiation exposure
caused by the medical use of radiation and its
development shall be prepared
Act of Ministry of Social Affairs and Health in Finland (423/2000)
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Kalvo 3
Measurements and comparison of results
• Radiation doses caused by X-ray examinations shall be regularly
measured or numerically assessed
• The activity to be administered to the patient in nuclear medicine
examinations shall be measured using an activity meter
• Dose and activity data shall be recorded and shall be systematically
compared to reference levels
• If a reference level is found to have been repeatedly exceeded,
then the reasons for this shall be investigated and necessary
measures shall be taken to reduce exposure to radiation
Act of Ministry of Social Affairs and Health in Finland (423/2000)
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Kalvo 4
Defining exposure
The exposure can either
• be measured for a group of patients or
• be assessed by calcutarory means for a
group of patients or
• be measured by using a phantom identical to
patient or
The meter used for the defining of exposure
needs to be calibrated
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Kalvo 5
Why to define patient exposure?
•
•
•
•
For comparison of reference values
For dose optimization
For organ dose and effective dose estimation
For comparison of best practices
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Kalvo 6
1,2
THORAX
Reference value 0,6 Gycm2
1
Mean value 0,34 Gycm2
Gycm2
0,8
0,6
0,4
0,2
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
X-ray units
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Kalvo 7
Different methods of defining exposure
• Measuring ESD from the surface of the
phantom
• Measuring ESD from the skin of the patient
• Calculting ESD on the basis of radiation product
• Useing DAP-meter
• Calculating ESD from the DAP value
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Kalvo 8
Measures in x-ray diagnostics
Entrance Surface Dose (ESD)
ESD
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Kalvo 9
Dose Area Product (DAP)
DAP ≈ D · A
Area A
Dose D
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Kalvo 10
Dose Area Product (DAP)
0.5 m
1m
2m
Dose:
Area:
DAP:
40 mGy
25 cm2
1000 mGy cm2
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10 mGy
100 cm2
1000 mGy cm2
2.5 mGy
400 cm2
1000 mGy cm2
STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 11
Dose Area Product (DAP)
• DAP is surface integral of absorbed
dose in air.
• DAP is almost independent of
measuring distance.
• DAP does not include scattered
radiation from the patient or
phantom.
• Approximately DAP ≈ Da*A.
DAP-meter
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STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 12
Why to calibrate DAP meters?
• The DAP-meter usually shows the ’right’ DAP
measure only in some spesific situations
• In some cases this level of exactitude might be
enough
• To improve the exactitude of measures the DAPmeter must be calibrated
• The calibration must be done for different radiation
beams commonly used
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Kalvo 13
DAP meters dependence of the radiation beam
quality
1,15
2,5 mmAl
Calibration Factor
1,1
1,05
1
0,95
0,9
0,85
40
60
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80
100
Tube Voltage (kV)
120
140
160
STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 14
DAP meters dependence of the radiation beam
quality
1,15
2,5 mmAl
4 mmAl
4 mmAl+0,1 mmCu
Calibration Factor
1,1
3 mmAl
5 mmAl
4 mmAl + 0,2 mmCu
1,05
1
0,95
0,9
0,85
40
60
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80
100
Tube Voltage (kV)
120
140
160
STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 15
Calibration of the DAP-meter
• The DAP-meter should be calibrated with several
radiation beam qualities
• The specificity and variability of the calibration
determines the accuracy of the dose
measurements on patients
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Kalvo 16
Use of DAP-meter
Patient measurements
Calibration
Results
Calibration factors
Use of factors
Real DAP-values
Comparision with the reference values
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STUK: Teuvo Parviainen, VM, PP, TKo
Kalvo 17
Use of the Calibration Factors and
DAP-value
• There is no need to use the calibration factors in the
measuring situations
• The calibration factors are used when processing and
analyzing the results
• The use of the calibration factors improves the
accuracy of the measurement
DAP  NKA  k pT  MDAP
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Kalvo 18
Grounds for the Calibration
• The value displayed by the DAP meter often has to be
corrected with the calibration factor in order to get the
accurate dose
• The calibration factor can be aquired by comparing the
results of the meter in use at the site and the calibration
meter
• It is recommended to perform the calibration simultaneously
with those two meters
• The ionization chamber of the reference meter has to be far
enough from the ionization chamber of the other meter and
from other scatter causing structures
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Kalvo 19
Calibration Methods
• The reference meter must be calibrated by a reliable
facility
• The reference meter can be either
• an air kerma meter (area method) or
• a DAP-meter (tandem method) or
• a thermoluminescent dosimeter (TLD)
A removable DAP-meter can be sent for calibration to a
reliable facility (such as a dosimetry laboratory)
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Kalvo 20
Area Method
Ion Camber of DAP meter
Measurements to be taken
• Air Kerma
• In the center axis of primary radiation
beam
•
• Distance from the focus approximately
50 cm to 150 cm
Field size
• about 15 cm * 15 cm in the ionization
chamber of the reference meter
• accurately measured from film or a
digital image
Easiest way to measure is to use the same
distance from focus with both
measurements
Ion Camber of Air Kerma Meter
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Kalvo 21
Area Method
• Air kerma can usually be measured with most of the
commonly used voltages and filterings
• Area method results an approximate value of the true DAP
value
• Inaccuracies can be created by penumbra, measurement
mistakes of the field size, a very small field size, scattered
ratiation etc
• Remember to measure the field size accurately
• When using the film system the density of the x-ray must
not be too high (OD 1 – 1,5)
• Use the midpoint of the penumbra when measuring from a
film
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Kalvo 22
Tandem Method
• The reference value is measured by using a
reference DAP-meter (calibrated for this
purpose)
• Distance between the two meters should be
about 30 cm
• Field size is about 8cm*8cm at the level of the
reference meter
• The radiation beam has to fit completely in the
midarea of the ionization chamber of the
reference meter
Ion Camper of DAP Meter
Ion Camber of Reference Meter
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Kalvo 23
Tandem Method
• With this method there is no need to measure field size and
distance separately
• There is no need to measure the field size accurately
• A reference DAP-meter is needed
• The reference meter has to be calibrated beforehand by a
reliable facility
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Kalvo 24
The Maintenance of the DAP-Meter
• It is useful to check the calibration at least once a year by
using one commonly used radiation value set, and once in
three years by using several different radiation values
• If the meter is repaired or adjusted, it must also be
recalibrated
• The electrical function of the meter can be checked up
with a test button
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Kalvo 25
References
• STUK-TR 4, 2008 (tekninen raportti): Annoksen ja
pinta-alan mittaaminen DAP-mittarin kalibrointi
röntgensäteilykeilassa
• Act of Ministry of Social Affairs and Health in Finland
(423/2000)
• The Directive 69/29/Euratom
• See more in course references
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Kalvo 26