kap meter calibration - linköping university · patient dosimetry for x rays used in medical...
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Calibration of KAP metersAlexandr Malusek
!Division of Radiological Sciences
Department of Medical and Health Sciences Linköping University
!2014-04-15
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Outline
1. KAP meter construction
2. Air kerma-area product
3. Calibration methods (NRPB, tandem, IAEA)
4. IAEA’s NK based formalism
5. Calibration corrections
6. Suggestions for improvement
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Introduction
• A KAP meter is a plane-parallel ionization chamber calibrated to measure the air kerma-area product.
• Old calibration methods (NRPB) cannot be used as films are no longer used in clinics
• New calibration methods (IAEA) have been developed but issues with accuracy still exist.
• Work on improvements continues
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Accuracy of KAP meters
• Large uncertainties in PKA values measured by KAP meters in clinics are common owing to the strong energy dependence of these chambers. Manufacturers typically guarantee the accuracy of 25% (k=2) required by IEC specifications.
• In diagnostic radiology, IAEA’s [3] and ICRU’s [4] specifications require accuracy better than 7% (k=2 ). This holds true for PKA measurements too.
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KAP meters
inner electrode
air cavity
outer electrode
air cavity
outer electrode
conductive coating
1.5 mm
5.9 mm
1.0 mm
5.9 mm
1.5 mm
• sensitive area ~ 14 cm x 14 cm • thickness ~ 1.6 cm • conductive coating ~ 10 nm • transparent to visible light
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X-ray tubes with collimator housing
C-arm x-ray units6
Air kerma-area product, PKA
beamaxisreference
plane
∆ΑKair
x−ray tube
Α
∆A
x−ray tube
PKA does not depend on the position of the reference plane if photons in the beam are neither scattered nor absorbed
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PKA =
Z
AKair dA
�A ⇠ d2
d is the distance from focus
Kair ⇠1/d2
Photon scatter and absorption
KAP meter
x−ray tube
planepatient
Φ1
Φ1Φ2 >
• some photons registered by the KAP meter may not reach the patient plane owing to scatter or absorption
• some scattered photons may increase PKA at the patient plane
• Scatter increases fluence of particles
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Calibration methods
• National Radiological Protection Board (NRPB) [1]
• Tandem calibration (developed at STUK) [2]
• International Atomic Energy Agency (IAEA) [3]
• modification of NRPB’s method for screen-film systems and computed radiography systems
• tandem calibration in IAEA’s geometry
• laboratory calibration of the reference KAP meter
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axisbeam
Kair
opticaldensity
dm/2
dm
collimator
KAP−meter
focal spot
ionization chamberfilm
NRPB calibration method
Features:!• Kair is measured with an
ionization chamber at the beam axis
• A is determined as the area within 50% of the maximum optical density
• PKA = Kair A
Problems:!• Kair measured at one point
only • Films are no longer used
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dcr
collimator
focal spot
clinicalKAP−meter
referenceKAP−meter
Tandem calibration method
Features:!• Reference KAP meter
measures PKA for incident radiation
• dcr ~ 30 cm to 40 cm • Attenuation in air is neglected
Problems:!• A difference between clinical
and standards laboratory beam qualities may result in systematic error
• A KAP meter holder is needed
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95 cm
100 cm
4 −
6 c
m
KAP−meter
x−ray tube
10 cm
20
cm
couchstyrofoam
diagnosticdetector
laboratory calibration
IAEA calibration methods
Features!• Kair is measured with detector at the
beam axis • Beam size 10 cm x 10 cm at the
position of the detector • The nominal area is determined as the
area contained within 50% of the maximum optical density / pixel value
• PKA = Kair A • Alternatively: PKA is measured using a
reference KAP meter
Problems:!• Kair is measured in one point only in the
beam-area method
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The effect of distance
radius / cm
PK
Ap(r
)P
KA
r
0.970
0.975
0.980
0.985
0.990
0.995
1.000
5 10 15 20 25 30
40 kV80 kV140 kV
PKA,r
30.0 cm
30.0 cm
100 cm
KAP−meterclinical
x−ray tube
PKA,pr (r)
Method:!• Monte Carlo simulations of PKA
using MCNP • Ring detectors were used to
score Kair • X-ray spectra for 40, 80 and
140 kV filtered with 5 mm Al • Cylindrical KAP meter based
on VacuTec 70157 !Results:!• Beam attenuation in air • Larger beam radius resulted
in lower relative difference13
From [7]
Stray radiation
KAP meter
or ?
stray radiation
• Relative difference in PKA between the two configurations was less than 3% for considered tube voltages. Attenuation in air was the main factor.
• Stray radiation may be undetected in the IAEA calibration method.
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NK formalism of IAEA’s TRS-457
dosimetric quantity calibration coefficient
dosimeter reading, reference conditions
dosimeter reading, no beam
correction factors
z}|{K = (MQ0|{z}
� M0|{z})z }| {NK,Q0
Y
i
ki|{z}
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Correction for temperature and pressure
kTP =
✓273.2 + T
273.2 + T0
◆✓P0
P
◆
reference pressure P0 = 101.3 kPa
pressure of air in kPa
reference temperature T0 = 20 oC
temperature of air in oC
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Correction for humidity
• Reference value of relative humidity of air is 50%
• No correction is needed in the range 30% - 80% (TRS-457)
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Correction for radiation quality
c.c. obtained from standards laboratory
unknown calibration coefficient
beam quality correction factorNK,Q = NK,Q0 kQ,Q0
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Thus
KQ = MQ
z }| {NK,Q = MQ
NK,Q
NK,Q0
z }| {NK,Q0 = MQ NK,Q0 kQ,Q0| {z }
Suggestions for improvement
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Problem 1: Large energy dependence of KAP meters
Calibration coefficients (RQA)
U (kV)
NK
A (
Gym
2 C<1 )
1000
1200
1400
1600
1800
2000
40 60 80 100 120 140
10 nm15 nm20 nmKAP 1KAP 2
Calibration coefficients (RQR)
U (kV)
NK
A (
Gym
2 C<1 )
1000
1200
1400
1600
1800
40 60 80 100 120 140
10 nm15 nm20 nmKAP 1KAP 2KAP 3
Figure: Calibration coefficient as a function of tube voltage for RQR and RQA beam qualities. Measured (markers) and simulated (lines) values.
Suggestions for improvement
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Problem 2: Beam qualities at clinics differ from beam qualities at standards laboratories. !!As a consequence, transfers of calibration coefficients from the standards laboratory to clinics are associated with uncertainties.
Our approach
• Calibration coefficients of built-in KAP meters should be beam-quality specific. This is a task for manufacturers.
• Before it happens, hospital physicists can determine factors correcting PKA values reported by the built-in KAP meters. To do so, three beam qualities have to be considered: • Q0: reference beam quality at standards laboratory • Q1: reference beam quality at the clinic • Q: any diagnostic beam quality at the clinic
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Our approach
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PKA = N refPKA,QM
refQ =
N refPKA,Q
N refPKA,Q1
N refPKA,Q1
N refPKA,Q0
N refPKA,Q0
M refQ
PKA = krefQ,Q1krefQ1,Q0
N refPKA,Q0
M refQ
where the beam quality correction factors are:
krefQ,Q1
Clinical reference beam quality Q1 to diagnostic beam quality Q. E.g. 70 kV and 0.1 mm Cu to 140 kV and 0.3 mm Cu.
krefQ1,Q0
Standards laboratory reference beam quality Q0 to clinical reference beam quality Q1. E.g. RQR 5 to 70 kV and 0.1 mm Cu.
Our approach
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U (kV)
NP K
A, Q
NP K
A, Q
1
1.0
1.1
1.2
60 80 100 120 140
0.0 mm Cu, 10 nm0.0 mm Cu, 15 nm0.0 mm Cu, 20 nm0.1 mm Cu, 10 nm0.1 mm Cu, 15 nm0.1 mm Cu, 20 nm0.3 mm Cu, 10 nm0.3 mm Cu, 15 nm0.3 mm Cu, 20 nm0.0 mm Cu, measured0.1 mm Cu, measured0.3 mm Cu, measured
Figure: Simulated and measured values of the beam quality correction factor kQ,Q1. (Q1: 70 kV, 0.1 mm Cu, Q: 0.0, 0.1, and 0.3 mm Cu, any U).
Our approach
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Thickness (nm) k
10 0.913
15 0.892
20 0.878
Table: Simulated values of the beam quality correction factor kQ1,Q0 as a function of the KAP meter’s conductive coating thickness.
Our approach
Result for an x-ray stand at the clinic:
• Manufacturer’s calibration was in error by up to 30%.
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376 pages 20.68 mm
Primary references
• [1] NRPB. National Protocol for Patient Dose Measurements in Diagnostic Radiology. Chilton: National Radiological Protection Board, 1992.
• [2] Toroi, P, T Komppa, and A Kosunen. “A Tandem Calibration Method for Kerma–area Product Meters.” Physics in Medicine and Biology 53 (September 21, 2008): 4941–58.
• [3] International Atomic Energy Agency. Dosimetry in Diagnostic Radiology: An International Code of Practice. Vienna: International Atomic Energy Agency, 2007.
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Secondary references• [4] ICRU. Patient Dosimetry for X Rays Used in Medical Imaging (Report 74).
International Comission on Radiation Units & Measurements, 2005. • [5] Toroi, P, T Komppa, A Kosunen, and M Tapiovaara. “Effects of Radiation
Quality on the Calibration of Kerma-Area Product Meters in X-Ray Beams.” Physics in Medicine and Biology 53 (September 21, 2008): 5207–21.
• [6] Malusek, A, J P Larsson, and G Alm Carlsson. “Monte Carlo Study of the Dependence of the KAP-Meter Calibration Coefficient on Beam Aperture, X-Ray Tube Voltage and Reference Plane.” Physics in Medicine and Biology 52 (February 21, 2007): 1157–70.
• [7] Malusek, Alexandr, and Gudrun Alm Carlsson. “Analysis of the Tandem Calibration Method for Kerma Area Product Meters via Monte Carlo Simulations.” In Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS), 1:129–36. Vienna: IAEA, 2011.
• [8] Larsson, J P, J Persliden, and G Alm Carlsson. “Ionization Chambers for Measuring Air Kerma Integrated over Beam Area. Deviations in Calibration Values Using Simplified Calibration Methods.” Physics in Medicine and Biology 43 (March 1, 1998): 599–607.
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