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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