Calibration of KAP meters

Transcription

1 Calibration of KAP meters Alexandr Malusek! Division of Radiological Sciences Department of Medical and Health Sciences Linköping University!

2 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 2

3 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 3

4 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. 4

5 KAP meters sensitive area ~ 14 cm x 14 cm thickness ~ 1.6 cm conductive coating ~ 10 nm transparent to visible light 1.5 mm 5.9 mm 1.0 mm 5.9 mm 1.5 mm conductive coating outer electrode air cavity inner electrode air cavity outer electrode 5

6 X-ray tubes with collimator housing C-arm x-ray units 6

7 Air kerma-area product, PKA x ray tube Z P KA = A K air da reference plane Α Kair beam axis x ray tube Α PKA does not depend on the position of the reference plane if photons in the beam are neither scattered nor absorbed K air 1/d 2 A d 2 A d is the distance from focus 7

8 Photon scatter and absorption x ray tube KAP meter patient plane 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 Φ 1 Φ 1 Φ 2 > Scatter increases fluence of particles 8

9 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 9

10 NRPB calibration method focal spot Features:! collimator KAP meter Kair is measured with an ionization chamber at the beam axis beam axis A is determined as the area within 50% of the maximum optical density optical density d m K air ionization chamber film PKA = Kair A Problems:! Kair measured at one point only d m /2 Films are no longer used 10

11 Tandem calibration method focal spot collimator clinical KAP meter Features:! Reference KAP meter measures PKA for incident radiation dcr ~ 30 cm to 40 cm d cr Attenuation in air is neglected Problems:! reference KAP meter A difference between clinical and standards laboratory beam qualities may result in systematic error A KAP meter holder is needed 11

12 IAEA calibration methods 20 cm 10 cm 95 cm 100 cm x ray tube KAP meter diagnostic detector styrofoam couch laboratory calibration 4 6 cm Features! K air 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 P KA = K air A Alternatively: P KA is measured using a reference KAP meter Problems:! K air is measured in one point only in the beam-area method 12

13 The effect of distance 30.0 cm 30.0 cm 100 cm r x ray tube clinical KAP meter P KA,r P KA,p (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 PKAp(r) PKAr kv 80 kv 140 kv radius / cm From [7] Cylindrical KAP meter based on VacuTec 70157! Results:! Beam attenuation in air Larger beam radius resulted in lower relative difference 13

14 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. or? KAP meter stray radiation 14

15 NK formalism of IAEA s TRS-457 dosimetric quantity calibration coefficient z} { K =(M Q0 {z} z } { Y M 0 {z} ) N K,Q0 i k i {z} dosimeter reading, reference conditions dosimeter reading, no beam correction factors 15

16 Correction for temperature and pressure temperature of air in o C reference pressure P0 = kpa k TP = T T 0 P0 P reference temperature T0 = 20 o C pressure of air in kpa 16

17 Correction for humidity Reference value of relative humidity of air is 50% No correction is needed in the range 30% - 80% (TRS-457) 17

18 Correction for radiation quality unknown calibration coefficient c.c. obtained from standards laboratory K Q = M Q z } { N K,Q = M Q N K,Q N K,Q0 z } { N K,Q0 = M Q N K,Q0 k Q,Q0 {z } Thus N K,Q = N K,Q0 k Q,Q0 beam quality correction factor 18

19 Suggestions for improvement Problem 1: Large energy dependence of KAP meters Calibration coefficients (RQR) Calibration coefficients (RQA) 2000 NKA (Gym 2 C 1 ) nm 15 nm 20 nm KAP 1 KAP 2 KAP 3 NKA (Gym 2 C 1 ) nm 15 nm 20 nm KAP 1 KAP U (kv) U (kv) Figure: Calibration coefficient as a function of tube voltage for RQR and RQA beam qualities. Measured (markers) and simulated (lines) values. 19

20 Suggestions for improvement 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. 20

21 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 21

22 Our approach P KA = NP ref KA,QMQ ref N ref P KA,Q 1 N P KA,Q ref NP ref KA,Q M 0 Q ref 1 P KA,Q 0 = N ref P KA,Q N ref P KA = kq,q ref k ref 1 Q 1,Q N ref 0 P KA,Q M ref 0 Q where the beam quality correction factors are: k ref Q,Q 1 k ref Q 1,Q 0 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. Standards laboratory reference beam quality Q0 to clinical reference beam quality Q1. E.g. RQR 5 to 70 kv and 0.1 mm Cu. 22

23 Our approach NPKA, Q NPKA, Q mm Cu, 10 nm 0.0 mm Cu, 15 nm 0.0 mm Cu, 20 nm 0.1 mm Cu, 10 nm 0.1 mm Cu, 15 nm 0.1 mm Cu, 20 nm 0.3 mm Cu, 10 nm 0.3 mm Cu, 15 nm 0.3 mm Cu, 20 nm 0.0 mm Cu, measured 0.1 mm Cu, measured 0.3 mm Cu, measured U (kv) 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). 23

24 Our approach Table: Simulated values of the beam quality correction factor kq1,q0 as a function of the KAP meter s conductive coating thickness. Thickness (nm) k

25 Our approach Result for an x-ray stand at the clinic: Manufacturer s calibration was in error by up to 30%. 25

26 Primary references [1] NRPB. National Protocol for Patient Dose Measurements in Diagnostic Radiology. Chilton: National Radiological Protection Board, [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): [3] International Atomic Energy Agency. Dosimetry in Diagnostic Radiology: An International Code of Practice. Vienna: International Atomic Energy Agency,

27 Secondary references [4] ICRU. Patient Dosimetry for X Rays Used in Medical Imaging (Report 74). International Comission on Radiation Units & Measurements, [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): [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): [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: Vienna: IAEA, [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):