Measurement Uncertainty. Measurements Accuracy - hitting the center of the target Precision - tight...
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![Page 1: Measurement Uncertainty. Measurements Accuracy - hitting the center of the target Precision - tight pattern of hits Bias - all two inches high Uncertainty.](https://reader035.fdocuments.us/reader035/viewer/2022070415/56649e8a5503460f94b8fe88/html5/thumbnails/1.jpg)
Measurement Uncertainty
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Measurements
• Accuracy - hitting the center of the target
• Precision - tight pattern of hits
• Bias - all two inches high
• Uncertainty - circle containing 95% hits
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![Page 4: Measurement Uncertainty. Measurements Accuracy - hitting the center of the target Precision - tight pattern of hits Bias - all two inches high Uncertainty.](https://reader035.fdocuments.us/reader035/viewer/2022070415/56649e8a5503460f94b8fe88/html5/thumbnails/4.jpg)
Traceability
• Accuracy - close to system limitation
• Precision - range
• Bias - identified and corrected for
• Acceptable Uncertainty - 2 sigma
• Unbroken links to national standards
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Calibration Traceability
• Unbroken linkage to national standards
• Uncertainty - elements identified and quantified
• Strong QC/QA program
• Periodic MQA testing
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Measurement Traceability
• Starts with calibration uncertainty
• Identifies and quantifies all factors that affect the quality of a measurement
• Requires reproducibility
• Testing usually required to validate values
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How this affects you
• Did I make the measurement right?
• Is it reproducible?
• What uncertainty is involved?
• Can I cite them for a violation?
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2 cases, more than 2 results
• Package survey of radioactive material
• Mammography machine measurements
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Package
• 31x31x27 cm
• Bicron RSO-50E
• Calibrated by secondary laboratory
• Maximum surface dose rate measured is 0.4 uSv/h (0.4 mR/h)
• Traveling by air
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IATA (ST-1 Requirements)
• Accuracy - so that correctly labeled
• Precision/uncertainty - based on package surface to detector center distance
• Bias - as high as a factor of 4 low
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ST-2 Measurement Guidance
a
D/P Distance Package Length CF
5 cm 20 - 40 cm
40 - 100 cm
>100 cm
2.3
1.6
1.0
Correction factors for package and detector sizes
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Mammography Measurements
• Exposure at skin entrance
• Half value layer
• Non-invasive kVp measurements
• Mean glandular dose calculation
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X-ray vs. Mammo Probes
Energy Response 10x5-6 vs 105-6M
0.8
0.85
0.9
0.95
1
1.05
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
HVL (mm Al)
Ind
ica
ted
vs.
Tru
e
10x5 -6 probe
10x5 -6M
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Phantom Variability
0.94
0.95
0.96
0.97
0.98
0.99
1.00
1.01
1.02
1.03
1.04
1.05
Normalization Factor
1202 2202 2203 1200 2790 8665 2791
Serial Number
Phantom Attenuation Variability
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Temperature Effects
MDH 1515 Temperature Effect
1860
1880
1900
1920
1940
1960
1980
2000
2020
0 10 20 30 40 50 60 70
Minutes since brought indoors
mR/
min
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HVL MeasurementsHVL Variation Due to Curve Fit
5.41
5.51
5.61
0.29 0.3 0.31 0.32 0.33 0.34 0.35 0.36 0.37 0.38 0.39 0.4 0.41
mm Al
ln o
f exp
osur
e
6 Points(FDA method - 0.3095 mm Al)
3 points (drop 0.2 mm Al - 0.3027 mmAl) (ACR method)
data points
MQSA method
ACR method
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Non Invasive kVp Measurements
kVp Filter Dependence
-2
-1.5
-1
-0.5
0
0.5
1
22 24 26 28 30 32
Measured kVp
kV C
orr
ect
ion 0.015 mm Mo
0.02 mm Mo
0.025 mm Mo
0.03 mm Mo
0.035 mm Mo
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Example of How Combined Uncertainties EffectMeasurement
0.03Machine Repro.
0.01Rate Dependence
0.01Energy Dependence
0.01Temp/Pressure
0.015Meter Calibration
Estimated Std. Dev.Component
(Cont.)
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Example of How Combined Uncertainties EffectMeasurement
0.0070.02kVp (+/- 1 kVp)
0.020.06HVL(+/-0.02mmAl)
0.020.06Phantom Variability
0.020.06Probe Placement
Estimated Std. Dev.
Estimated Max. Dev.Component
Estimate Std Deviation as Max Deviation/3 (NCRP Report 112)
(Cont.)
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Combined Uncertainty
• Total estimated uncertainty
• ( 1.52+ 12+ 12+12+ 32+ 22 + 22 + 22 + 0.72)1/2 = 5.2
• To estimate a confidence level of 95% use a coverage factor of 2 = 2 x 5.2 = 10.4 %