uwmrrc’s new kvp calibration system - aapm...
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UWMRRC’s New kVpCalibration System
Lindsay Jones and Dan AndersonUWMRRC
At NCCAAPM04/25/08
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Diagnostic Radiology• Use x-rays to image the
body
• Factors:– kV - voltage applied to the
x-ray tube– mA - current, or rate of
electrons emitted from the anode
– s - exposure time
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kVp Meters
• Routinely used for quality control• Consist of closely spaced diodes, filtered
by different thicknesses of material• Current calibration protocol:
– Place meter in the center of radiation field– Deliver a nominal kV and mAs– Compare the reference value from the voltage divider to
the value displayed by the meter– Turn calibration potentiometer on meter and redeliver
radiation and compare, repeating until the displayed value is within 2% of the reference from the voltage divider
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Definitions of kVp• kVp(abs) - Maximum value of the x-ray tube
potential• kVp(ave) - Mean value of all x-ray tube potential
peaks during a specified time interval• kVp(eff) and PPV - The x-ray tube potential giving
the same image contrast as when using a constant potential tube voltage
keV
time
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Newer Mammography and Radiography Machines
• Can not install a high voltage divider• We need a non-invasive way to determine
what voltage was actually applied to the x-ray tube
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Is spectrometry an option?• Terini et al. (1999) used two Si-PIN photodiodes to
obtain kVp of an x-ray tube by determining the end point of the energy spectrum emitted from the tube.
• Terini et al. (2004) used a commercial Si-PIN spectrometer (Amptek XR-100CR) and compared the results to their two homemade Si-PIN photodiode setups.
Terini et al. Applied Radiation and Isotopes, 50 (1999), 343-353.
Terini et al. The British Journal of Radiology, 77 (2004), 395-404.
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Measured radiographic spectrum
ANKERHOLD, U.: Catalogue of X-ray spectra and their characteristic data -ISO and DIN radiation
qualities, therapy and diagnostic radiation qualities, unfiltered X-ray spectra-, PTB-Bericht: PTB
Dos-34 (2000), ISBN: 3-89701-513-7
50 kV radiographic spectrum
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0.00 10.00 20.00 30.00 40.00 50.00 60.00
Energy (keV)
Normalized spectrum
Filtered normalized spectrum
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End-of-spectrum linear fit
ANKERHOLD, U.: Catalogue of X-ray spectra and their characteristic data -ISO and DIN radiation
qualities, therapy and diagnostic radiation qualities, unfiltered X-ray spectra-, PTB-Bericht: PTB
Dos-34 (2000), ISBN: 3-89701-513-7
50 kV radiographic spectrum - zoomed
y = -0.104x + 5.2187
y = -0.031x + 1.5571
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
49.30 49.40 49.50 49.60 49.70 49.80 49.90 50.00 50.10 50.20 50.30Energy (keV)
Normalized spectrumFiltered normalized spectrum
x = 50.229
x = 50.179
9IPEM Report 78 – 28 kV MoMo, 0.03 mm Mo filtration, 12º anode
28 kV Simulated Mammography Spectrum
0.0
0.1
0.2
0.3
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1.0
0.00 5.00 10.00 15.00 20.00 25.00 30.00
Energy (keV)
Normalized simulated spectrumFiltered normalized simulated spectrum
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Detector and Multi-Channel Analyzer (MCA)
• Amptek XR100 – 7 mm2 Si-PIN photodiode– Peltier-cooled
• Amptek PX4 MCA• Amptek ADMCA• Microsoft Excel• MATLAB
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Calibration
0
100
200
300
400
500
600
700
800
900
200 300 400 500 600 700 800Channel Number
Am-241Multinuclide SourceBa-133
13.95 keV
17.75 keV
22.16 keVCd-109
30.97 keV
34.99 keV
Spectrometer Energy Calibration
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MATLAB Linear regression code
• Eliminate unnecessary points– Low energy– Tail
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MATLAB, continued
– Fit line to end of spectrum• Repeat
– Load and fit background– Find the intersection of the two lines
Data Reduction and Error Analysis for the Physical Sciences, 2nd ed. Bevingtonand Robinson. McGraw-Hill, New York, 1992.
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Old mammography tube
Spectrometer
Tilt stageMCA
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Stainless steel collimation kit
Machined connection plates
Diode (behind extender)
Peltier cooler & electronics
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Overview of new protocol• Use a calibrated spectrometry system for end-of-
spectrum kVp determination• Use a two-diode apparatus (whose output is
scaled by the spectrometry output) to find the value for each exposure, side-by-side with the kVp meter that is being calibrated
• Calibrate the customer’s kVp meter to within 2% of the reference value from the two-diode apparatus
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Summary
• Developed linear regression analysis program
• Constructed alignment components• Performed energy calibration
• Benchmark spectrum • Compile new protocol
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Acknowledgements• My adviser, Dr. Larry DeWerd
• UW MRRC staff members:– Dan Anderson– John Micka
• Fellow graduate students:– Brian Rasmussen– Jackie Usher
• UW ADCL and RCL customers, who help fund research in the UW MRRC
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