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

0.4

0.5

0.6

0.7

0.8

0.9

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