kVp Measurement Mamdouh Alenazi

IAEAInternational Atomic Energy Agency

RADIATION PROTECTION INDIAGNOSTIC AND

INTERVENTIONAL RADIOLOGY

Part 15.1: Optimization of protection in radiography

Practical exercise

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 15.1: Optimization of protection in radiography 2

Overview / Objectives

• Subject matter : quality control of general radiography system

• Step by step procedure to be followed to implement the considered QC test:• kVp Accuracy and Reproducibility

• Interpretation of results

IAEAInternational Atomic Energy Agency

Part 15.1 : Optimization of protection in radiography

kVp measurement (Accuracy and Reproducibility)

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology

IAEA 15.1: Optimization of protection in radiography 4

kVp measurement

Purpose :• Image quality and patient dose are dependent

on any variation in the generator kilovoltage (kV) of the x-ray set. Therefore an accurate kV calibration is required.

• A non-invasive tube voltage check over the whole used kV-range should be performed

Equipment :• An electronic device (like multi-function meter)

to measure : kVp, time, dose.

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Multi-Function Meters

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Ion Chamber Dosemeters

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kVp Accuracy

• Before making measurements, one must understand what “kVp” means

• There are 3 definitions of kV, only one is commonly used

• CAUTION : some mobile x-ray units use a capacitor, and the kVp decreases during the exposure

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kVp Definitions

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kVp Accuracy

Method :• set meter at about 100 cm from focus or as

prescribed by the manufacturer

• set ~ 20 mAs

• measure kVp from about 60 - 120 kVp (eg. 60, 70, 80 100, 120)

• calculate error between set and measured value

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kVp Accuracy

Analysis :• The difference between set and observed kVp

should be within 5%

Frequency :• acceptance

• tube change, generator maintenance

• annual

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kVp Reproducibility

• This test checks that the kVp will stay the same from exposure to exposure

Method :• same setup, but at fixed 70 kVp set

• perform 5 exposures (be careful of tube !)

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kVp Reproducibility

Analysis :• calculate coefficient of variation (CV)

• CV = (standard deviation/mean) of the 5 readings

• CV must be less than 2%

Frequency :• as for accuracy

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kVp Reproducibility

Further test :• older x-ray units may have a mechanical

selector for kVp

• test this by moving the kVp dial up, down, then back to 70 before each exposure

• a variation of 3 kVp is permitted