4. Measurement Principles & Theory
HVL & Total Filtration
2014-06/5.5C
85
Piranha & QABrowser Reference Manual
Linearity4.8
The output in mGy/mAs for different mAs stations should remain constant if the kVp and
distance are maintained constant. This can be checked by measuring the coefficient of
linearity. The coefficient of linearity is defined as:
where: X = Dose/mAs, and X
1
and X
2
(X1 and X2) are measured at adjacent mAs
settings
To check the mAs linearity:
1. Place the detector on the table-top. If patient-equivalent phantom should be used it is
recommend to use the Piranha HVL stand to simplify the set up. Use 2 pieces of
10 mm Al filter as "patient -equivalent" filter in the beam.
2. Set the X-ray generator to technique factors commonly used clinically.
3. If the display is unstable; press the Reset button.
4. Use some form to record your measurements.
5. Make exposures at different mAs settings and both for small and large focuses.
6. Calculate the coefficient of linearity for adjacent measurements.
An acceptable value for the coefficient of linearity is less than 0.10.
A short example:
measurement 1 : X
1
= 13 µGy/mAs
measurement 2 : X
2
= 14.6 µGy/mAs
measurement 3 : X
3
= 12.8 µGy/mAs
Then:
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