4. Measurement Principles & Theory
HVL & Total Filtration
2012-10/4.3A
105
Barracuda & QABrowser Reference Manual
will be displayed in a pop-up screen. Tap Keep and the value is stored and will be
used on all further measurements. The pop-up screen will be displayed whenever the
value of the total filtration is changed.
Manually you can do this by entering the settings screen by tapping the icon, as
shown below. As default value the total filtration is set to Estimate. Measure the total
filtration and then enter the settings window again. Now choose Set and enter the
measured value.
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 Barracuda 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.
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