RTI Piranha, Reference Manual, Page: 90 / 181

4. Measurement Principles & Theory
Linearity
Piranha & QABrowser Reference Manual 2014-06/5.5C
86
0.05797 1.6/27.6
14.6 13.0
14.6 13.0
2
- 1
L
and:
0.06569 1.8/27.4
12.8 14.6
12.8 - 14.6
3
- 2
L
Of which both is below the 0.10 limit.
Reproducibility 4.9
Reproducibility is checked to find out how constant the output is when an X-ray
exposure is repeated many times. One method is to check the coefficient of variation.
The coefficient of variation is defined as:
x
n
x x
s
n
i
i
1
1
2
where
x
i
= Individual exposure readings
n
= Number of readings
= Mean value of readings
To check the output reproducibility:
1. Place the dose detector on the table-top (the patient-equivalent phantom is not
necessary since this measurement can be made as a relative measurement).
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 5 to 10 exposures and record the reading for each exposure.
6. Calculate the mean value, difference from mean value and square of differences.
7. Add all squared values and divide by (n-1) to get the variance. In this case n=10.
8. Calculate the square of the variance, i.e. the standard deviation, and divide it by the
mean value of the n measurements
An acceptable value for the coefficient of variation is less than 0.05.