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5. Measurements with the Piranha System
Mammography
Piranha & QABrowser Reference Manual 2014-06/5.5C
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5.5.7.1 Corrections for the Compression Paddle
The Piranha is well collimated above its small detector area, and will measure the same
whether the compression paddle is placed directly on top of the Piranha or high above.
This is NOT true for an ion chamber.
A factor has been introduced which enables the Piranha to take the scattered radiation
into consideration and produce measurement results as if it was an ion chamber which
senses the scattered radiation directly.
When an ion chamber is placed directly below the compression paddle, a relatively
constant scatter factor of 6 % is found. The factor is typical for ion chambers such as
Radcal 6M, PTW N23344, and Standard Imaging Magna 1cc.
Typically for a Mo/Mo beam energy, a 0.10 mm Al
equivalent compression paddle is used. That is
equal to approximately 3 mm of plexiglass (PMMA).
For W/Al beam energy, an equivalent compression
paddle of 0.18 mm Al is typically used instead.
If you tap the symbol you can see these two
settings under Conditions.
When comparing with typical mammographic ion chambers listed above,
you should multiply the Piranha dose value with a scatter factor of 1.06 to
make it measure as an ion chamber directly below the compression
paddle.
More info about the correction for compression paddle can be found in Application Note
1-AN-52020-2 from RTI Electronics AB. Please also see section
Angular Sensitivity,
Piranha for details on sensitivity in different directions.
5.5.7.2 Normalization
A normalization function is available which enables all measurements to be virtually
performed at the same distance, increasing productivity. According to European
protocol (ECR 16263 EU, 1996), ESAK should be measured 45 mm above the breast
support. The QABrowser supports calculation of the dose at a user set virtual distance.
An example:
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