5. Measurements with the Barracuda System
Mammography
2012-10/4.3A
141
Barracuda & QABrowser Reference Manual
5. When the dose value has been reduced to
less than half the HVL is calculated and the
graph can be selected.
5.6.6 Mammo Compensations and Corrections
Here various corrections and compensations are described, that are of special
importance for mammography.
5.6.6.1 Corrections for the Compression Paddle
The MPD is well collimated above its small detector area, and will measure the same
whether the compression paddle is placed directly on top of the Barracuda or high
above. This is NOT true for an ion chamber.
A factor has been introduced which enables the MPD 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.
Conclusion: When comparing with typical mammographic ion chambers listed above,
you should multiply the MPD dose value with a scatter factor of 1.06 to make it
measure as an ion chamber directly below the compression paddle.
When the compression paddle is not used, the scatter factor is automatically set to
1.00. This since free in air, the ion chamber measures the same as the MPD.
More info about the correction for compression paddle can be found in Application
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