RTI Piranha, Reference Manual, Page: 87 / 181

4. Measurement Principles & Theory
Measurement Principle for the Piranha
2014-06/5.5C
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Piranha & QABrowser Reference Manual
Measurement Principle for the Piranha 4.6
The following are the key features of the Piranha design:
Small size
Optimized filter packages for five different kV ranges
Very sensitive and wide dynamic range
Check filter for measurement geometry verification
Single exposure estimation of total filtration and Quick-HVL
Single exposure estimation of generator waveform type
The design of the detector package is very important to be able to measure kV and
dose correctly in the whole range of 20 to 155 kV.
The Piranha design makes it possible to measure small field sizes, less than 3 mm
width, and low output levels down to approximately 1 µGy/s. Basically the detector
packages consist of four separate electrometer channels connected to detectors D1,
D2, D3, and D4 and a moveable filter package that can change to one of six positions,
each a combination of different filters for the detectors. One of these positions is used
as a "check-filter". It has the same filter thicknesses for both D1 and D2. When the
detector is perfectly positioned and both detectors have the same radiation the ratio
between the two signals should thus be exactly "1.000". This is very useful information,
and testing this makes sure that your measurement geometry is fine, giving reproducible
readings. The other 5 filter pairs have different thicknesses all optimized for different
ranges of the tube voltage; two (1 and 2) are used for the low mammography energy
range 20 to 45 kV, and three filters (3 - 5) are used for the radiography range 35 to
155 kV (35 - 75, 55 - 105, and 80 - 155 kV).
Using these four signals S1-S4 (from detectors D1 to D4) the Piranha can accurately
calculate the corresponding tube voltage. The signal S3 is not affected by the moveable
filters and is designed to measure the dose. This detector is marked by a square inside
the rectangular detector area on the top panel. The reference depth for the sensitive
area of the dose detector is 10 mm under the Piranha top panel surface.
The detector D4 is placed directly under D3 with additional filter in between. The ratio
between S3 and S4 is used to estimate the total filtration for the radiography range.
Using these signals together more accurate dose and tube voltage readings can be
obtained.
Since all signals is measured simultaneously and with a relative high speed, the Piranha
can thus automatically compensate the kV and dose for the dependence of the
waveform and inherent/added tube filtration.
HVL & Total Filtration 4.7
HVL is a method of specifying the radiation quality. The half-value layer is defined as
the thickness of a specified material that attenuates the X-ray beam to one-half of its
value in absence of that material, usually aluminium. See also Application Note 03-
009/01 that can be downloaded from RTI Electronics web page at
http://www.rti.se .
From the HVL-value the total filtration value can be estimated. See Application Note 1-
AN-52020-11 from RTI Electronics AB.