4. Measurement Principles & Theory
Measurement Principle for the MPD
Barracuda & QABrowser Reference Manual
2012-10/4.3A
104
can thus automatically compensate the kV and dose for the dependence of the
waveform and inherent/added tube filtration.
HVL & Total Filtration4.7
HVL is a method of specifying the beam 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.
To measure the HVL:
1. Use the Barracuda HVL stand on the table-top.
2. Set the generator to 80 kV/25 mAs.
3. If the display is unstable; press the Reset button.
4. Use some form to record your measurements.
5. Begin with 3 exposures without any added aluminum filter to get the zero-point and
to check the consistency of the generator.
6. Add the aluminum filters, 1, 2, 3, and 4 mm, and record the readings. Make a
measurement for each thickness and use the mean value.
7. Plot the results in a semi-logarithmic graph, set the value for 0 mm of added
aluminium to 1.0.
8. Join the measured points with a curve and find the value of added filtration required
to reduce the exposure to 0.5.
9. We find from the semi-logarithmic plot that the measured half-value layer is 3.3 mm
Al.
To measure Total Filtration:
In order to measure total filtration with one exposure optimally, there are some settings
to be aware of. Since the highest accuracy is obtained between 60 and 120 kV, we
recommend to do the measurement of the total filtration in between, at 80 kV. The
MPD is also calibrated for the total filtration at this kV. Use a high signal level, i.e.
200 mA during 200 ms to get a stable result. It does not matter if you intend to do
measurements at a higher or lower kV than this, this measurement aim to get a correct
value of the total filtration.
See the graph below for optimum choice of method.
Hence, choose 80 kV, 200 mA and 200 ms and make an exposure. The total filtration
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