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Technical Note
D606.200.1/0
PPV – A new measuring quantity in diagnostic radiology
1 Introduction
As mentioned in the commercial note
D728.208.0/0 each kV meter of the DIAVOLT
series is able to measure the new IEC quantity
termed practical peak voltage (PPV).
3.1 kVpmax
kVpmax characterizes the maximum voltage
applied across an X-ray tube in a specified
time interval (t0 - t1) and affects the maximum
energy of radiation produced.
This note gives an overview about
the
meanings and differences of the customary
quantities such as kVpmean, kVpmax and of the
new quantity PPV according to IEC 61676.
2 Basics
The voltage applied across an X-ray tube can
be generated by various types of generators.
The temporal run of the voltage during the
current flow depends on the rectifier type and
affects the produced dose.
The emitted bremsstrahlung covers a spectrum
in which the highest energetic component
accords to the maximum voltage of the tube.
Figure 2: kVpmax
For acceptance tests kVpmax can be of special
interest. In terms of quality imaging kVpmax
plays normally not an important role.
3.2 kVpmean
The determination of kVpmean takes place over
a specific weighting function and constitutes simplified expressed - the mean value of all Xray tube voltage peaks during a specified time
interval.
Figure 1: X-ray spectrum
The tube voltage is one crucial factor in quality
imaging and affects the quantity and quality of
X-rays produced during an exposure. Different
types of generators (1, 2, 6, 12 pulse high
frequency generator) used to power X-ray
tubes create again diverse waveforms.
Accurate and consistent waveforms are
important in diagnostic imaging in order to
control density and contrast of the X-ray image
as well as radiation dose to the patient [1].
3 The peak kilovoltage
The current standard used to measure the
energy of the photons produced by an X-ray
tube is the peak kilovoltage (kVp). The term
and its definition are derived from the fact that
in some systems the accelerating potential is
not
constant
and
varies
over
time
(voltage ripple).
Figure 3: kVpmean
3.3 DIAVOLT kV meters
The non-invasive DIAVOLT kV meters provide
a kV measurement based on the change in
X-ray absorption through varying thicknesses
of filtration.
Each DIAVOLT kV meter allows to select a
delay to eliminate the power-up effects of the
installation from the measuring result. With the
delay function of the DIAVOLT devices a
period of time between radiation start (t0) and
activation (t1) of the measurement from
0 ms to 9999 ms can be set [2].
Technical Note
4 The new quantity termed PPV
D606.200.1/0
The result is a histogram:
Due to various types of generators used to
power X-ray tubes, it is possible to obtain
differing radiographic contrast values for
machines measuring the same kVp. This
`problem` led to the creation of PPV.
4.1 Definition
The practical peak voltage (PPV) is based on
the concept that radiation generated by a high
voltage of any waveform produces the same
contrast as radiation generated by an
equivalent constant potential.
The constant potential producing the same
contrast for a specified contrast configuration
and specified X-ray tube properties as the
waveform under test is the PPV [4].
The PPV is derived by equating the low level
contrast in an exposure produced with an
X-ray tube connected to a generator delivering
any arbitrary waveform, to the contrast
produced by the same X-ray tube connected to
a constant potential generator.
One contrast configuration is selected as being
suitable for diagnostic radiology and a direct
link is established between the electrical
quantity X-ray tube voltage and the PPV. It is
shown that the spread in total X-ray tube
filtration as encountered in medical diagnostic
radiology can influence the result of a
measurement of the PPV only marginally [3].
4.2 PPV Calculation
For the PPV determination of an arbitrary
waveform the distribution for the occurence of
a voltage value during an interval must be
calculated.
Figure 5: Corresponding histogram of an
arbitrary waveform
The function of the determined histogram is
allocated with a specific weightening function.
The result is the practical peak voltage, an
algorithm that determines the equivalent
voltage applied to a constant potential X-ray
machine that would produce the same contrast
on a radiograph as the generator under test.
4.3 Advantage
In comparison to other quantities such as
kVpmean and kVpmax a direct conclusion about
imaging properties can be taken:
The PPV describes the constant potential
delivering the identic image contrast as the
waveform under test and consequently
enables users to compare the imaging quality
delivered by different generators.
PTW`s new non-invasive DIAVOLT kV meters
are able to measure this new quantity.
Literature
[1] Conference of Radiation Control Program Directors,
Inc. (CRCPD), Q. A. Collectible, Frankfort, KY (1994)
[2] For further information ask for document D606.219.0/2
[3] Kramer, Selbach, Iles, The practical peak voltage of
diagnostic X-ray generators, BJR, 71, (1998) 200-209
[4] IEC 61676: Dosimetric instruments for non- invasive
measurements of X-ray tube voltage in diagnostic
radiology
Figure 4: Arbitrary waveform
PTW-Freiburg
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Phone: +49 761 49055-0 • FAX: +49 761 49055-70
[email protected] • www.ptw.de