Fluke Biomedical 07-492 - Manual (Page 15)

Theory of Operation
General
4
4-1
Section 4
Theory of Operation
4.1 General
The kVp measurement is computed basically from a measurement of the linear absorption coefficient
(MU) of the hardened x-ray beam. As the kV increases, the linear absorption coefficient increases.
An x-ray beam is composed primarily of two parts, the bremsstrahlung radiation and the characteristic
radiation. For a sufficiently hardened beam, if a plot is made of the log of the kV vs. the ratio of the
bremsstrahlung part of the beam, a nearly straight line would result.
4.2 Filtration Effects
A slight change in the beam spectrum being measured will cause a change in the linear absorption
coefficient. Such a change can be caused by filtration differences with respect to the calibration beam.
With lower filtration, the x-ray beam will not be as hard as the calibration beam and the results will be
lower. With more filtration, the beam will be harder than the calibration beam and the results will be
higher.
4.3 Waveform Effects
Beam spectrum changes occur with different waveforms. A single-phase waveform, as opposed to a
three-phase waveform, displays an approximate 5% change in readings. Correction for this is made by
selecting the appropriate phase switch position. The Phase switch is located on the rear panel. If loading
on the generator causes the waveform to appear in the classic manner, the switch should be positioned in
the three-phase position. In some situations, the waveform may be somewhat between the single-phase
and the three-phase positions and the results may be compromised.
4.4 Low Battery
The low battery indication is displayed when the battery voltage drops below a predetermined value. This
value is selected such that there is not a noticeable effect on the measurement results. Use of the
instrument below this value (i.e. while the LO BAT indication is displayed) may result in erroneous
measurements due to the loss of an accurate voltage reference.
4.5 Positioning Error
The kVp meter should be positioned in the center of the beam. The measurement of the meter is two
square inches. If measurements are made in other parts of the beam, different results will occur since the
beam spectrum is different at different locations. The accuracy of the kVp meter will be compromised.
The heel effect is an example of such a beam spectrum change. To view the effects of beam positioning
on the measurement, the user may want to position the meter in various parts of the beam for illustration
purposes.