X-Ray Tube Feedback System - Planmeca Proline XC Technical Manual

Chapter F - PREVENTIVE MAINTENANCE

2.2 X-ray tube feedback system

CAUTION Radiation is emitted during all these tests. Proper protection against unnecessary
exposure to radiation must be considered.
2.2.1 NON-INVASIVE TESTING (directly from the radiation)
A non-invasive method can be used for checking the kVp, radiation quality (half-value layer)
and the exposure time. This method is efficient since no covers need to be opened, and it
gives a "second" opinion on the measured parameters. However, care must be taken when
selecting the appropriate non-invasive X-ray meter; older meters calculate the kVp avg based
on the assumption that the kV waveform is AC. The Planmeca Proline XC has DC high volt-
age with very small high frequency ripple, so the accurate measurement of kV waveform can
be impossible if, for an example, the meter's sampling frequency isn't high enough. If in any
doubt whether or not the meter is suitable for ProMax, please consult the meter manufacturer
for additional information. Otherwise, please refer to the radiation meter manufacturers user
manual of how to use the meter.
Peak tube potential (kVp) measurement
When a non-invasive meter is used for kVp measurement, following things should be noted:
1) The sensor should be placed exactly in the middle of the X-ray field in both horizontal and
vertical directions (very important especially if the measurements are made in the panoramic
mode!), use a fluorescent screen to determine the X-ray field area if you are not totally sure
about it.
2) The sensor distance from the focal spot should be as short as possible to maximise the sig-
nal / noise ratio.
3) The whole sensor area must be within the radiation field.
4) The meter must be properly calibrated and, when necessary, appropriate calibration/cor-
rection factors must be used when interpreting the results.
The measured kVp must be within ±5% of the value displayed on the user interface.
Half-value layer measurement
There are different recommended procedures for measuring the HVL. The HVL is defined as
the thickness of a specified material (generally expressed in mm Al) which attenuates x-radia-
tion with a particular spectrum to an extent such that the value of air kerma (or exposure or
absorbed dose) rate is reduced to one half of the value that is measured without the material.
The simplest method to ensure that the unit complies with the requirement (With Promax, the
first permissible HVL must be at least 2.5 mm Al at 80 kV) is to measure the air kerma rate
first without any additional material in the radiation field, then add 2.5 mm Al to the radiation
field, measure the air kerma rate again and check that the air kerma rate with additional 2.5
mm Al is more than one half of the one measured without the added material. That is,
(Dose rate with added 2.5 mm Al eq filtration) / (Dose rate without added filtration) > 0.5
This is sufficient to ensure that the HVL is at least 2.5 mm Al. Depending on the type of the
radiation meter used, it is possible that a correction factor needs to be applied to the result
measured with added material in the radiation field.
F-4 Planmeca Proline XC X-ray unit
PREVENTIVE MAINTENANCE CHECKS
Technical Manual