Planmeca ProMax X-ray Instruction Manual page 83

PREVENTIVE MAINTENANCE CHECKS
Half-value layer measurement
Exposure time measurement
2.2.2 INVASIVE TESTING (directly from the units own feedback signals)
NOTE
NOTE
Technical Manual
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 84 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.
The exposure time is controlled by the microprocessor, so the exposure time accuracy is
exactly the same, no matter if the set exposure time is 200 ms (in film-based cephalostatic
mode) or 16 s (in panoramic / tomographic mode). The exposure time is defined automatically
based on the selected program and is displayed in the upper right corner of the graphical user
interface. In panoramic mode, attach the non-invasive sensor to the front panel of the cas-
sette holder (or Dimax3 sensor) and make sure its whole area is in the radiation field. Select
70 kV / 8mA and press ready. Take an exposure and record the measured exposure time.
The measured exposure time must be within ±10% of the exposure time displayed in the user
interface.
In film-based cephalostatic mode, the exposure time is selected by the user. Select the short-
est possible exposure time, attach the non-invasive sensor to the cephalostatic cassette
holder in the middle of the radiation field, take an exposure and record the measured expo-
sure time. When placing the sensor, note that if the unit is equipped with auto-ceph, the cas-
sette size is automatically detected when the cassette is on its place and the area of radiation
field is adjusted accordingly.
The manufacturer does not require the invasive testing. The invasive test must only be
performed if the local authorities require it.
An invasive method should be used for checking the tube current (mA), and can be used for
checking the kVp and exposure time. This method requires that the covers around the tube
head assembly are removed, and a special measurement adapter cable, Planmeca order
code 10008320, is connected to the connector J2 in the FBK PCB. The FBK PCB is perma-
nently fastened to the front side of the tube head assembly. The analog feedback voltage sig-
nals can be measured with a calibrated multimeter from the adapter cable connectors
(labelled kVpos, kVneg, mApos and mAneg). An oscilloscope is required if kV and mA wave-
forms need to be observed, for an example when determining the exposure time.
The feedback signals are differential, so measuring only one polarity signal (e.g. kVpos
with respect to the X-ray units ground potential) will give false results. The feedback
signals must always be measured differentially, kV feedback voltage = (kVpos – kVneg)
and mA feedback voltage = (mApos – mAneg).
Chapter D - PREVENTIVE MAINTENANCE
ProMax X-ray unit with Dimax3 D-3