• From the unit's feedback signals
Both methods are presented here.
NOTE
Not all tests can be performed both ways. Radiation is emitted during all
these tests. Proper protective measures against unnecessary exposure to
radiation must be observed.
4.5.1 Non-invasive testing
This method can be used for checking the kVp, radiation quality (half-value
layer) and the exposure time.
Peak tube potential (kVp) measurement
When a radiation measurement device is used for kVp measurement, the
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 sure about it.
2. The sensor distance from the focal spot should be as short as possible
to maximise the signal / 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/correction 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
Measure the air kerma rate again and check that the air kerma rate with
additional 2.5mm Al is more than one half of the one measured without the
added material. That is:
(Dose rate with added 3 mm Al eq filtration) / (Dose rate without added
filtration) > 0.5
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.
Exposure time measurement
Attach the radiation measurement device to the front panel of the sensor and
make sure its whole area is in the radiation field. The measured exposure
time must be within ±10% of the exposure time displayed in the user
interface.
4.5.2 Direct-from-unit-signals testing
NOTE
The manufacturer does not require this testing. Only perform the test 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
4 Annual maintenance
54 Planmeca Viso Technical manual
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