System information

The general comments
of Section I-7-C on measuring
radiation exposures apply to dental x-ray units. The
ionization chamber and electrometer described in Section
I-7-D are appropriate for these
measurements on
Cephalometric or intraoral units.
Most dental units are
single phase halfwave or full wave rectified units.
The
expected exposures at 24" from the x-ray source as a
function of kV are
listed in Table
III, Section I-7-C.
Due to the geometry of the Panoramic unit, the
patient tissue exposed to
radiation is
continually
changing. Therefore, the
patient entrance
exposure
(mR/mAs) is less than the expected radiation output
(mR/mAs) within the slit beam. The patient
entrance
exposure per film
can
be measured
by placing the
ionization chamber at the location of the patient's skin
(at chin support) and by scanning across it using clinical
kV and mA settings. The manufacturer's tube rating chart
should be consulted to
avoid overheating the tube when
making multiple exposures.
To measure the exposure within the slit x-ray field
(mR/mAs) for comparison with the values in Table III,
section I-7-C, use the
ionization chamber geometry
described in section III-6 and use short exposure times to
collect exposure rate date.
Correct these readings to
mR/mAs values and for error due to partial coverage of the
ionization chamber. The correction factor for partial
coverage error is the ratio of the total volume of the
chamber to the radiated chamber volume.
The actual width
of the x-ray slit field should be known from measurements
in Section III-5.B.
Patient entrance exposures should be measured at
least annually or after replacement of the x-ray tube,
generator recalibration, filter thickness change or a
change in film or development techniques.
III-8. Multiple Tube Configuration
Multiple tube configurations
are
commonly found in
many dental offices and clinics.
One generator may be
installed to power as many as four x-ray tubes. Only one
exposure switch may be present to initiate an exposure
from any one of the x-ray tubes, or each x-ray tube may be
equipped with its own exposure
switch. In such
configurations, the x-ray
tubes must be interlocked to
prevent exposure from more than one x-ray tube at a time.
The x-ray tube selected for the exposure should be clearly
identified (Reference #1).
III-9. Mechanical Support
Certain states require checks of mechanical supports.
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