these interchangeable cones. or
apertures
shall have
permanent labels which indicate the image receptor size
and SID for
which the beam restrictor is
designed
(Reference #1).
III-6. HVL Measurement
Measurement of
radiation beam
quality on
Cephalometric and Intraoral units is a straight forward
process.
The comments of Sections I-7, I-7-A, and I-7-B
apply.
Special care must be used to make HVL measurements on
Panoramic units. These x-ray units have long exposure
times.
In general, one must
wait 5 minutes between
Panoramic exposures of 15-25 seconds to ensure that the
heat load of the tube's anode is not exceeded. This heat
load can be minimized by making exposure rate measurements
and terminating the exposure-
immediately after
the
electrometer reading stabilizes.
IN EMPLOYING
EITHER
METHOD, CARE SHOULD BE EXERCISED AND THE MANUFACTURER'S
TUBE RATING CHARTS SHOULD BE CONSULTED TO INSURE THAT TUBE
LOADING LIMITATIONS ARE NOT EXCEEDED.
One desires stationary geometry of the Panoramic unit
when making HVL measurements.
Depending on
the
manufacturer, the rotating
motion
of the panoramic unit
can be disabled by removing the appropriate fuse. The
recommended method for this
is usually
noted in the
installation manual.
This allows
one to position the
ionization chamber on a stand in front of the post patient
slit aperture.
One more
geometry
concern for HVL measurements is
presented by the slit aperture of the x-ray tube cone.
The slit aperture produces a typical x-ray field size of
13 cm x 0.6 cm (5" x 0.25").
The 0.6 cm dimension results
in approximately the same partial coverage of the typical
ionization chamber regardless
of the thickness
of test
filter in front
of the chamber.
While each measured
exposure is too
small,
the same test filter thickness is
required to reduce the measured value
to one half of its
measured zero filter value.
Therefore,
provided
the
position of the
ionization chamber
"within"
the x-ray
field does not change,
a sufficiently accurate measurement
should result.
Half value layer measurements should be completed at
least annually or
after replacement of
x-ray
tube,
generator recalibration,
or filter thickness change.
III-7. Patient Exposure
Patient exposure is
greatly influenced by film
processing conditions.
The Dental Exposure Normalization
Technique (DENT) program, an ongoing study by the Center
41
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