SSD shall be 10 cm (4") (Reference #L). The distance
between the end of the cone attached to the tube head
assembly and focal
spot usually can be measured with a
tape measure.
If the location of the focal spot is not
indicated or if the tube has been replaced since
the
previous survey, see
Section
I-5-A for
measurement
details.
Federal standards do not
specify
an SSD for
cephalometric or panoramic
units. The
geometrical
requirements of these two units result in SSDs
in excess
of the specifications for intraoral units.
III-4. Source to Image Receptor Distance: Cephalometric
Unit
The measured Source to Image Receptor Distance (SID)
should be within 2%
of the SID indicated
the
cephalometric unit (Reference
#1).
This distance usually
can be measured with a tape measure. The location of the
focal spot, if unmarked,
can be
located using
triangulation (Section I-5-A). Since this SID requirement
does not apply to fixed SID units, it does not apply to
Panoramic units. It is also not applicable to intraoral
units because operationally the SSD is fixed and the SID
is variable due to varying patient anatomy.
III-5. Radiation Beam Restrictors
Beam restriction on a dental unit usually consists of
a circular,
rectangular, or slit diaphragm mounted at the
x-ray tube/cone junction of an intraoral, cephalometric or
panoramic unit, respectively.
Since light localizers are
not used, beam restrictor tests are straight forward. The
method is discussed in III.5.A, B, and C.
These tests
should be completed at least annually or after removal
and/or adjustment of the diaphragm.
III-5-A. Intraoral System
For a unit with an SSD equal to or greater than 18 cm
(7"), the x-ray field size at the plane of the SSD must be
contained within a maximum diameter circle of 7 cm (2.8")
(Reference #l).
If the SSD is less than 18 cm (7"), the
maximum diameter circle must be less than 6 cm (2.4")
(Reference #l).
"Contained" in this context means that no
part of the cross-sectional area of a different shaped x-
ray field (eg. square, rectangle, etc.) falls outside the
boundary of the circle,
is defined as the "edge"
The boundary of the x-ray field
of the field where the exposure
rate is 25% of the maximum exposure rate within the field
(Reference #1).
This parameter is measured by making an exposure on
39
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