an appropriate image receptor which is larger than the x-
ray field placed at the distal end of the cone.
The image
receptor can be film, fluorescent screen, etc.
III-5-B. Panoramic System
The rectangular slit within the cone mounted on the
x-ray tube assembly should restrict the size
of the x-ray
beam to a size
smaller than the slit opening on the image
receptor support housing. The alignment of the x-ray tube
assembly with respect to the
image
receptor support
housing shall ensure that the entire x-ray beam falls
within the slit opening on the image receptor support
housing.
Vertical beam misalignment results in either top
or bottom cone cut on the film.
Horizontal misalignment
requires increased radiographic technique factors
due to
attenuation of the x-ray beam by the edge of the slit in
front of the image receptor. Both horizontal and vertical
misalignment of the slits results
in unnecessary patient
exposure.
The accuracy of the slit's size and alignment
can be verified by exposing a film (with appropriate
marks) or a fluorescent screen
taped on the slit of the
image receptor support housing.
III-5-C. Cephalometric System
The alignment
of the central ray of the x-ray beam
with respect to the center of the image receptor shall be
within 2 percent of the SID (Reference
#1).
The x-ray
field shall be restricted so that each dimension
does not
exceed the image receptor dimension by more than 2 percent
of the SID (Reference #1). Since the cassette holder
generally permits the image receptor to move horizontally
to compensate for different patient mandibular and cranial
profiles,
the image receptor should be placed in the
center of the
cassette holder when the alignment is
checked.
To verify the alignment and size of the x-ray field,
place a 14" x 17" (35 cm x 43 cm) cassette distal to the
cassette holder.
Measure the source to film distance.
Expose the film.
The field size in the image is scaled to
the field size at the cassette holder.
A shadow image of
the cassette holder should also be present on the film.
This may be used to verify x-ray beam to image receptor
alignment.
III-5-D. Labeling of Cones or Apertures
Cephalometric units may use more than one size of
image receptor.
In this case the system usually uses
removable cones or apertures to restrict the x-ray beam to
meet the federal requirements listed in III-5-C. Each of
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