on the x-ray tube assembly.
X-ray Field/Numerical Indicator Size:
I-6(c)
Measure the length and width of the x-ray field on
each film.
These dimensions
should
agree with the
numerical values originally set in step 3 above to within
2% of the SID (Reference #l). Most units have more than
one numerical indicator
scale.
Each of these scales
references a specific SID.
The units displayed on the
scales (metric vs English) should match the units used to
size the typically used cassettes. This parameter should
be checked-at least annually or after repairs to the
collimator blade assembly or dial indicator assembly.
Other Test Methods
Hendee and Rossi (Reference #3) and Gray et al.
(Reference #4) have described similar collimator tests.
The X-ray Field/Image Receptor Alignment can also be
evaluated by closing one pair of the collimator blades to
obtain a slit beam and making
an exposure.
Prior to
moving the cassette, a second
exposure is made with the
opposing pair of
collimator blades now open and the
original pair closed.
This double exposure technique
produces a radiographic cross hair which indicates the
center of the x-ray field.
The X-ray/Light Field Congruence of the collimator
can also be measured without exposing x-ray film if film
processing is unavailable.
Four fluorescent strips (2 x
10") with appropriate reticule lines
drawn in the
lengthwise direction can be used. The lines are placed
at the edge of the collimator
light field. This method
is faster than the above method,
but may require an
observer at table side to view the fluorescent emission
during the radiographic exposure.
(Appropriate shielding
should be used.) A misalignment
of l/8" can be detected
depending on how the reticule lines are drawn on the
fluorescent strips. With care one can also directly
measure the distance between the reticule lines to check
the numerical field accuracy.
I-6-D. Positive Beam Limitation (PBL) Collimators
Both PBL and manual collimators must be tested
according to Section
I-6-A-C.
In
addition,
PBL
collimators must confine the radiation beam size to the
size of the image receptor.
The automatic setting of the
x-ray field size should also be accurate for any distance
between the x-ray tube focal spot and cassette which is
within the SID range of the PBL system.
8
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