19) If any question concerning alignment as a
function of SID exists, steps 13-18 should
be repeated with the tower positioned vertically
at the maximum SID.
Spot Film Field Sizing
20) Repeat steps l-4 above.
21)
22)
23)
24)
25)
26)
27)
Place an unloaded cassette in the spot film
device.
Place the collimator in the automatic mode
and select the full size spot film mode.
Place a ready pack film on top of the test
tool on the table top.
Make three exposures using the technique of
step 17.
Process the film.
density.
Verify the correct
Repeat steps 21-25 for each available
spot filmer format
Repeat steps 20-26 with the tower positioned
vertically at the maximum SID.
The above test method is used when the x-ray tube is
under the tabletop.
The test method, steps l-12 and 20-
27, also assumes automatic collimation is present and is
the routine choice for clinical work.
If this is not the
case,
the unit
should be evaluated in
the
manual
collimator mode.
If the x-ray tube is overhead, (eg.
remote unit) the ready pack
film
must be
placed
underneath the test
tool directly on the
tabletop.
Normally, a mobile fluoroscope can be evaluated with the
x-ray tube below the image intensifier. A stretcher can
be used in this case to provide a tabletop.
II-6-B. Evaluation of Congruence Between X-ray Field/
Field of View of Imaging Chain
Steps l-12 of Section II-6-A allow evaluation of the
congruence between the x-ray field and each field of view
of the imaging chain as a function of the SID.
On the
images,
the inside edges of the radiopaque slides mark
the field of view of the imaging chain.
The darkened
area on the film defines the actual radiation field size
and location.
The center of the radiation field is
determined by drawing diagonals from the corners of the
darkened area.
The center of the test tool marks the
center of the field of view of the imaging chain.
For certified
equipment,
the congruence of the
radiation field size with respect
to the imaging chain's
field of view shall be within an accuracy of 3 percent of
the SID.
The sum, without regard to sign,
of these
differences along any
two
orthogonal dimensions
27
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