The surveyor
shielding devices.
should verify the presence and use of
The surveyor should check or monitor
the checking of
protective
apparel. The
shielding
apparel should be examined fluoroscopically. The results
should be documented as required by the JCAH. These
checks should be completed at least annually.
I-5. The Source-to-Image Receptor Distance
Most general radiographic units have a
variable SID
(source-to-image receptor distance).
Head
units often
have a fixed 32" (81 cm) or 36" (91 cm) SID.
Chest units
usually have a fixed SID of 72" (180 cm).
On variable
SID units, the SID indicator should indicate
the distance
between the focal spot and
the film cassette in the
cassette holder in the table or on the wall.
Four basic types
of SID indicators are available.
The first type is interfaced with the telescopic motion
of the x-ray tube hanger assembly;
the SID distance is
displayed on the tube hanger carriage or on the x-ray
tube positioning handle.
The second-type of indicator is
a linear scale or single marks labeled 40" (100 cm), 72"
(180 cm), etc.,
attached to the telescopic support arm.
A magnetic or a mechanical lock/detent is the third type
of indicator. A marking on the overhead tube rail and or
a detent should be provided for all wall cassette
holders.
A tape measure is the fourth type of SID indicator.
It should be available to determine the SID whenever the
film cassette is placed in locations other than the table
or wall cassette holder.
The indicated SID on any and all of the types
employed on a unit should be within 2% of the measured
SID (Reference #l).
The range
of SIDs used should be within the focal
distance range of the grid.
For example, if a wall
cassette holder with a 72" (180 cm) focused grid designed
for chest imaging is used for 40" (100 cm) upright
abdomens, grid cut off occurs which needlessly increases
patient exposure,
I-5-A. Fixed Radiographic Units
To evaluate
any of
the
above SID indicator
accuracies, the location of the focal spot must be known.
On some x-ray tube housings,
the focal spot location is
marked with a dot or a line at the factory.
Thus the SID
indicator accuracy can be
determined by
direct
measurement,
If the focal
spot
location is
triangulation can be used to
locate it.
not marked,
Several simple
5
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