for the high
exposure rate
mode.
considered good practice to
However, it is
limit this mode to 10 R/min
(2.6 mC/(kgmin)) unless a specific clinical need has been
identified.
The entrance exposure rate of fluoroscopic equipment
without AERC, shall not exceed 5 R/min (1.3 mC/(kgmin))
during
fluoroscopy
unless
optional high exposure rate
the unit is provided with an
control (Reference #l). If
the unit is provided with this option the comments
about
high exposure rate control in the previous paragraph
apply.
The Federal Standards also specify the location of
the entrance plane of the patient at which the above
maximum exposure limits
apply
(Reference
#l). The
ionization chamber shall be positioned one cm above the
tabletop
or cradle if the x-ray source is installed under
the tabletop.
If the source is installed above the
table, the ionization chamber
shall be positioned 30 cm
(12") above the
tabletop
with the end of the beam-
limiting device positioned as closely as possible to the
ionization chamber.
If the fluoroscope has C-arm type
geometry, the ionization chamber should be positioned 30
cm (12") from the input surface of the image intensifier
assembly.
While the entrance plane for lateral fluoroscopes is
not defined in the
Federal Standard (Reference #l), an
"FDA Compliance Policy
Guide" was issued to
manufacturers, assemblers,
1977.
and field test personnel in
This guide
(Reference
#13)
states that the
ionization chamber shall be located 15 cm (6") from the
center line of the table in the direction of the lateral
x-ray source with the end of the beam limiting device or
spacer positioned as close as possible to the chamber.
Any movable table ton shall be positioned as close as
possible to the lateral x-ray source, with the end of the
beam-limiting device or spacer no closer than 15 cm (6")
to the table top centerline.
II-7-C. Entrance Exposure Rate Measurements
To test compliance, geometrical arrangement of the
ionization chamber, x-ray tube,
and image intensifier
specified in the previous section should be followed. If
it is not practical to do this, the geometry employed
should be as close as possible to that recommended, with
inverse square law corrections applied to
the exposure
rate readings.
Any clinically used grid should be in
position during these measurements.
Measurement of the maximum entrance exposure can
normally be achieved by placing a Lead plate of at least
3.2 mm (l/8") at the face of the image intensifier
32
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