IV. Measurement of Area Radiation Levels
IV-l. Introduction
Parts I,
II, and III, have concentrated on equipment
related radiation safety matters.
In addition to these
concerns,
a radiation survey
of the facility should be
performed.
The following three types of radiation levels
in the vicinity of the machine may be of interest:
(a) Scattered radiation inside the examination room,
(b) Stray radiation outside the examination room,
(c) Leakage radiation from the x-ray tube housing.
Measurement of scattered
radiation
inside the
examination room should be
checked
annually.
The
measurement of radiation levels outside the procedure room
is necessary prior to first clinical use and following
room modifications.
IV-2. Leakage Radiation
All certified diagnostic
medical
and
dental
diagnostic source assemblies
must
meet
the
Federal
Standard for the diagnostic source assembly (Reference
Cl).
This standard results from the definition of a
"diagnostic-type protective tube housing"
by the NCRP
(Reference #2):
"An x-ray diagnostic source assembly must
be so constructed and assembled that the
leakage radiation measured at a distance of 1
meter from the source does not exceed 100 mR (25.8
µC/kg) in 1 hour when the tube is
operated at its
maximum continuous rated current for the maximum
rated tube potential."
As stated in NCRP report No. 33 (Reference #2), "in
general, modern diagnostic
tube housings
incorporate
sufficient attenuating material to
radiation to that permitted in
the
limit the leakage
definition of a
diagnostic-type protective housing
and it is probably
unnecessary to perform leakage tests in the field on
modern x-ray machines".
Unless one suspects a faulty x-
ray tube housing is causing excessive leakage radiation as
a result of the housing's age, appearance, history, high
measured radiation levels inside the room as noted in
Section IV-3, or
unless it
is required by
State
Regulations, this survey is generally not necessary.
In general,
leakage radiation measurement conditions
cannot be precisely met and various
technical problems
must be overcome.
Examples are:
(a) The geometry of "1 meter from the source"
vs.
actual physical limitations in the
examination room,
48
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