high voltage cables contributes to the actual output of
each exposure (Reference #21).
While this contribution
to the measured mR/mAs is less than 5-10% for tube
currents above 200
mA
and exposure time settings greater
than 0.05 seconds, exposure measurements obtained with
tube currents less than 25
mA
and exposure time settings
less than 0.005 seconds can be two to three times higher
than the predicted values in the last column of Table III
(Reference #21). Due to the influence of timer setting
and tube current on expected radiation output,
the same
technique settings should be used from one survey to the
next to track the consistency
of radiation output over
the lifetime of the tube.
I-7-D. Instrumentation
A good quality ionization chamber and electrometer
capable of integrating the collected charge is necessary
for
HVL
and
output
measurements. Since
both
the
intensity and effective energy of the x-ray beam changes
as a function of added filtration, the ion chamber should
have a relatively constant energy response between 10-120
keV.
The collection efficiency should be 95% or greater
for the peak x-ray intensities
to be measured. In
addition, no linear dimension of the sensitive volume of
the chamber should exceed 6 cm.
This allows the x-ray
field cross section to approximate good geometry during
HVL
measurements.
I-8 Radiographic Technique Chart
Verify the availability of a radiographic technique
chart.
The chart should
contain
the
following
information:
(a) Exam and projection,
(b) Radiographic technique factors as a
function of anatomical size,
(c) Type and size of image receptor,
(d) SID, and
(e) Type and placement of
gonadal
shielding.
I-9 Identification of the Person Conducting the
Evaluation
The report should contain a section that identifies
the radiologic physicist conducting the survey,
his/her
title, and any appropriate professional certifications.
I-10. Recommendations and Suggestions
A section
which lists identified non-compliance
17
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