7) Compare the measured HVL against "Table I-
Minimum
HVL
Requirement" to determine
compliance.
Test Method II:
Units With No Manual Timing
Place the unit in Automatic Exposure
1)
2)
Control (AEC).
3)
4)
5)
6)
8)
10)
11)
12)
Place the ionization chamber a few inches
away from table top or wall cassette holder
to reduce backscatter. The chamber should be
positioned over the center of the AEC sensor.
Collimate the x-ray field to a size slightly
larger than the larger of the ionization
chamber or AEC sensor.
Select 80
kVp
or the closest available
kV
setting.
Achieve an exposure in the 10-20 mAs range
by placing an-appropriate thickness of
Aluminum or Copper attenuator in front of
AEC sensors, but behind the ionization chamber.
Place an additional 4-5 mm Aluminum
(1100 alloy) in front of the AEC sensors at the
location chosen in step #5.
7) Record the exposure for this 0 mm data point.
Remove 1 mm of the 1100 alloy Aluminum from
behind the ionization chamber and place it in
front of the chamber near the face of the
collimator.
Record the exposure.
Repeat steps 8-9 with additional thicknesses
of Aluminum removed from behind the chamber
and placed in front of it.
Use interpolation to estimate the HVL.
Compare the measured HVL against Table I
minimum HVL's to determine compliance.
I-7-B. Simplified Filtration Test
If one is interested only in compliance and not in
an accurate measurement of the HVL,
a two
exposure
technique can be employed.
In step (5) of Test Method I, Section I-7-A, use
Table 2 "Half-value layers as a function of filtration
and tube potential for diagnostic units",
Reference #8 to determine
on page 17 in
the selected tube potential.
the minimum required HVL for
Place this thickness of
Aluminum in the beam.
If the output of the unit is
greater than one half of the output measured without any
additional filters in the radiation beam,
the minimum
filtration requirement is met. If the exposure is less
than half the
zero filter
measurement, additional
14
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