concerning compliance with minimum
filtration
requirements. For example.
if the actual
kV
is lower
than indicated on the operator's console,
the measured
HVL
might not meet the minimum HVL specifications even
though adequate filtration is in the beam.
Likewise, if
the
kV
is higher than indicated, the measured HVL might
appear to be adequate even though the filtration in the
beam is inadequate.
I-7-A. Determination of
HVL
The
HVL
of the x-ray beam should be measured at
least annually, after replacement
of the x-ray
tube
assembly,
or change in the added filter. The measured
HVL
is affected by the amount of scatter radiation at the
ionization chamber (Reference #7). In three phase units
the choice of tube current and exposure time setting also
affect the measured HVL due to the effects of capacitance
of the high
voltage
cables (Reference #21).
These
influences can be minimized by using "good geometry"
(Reference #7), tube currents greater than 200 mA, and
exposure time settings greater than 0.050 sec
(Reference
#21).
Comparison of measured HVLs from survey to survey
are meaningless unless the chosen geometry, tube current,
exposure time settings, and
kVp
remain constant.
Test Method I: Manual Timing
1) Disable the Automatic Exposure Control (ARC)
and operate the unit in the manual mode.
2) Position the ionization chamber (which should
be calibrated for the diagnostic x-ray energy
range) 100 cm from the focal spot. If 100 cm is
not practical, use the largest distance allowed.
The ionization chamber should be free standing,
a few inches away from the table top or wall
cassette holder to minimize back scatter.
3) Collimate the x-ray field to a size slightly
larger than the ionization chamber.
4) Select an appropriate tube potential
(e.g. 80
kVp
for radiographic, 30
kVp
for
mammographic),
a typical clinically used tube
current (eg. 200-400 mA for radiographic, 50-100
mA
for mammographic),
and an exposure time
greater than 50 msec.
Make output measurements
with no additional attenuating material between
the focal spot and the ion chamber.
5) Make additional output measurements with varying
thicknesses of 1100
alloy Aluminum filters
located near the face of the collimator.
6) Use interpolation to estimate the HVL.
13
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