filtration may be necessary as discussed in Section 1-7-
A.
This abbreviated test method can also be applied to
step #8 in Test Method II in section I-7-A.
I-7-C. Radiation Exposure and Expected Output
After verifying correct filtration of the unit, the
radiation output of
the x-ray unit should be measured at
several tube potentials with
the
tube current stations
routinely employed clinically.
The ionization
chamber
can be placed in air 24"
(61 cm) from the focal spot for
the 40" (100 cm) SID (clinical) geometry. Use tube
potentials of 60, 80, and 100 kVp. These encompass the
most often employed
x-ray
tube potentials in Diagnostic
Radiology except for mammography
and
high
kV
chest
radiography.
If the unit operates only in the Automatic
Exposure Control (AEC) mode,
the mAs can be adjusted by
placing attenuators directly in
front of the AEC
detectors and noting the indicated mAs of each exposure.
These measurements may be used to estimate the patient
exposure when such a need arises.
The expected radiation output in mR/mAs [(1000/kg =
(µC/kg)/mAs] of diagnostic x-ray units with different
types of
kV
waveforms has been measured or calculated
(References 9, 21, 22, 23, and 24).
These expected
values are listed in Table III. The final column of
values applies to
three phase units, constant potential
generators, mid frequency units,
and battery
inverter
mobile units.
All the output data assumes
a total
filtration of 2.5
mm
Al and a 24-inch distance between
the ionization chamber
and x-ray
source.
The output
values in each column can be mathematically predicted by:
Output = C x kVp
n
[1]
(Reference #24) using the fitted values for the constants
n and C listed in Table III.
If the total filtration
exceeds 2.5 mm Al, the expected output values in Table
III should be reduced.
For example,
at 80 kVp, 15% or
10% reductions to
the listed values, single or
three
phase respectively, should be applied for every half
millimeter of Aluminum filtration
added (Reference #24).
Technically, radiation output (mR/mAs) is a function of:
(a) High Voltage (kVp) calibration,
(b) Tube current (mA) calibration,
(c) Exposure timer accuracy,
(d) The total filtration in the x-ray tube housing-
collimator assembly,
(e) The electrical phase,
method of rectification,
and capacitance of high tension cables,
(f) The distance from the focal spot, and
15
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