Fluke Biomedical Victoreen 07-494, Руководство оператора

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Theory and Applications
Applications
3
3-5
Table 3-4. kVp vs. Time Study at 300 mA
mA T (sec) ISF X FDD
MAX
FDD (Inches) Measured kVp
300 0.500 1.000 36.0
300 0.250 0.707 25.5
300 0.125 0.500 18.0
300 0.067 0.365 13.1
mAs Reciprocity Corrections
A mAs reciprocity examination of an x-ray system involves variation of the mA or time or both
simultaneously. An ionization chamber (RAD-CHECK Plus, Model 06-526) is used to measure the output
of the machine and mR values are then calculated. The mR values should be proportional to the mAs
quantity. Normally the kV is held constant on the generator.
As discussed above, the kV may not be a constant function of mA and time if there is a malfunction in the
machine.
When mAs reciprocity is being tested, noted variations in the mR value may be due to kV fluctuations as
well as mA and timer nonlinearities. A simple correction for mAs reciprocity fluctuations due to kV
changes can be made if the kV fluctuations are known. If the percent change in kV is small, the mR
output will change as the square function of the kVp. That is, the percent change in mR output is double
the percent change in kV over a small percentage range (Table 3-5).
Table 3-5. mAs Reciprocity
Measured kVp mA Time mAs mR mR/mAs
80 100 0.1 s 10 160 16.0
82 300 0.1 s 30 525 17.5
In Table 3-5 there is a 9.4% increase in mR/mAs values. There is a 2.5% increase in kVp value which
would contribute to an approximate 5% increase in mR output. The 94% non-linearity in mAs reduces to
9.4% -5% = 4.4% which is acceptable.
From this analysis, it is evident that the mA/kV compensation needs correction, whereas, without this
analysis, the mA or time looks suspect. If the changes in kV and mAs are in the opposite direction, the
mAs reciprocity results from the mR values may be very nearly linear and a simple reciprocity check
would indicate that the machine seems to be functioning properly. However, the use of the kVp Meter
would indicate the kV error and possibly an error in the mAs which was being cancelled out by the kV
error.
Waveform Diagnostics
If the mA is held strictly constant, and filtration does not change through the exposure, then variations in
the radiation waveform can be interpreted as variations in the kV waveform. It is difficult to reduce the kV
waveform from a single detector waveform even if the mA is held strictly constant. Relative judgments can
be made from the radiation waveform; however, absolute judgments should be avoided. For example,
the maximum observed point on a radiation waveform corresponds to the maximum kV point in the
waveform, but it may also be closely aligned to a maximum in the mA waveform.
The radiation waveform can exhibit symptoms of one or more type of malfunctions in an x-ray system as
listed in to Table 3-6.
Table 3-6. Waveform Diagnostics