Fluke Biomedical Victoreen 07-494, Руководство оператора

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Theory and Applications
Applications
3
3-5
To eliminate time as a variable, decrease the FDD and mA while holding time constant. Utilizing the
inverse square law:
FDD
NEW
= FDD
MAX
X SQRT (mA
NEW
/mA
MAX
):
where:
FDD
NEW
= FDD to be used with new mA,
FDD
MAX
= FDD used with the maximum mA, and
SQRT = Square root.
kVp vs. Time Studies
During an x-ray exposure, the kVp may vary as a function of time. This can be illustrated by keeping the
mA constant and changing the FDD to vary the measurement time.
The kVp Meter accumulates data until certain internal values are reached, and then it displays the result.
The measurement time is independent of the generator time. For example, if the minimum mAs
requirement for a particular kV position is 50, the mA is set to 500, and the exposure time is set to 1
second, the kVp meter will accumulate data only during the first 100 ms interval (=50 mAs). (Refer to the
discussion in Section 3.5, mAs Requirements.) Nine tenths of the shot is wasted and the displayed result
does not indicate what occurs during the tail end of the exposure. A greater portion of the total exposure
can be included by increasing the mAs requirement by increasing FDD. There is no limit on maximum
exposure time.
To implement this application:
1. Select the kV, the mA, and the longest exposure time to be studied. The product of the mA and
time should correspond to the minimum mAs requirements determined from Figure 1-2 (refer to
Section 1.2, Specification).
2. Set the FDD to correspond to the longest exposure time.
3. Make an exposure and record the results.
4. Decrease the FDD, which, in effect, decreases the exposure time according to the inverse square
law.
ISF = inverse square factor = SORT (TMAX)
where,
T= kVp measurement for the kVp, mA and distance values.
5. Make an exposure and record the results.
For a value of 65 kVp at a maximum FDD of 36 inches, using the low range filter set the minimum
required mAs is 150 (from Figure 1-2).
If TMAX = 2 sec, tube current = (150 mAs/2 s) = 75 mA (Table 3-3). If TMAX = 0.5 sec, tube current =
(150 mAs/0.5 s) = 300 mA (Table 3-4).
Table 3-3. kVp vs. Time Study at 75 mA
mA T (sec) ISF X FDD
MAX
FDD (Inches) Measured kVp
75 2.000 36.0
75 1.000 0.707 25.5
75 0.500 0.500 18.0
75 0.250 0.354 12.7
75 0.1250 0.250 9.0