Fluke Biomedical 07-494 - Manual (Page 15)

3
Theory and Applications
Detector Positioning
3-3
3.6 Detector Positioning
The kVp Meter should be positioned in the center of the beam. The measurement area of the meter is 2
square inches, and is located on the top panel of the unit. Measurements made on other parts of the
beam will result in inaccurate readings due to the fact that the beam spectrum is different for different
parts of the beam. The heel effect is an example of such a beam spectrum change. As an illustration of
the effect of detector positioning, place the kVp meter in a selected part of the beam, make an exposure,
and observe the displayed measurement. Then move the detector to another part of the beam, make
another exposure, and observe the displayed measurement. Notice that the two measurements are
significantly different.
3.7 Low Battery
A low battery indication is displayed when the battery voltage drops below a factory-preset value. The
preset value is high enough that there is not an immediate noticeable effect on the measurement results.
However, prolonged use of the Instrument in the low battery condition will result in decreased accuracy.
3.8 Applications
kVp Accuracy Determinations
With the Digital kVp Meter, it is possible to enter a diagnostic x-ray room and, within one minute,
determine kVp accuracy for several stations of the x-ray machine. As a result, a significant cause of poor
image quality can be immediately diagnosed without affecting patient throughput.
Calculating Generator Loading Effects
In an x-ray generator, when the load (mA) changes, the kV may also change. Often, the generator
circuitry has been designed to compensate for this effect so that there is not a change in kV as different
mA stations are selected. To test for this effect:
1. Determine the kV to be investigated and the maximum mA to be tested.
2. Determine the minimum mAs requirements for the kV from Figure 1-2 (see Section 1.2,
Specifications).
3. Set the distance (FDD) such that the high-test mA and 50 ms (1/20 sec.) correspond to the
minimum mAs requirements.
4. Make an exposure.
NOTE
If a low mAs indication appears on the kVp meter,
set the timer to the next station.
5. Record the value displayed on the kVp meter.
6. Set the mA to the next lower position.
7. Increase the exposure time so that the mAs remains greater than or equal to the value determined
in Step 3.
8. Make an exposure.
9. Record the value displayed on the kVp meter.
10. Compare the values recorded in Step 5 and Step 9 to determine how well the generator
compensates for mA changes.