Fluke Biomedical 07-494 - Manual (Page 13)

Theory and Applications
General
3
3-1
Section 3
Theory and Applications
3.1 General
The kVp measurement is computed from a measurement of the linear absorption coefficient (µ) of the
hardened x-ray beam. As the kV increases, µ increases, as discussed in the following paragraphs.
An x-ray beam is composed primarily of two parts - the bremsstrahlung radiation and the characteristic
radiation. Bremsstrahlung radiation predominates below 70 kV. When the beam potential crosses the 70
kV threshold, an apparent beam hardening takes place due to the sudden increase in the emission of
higher energy x-rays. The linear absorption coefficient, µ, increases at continually faster rates until
approximately 90 kV. Above 90 kV, the characteristic radiation becomes less important and contributes
less hardening to the beam. The bremsstrahlung radiation again dominates, and
μ increases at a slower
rate. Use of dual range filters above 90 kVp helps minimize this effect.
3.2 Filtration Effects
A slight change in the beam spectrum being measured results in a change in the µ as described above.
This change may be caused by filtration differences with respect to the calibration beam. With lower
filtration, the x-ray beam is as hard as the calibration beam and the results are lower. With more filtration,
the beam is harder than the calibration beam and the results are higher. Refer to Table 3-1.
Table 3-1. Filtration Effects
Selected Range kVp
Display Increase
(%/mm AI)
50 – 90 kVp
65
80 0.34
0.40
80 – 150 kVp
100
120 0.40
0.45
The results in Table 3-1 were reduced from data taken with a single-phase machine with 4 mm Al
inherent filtration. The values in the display increase column represents the % change in displayed kV for
each added mm Al filtration. The correlation coefficient (r) was above 0.99 for all curves. The filtration
varied from 0 to 8 mm Al.
To correct the display for tube filtration above 4 mm Al, reduce the display by F% where:
F = (%/mm Al) x (Tube Filtration - 4 mm Al).
Use the %/mm Al value closest to the kVp results.
Tube filtration effects below 4 mm Al have not been studied. It is suggested for the best accuracy in kVp
measurements that the tube filtration be increased to 4 mm Al if it is less than that. To test the filtration
present on the tube, measure the HVL of the beam with 80 kVp set on the tube. For 80 kVp and 4 mm Al
filtration, the HVL should be 3.5 mm Al. An HVL less than 3.5 mm Al indicates that the filtration is less
than 4 mm Al.