Ionization chambers for measuring air kerma integrated over beam area. Deviations in calibration values using simplified calibration methods

Abstract

Calibrations of kerma-area product meters (KAP meters) are often performed using simplified methods. The accuracy thus obtained can be insufficient, especially when the KAP meters are used for optimizing radiological procedures. The deviations between the best available calibration factor (k) and the simplified calibration factor (k s ) were measured at different clinical x-ray installations. Depending on the type of x-ray installation and calibration method, the quotient k s /k ranged from 0.83 to 1.19, reflecting the error made in practice using these methods. A simple alternative calibration method based on comparison with a KAP meter calibrated by the best available method is described. Depending on tube potential and the stability of the electrometers, the uncertainty in the calibration factor derived with this method was between 3.8% and 5.6% (at 95% confidence level).

Effect of fluoroscopic X‐ray beam spectrum on air‐kerma measurement accuracy: implications for establishing correction coefficients on interventional fluoroscopes with KAP meters

“…For the Philips systems, the CCs decreased with increasing beam quality (similar to previous reports), (8) whereas for the Siemens systems, the CCs increased with increasing beam quality. Both the Philips and Siemens systems exhibited a convergence of the CCs at a given beam quality, something not reported by others who have investigated standalone KAP meter responses 5 , 6 , 7 , 8 . These results indicate that the fluoroscope vendors are likely normalizing or otherwise influencing the KAP meter output data.…”

“…The accuracy and variability of stand‐alone air kerma‐area product (KAP) meters have been investigated by various groups since these meters came into common use a couple of decades ago 5 , 6 , 7 , 8 . Toroi et al (8) investigated the response of various standalone KAP meters over a range of X‐ray beam qualities, varying both the kVp and filtration.…”

Evaluation of patient dose during fluoroscopically guided cervical spinal fusion

“…In this case, in-situ calibration is the only method available. In situ calibration is needed, in order to manage suitably the heterogeneous distribution of air kerma in the beam caused by the heel effect and extrafocal radiation [314,316,372,373]. Procedures for in situ calibration of the KAP meter are described in Appendix C.…”

“…Each system may involve different screws, hooks, plates, and rods to stabilize the spine. KAP reference values are usually determined by approximating the surface integral by the product of the nominal area A of the x-ray field and the air kerma measured in the centre of the field (beam area method) [277,296,316,368,373]. However, non-uniformity of the Xray field may cause inaccuracy in this method.…”

“…However, non-uniformity of the Xray field may cause inaccuracy in this method. Other important sources of uncertainty are the measurements of field size and the location of the planes of air kerma and field size determination [316,373,525,545]. The dependence of calibration coefficients on the nonuniformities of the x-ray beam results in problems for in situ calibrations when using the beam area method at clinical x-ray systems.…”