Room: AAPM ePoster Library
Purpose: identify the limitations and potential complications in evaluating Exposure Index (EI) values in digital radiography imaging systems when adhering to the standard beam spectrum guidelines of AAPM Task Group Report 116 (TG116).
Methods: TG116 outlines conditions for standardizing beam spectrum for use in evaluating the Exposure Indices of digital radiography systems. For this study, exposure index values were recorded across all available combinations of kVp, half-value layer (HVL), and added filtration which met the conditions of TG116. These conditions state that kVp may range from 66-74 kVp, HVL may range from 6.55mm Al to 7.05mm AL, and filtration may include 0.5mm Cu with up to 4mm of additional Aluminum filtration. The exposure indices were then evaluated on parameters including relationship between EI and exposure, linearity, and the effects of HVL on EI linearity.
Results: EI per exposure was plotted against exposure, where EI per milliroentgen (mR) was compared to the mean. For the DR system used for these measurements, an average EI/mR of 866.8 was obtained with a standard deviation of 62.7 (coefficient of variation 7.2%). For these measurements, where the mean of the sample is taken to be the expectation, 17.18% of recorded EI/mR values fell outside of the +/-10% tolerance (vendor normative). Additionally, the EI/mR is expected to be a constant, as EI should be linear with exposure, however, there appears to be variations in EI linearity as beam qualities and exposures change.
Conclusion: This study shows that the beam spectrum conditions of TG116 may be too broad for consistent evaluation of EI values. The results display some nonlinear characteristic of EI with respect to exposure, as well as potentially unwarranted failures in EI calibration.