A suitable procedure of dose reduction factor measurements of X-ray shields during computed tomography examination - The importance of considering positional changes of an X-ray tube

Abstract

During computed tomography (CT) examinations, radio-sensitive organs located outside the field of view (FOV) are usually exposed to radiation caused by both direct and scattered X-rays. Traditionally, the use of radiation protection products has been an option for reducing exposure doses. The catalog value of the dose reduction factor (DRF) for a commercial X-ray shield is generally determined by the manufacturer using a plain X-ray irradiation system to measure the difference in dose with and without the shield. In contrast, actual shielding ability measurements during clinical CT examinations are usually evaluated using a similar equation, but it is not guaranteed that the incident directions of X-rays will be the same between the measurement data with and without a shield. The purposes of this study are to introduce a novel method for evaluating DRF by accounting for the influence of the X-ray incident direction in helical scanning and to obtain knowledge about the correctness of previously reported DRF values. Experiments pertaining to chest CT examinations were performed using a human body phantom and small-type dosimeters which consist of an optically stimulated luminescence element using Al2O3:C. A total of 100 examinations were iteratively performed, both with and without a shield. The corresponding DRF was then calculated. The novelty of the proposed analysis procedure lies in a paired analysis method, where the DRF is calculated by reorganizing numerous acquired data sets with and without the shield to pair similar incident angles of X-rays. As a result, the DRF values were estimated to be 48.5 ± 4.3%, 48.7 ± 2.7%, and 48.9 ± 1.5% based on the data results within the 10–40 data sets, 40–70 data sets, and 70–100 data sets, respectively. When the DRF values were obtained by the conventional method without applying our sorting procedure, the results were 48.4 ± 8.1%, even when the 70–100 data sets were used. This result indicates that the proposed method can analyze DRF with higher accuracy. In the example of thyroid X-ray shield, while there was no significant difference in the most probable values of DRF between the conventional and proposed methods coincidentally, we believe it is important to evaluate DRF based on a scientifically correct approach because the mathematical formulas for calculating these two values differ. In conclusion, when calculating the DRF of shielding materials during CT examination, it is imperative that data analysis considers the significant impact of differences in incident X-ray directions on the measured exposure dose data.