Determinants and expected differences in the calculation of CT effective dose conversion factors for various phantom sizes and scan parameters.
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BACKGROUND: Estimating effective dose (E) for CT is often done with dose-length product (DLP) and effective dose conversion factors, a.k.a. k-factors. Previous studies have primarily focused on k-factor values derived from certain fixed-sized phantoms but have not extensively explored all the contributing factors and associated variations in E. PURPOSE: In this study, we investigated the contributing factors and variations of E using k-factors as surrogates in adult and pediatric CT exams and provided empirical formula to estimate a comprehensive new set of k-factors. METHODS: Effective dose was estimated using NCICT (National Cancer Institute, USA), including organ and effective dose conversion coefficient library estimated by Monte-Carlo radiation transport techniques. The k-factors were derived as a surrogate of DLP-normalized E, via E/DLP (unit: mSv/mGy·cm). We examined the variations of effective dose estimates for different phantom and scan parameters represented by the calculations of k-factors. This included variations in patient sex, weight, height, kVp, and scan length across typical adult and pediatric CT examinations of anatomical regions such as the head, neck, chest, abdomen, and pelvis. Multivariable regression analyses with/without standardization were conducted to evaluate the significance of the dependency of k-factors on the studied parameters. Tube current modulation (TCM) parameters, modeled as a function of patient attenuation using variable modulation strengths, were also incorporated to reflect realistic clinical CT dose variations. RESULTS: The variations of DLP-normalized E were substantially affected by multiple factors, exceeding 100% for adults (weight for adult pelvis) and more than 200% for pediatrics (height for pediatric head). The analysis of standardized regression coefficients revealed that patient weight is the most critical factor, except for head scans in adults, where scan length and patient height are the main determinants. In pediatric CT, both patient weight and height had critical impact. Similarly, for pediatric head CT scans, patient height and scan length are the dominant factors, as observed in adults. Consequently, the k-factors have a wide range, from 0.0099 to 0.0357 mSv/mGy·cm for adult chest and from 0.0085 to 0.0363 mSv/mGy·cm for adult abdomen. Empirical formulas to estimate k-factors were obtained based on multi-variable regression with adjusted R square ranging from 0.71 to 0.93. Based on NCICT's TCM model, the use of TCM resulted in an additional average variation of 110% in the k-factor compared to non-TCM results. CONCLUSIONS: Our study demonstrated the large variations of DLP-normalized effective dose. Standardized coefficients revealed that patient weight, height and/or scan length were the most crucial determinants. Depending on the specific TCM model used, even greater variations may be observed. This work also provided a practical framework for estimating CT effective dose using the traditional k-factor method with updated new conversion factors.