Mihailovic, Jelena M

Abstract 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.

AbstractPurposeA dual‐source CT system can be operated in a high‐pitch helical mode to provide a temporal resolution of 66 ms, which reduces motion artifacts in CT pulmonary angiography (CTPA). It can also be operated in a multi‐energy (ME) mode to provide iodine maps, beneficial in the evaluation of pulmonary embolism (PE). No energy‐integrating detector (EID) CT can perform simultaneous ME and high‐pitch acquisition. This phantom study aimed to evaluate the ability of a photon‐counting‐detector (PCD) CT to perform simultaneous high‐pitch and ME imaging for CTPA.MethodsA motion phantom was used to mimic the respiratory motion. Two tubes filled with iodine with intravascular thrombus mimicked by injecting glue within the tubes were placed along with 5, 10, and 15 mg/mL iodine samples, on a motion phantom at 20 and 30 revolutions per minute. Separate high‐pitch and ME EID‐CT scans and a single high‐pitch ME PCD scan were acquired and virtual monoenergetic images and iodine maps reconstructed. Percent thrombus occlusion was measured and compared between static and moving images.ResultsWhen there was motion, EID‐CT ME suffered from significant motion artifacts. The measured iodine concentrations with PCD‐CT in high‐pitch ME were more stable when there was a motion, with a lower standard deviation than EID‐CT in ME mode. The estimated percent thrombus occlusion dropped significantly with applied motion on EID‐CT, while PCD‐CT high‐pitch ME mode showed good agreement between measurements on static or moving images.ConclusionPCD‐CT with combined ME and high‐pitch mode facilitates simultaneous accurate iodine quantification and assessment of intravascular occlusion.