The relationship between organ dose and patient size in tube current modulated adult thoracic CT scans

Abstract

Recently published AAPM Task Group 204 developed conversion coefficients that use scanner reported CTDI vol to estimate dose to the center of patient undergoing fixed tube current body exam. However, most performed CT exams use TCM to reduce dose to patients. Therefore, the purpose of this study was to investigate the correlation between organ dose and a variety of patient size metrics in adult chest CT scans that use tube current modulation (TCM). Monte Carlo simulations were performed for 32 voxelized models with contoured lungs and glandular breasts tissue, consisting of females and males. These simulations made use of patient's actual TCM data to estimate organ dose. Using image data, different size metrics were calculated, these measurements were all performed on one slice, at the level of patient's nipple. Estimated doses were normalized by scanner-reported CTDI vol and plotted versus different metrics. CTDI vol values were plotted versus different metrics to look at scanner's output versus size. The metrics performed similarly in terms of correlating with organ dose. Looking at each gender separately, for male models normalized lung dose showed a better linear correlation (r 2 =0.91) with effective diameter, while female models showed higher correlation (r 2 =0.59) with the anterior-posterior measurement. There was essentially no correlation observed between size and CTDI vol -normalized breast dose. However, a linear relationship was observed between absolute breast dose and size. Dose to lungs and breasts were consistently higher in females with similar size as males which could be due to shape and composition differences between genders in the thoracic region.

The impact on CT dose of the variability in tube current modulation technology: a theoretical investigation

“…Provided that α value can be determined for a given TCM scan, the relationship between h and α may be used to obtain the organ dose conversion factors appropriate for the TCM scheme used. Alternatively, it was postulated that approximation methods may be used to take advantage of the existing organ dose conversion factors for fixed-mA scans, h FIXED (Khatonabadi et al 2011(Khatonabadi et al , 2013aLi et al 2011a). The accuracy of three approximation methods was evaluated.…”

“…The second method was motivated by recent research (Khatonabadi et al 2011(Khatonabadi et al , 2013a; Li et al 2011a), which suggested that organ dose associated with a TCM scan may be approximated as…”