Comparison of methods to estimate water-equivalent diameter for calculation of patient dose

Modern CT systems seek to evaluate patient-specific dose by converting the CT dose index generated during a procedure to a size-specific dose estimate using conversion factors that are related to patient attenuation properties. The most accurate way to measure patient attenuation is to evaluate a full-field-of-view reconstruction of the whole scan length and calculating the true water-equivalent diameter (D_w ) using CT numbers; however, due to time constraints, less accurate methods to estimate D_w using patient geometry measurements are used more widely. In this study we compared the accuracy of D_w values calculated from three different methods across 35 sample scans and compared them to the true D_w . These three estimation methods were: measurement of patient lateral dimension from a pre-scan localizer radiograph; measurement of the sum of anteroposterior and lateral dimensions from a reconstructed central slice; and using CT numbers from a central slice only. Using the localizer geometry method, 22 out of 35 (62%) samples estimated D_w within 20% of the true value. The middle slice attenuation and geometry methods gave estimations within the 20% margin for all 35 samples.