RESUMO
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 (Dw ) using CT numbers; however, due to time constraints, less accurate methods to estimate Dw using patient geometry measurements are used more widely. In this study we compared the accuracy of Dw values calculated from three different methods across 35 sample scans and compared them to the true Dw . 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 Dw within 20% of the true value. The middle slice attenuation and geometry methods gave estimations within the 20% margin for all 35 samples.