Maryam Bostani PhD, Presenter: Research support, Siemens AG

Kyle McMillan, Abstract Co-Author: Institutional research agreement, Siemens AG Research support, Siemens AG

Christopher H. Cagnon PhD, Abstract Co-Author: Nothing to Disclose

John J. Demarco PhD, Abstract Co-Author: Nothing to Disclose

Michael F. McNitt-Gray PhD, Abstract Co-Author: Institutional research agreement, Siemens AG Research support, Siemens AG

The purpose of this study was to compare several methods of reporting water equivalent diameter (WED) in terms of the ability to correlate with radiation doses to organs from thoracic and abdominal CT exams performed with TCM. 

101 thoracic and 82 abdomen/pelvis scans from clinically indicated CT exams were collected from a 64 slice MDCT (Sensation 64, Siemens Healthcare) with Institutional Review Board approval. All scans were performed with TCM (CareDose4D) and image data were used to create voxelized patient models. Relevant organs for each scan type were segmented and used as tally regions in Monte Carlo simulations for calculating organ doses. Raw projection data were also collected to obtain tube current information for simulating TCM. WED was calculated per image for each patient and reported as: (a) an average WED over the entire length of the exam (WEDglobal), (b) an average WED over the region containing the organ of interest (WEDregional) (e.g. chest excluding shoulder for lung dose) and (c) a single value calculated from only the image in the middle of the scan length (WEDmiddle). Organ doses were normalized by CTDIvol-Regional, CTDIvol weighted by the average effective mAs over the anatomical region of interest, to reflect regional variation of tube current in TCM scans. To compare the different WED metrics, each was correlated with normalized organ doses using linear regression analysis.  

For lung dose, WEDregional and WEDmiddle had stronger correlations with normalized organ dose than WEDglobal : 0.70, 0.70 and 0.51, respectively. For abdominal organs, all three methods had similar correlations (0.85 and above) with normalized organ dose. 

Due to homogeneity in the abdominal region, all three methods of reporting WED resulted in similar and reasonable correlations with normalized organ dose. On the contrary, due to thoracic heterogeneous characteristics, WEDglobal performed poorly (R2=0.51) compared to the other two methods. While both WEDregional and WEDmiddle performed similarly in chest, it is worth noting that WEDmiddle can be less reliable due to scan length variations among thoracic CT examinations.  

WEDregional and WEDmiddle were shown to be robust metrics of patient size and can be used, with a regional measure of scanner output, to estimate organ doses from thoracic and abd/pel CT exams.    

Bostani, M, McMillan, K, Cagnon, C, Demarco, J, McNitt-Gray, M, A Comparison of Methods for Reporting Water Equivalent Diameter (WED) to Predict Organ Dose from Tube Current Modulated (TCM) Thoracic and Abdominal CT Examinations .  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14014055.html