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Generating K-Factors to Estimate Effective Dose for CT Exams That Account for Patient Size and Tube Current Modulation
A Hardy1*, K McMillan1 , M Bostani1 , M Zankl2 , C McCollough3 , C Cagnon1 , M McNitt-Gray1 , (1) University of California, Los Angeles, Los Angeles, CA, (2) Helmholtz Zentrum Munchen, Neuherberg, Bavaria, (3) Mayo Clinic, Rochester, MN
Presentations
TH-EF-601-7 (Thursday, August 3, 2017) 1:00 PM - 3:00 PM Room: 601
Purpose: The purpose of this work was to determine conversion coefficients (i.e. k-factors) that could be used to estimate size-specific effective dose (SSED) from body CT exams using tube current modulation (TCM) with water equivalent diameter (Dw) as the patient size metric.
Methods: Exponential models were developed and tested that expressed SSED normalized by dose-length product (i.e. k-factors) as a function of patient size, using Dw. MC simulation was used to model CT scans performed with TCM for chest, abdomen, abdomen/pelvis, and chest/abdomen/pelvis exams for eight different sizes of voxelized GSF patient models with all radiosensitive organs identified and a validated MC model of a 128-slice CT scanner. For each region and patient, TCM schemes were generated using a validated method that accounts for patient attenuation and scanner tube current limitations. Organ doses were estimated and effective doses were calculated using ICRP 103 tissue weighting factors. Effective doses were normalized by scan-specific DLP based upon 32 cm CTDIvol values to obtain size-specific, scanner-independent k-factors. Dw was determined for each region by estimating the attenuation at the center of the scan volume and DLP-normalized SSED were plotted against Dw. Exponential models were used to fit the data and generate size-specific k-factors for each protocol. These factors were evaluated for the ICRP 110 voxelized female (Regina) and male (Rex) patient models by comparing SSED from the derived k-factors to the effective dose estimates from MC-simulated TCM exams.
Results: The R² values for the abdomen, abdomen/pelvis, chest, and chest/abdomen/pelvis protocols were 0.87, 0.78, 0.84, and 0.86, respectively. When tested with Regina and Rex, errors for abdomen were 44.7% and 10.2%, 6.7% and 3.1% for A/P, -3.2% and -3.8% for chest, and 26.9% and 22.9% for CAP.
Conclusion: SSED may be reasonably estimated for TCM CT exams using DLP, Dw, and the derived k-factors.
Funding Support, Disclosures, and Conflict of Interest: Dr. Cynthia McCullough receives research support from Siemens Healthcare. National Institute of Biomedical Imaging and Bioengineering of the National Institutes of Health under award number R01EB017095. Dr. Michael McNitt-Gray's department has a master research agreement with Siemens Healthcare
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