Program Information
Determining Size-Specific Dose Estimates for Head CT Examinations
A Hardy1*, 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-601-4 (Thursday, August 3, 2017) 1:00 PM - 3:00 PM Room: 601
Purpose: Size Specific Dose Estimates (SSDE) conversion factors have been determined by AAPM TG 204 to account for patient size but only for body CT exams. The purpose of this work was to determine SSDE conversion factors for head CT examinations.
Methods: Validated Monte Carlo (MC) simulation methods were used to estimate dose to the center of the scan volume within the brain from a routine head exam for a group of patient models representing a range of ages and sizes. Patients included ten GSF voxelized models with all radiosensitive organs identified. CT scans were simulated using a MDCT source model. Scan parameters were taken from the AAPM routine head protocols for a helical protocol. MC simulations were performed using mesh tallies to produce voxelized dose distributions for the head of each patient. To estimate the dose to the middle of the scan volume, a region of approximately 0.6 cc was positioned at the center of the scan volume within each patient. The dose values within the volumes were averaged and the standard deviations were recorded. Average dose values within the central volumes were then normalized by CTDIvol (16 cm phantom). SSDE conversion factors were determined by relationship between normalized dose and water equivalent diameter (Dw).
Results: For all patients, the doses within the 0.6 cc volumes were homogenous with each model having coefficients of variation below 2.5% for the doses within the central tally region. An exponential relationship between CTDIvol normalized dose within the scan volume and Dw was observed with a coefficient of determination of 0.81.
Conclusion: Conversion factors for head CT exams were determined that represent normalized dose to the center of the scan volume as a function of head size using water equivalent diameter. These may serve as the basis for a SSDE for head exams.
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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