Kyle McMillan, Presenter: Institutional research agreement, Siemens AG Research support, Siemens AG

Maryam Bostani PhD, Abstract Co-Author: Research support, Siemens AG

Maria Zankl PhD, Abstract Co-Author: Nothing to Disclose

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

To evaluate the impact of patient size, scanner selection and scan start location on brain and lens of the eye dose for contiguous axial head CT examinations.

Using Monte Carlo simulations of contiguous axial scanning for 64-slice multi-detector row CT scanners from four major manufacturers, brain and lens of the eye dose were estimated for eight patient models from the GSF family of voxelized phantoms. Simulations were initially performed with a scan from the top of the C1 lamina through the top of calvarium. Additional simulations were performed with start locations 1 cm, 2 cm, 3 cm and 4 cm inferior to the C1 lamina. CTDIvol-to-organ-dose conversion coefficients were calculated for each combination of patient model, scanner and start location by normalizing brain and lens of the eye dose by scanner-specific 16 cm CTDIvol values. These scanner-specific conversion coefficients were averaged across all scanners and start locations to determine scanner-independent CTDIvol-to-organ-dose conversion coefficients for each patient model. Scanner-independent conversion coefficients were then correlated with patient size, and variation between scanner-specific and scanner-independent conversion coefficients was assessed.

An exponential relationship between scanner-independent CTDIvol-to-organ-dose conversion coefficients and patient size was observed with correlation coefficients of 0.92 and 0.85 for the brain and lens of the eye, respectively. For the lens of the eye, scanner-specific and scanner-independent conversion coefficients for each patient model varied up to 26.1%. For the brain, variation upwards of only 8.9% was observed.

Patient size, scanner selection and scan start location all influence organ dose in contiguous axial head CT examinations. Scan start location causes surface dose variation in a manner similar to tube start angle for helical scanning. This effect can be enhanced by the scanner-specific dose efficiency of beam collimations. Dose to small, superficial organs like the lens of the eye may have pronounced variation due to these start location effects, while brain dose is relatively constant.

For contiguous axial head CT exams of a given patient size, scanner selection and scan start location may have a noticeable impact on lens of the eye dose, while brain dose is relatively constant.

McMillan, K, Bostani, M, Zankl, M, Cagnon, C, Demarco, J, McNitt-Gray, M, Quantifying the Effects of Patient Size, Scanner Selection and Scan Start Location on Organ Dose Estimates in Contiguous Axial Head CT Examinations.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14011027.html