Maryam Khatonabadi, Presenter: Nothing to Disclose

Di Zhang PhD, Abstract Co-Author: Employee, Toshiba Corporation

John Hoffman, 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 Grant, Siemens AG Instructor, Medical Technology Management Institute

The purpose of this study was to investigate the influence of patient centering for the CT planning radiograph on organ dose; exploring two extremes of patient’s mis-centering.

An anthropomorphic thorax phantom was used to acquire 3 scans on a Siemens Sensation 64 scanner. Three topograms were acquired in the Anterior-Posterior (AP) direction at several different relative table height positions: (a) centered in the AP direction; (b) at the highest possible table height (UP), which was 74 mm above center, and (c) at the lowest possible table height (DOWN), which was 86 mm below center. After each topogram, the phantom was scanned at that table height with 120 kVp, CareDose4D on, 250 Quality Reference mAs, pitch of one, and 32x0.6 collimation. The resulting images were used to create voxelized models where lungs and breasts were contoured for use in Monte Carlo simulations. A previously developed Monte Carlo simulation package using MCNPX was utilized to estimate organ doses from each scan, making use of each scans’ detailed tube current modulation function extracted from the raw projection data.

When the topogram was acquired at the highest possible table height, the dose to lungs and breasts increased by 35% and 41%, respectively, compared to values obtained with a centered topogram; in addition the scanner reported CTDIvol increased by about 20%, from 14.02 to 16.81 mGy. When the topogram was acquired at the lowest table height, the dose to lung and breasts decreased by 31% and 23%, respectively, compared to values obtained with a centered topogram. The scanner reported CTDIvol decreased by 18%, reducing from 14.02 to 11.45 mGy.

In this study we have quantified the influence of patient centering in terms of organ dose and also provided an upper and lower boundary which could be scanner-specific. Additionally we have shown that scanner-reported CTDIvol dramatically underestimates the influence of patient centering on organ dose.

In terms of organ dose the influence of patient centering is even higher than the reported CTDIvol. We have shown the maximum achievable dose increase and reduction due to mis-centering.

Khatonabadi, M, Zhang, D, Hoffman, J, Cagnon, C, Demarco, J, McNitt-Gray, M, The Influence of Patient Centering on Organ Dose in Computed Tomography.  Radiological Society of North America 2012 Scientific Assembly and Annual Meeting, November 25 - November 30, 2012 ,Chicago IL. http://archive.rsna.org/2012/12043870.html