Xiang Li PhD, Presenter: Research grant, General Electric Company

Ehsan Samei PhD, Abstract Co-Author: Advisory Board, Ion Beam Applications, SA Consultant, Siemens AG Research grant, Siemens AG Research grant, General Electric Company Research grant, Carestream Health, Inc

Rainer Raupach PhD, Abstract Co-Author: Employee, Siemens AG

Bernhard Schmidt PhD, Abstract Co-Author: Employee, Siemens AG

Xiaodong Zhou PhD, Abstract Co-Author: Employee, Siemens AG

Cameron Hope Williams BA, Abstract Co-Author: Nothing to Disclose

William Paul Segars PhD, Abstract Co-Author: Nothing to Disclose

Erik K. Paulson MD, Abstract Co-Author: Research Consultant, Siemens AG Stockholder, ZONARE Medical Systems, Inc

Donald P. Frush MD, Abstract Co-Author: Nothing to Disclose

To assess the effects of longitudinal (Z) and combined longitudinal and angular (XYZ) tube current modulation on CTDIvol-to-organ dose, DLP-to-effective dose, and DLP-to-risk index conversion coefficients.

The study included a normal-weight (66 kg), an overweight (93 kg), and an obese (117 kg) patient. Full-body computer models were created from the patients’ clinical CT data. A method was developed to generate Z and XYZ modulation profiles based on patient attenuation, emulating the principle of typical commercial software used clinically (CARE Dose 4D, Siemens, Germany). Using this method, modulation profiles were generated for each patient model for a chest and an abdomen-pelvis exam. A validated Monte Carlo program, modeling a clinical CT system, was used to simulate organ dose under the conditions of no modulation, Z modulation, and XYZ modulation. The two modulation methods were compared against no modulation in terms of CTDIvol-to-organ dose, DLP-to-effective dose (k factor), and DLP-to-risk index (q factor) conversion coefficients.

When using Z modulation in a chest exam, CTDIvol-to-organ dose conversion coefficient decreased 12-24% for lung and 21-40% for breast, but increased 26-101% for thyroid, due to the higher photon flux needed for the shoulder region. The k and q factors decreased 9-19% and 6-15%, respectively. Similar percent changes were found when using XYZ modulation, with the exception that the conversion coefficient for thyroid dose decreased 3-11%. When using Z modulation in an abdomen-pelvis exam, CTDIvol-to-organ dose conversion coefficient decreased 2-6% for colon and 5-21% for stomach, but increased 25-43% for testes. The k and q factors decreased 0-7% and 0-2%, respectively. Similar percent changes were found for XYZ modulation, with the exception that the conversion coefficient for testes dose changed by -4% to 6%. The changes in conversion coefficients were generally smaller for larger patients.

The effect of tube current modulation on dose and risk conversion coefficients is the largest for individual organs (< ~100%), smaller for effective dose (< ~20%), and smallest for risk index (< ~15%). The effect is larger for a chest exam than for an abdomen-pelvis exam and generally smaller for larger patients.

Accurate dose and risk estimation in CT requires conversion coefficients that take into account the effects of tube current modulation.

Li, X, Samei, E, Raupach, R, Schmidt, B, Zhou, X, Williams, C, Segars, W, Paulson, E, Frush, D, The Effect of Tube Current Modulation on Dose and Risk Conversion Coefficients in CT.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11008378.html