Di Zhang MS, Presenter: Nothing to Disclose

Christopher H. Cagnon PhD, Abstract Co-Author: Nothing to Disclose

John DeMarco, Abstract Co-Author: Nothing to Disclose

Cynthia H. McCollough PhD, Abstract Co-Author: Research grant, Siemens AG

Dianna Cody PhD, Abstract Co-Author: Nothing to Disclose

Michael F. McNitt-Gray PhD, Abstract Co-Author: Institutional research agreement, Siemens AG

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

Adam Turner BS, Abstract Co-Author: Nothing to Disclose

Maryam Khatonabadi, Abstract Co-Author: Nothing to Disclose

In brain perfusion studies, the patient’s head is scanned repeatedly at one location over a short period of time to monitor contrast wash in and wash out, which may result in high radiation doses to the skin and the eye lens. This could possibly cause deterministic effects. The purpose of this study is to estimate how well CTDI and TG111 dose metrics predict the radiation dose to skin and eye lens from brain perfusion studies for patients of different sizes.

Skin dose and eye lens dose were estimated using Monte Carlo simulation methods applied to the eight GSF patent models which represent a range of patient sizes and ages. The simulated brain perfusion scans used a Siemens Sensation 64 CT source model at 24 x 1.2mm collimation for all four tube voltage settings. All simulations were performed at the locations where the x-ray primary beam completely covers the eye lens. Meanwhile, CTDIvol and small ion chamber measurements (based on AAPM TaskGroup 111 methods) under each condition were performed to investigate how well each index predicts these doses for each patient model.

Dose to eye lens and skin were estimated using Monte Carlo simulations on a per mAs basis. They were used as a ground truth and compared with predictions from CTDI, as well as predictions from small chamber measurement at 12:00 position of a 16 cm PMMA phantom. The estimation performances were plotted as a function of patient head perimeter. CTDI values overestimate eye lens dose by 10% to 80% and overestimate skin dose by 4% to 49%. Small chamber measurements perform better and it generally predict eye lens dose to within 20%, and predict skin dose to within 10%.

CTDI consistently overestimates dose to eye lens and skin. Small chamber measurements provide values that are closer to the simulated for both eye lens and skin dose. Since smaller patients receive higher dose if the same scanning technique is used, CTDI overestimates less for pediatric patients. Due to the same reason, small chamber measurement can underestimate dose for pediatric patients. Therefore, for adult patients, small chamber measurement is a better estimate. However, for pediatric patients, CTDI is a more conservative estimator.

CT brain perfusion scans sometimes cause deterministic effects such as skin burn or even cataract. This study proposes appropriate dose estimators for patients with different sizes.

Zhang, D, Cagnon, C, DeMarco, J, McCollough, C, Cody, D, McNitt-Gray, M, Zankl, M, Turner, A, Khatonabadi, M, How Do CTDI and TG111 Small Chamber Dose Perform in Estimating Radiation Dose to Eye Lens and Skin from CT Brain Perfusion Examinations for Patients with Various Sizes: A Monte Carlo Study.  Radiological Society of North America 2010 Scientific Assembly and Annual Meeting, November 28 - December 3, 2010 ,Chicago IL. http://archive.rsna.org/2010/9012286.html