Maryam Khatonabadi, Presenter: Nothing to Disclose

Di Zhang MS, Abstract Co-Author: Nothing to Disclose

Kelsey Mathieu, Abstract Co-Author: Nothing to Disclose

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

Dianna D. Cody 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

John Demarco, Abstract Co-Author: Nothing to Disclose

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

Donna M. Stevens MS, Abstract Co-Author: Nothing to Disclose

Low kVp imaging is being recommended not only for pediatric patients but also when contrast studies are being performed on adult patients. The purpose of this study is to estimate organ doses resulting from scans at different kVps and to use these to guide the adjustment of mAs at the lower kVp that results in reduced organ dose compared to the original120kVp scan.

A Monte Carlo based simulation package was used to model the GE LightSpeed VCT scanner. Three fixed tube current chest exams with 120, 100 and 80kVp were simulated for 40 voxelized female patient models with contoured lungs and breast tissue (18 adults and 22 pediatric patients ranging from 3 to 17 years old). The resulting organ doses for all kVps were compared for a given mAs. Comparisons were made to identify the ratio of mAs values that would result in the same organ dose for all kVps; these could serve as upper limits when adjusting mAs between kVps. In addition these ratios were compared to CTDIw values obtained at all kVps for both 16cm and 32cm CTDI phantoms.

The ratio of 120 and 80kVp doses for pediatric patients for a given mAs was 3.07 with a standard deviation (std) of 0.11 and 3.41 with a std of 0.16 for lungs and breasts, respectively. In adult models these ratios were 3.08 (std 0.10) and 3.00 (std 0.04) for lungs and breasts, respectively. The ratio of mAs values used to attain equivalent organ doses from 120 and 80kVp was 3.23 and 3.04 for pediatric and adult models, respectively. The ratio of CTDIw values for 120 to 80kVp were 2.91 and 3.06 for 16cm and 32cm phantoms, respectively. These ratios result in a range of values from 2.63 to 3.02 for the power n in (kVp)n ∝ exposure.

Monte Carlo simulation results point out that lowering tube voltage from 120 to 80kVp results in more than 3 times reduction in organ dose for a given mAs. These results also indicate that the mAs value for the 80kVp scan can be up to 3 times higher than the mAs value used for a 120kVp scan without exceeding the dose incurred from the original 120 kVp scan. Comparison of CTDIw values for 16cm and 32cm phantoms resulted in similar ratios for 120 and 80kVp.

This study investigates the dose savings of low kVp imaging and quantitatively compares doses from different kVp settings to guide adjustment of mAs when implementing low kVp imaging in the clinic.

Khatonabadi, M, Zhang, D, Mathieu, K, Turner, A, Cody, D, McNitt-Gray, M, Demarco, J, Cagnon, C, Stevens, D, Organ Dose-based Adjustment of mAs When Using Lower kVp for Adult and Pediatric Chest CT.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11034367.html