Program Information
Evaluation of Rho/Z Images for Proton Relative Stopping Power Calculations
J Roeske1*, R Patel1 , G Coutrakon2 , M Pankuch3 , R Schulte4 , (1) Loyola Univ Medical Center, Maywood, IL, (2) Northern Illinois University, Dekalb, Illinois, (3) CDH Proton Center, Warrenville, IL, (4) Loma Linda University, Loma Linda, CA
Presentations
SU-F-601-3 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 601
Purpose: To evaluate a commercial application that produces atomic number (Z) and electron density images (Rho) from dual energy CT images, and how these may be used to directly calculate relative stopping power (RSP) images for proton dose calculations.
Methods: A customized phantom was designed for this study. Materials with known atomic number and electron density (Gammex RMI, Middleton, WI) were individually inserted into the phantom and scanned with a Somatom Open AS (Siemens Healthcare, Erlangen, Germany), equipped with sequential dual energy (DE) imaging capabilities (80 kVp and 140 kVp). The Rho/Z software application (Siemens) then utilized the DE images to produce individual atomic number and electron density images. These images were subsequently used within the Bethe-Bloch equation to perform a pixel-by-pixel RSP calculation and compared with values calculated from the known material properties.
Results: Based on 11 material inserts, the average difference between RSP values based on known material values and those obtained from the Rho/Z images was -0.13% +/- 1.00%. The largest discrepancy occurred for inserts with lung type properties. In these cases, the differences between the CT numbers between the 80 kVp and 140 kVp images were minimal, resulting in an underestimate of the atomic number. Substitution of the atomic number with a nominal value resulted in closer agreement between the RSP values.
Conclusion: Use of the Rho/Z images obtained directly from the CT scanner results in good agreement with RSP values of most materials. The advantage of this method is that it allows direct calculation of RSP using the Bethe-Bloch equation, independent of the stoichiometric method, and without the need for iterative algorithms to estimate atomic number and electron density. This approach may increase the accuracy of proton dose calculations.
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