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Spectral Separation of Clinical Megavoltage Beams with a Multi-Layer Imager (MLI) Design

M Myronakis

M Myronakis1*, J Rottmann1 , Y Hu2 , P Baturin3 , A Wang3 , P Huber4 , R Fueglistaller4 , D Morf4 , J Star-Lack3 , R Berbeco1 , (1) Brigham and Women's Hospital, Boston, MA, (2) Dana Farber Cancer Institute, Boston, MA, (3) Varian Medical Systems, Palo Alto, California, (4) Varian Medical Systems, Baden, Dattwil


WE-F-605-10 (Wednesday, August 2, 2017) 1:45 PM - 3:45 PM Room: 605

Purpose: We hypothesize that material separation can be accomplished in MV imaging using a multi-layer imager (MLI) design. The proposed clinical tasks, bone suppression and fiducial enhancement, can be approximated using common materials like aluminum, copper and gold.

Methods: Energy absorption, Compton scattering and energy separation between MLI components were estimated through Monte Carlo simulations. Material separation was evaluated experimentally using solid water and sheets of aluminum (Al), copper (Cu) and gold (Au) for 2.5MV, 6MV and 6MV flattening filter free (FFF) clinical photon beams. Bone separation was evaluated using a pelvis phantom in clinical beams. Images from each layer were weighted subtracted; the weighting factor (w) was iteratively estimated. A conventional electronic portal imaging device (EPID) and a lead (Pb) plate were used to emulate energy separation in conventional imagers. The optimal w value was determined by minimization of the relative signal difference (ΔSR) and signal-difference-to-noise ratio (SDNR) between material (or bone) and the background.

Results: Energy separation was attributed to beam hardening between components; the average energy separation was between 34 and 47 keV. The phosphor and build-up layers were the main source of scatter. The w values that minimized for Al, Cu and Au were equal to 0.89, 0.76 and 0.64 at 2.5MV. The w value for bone removal using the MLI was equal to 0.90, 1.05 and 1.00 for 2.5MV. The w value that minimized for pelvic imaging with the Pb/EPID was 0.96, 0.99 and 1.00 for 2.5MV. w values were also calculated for 6 and 6MV FFF beams.

Conclusion: MV spectral imaging using the MLI prototype or the conventional EPID with additional filtration was feasible for 2.5MV. Further optimization of the MLI design is required to achieve significant energy separation for 6 MV beams.

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