(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: evaluate the impact of spectral gap on achieved theoretical prediction accuracy of proton stopping power using imaging-domain decomposition method via dual-energy CT technique when phantom size deviates from calibration phantom.
Methods: geometry of the virtual phantoms was fabricated according to the Gammex RMI 467 phantom, which consists of a cylindrical solid water background of 310 mm diameter with 17 cylindrical inserts of 30 mm diameter. The standard Gammex tissue substitutes were used as the calibration materials. A total of 34 ICRU standard tissue are simulated as testing materials. The size of test phantom was varied from 80% to 120% to represent the volume discrepancy between calibration and test phantoms. The low energy spectra of CT scanner in the study were from 80 to 100 kVp in the increments of 5 keV; while the high energy spectra ranged from 120 to 150 kVp and 1.5 mm Sn filter with increment of 0.1 mm. The CT images were reconstructed via conventional filtered backprojection algorithm (FBP) on noiseless projection data. The image domain method used is adapted from Hünemohr et al. The root-mean-squared-errors (RMSE) of stopping power (SP) of 34 tissues within different size of test phantoms were reported.
Results: For the case of test phantom is 20% smaller than calibration phantom, the spectra pair of 80 kVp and 150 kVp with 0.4 mm Sn filter thickness, produces the lowest of RMSE (0.23%) of SP. While for 20% difference, the largest spectra pair of 80 and 150 kVp with 1.5 mm Sn produces lowest RMSE 0.26%.
Conclusion: on size deviations of test phantom, the most accurate estimation of SP is not necessarily yielded from the widest spectra gap. With added tin filter, the most accurate estimation of SP could be as low as 0.2% with moderate dual-energy spectra gap.
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