Prediction of proton beam range in phantom with metals based on monochromatic energy CT images

The purpose of the study was to evaluate the accuracy of monochromatic energy (MonoE) computed tomography (CT) images reconstructed by spectral CT in predicting the stopping power ratio (SPRw) of materials in the presence of metal. The CIRS062 phantom was scanned three times using spectral CT. In the first scan, a solid water insert was placed at the center of the phantom (CTno metal). In the second scan, the solid water insert was replaced with a titanium alloy femoral head (CTmetal). The metal artifact reduction (MAR) algorithm was used in the last scan (CTmetal+MAR). The MonoE-CT images of 40 keV and 80 keV were reconstructed. Finally, the single-energy CT method (SECT) and the dual-energy CT method (DECT) were used to calculate the SPRw. The mean absolute error (MAE) of the SPRw of the inner layer inserts calculated by the SECT method were 3.19%, 13.88% and 2.71%, corresponding to CTno metal CTmetal and CTmetal+MAR, respectively. For the outer layer inserts, the MAE of SPRw were 3.43%, 5.42% and 2.99%, respectively. Using the DECT method, the MAE of the SPRw of the inner layer inserts was 1.30%, 3.69% and 1.46% and the MAE of the outer layer inserts-was 1.34%, 1.36% and 1.05%. The studies shows that, compared with the SECT method, the accuracy of the DECT method in predicting the SPRw of a material is more robust to the presence of metal. Using the MAR algorithm when performing CT scans can further improve the accuracy of predicting the SPR of materials in the presence of metal.