Application of Dual-Energy CT Virtual Monochromatic Images Combined with the iMAR Algorithm in Metal Artifact Reduction and Radiotherapy Dose Calculation

Objective To evaluate the effectiveness of dual-energy CT (DECT) derived virtual monochromatic images (VMI) combined with an iterative metal artifact reduction (iMAR) algorithm in correcting metal artifacts,and to assess its accuracy in radiotherapy dose calculation, thereby providing evidence for clinical application.Methods An intensity-modulated radiotherapy phantom with three sets of titanium alloy rods inserted to simulate patients with metal implants was used. It underwent a conventional CT scan (120 kV) and a DECT scan (80 kV/Sn140 kV). VMI sequences at energies from 40 to 180 keV (at 20 keV intervals) were reconstructed from the DECT data. Both conventional CT images and VMI at each energy level were reconstructed with and without iMAR algorithm. The artifact index (AI) was analyzed for all images before and after iMAR processing. A radiotherapy plan was generated on the conventional CT images following density override in the metal artifact regions, which served as the reference plan. This reference plan was then copied to all other image sets for dose recalculation. Dosimetric parameters for both the planning target volume (PTV) and the metal artifact-affected regions were compared with those of the reference plan. Three-dimensional gamma pass rate analysis was performed using dedicated dose analysis software.Results The AI decreased with increasing VMI energy levels. A further significant reduction in AI was achieved when combined with iMAR processing, with the most substantial reduction (>70%) observed for low-energy VMIs (40-60 keV). A statistically significant difference in AI was found between images with and without iMAR processing across all energy levels (p=0.008). Dosimetric analysis showed that plans based on medium- to high-energy VMI images (80-140 keV) had PTV dose parameters differing from the reference plan by within 0.3%. In contrast, even with iMAR correction, plans based on low-energy VMIs (e.g., 40 keV) showed significant deviations from the reference values. For the metal artifact evaluation region, the mean dose differences for plans based on 80–180 keV VMIs were all within 0.5%.The gamma pass rates (2%/2mm criteria) were all above 95%, with rates for plans based on 80-140 keV images generally exceeding 97%.Conclusion The combination of DECT-VMI and iMAR technology can effectively reduce CT metal artifacts. Medium-to high-energy VMIs (80-140 keV) provides high accuracy for dose calculation, and its combination with iMAR further improves image quality. Therefore, this approach is recommended as the preferred imaging protocol for radiotherapy dose calculation in patients with metal implants.