Impact of dose allocation between high‐ and low‐kV scans on virtual monoenergetic image quality in developmental dual‐energy cone‐beam CT for image‐guided radiotherapy

BACKGROUND: Dual-energy cone-beam CT (DE-CBCT) offers potential as an advanced imaging technique for image-guided radiation therapy (IGRT) through the production of virtual monoenergetic images (VMI). CBCT-VMI can show enhanced image quality, with improved CNR and reduced nonuniformity artifacts, compared to polychromatic images. However, optimized DE-CBCT protocols are required for clinical application. In particular, the impact of the dose allocation between the two acquisition energies on the resulting VMI image quality remains to be assessed. PURPOSE: This study evaluates the impact of different dose allocation strategies between the low- and high-energy scans on the image quality of DE-CBCT derived VMI. METHODS: A series of seven DE-CBCT scans were performed using various dose allocations ranging from 10% Low-energy/90% High-energy (LE/HE) to 70%/30% LE/HE in 10% intervals while maintaining a constant total imaging dose. To assess the impact of the dose allocation strategies, VMI were reconstructed at 40, 60, 80, and 100 keV and evaluated for Hounsfield Unit (HU) accuracy, relative contrast-to-noise ratio (rCNR) enhancement, and HU uniformity compared to a dose-matched clinical CBCT protocol. RESULTS: DE-CBCT protocols between 20%/80% and 70%/30% LE/HE showed significant improvements over the clinical protocol, with CNR values improving by an average of 18%-32% at 60 keV and non-uniformity artifacts reduced to negligible levels without loss of HU accuracy. Statistical comparison between these DE-CBCT protocols further showed that the specific dose allocation had statistically insignificant impact on the resulting VMI image quality between 20%/80% and 50%/50% LE/HE at any energy. Of the remaining DE protocols, 70%/30% showed a significant decrease in HU accuracy while 10%/90%, 60%/40%, and 70%/30% showed reduced rCNR enhancement at some energies. CONCLUSIONS: Overall, a wide range of dose allocations are shown to produce comparably near-optimal VMI quality. This finding supports protocol design flexibility for future clinical DE-CBCT implementations.