Purpose: Cone beam CT scans at typical kV energies produce metal implant artifacts that may obscure anatomy of interest in IGRT. As a remedy, we propose an artifact-reducing scan technique that incorporates low dose, full field of view projections acquired with a high DQE, multi-layer MV-imager. The supplementary MV data serves two functions: (1) to provide additional data for the final reconstruction (2) to provide an initial low-artifact reconstruction in which metal implants are easily segmented, giving a prior 3D geometric model of the implants.
Methods: The prior geometric model facilitates an adaptation of Joseph & Spitalâ€™s beam hardening technique (1978, J. Comp. Assist. Tomogr., 2:100-108), and is used to remove poly-energetic effects from the projections. Photon-starved kV projections corresponding to long path lengths through metal are also pre-detected using the prior model and inpainted with MV measurements. The corrected MV and kV data are FDK reconstructed and consolidated into a single image via weighted averaging. Performance tests were based on simulated acquisitions of phantoms with titanium implants: a thorax phantom with pedicle screws and a pelvis phantom with a hip replacement. The simulations captured x-ray source spectrum, noise, and detector efficiency characteristics observed in actual scans with the MV-imager prototype, as integrated into the Varian TrueBeam. The simulated dose of the kV and MV scans were each 12.5 mGy, the CTDI of the TrueBeamâ€™s clinical Spotlight mode protocol. The proposed method was compared to images generated from TrueBeamâ€™s simpler water-equivalent beam hardening correction algorithm.
Results: The proposed technique demonstrated artifact suppression superior to the TrueBeamâ€™s usual image reconstruction chain, while nearly preserving soft tissue contrast typical of kV-only set-up CBCT scans.
Conclusion: Supplementing kV-imager projections with measurements from a high efficiency, multi-layer MV-imager promises to improve image guidance in patients with metal implants at modest additional dose.
Funding Support, Disclosures, and Conflict of Interest: NIH/NCI R01CA188446