  During the process of computed tomography (CT), metallic implants often cause
disruptive artifacts in the reconstructed images, impeding accurate diagnosis.
Several supervised deep learning-based approaches have been proposed for
reducing metal artifacts (MAR). However, these methods heavily rely on training
with simulated data, as obtaining paired metal artifact CT and clean CT data in
clinical settings is challenging. This limitation can lead to decreased
performance when applying these methods in clinical practice. Existing
unsupervised MAR methods, whether based on learning or not, typically operate
within a single domain, either in the image domain or the sinogram domain. In
this paper, we propose an unsupervised MAR method based on the diffusion model,
a generative model with a high capacity to represent data distributions.
Specifically, we first train a diffusion model using CT images without metal
artifacts. Subsequently, we iteratively utilize the priors embedded within the
pre-trained diffusion model in both the sinogram and image domains to restore
the degraded portions caused by metal artifacts. This dual-domain processing
empowers our approach to outperform existing unsupervised MAR methods,
including another MAR method based on the diffusion model, which we have
qualitatively and quantitatively validated using synthetic datasets. Moreover,
our method demonstrates superior visual results compared to both supervised and
unsupervised methods on clinical datasets.
