Diffusion probabilistic priors for zero‐shot low‐dose CT image denoising

Abstract Background Denoising low‐dose computed tomography (CT) images is a critical task in medical image computing. Supervised deep learning‐based approaches have made significant advancements in this area in recent years. However, these methods typically require pairs of low‐dose and normal‐dose CT images for training, which are challenging to obtain in clinical settings. Existing unsupervised deep learning‐based methods often require training with a large number of low‐dose CT images or rely on specially designed data acquisition processes to obtain training data. Purpose To address these limitations, we propose a novel unsupervised method that only utilizes normal‐dose CT images during training, enabling zero‐shot denoising of low‐dose CT images. Methods Our method leverages the diffusion model, a powerful generative model. We begin by training a cascaded unconditional diffusion model capable of generating high‐quality normal‐dose CT images from low‐resolution to high‐resolution. The cascaded architecture makes the training of high‐resolution diffusion models more feasible. Subsequently, we introduce low‐dose CT images into the reverse process of the diffusion model as likelihood, combined with the priors provided by the diffusion model and iteratively solve multiple maximum a posteriori (MAP) problems to achieve denoising. Additionally, we propose methods to adaptively adjust the coefficients that balance the likelihood and prior in MAP estimations, allowing for adaptation to different noise levels in low‐dose CT images. Results We test our method on low‐dose CT datasets of different regions with varying dose levels. The results demonstrate that our method outperforms the state‐of‐the‐art unsupervised method and surpasses several supervised deep learning‐based methods. Our method achieves PSNR of 45.02 and 35.35 dB on the abdomen CT dataset and the chest CT dataset, respectively, surpassing the best unsupervised algorithm Noise2Sim in the comparative methods by 0.39 and 0.85 dB, respectively. Conclusions We propose a novel low‐dose CT image denoising method based on diffusion model. Our proposed method only requires normal‐dose CT images as training data, greatly alleviating the data scarcity issue faced by most deep learning‐based methods. At the same time, as an unsupervised algorithm, our method achieves very good qualitative and quantitative results. The Codes are available in https://github.com/DeepXuan/Dn‐Dp .