Unsupervised Training of Denoisers for Low-Dose CT Reconstruction Without Full-Dose Ground Truth

Recently, deep neural network (DNN) based methods for low-dose CT have been investigated to achieve excellent performance in both image quality and computational speed. However, almost all methods using DNNs for low-dose CT require clean ground truth data with full radiation dose to train the DNNs. In this work, we attempt to train DNNs for low-dose CT reconstructions with reduced tube current by investigating unsupervised training of DNNs for denoising sensor measurements or sinograms without full-dose ground truth images. In other words, our proposed methods allow training of DNNs with only noisy low-dose CT measurements. First, the Poisson Unbiased Risk Estimator (PURE) is investigated to train a DNN for denoising CT measurements, and a method is proposed for reconstructing the CT image using filtered back-projection (FBP) and the DNN trained with PURE. Then, the CT forward model-based Weighted Stein's Unbiased Risk Estimator (WSURE) is proposed to train a DNN for denoising CT sinograms and to subsequently reconstruct the CT image using FBP. Our proposed methods achieve excellent performance in both fast computation and reconstructed image quality, which is more comparable to the results of the DNNs trained with full-dose ground truth data than other state-of-the-art denoising methods such as the BM3D, Deep Image Prior, and Deep Decoder.