A novel approach is proposed to improve low-dose CT image quality by a non-local weighs prior which is acquired from previous normal-dose scan.First,we align two images—the low-dose and the previous normal-dose scan image,and filter the low-dose image to suppress noise.Then,the weight matrix of the registered normal-dose image is calculated by the non-local means weighting formula,and the low-dose image is restorated using this weight matrix as a prior.Simulated phantom and perfusion data experiment results demonstrate a highly quality improvement in terms of suppressing noise-induced streak artifacts and preserving resolution of the low-dose image using the proposed method.The proposed method is especially effective for clinical short-term repeated CT scan,such as perfusion and radiation therapy.